OS1.5 | The North Atlantic: natural variability and global change
EDI
The North Atlantic: natural variability and global change
Co-organized by CL2
Convener: Richard Greatbatch | Co-conveners: Damien Desbruyeres, Caroline Katsman, Monika Rhein, Bablu Sinha
Orals
| Tue, 25 Apr, 14:00–17:55 (CEST)
 
Room L3, Wed, 26 Apr, 08:30–10:10 (CEST), 10:45–12:25 (CEST)
 
Room L3
Posters on site
| Attendance Wed, 26 Apr, 14:00–15:45 (CEST)
 
Hall X5
Posters virtual
| Attendance Wed, 26 Apr, 14:00–15:45 (CEST)
 
vHall CR/OS
Orals |
Tue, 14:00
Wed, 14:00
Wed, 14:00
The North Atlantic exhibits a high level of natural variability from interannual to centennial time scales, making it difficult to extract trends from observational time series. Climate models, however, predict major changes in this region, which in turn will influence sea level and climate, especially in western Europe and North America. In the last decade, several observational projects have been focused on the Atlantic circulation changes, for instance ACSIS, OSNAP, OVIDE, RACE and RAPID, and new projects have started such as CANARI and EPOC. Most of these programs include also a modelling component. Another important issue is the interaction between the atmosphere and the ocean as well as the cryosphere with the ocean, and how this affects the climate.

We welcome contributions from observers and modelers on the following topics:

-- climate relevant processes in the North Atlantic region in the atmosphere, ocean, and cryosphere
-- response of the atmosphere to changes in the North Atlantic
-- atmosphere - ocean coupling in the North Atlantic realm on time scales from years to centuries (observations, theory and coupled GCMs)
-- interpretation of observed variability in the atmosphere and the ocean in the North Atlantic sector
-- comparison of observed and simulated climate variability in the North Atlantic sector and Europe
-- dynamics of the Atlantic meridional overturning circulation
-- variability in the ocean and the atmosphere in the North Atlantic sector on a broad range of time scales
-- changes in adjacent seas related to changes in the North Atlantic
-- role of water mass transformation and circulation changes on anthropogenic carbon and other parameters
-- linkage between the observational records and proxies from the recent past

Orals: Tue, 25 Apr | Room L3

Chairperson: Monika Rhein
14:00–14:05
14:05–14:15
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EGU23-987
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OS1.5
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ECS
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On-site presentation
Dipanjan Dey, Robert Marsh, and Sybren Drijfhout

The Arctic region is warming four times quicker than the global average, a phenomenon known as the Arctic amplification. Some studies suggested that this warming may lead to seasonally ice-free Arctic Ocean by 2050 which will have potentially devastating consequences for Arctic oceanography, marine ecosystems and the Atlantic Meridional Overturning Circulation (AMOC). The relation between the slowdown of the AMOC and the Arctic Ocean is believed to be linked with enhanced freshwater outflow primarily through the Fram Strait which increases the stratification over sites of deep convection in the Irminger Sea. Earlier studies have also confirmed a link between deep water formation and freshwater release from the Arctic. In the current study, our objectives are to understand how and where the Arctic outflow is changing temperature, salinity and density, moving into the North Atlantic, during the historical period and in a warmer future climate. We use the Lagrangian parcel tracing algorithm, TRACMASS, to trace both the southward flows from Fram Strait and North Atlantic flows into the Nordic Sea. The results quantify how and where Arctic outflow increases temperature and salinity, and decreases density, in transit. This is primarily associated with mixing between the cold, fresh outflow and the relatively warmer, saltier Atlantic waters at Denmark Strait, despite some surface cooling in transit from Fram to Denmark Straits that is due to net surface heat loss and sea ice melting.

How to cite: Dey, D., Marsh, R., and Drijfhout, S.: Tracing Arctic outflow through the Fram Strait and its interaction with North Atlantic waters, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-987, https://doi.org/10.5194/egusphere-egu23-987, 2023.

14:15–14:25
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EGU23-1359
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OS1.5
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ECS
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Virtual presentation
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Nora Fried, Caroline A. Katsman, and M. Femke de Jong

The Irminger Current (IC), located over the western flank of the Reykjanes Ridge, is a contributor to the northward volume transport related to the Atlantic Meridional Overturning Circulation.

Previous studies showed that the IC is associated with a region of enhanced eddy kinetic energy. Using high-resolution mooring data from 2014 – 2020 combined with satellite altimetry, a strong intensification in volume transport of the IC in August 2019 could be attributed to the presence of mesoscale eddies in the vicinity of the moorings. At this time, altimetry showed an anticyclone lingering next to a cyclone in the mooring array, which intensified northward velocities within the IC. This example shows that mesoscale variability can directly impact the transport variability of the IC.

Further research presented here uses the high-resolution model POP (Parallel Ocean Program, 1/10°) to investigate the pathways of the IC up- and downstream of the mooring array. Here, the focus lies on determining the origin of waters feeding the IC and the role of mesoscale eddies in shaping the current and its pathways using Lagrangian particle tracking with the Ocean Parcels software. First results from a backtracking experiment reveal different origins for the water masses feeding the respective cores of the IC. Waters of the eastern core mostly originate from the eastern side of the Reykjanes Ridge. The western core appears to contain a substantial amount of waters from the interior Irminger Sea that partly recirculate from the Labrador Sea.

Additionally, we explore the mesoscale variability within the whole eastern Irminger Sea to investigate the potential impact of mesoscale eddies on restratification in the central Irminger Sea.  We focus on identifying the characteristics of variability along the ridge using the output of the Lagrangian particle tracking in POP. First results from a forward experiment show stronger mesoscale activity in the western core than the eastern core, which is in line with available mooring observations.

How to cite: Fried, N., Katsman, C. A., and de Jong, M. F.: A Lagrangian study on the structure and pathways of the Irminger Current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1359, https://doi.org/10.5194/egusphere-egu23-1359, 2023.

14:25–14:35
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EGU23-3170
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OS1.5
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ECS
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On-site presentation
louise abot

We investigated wintertime convection evolution in the past two decades over the Greenland Sea. This area is a major location regarding dense water production and supply of the lower limb of the Atlantic Meridional Overturning Circulation, a key component of the global climate.
Previous studies mentioned an increase in Greenland Sea wintertime convection intensity during the 2000s in comparison with the previous decade till the mid 2010s. Here, we further document the ongoing oceanic changes within the Greenland Sea using the Mercator Ocean Physical System, an operational ocean model with data-assimilation.
The model shows a large interannual variability, a later start and a decline of convection in the Greenland Sea in recent years. In particular, the depth of the annual maximum mixed layer diminished by 52 % between 2008/2014 and 2015/2020, from 1168 m to 559 m, over the convective area. There, hydrographic changes, especially a temperature increase, have led to isopycnal deepening and stratification strengthening at a larger rate in the north and east of the area (namely the Boreas Basin).
Atlantic Water spreading over the Boreas Basin and the eastern part of the Greenland Basin contributes to the changes of the Greenland Sea hydrography. The model also indicates a decrease in the intensity of the gyre in accordance with the isopycnal deepening while local surface winds and fluxes do not exhibit neither significant trends nor significant interannual variations.

How to cite: abot, L.: Recent Convection Decline in the Greenland Sea - insights from the Mercator Ocean System over 2008-2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3170, https://doi.org/10.5194/egusphere-egu23-3170, 2023.

14:35–14:45
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EGU23-2035
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OS1.5
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ECS
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Virtual presentation
Oliver J. Tooth, Helen L. Johnson, Chris Wilson, and Dafydd G. Evans

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the uptake and redistribution of heat, freshwater and carbon. At subpolar latitudes, recent observations show that the strength of the AMOC is dominated by water mass transformation in the eastern North Atlantic Subpolar Gyre (SPG). Both observations and ocean reanalyses show a pronounced seasonality of the AMOC within this region. However, the distribution of the strength and seasonality of overturning across the individual circulation pathways of the eastern SPG remains poorly understood. To investigate the nature of this seasonal overturning variability, we use Lagrangian water parcel trajectories evaluated within an eddy-permitting ocean sea-ice hindcast simulation.

By introducing a novel Lagrangian measure of the density-space overturning, we show that water mass transformation along the circulation pathways of the eastern SPG accounts for 8.9 ± 2.2 Sv (55%) of the mean strength of AMOC in the eastern subpolar North Atlantic. Our analysis highlights the crucial role of water parcel recirculation times in determining the magnitude of the strength and seasonality of overturning. We find that upper limb water parcels flowing northwards into the eastern SPG participate in a recirculation race against time to avoid wintertime diapycnal transformation into the lower limb of the AMOC. Upper limb water parcels sourced from the central and southern branches of the North Atlantic Current typically recirculate on interannual timescales (1-5 years) and thus determine the mean strength of overturning within this region. The seasonality of Lagrangian overturning is explained by a small collection of water parcels, recirculating rapidly (≤ 8.5 months) in the upper Central Iceland and Irminger Basins, whose along-stream transformation is dependent on their month of arrival into the eastern SPG.

How to cite: Tooth, O. J., Johnson, H. L., Wilson, C., and Evans, D. G.: A Lagrangian view of seasonal overturning variability in the eastern North Atlantic subpolar gyre., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2035, https://doi.org/10.5194/egusphere-egu23-2035, 2023.

14:45–14:55
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EGU23-5281
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OS1.5
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ECS
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On-site presentation
Finn Ole Heukamp, Lars Aue, and Torsten Kanzow

The Barents Sea Opening (BSO) is one of two Atlantic gateways connecting the North Atlantic Ocean to the Arctic Ocean. The ocean transport through the BSO is composed of warm and saline Atlantic Water inflow in the central and southern parts of the section and cold Polar and modified Atlantic Water outflow in the north. The variability of strengths of both inflow and outflow largely controls the evolution of the net ocean heat transport into the Barents Sea, locally impacting e.g., ocean-atmosphere heat fluxes, sea ice extent, and deep-water formation. Moreover, changes in heat fluxes and sea ice extent have been shown to impact remote properties such as wintertime weather in northern Europe and water properties in the central Arctic Ocean.


In this study, we identified and disentangled the contributions of local and remote atmospheric forcing mechanisms of the wintertime volume transport through BSO from 1970-2020. In order to understand the variability and co-variability of the local and remote forcing mechanisms and the linked transport anomalies, we performed dedicated model experiments with the unstructured ocean and sea ice model FESOM2. In addition to a hindcast control simulation using JRA55 reanalysis forcing, we performed two additional model experiments in which we combined JRA55 forcing with CORE1 normal year forcing in a way that the simulations are forced with JRA55 (CORE1) in the Arctic domain and CORE1 (JRA55) outside the Arctic domain. This setup allows the separation of local and upstream forced transport variability. Our experiments show, that both BSO inflow and outflow exhibit strong variability on interannual to decadal timescales. While inflow variability is forced to a similar degree by local alongshore winds and alongshore winds upstream in the Norwegian Sea, the outflow variability is almost entirely forced by wind stress curl anomalies over the northern Barents Sea shelf. Moreover, the inflow anomalies forced upstream are highly correlated with the North Atlantic Oscillation (NAO) while the transport anomalies forced locally exclusively correlate with the NAO during periods of a negative NAO. Furthermore, we observe a drastic drop in the correlation of inflow anomalies forced upstream and the NAO around the year 2000 - the same period in which winters with strongly enhanced outflow anomalies (97/98, 03/04) are found. By expanding our analysis to cyclone activity in the northern North Atlantic, we link the loss of co-variability of NAO and BSO inflow to an anomalous southward deflection of cyclones in these winters, affecting the alongshore winds in the Norwegian Sea as well as the wind stress curl over the northern Barents Sea shelf.


In general, this study aims to improve our understanding of the drivers of volume and heat transport variability in the BSO as a key factor for (sub-)Arctic, ocean, weather, and climate variability. 

How to cite: Heukamp, F. O., Aue, L., and Kanzow, T.: Decadal Variability of Transports through Barents Sea Opening: Changing impact of Large-Scale Wind Forcing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5281, https://doi.org/10.5194/egusphere-egu23-5281, 2023.

14:55–15:05
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EGU23-15913
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OS1.5
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ECS
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On-site presentation
Alice Marzocchi, George Nurser, Louis Clement, and McDonagh Elaine

The ocean takes up 93 % of the excess heat in the climate system and approximately a quarter of the anthropogenic carbon via air–sea fluxes. Ocean ventilation and subduction are key processes that regulate the transport of water from the surface mixed layer to the ocean's interior, which is isolated from the atmosphere for a timescale set by the large-scale circulation. Using numerical simulations (NEMO framework), we assess where the ocean subducts water and takes up properties from the atmosphere, and how ocean currents transport and redistribute these properties. This is achieved by adding a set of simulated seawater vintage dyes (passive tracers) that are released annually from different ocean surface “patches”, representing water masses’ source regions. The dyes’ distribution captures years of strong and weak convection at deep and mode water formation sites in both hemispheres, showing good agreement with observations in the subpolar North Atlantic. We show that interannual variability in subduction rates, driven by changes in surface forcing, is key in setting the different sizes of the long-term inventory of the dyes. The Northern and Southern Hemispheres are characterised by different ventilation pathways and timescales, but our analysis highlights a strong correlation between the strength of ventilation in recently subducted waters and the longer-term dye inventory in each hemisphere. This means that the conditions close to the time of dye injection are driving the amount of seawater being subducted, but also that this signal persists over time and the longer-term tracer inventory is strongly related to the initial surface conditions. The correlation still holds for the different source regions, where it is even stronger, but the slope of the correlation does vary. Export and isolation of subducted waters is shown to be faster in the Northern Hemisphere, defining a stronger ventilation “persistence” – represented by the slope of the correlation between subduction and the longer-term inventory. The highest ventilation persistence is found in the subpolar North Atlantic and specifically in the Labrador and Irminger Seas, which are the dominant regions in retaining tracer on multi-decadal time scales.

How to cite: Marzocchi, A., Nurser, G., Clement, L., and Elaine, M.: The role of surface forcing in driving pathways and time scales of ocean ventilation in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15913, https://doi.org/10.5194/egusphere-egu23-15913, 2023.

15:05–15:15
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EGU23-7805
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OS1.5
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ECS
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On-site presentation
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Raphaël Bajon, Lidia Carracedo, Herlé Mercier, Fiz F. Pérez, Anton Velo, Rémy Asselot, and Virginie Thierry

The ocean is the largest carbon reservoir on Earth, and a major sink for the excess of CO2 (anthropogenic carbon) emitted to the atmosphere by human activities. Having removed about a quater of these emissions since the beginning of the industrial era, ocean’s key role in climate is particularty outstanding in the North Atlantic (NA). A combination of physical and biological processes makes the NA a key-role region for the natural and anthropogenic carbon uptake and storage, and hence for the global carbon cycle. Traditionally, the seasonal carbon cycle has been assumed to respond to natural variability, unnafected by the ongoing anthropogenic increase of atmospheric CO2. Recent model projections, however, point otherwise, yet observational evidence to verify these predictions is still missing. Here we examine seasonal cycle in dissolved inorganic carbon (DIC) and its (surface-2000 dbar) transport, estimated using in-situ data and neural networks, across the OVIDE (GO-SHIP A25) section, from 1993 to 2021 at a monthly resolution. Our results highlight that changes in temperature, dissolved oxygen and ocean circulation are key components driving the seasonal DIC variability. DIC concentrations are higher in years with strong winter mixing regimes (which bring more nutrient-rich waters to the surface, favouring photosynthesis, and more (remineralized) carbon back to the surface). Seasonal DIC transport fluctuations are found significant compared to the mean (e.g. +/- 25% in the upper branch of the meridional overturning circulation), putting into relevance that caution is needed if assuming that single-cruise occupations are representative of the annual state. We also observe a yearly variant seasonal imbalance, with a significant reduction over the past two decades in the upper branch of the meridional overturning circulation. These results underscore the importance of considering intra-annual variability in the North Atlantic's carbon cycle when addressing climate change.

 
 
 

How to cite: Bajon, R., Carracedo, L., Mercier, H., Pérez, F. F., Velo, A., Asselot, R., and Thierry, V.: Intra-annual variability of carbon signature and transport in the North Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7805, https://doi.org/10.5194/egusphere-egu23-7805, 2023.

15:15–15:25
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EGU23-7535
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OS1.5
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On-site presentation
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Anne-Sophie Fortin and Susan Lozier

The Atlantic Meridional Overturning Circulation (AMOC) exports cold, fresh, dense waters formed in the subpolar North Atlantic to equatorward latitudes along the western boundary and interior pathways. The properties of the water formed in the North Atlantic vary from year to year, however the strength and time scale for the downstream communication of this variability is still unclear. While several past studies have focused on tracking specific property anomalies, particularly from the Labrador Sea, we approach our study by investigating property variance downstream of the water mass source region. In effect, we aim to understand the downstream memory of water mass property variability in the North Atlantic along western boundary and interior pathways. To do so, we analyze hydrographic properties on neutral density isopycnal surfaces in the subpolar North Atlantic and along the western boundary and interior pathways with two reanalysis products from the Met Office, the hydrographic dataset (EN4) and ensemble prediction system (GloSea5), over their overlapping time period (1993-2019). Our results show different patterns of downstream variance for the interior compared to the western boundary, which we interpret in terms of known circulation features in the deep North Atlantic and what we have learned from past Lagrangian studies.

How to cite: Fortin, A.-S. and Lozier, S.: Variability of North Atlantic Water Mass Properties along Western Boundary and Interior Pathways, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7535, https://doi.org/10.5194/egusphere-egu23-7535, 2023.

15:25–15:35
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EGU23-6539
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OS1.5
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Virtual presentation
Ben Moat, Bablu Sinha, Neil Fraser, Leon Hermanson, Simon Josey, Claire MacIntosh, David Berry, Simon Williams, Marilena Oltmanns, Dan Jones, and Rachael Sanders

We synthesize observational datasets and a state of the art forced global ocean model to construct a multidecadal upper ocean heat budget for the North Atlantic for the period 1950 to 2020. Using multiple independent estimates of the variables allows us to provide robust uncertainty estimates for each term. Time-varying ocean heat transport convergence dominates the budget on multidecadal timescales in all regions of the North Atlantic. In the subpolar region (north of 45N) we find that the heat transport convergence is dominated by geostrophic currents whereas in the subtropics (26-45N) advection by ageostrophic currents is also significant. The geostrophic advection is dominated (especially in the subpolar regions) by anomalous geostrophic currents acting on the mean temperature gradient. The timescale and spatial distribution of the anomalous geostrophic currents are consistent with basin scale ‘thermal’ Rossby waves propagating westwards/northwestwards in the subpolar gyre. Multidecadal changes in North Atlantic Changes in ocean heat storage directly affect the climate of the surrounding continents, and hence it is important to understanding the mechanism behind these.

How to cite: Moat, B., Sinha, B., Fraser, N., Hermanson, L., Josey, S., MacIntosh, C., Berry, D., Williams, S., Oltmanns, M., Jones, D., and Sanders, R.: Mechanism of observed North Atlantic multidecadal upper ocean heat content changes 1950-2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6539, https://doi.org/10.5194/egusphere-egu23-6539, 2023.

15:35–15:45
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EGU23-15631
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OS1.5
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ECS
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On-site presentation
Neil Fraser, Alan Fox, and Stuart Cunningham

In the subtropics, the Atlantic meridional overturning circulation (MOC) has the same strength and variability whether measured in depth- or density-space. Two different continuity budgets must therefore be satisfied north of the subtropics, one via diapycnal volume transport and the other via downward volume transport. However, as water can get denser without getting deeper (and vice versa), it is unclear why the integrated effect of these processes, the MOC, should have the same strength and variability in both depth- and density-space, provided one integrates these terms sufficiently far south (e.g. to 26 °N). Previous work has investigated the surface buoyancy forcing and mixing processes which drive diapycnal volume transport. Here, we use a suite of observational products and new analyses in a vorticity framework to study the magnitude and distribution of the various terms responsible for vertical volume transport, and gain further insight by also evaluating these terms using VIKING20X model output. We conclude that bottom Ekman transport and advection curl around the boundaries of the subpolar gyre, particularly around Greenland, are dominant drivers of downward vertical transport and hence crucial for closing MOC streamlines in depth-space, with much of the variability also projecting onto the MOC in density-space. As these processes are “spun-up” by the sub-polar gyre yet project onto the overturning, our results offer new insights into the coupling between the overturning and gyre circulations.

How to cite: Fraser, N., Fox, A., and Cunningham, S.: Continuity Constraints on the Atlantic Meridional Overturning Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15631, https://doi.org/10.5194/egusphere-egu23-15631, 2023.

Coffee break
Chairperson: Damien Desbruyeres
16:15–16:25
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EGU23-2883
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OS1.5
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ECS
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solicited
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Virtual presentation
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Isabela Le Bras, Joshua Willis, and Ian Fenty

From 2004 to 2014, the Line W moorings measured a 0.7 Sv/yr slowing of the Deep Western Boundary Current (DWBC) offshore of Cape Cod, Massachusetts. Here, we combine these deep mooring observations with float and satellite altimeter data and find that this DWBC change corresponded to a slowing of the cross-basin Atlantic Meridional Overturning Circulation (AMOC) of about 0.3 Sv/yr. Our AMOC transport time series corresponds well with the ECCO state estimate, particularly when the Line W mooring data influences our reconstruction of upper ocean volume fluxes. We compare our 35N time series with a similar time series at 41N as well as with the 26N RAPID AMOC, and find AMOC declines across datasets during this time period. The relative magnitudes of these declines are consistent with interdecadal variability originating in the Labrador Sea. We find that though our integrated overturning estimate agrees well with ECCO, the structure of the deep flow differs substantially. While we cannot rule out a decreasing AMOC trend during the 20th century, we find that natural variability is too large to detect a net AMOC decrease in direct observations or the ECCO ocean model since 2004.

How to cite: Le Bras, I., Willis, J., and Fenty, I.: The Atlantic meridional overturning circulation at 35N from deep moorings, floats, and satellite altimeter, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2883, https://doi.org/10.5194/egusphere-egu23-2883, 2023.

16:25–16:35
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EGU23-5053
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OS1.5
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ECS
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On-site presentation
Simon Wett, Monika Rhein, Dagmar Kieke, Christian Mertens, and Martin Moritz

The Atlantic Meridional Overturning Circulation (AMOC) plays a key role for the climate system of Europe and the Arctic by redistributing heat and freshwater in the Atlantic. Since climate model studies project a likely decline of the AMOC under climate change in the 21st century, monitoring AMOC changes remains an important task. Several moored arrays in the Atlantic deliver estimates of the AMOC volume transport. The longest of these observational AMOC records is the RAPID array in the subtropical North Atlantic. The depiction of the AMOC as a global ocean conveyor assumes that the AMOC variability is consistent across latitudes. This concept has been questioned by model studies. However, model studies and estimates based on altimetry and Argo data disagree on the regions and timescales of meridional connectivity. From measurements of the North Atlantic Changes (NOAC) array in the subpolar North Atlantic at 47°N we calculate the AMOC volume transport timeseries. Our approach combines data from moored instruments with hydrography (from Argo floats and shipboard measurements) and satellite altimetry. Here, we present this 25-year (1993-2018) purely observational AMOC record in monthly resolution and analyze its meridional connectivity with the subtropical RAPID AMOC.

How to cite: Wett, S., Rhein, M., Kieke, D., Mertens, C., and Moritz, M.: Meridional Connectivity of a 25-year Observational AMOC Record at 47°N, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5053, https://doi.org/10.5194/egusphere-egu23-5053, 2023.

16:35–16:45
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EGU23-4077
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OS1.5
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ECS
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On-site presentation
Yao Fu and M. Susan Lozier

Variability of the Atlantic Meridional Overturning Circulation (MOC) has drawn extensive attention due to the MOC’s impact on global heat and freshwater redistribution. The Overturning in the Subpolar North Atlantic Program (OSNAP) array, consisting of an OSNAP West section covering the Labrador Sea and an OSNAP East section covering the eastern subpolar basins (Irminger and Iceland Basins), has continuously observed the MOC and meridional heat and freshwater transports since 2014. The OSNAP observations have contributed substantially to the understanding of the mean state and sub-seasonal to seasonal variability of the subpolar MOC. In this study, we present the latest OSNAP observational results and investigate interannual variability of the subpolar MOC with respect to water mass transformation and formation in the Labrador Sea and eastern subpolar basins. We detail the differences between formation and transformation in each of these basins and discuss their relationships to overturning on monthly and interannual time scales. Finally, we explore the mechanism(s) responsible for these differences.

How to cite: Fu, Y. and Lozier, M. S.: Interannual variability of the meridional overturning circulation in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4077, https://doi.org/10.5194/egusphere-egu23-4077, 2023.

16:45–16:55
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EGU23-14637
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OS1.5
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ECS
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On-site presentation
Kristin Burmeister, Neil Fraser, Lewis Drysdale, Sam Jones, Stuart Cunningham, Mark Inall, and Alan Fox

The Rockall Trough (RT) is a key pathway for warm and salty water flowing northward, a process which plays a key role in dictating the western European climate. The picture of the mean circulation and variability in the RT is still emerging, as the record of continuous transport observations has only recently been extended to eight years. Here, for the first time, we present the temporally extended record of RT volume, heat and freshwater transports. An important feature of the RT circulation is the European Slope Current (ESC) which is poorly constrained by ship-based, mooring, and satellite observations. To tackle this, we gathered around 150 glider transects over 2.5 years which capture the ESC velocity field in unprecedented detail. The data are sufficient to characterise both the mean state and the emergent seasonal variability of the ESC, and reveal the year-round presence of a southward countercurrent at depth. Variability in the strength and structure of this previously unstudied feature modulates net northward transport in the eastern boundary current system.

We also utilise these observations for monitoring the basin-wide overturning circulation as part of the newly developed OSNAP_I transect. We will present the first results from that programme.

How to cite: Burmeister, K., Fraser, N., Drysdale, L., Jones, S., Cunningham, S., Inall, M., and Fox, A.: Eight years of continuous Rockall Trough transport observations from moorings and gliders, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14637, https://doi.org/10.5194/egusphere-egu23-14637, 2023.

16:55–17:05
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EGU23-1821
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OS1.5
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On-site presentation
Sijia Zou, Amy Bower, M. Susan Lozier, and Heather Furey

As part of the Overturning in the Subpolar North Atlantic Program, 122 acoustically-tracked subsurface floats were deployed at 1800-2800 dbar to understand the deep ocean circulation in the subpolar North Atlantic. Gridded mean velocity and eddy kinetic energy (EKE) maps have been constructed using velocity vectors derived from the floats. The mean velocity field reveals a relatively strong deep boundary current around Greenland and in the Labrador Sea, with a weaker deep boundary current over the eastern flank of the Reykjanes Ridge, and near-zero mean flow over the western flank, implying a discontinuous deep boundary current across the subpolar basin. Over most of the subpolar basin, deep EKE resembles that at surface, albeit with smaller magnitudes. A surprising finding about deep EKE is an elevated EKE band east of Greenland. This high EKE band is possibly attributed to the combined influence from propagating Denmark Strait Overflow Cyclones, variability of the wind-driven recirculation offshore of southeast Greenland, and/or topographic waves. The float-based flow fields constructed in this study provide an unprecedented view on the kinematic properties of the large-scale deep circulation in the subpolar North Atlantic.

How to cite: Zou, S., Bower, A., Lozier, M. S., and Furey, H.: Deep Ocean Circulation in the Subpolar North Atlantic Observed by Acoustically-tracked Floats, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1821, https://doi.org/10.5194/egusphere-egu23-1821, 2023.

17:05–17:15
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EGU23-7809
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OS1.5
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ECS
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On-site presentation
Seasonal to interannual variability of salinity on the Northeast U.S. continental shelf and links to large-scale North Atlantic variability
(withdrawn)
Svenja Ryan, Glen G. Gawarkiewicz, and Caroline C. Ummenhofer
17:15–17:25
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EGU23-9765
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OS1.5
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ECS
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On-site presentation
Elodie Duyck and Femke De Jong

The Atlantic Meridional Overturning Circulation (AMOC) is predicted to weaken in the 21st century as a result of climate change. One of the proposed drivers for such a weakening is the dampening of deep convection in the Subpolar North Atlantic following an increase in freshwater fluxes from the Greenland ice sheet. However, the fresh waters that flow from Greenland and the Arctic to the Subpolar North Atlantic are primarily found over the Greenland shelf, and it is unclear where and how much freshwater is exported from the shelf to the interior seas where deep convection occurs. While the main export of freshwater off the Greenland shelf is likely to occur west of Greenland, the importance of water mass transformation and overturning east of Greenland in the total subpolar AMOC makes it essential to better understand freshwater exchanges between the east Greenland shelf and deep convection regions of the Irminger and Nordic Sea.

We investigate these exchanges using drifter data from five deployments carried out at different latitudes along the east Greenland shelf in 2019, 2020 and 2021, as well as satellite data and an atmospheric reanalysis. We compute Ekman transport (from winds) and geostrophic velocity (from satellite altimetry) at the shelfbreak and find that the Blosseville Basin, just upstream of Denmark Strait, and Cape Farewell, are particularly favorable to cross-shelf exchanges. We further investigate exchange processes in these regions using drifter data. In the Blosseville Basin, drifters are brought off-shelf towards the Iceland Sea and into the interior of the Basin, possibly joining the separated EGC. As they flow downstream, they re-enter the shelf and most are driven towards the coast. This exchange appears to be mainly driven by the shape of the bathymetry. At Cape Farewell, the wind appears to be the main driver, although occasionally an eddy seems to turn drifters away from the shelf. The drifters brought off-shelf at Cape Farewell mostly continue around Eirik Ridge, where they re-enter the West Greenland Current. How much of the freshwater signature is lost between leaving the East Greenland Current and entering the West Greenland Current is not clear and will need further study.

How to cite: Duyck, E. and De Jong, F.: Cross-shelf exchanges between the East Greenland shelf and interior seas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9765, https://doi.org/10.5194/egusphere-egu23-9765, 2023.

17:25–17:35
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EGU23-7698
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OS1.5
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ECS
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On-site presentation
Fraser Goldsworth, Isabela Le Bras, Helen Johnson, and David Marshall

Observations show that strong southerly winds over the Irminger Sea can excite symmetric instability in the East Greenland Current, resulting in the generation of a low potential vorticity layer below the convectively mixed layer (Le Bras et al., 2022). The role of these downfront wind events on the formation of dense waters is not yet well understood.

Using an ensemble of ultra-high resolution models (25 m in the horizontal)  we show that the low potential vorticity layer is virtually indistinguishable from the convectively mixed layer, implying the absence of symmetric instability in coarse models may lead to underestimates in the mixed layer depth and baroclinicity of the East Greenland Current. We explore the hypothesis that symmetric instability acts as the short time-scale response of the current to these southerly wind events and pre-conditions the mixed layer, making it more susceptible to baroclinic instability over longer time-scales. We then investigate whether baroclinic eddy activity is enhanced following these wind events and examine the implications of this on lateral and diapycnal mixing, including by calculating water mass transformation rates.

How to cite: Goldsworth, F., Le Bras, I., Johnson, H., and Marshall, D.: Water mass transformation following instability in the mixed layer of the East Greenland Current, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7698, https://doi.org/10.5194/egusphere-egu23-7698, 2023.

17:35–17:45
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EGU23-9981
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OS1.5
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Virtual presentation
Louis Clement, Eleanor Frajka-Williams, Nicolai von Oppeln-Bronikowski, Ilona Goszczko, and Brad de Young

By ventilating the deep ocean, deep convection in the Labrador Sea plays a crucial role in the climate system. Unfortunately, the mechanisms leading to the cessation of convection and, hence, the mechanisms by which a changing climate might affect deep convection remain unclear. In winter 2020, three autonomous underwater gliders sampled the convective region and both its spatial and temporal boundaries. Both boundaries are characterised by higher sub-daily mixed-layer depth variability than the convective region. At the convection boundaries, buoyant intrusions--including eddies and filaments--primarily drive restratification by bringing freshwater, instead of warm warmer, and instead of atmospheric warming. At the edges of these intrusions, submesoscale instabilities, such as symmetric instabilities and mixed-layer baroclinic instabilities, seem to contribute to the decay of the intrusions. In winter, strong destabilising surface heat flux and along-front winds can enhance the lateral stratification, sustaining submesoscale instabilities. Consequently, winter atmospheric conditions and freshwater intrusions participate in halting convection by adding buoyant freshwater into the convective region through submesoscale flows. This study reveals freshwater anomalies in a narrow area offshore of the Labrador Current and near the convective region; this area has received less attention than the more eddy-rich West Greenland Current, but is a potential source of freshwater in closer proximity to the region of deep convection. Freshwater fluxes from the Arctic and Greenland are expected to increase under a changing climate, and our findings suggest that they may play an active role in the restratification of deep convection.

How to cite: Clement, L., Frajka-Williams, E., von Oppeln-Bronikowski, N., Goszczko, I., and de Young, B.: Cessation of Labrador Sea Convection by Freshening through (Sub)mesoscale Flows, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9981, https://doi.org/10.5194/egusphere-egu23-9981, 2023.

17:45–17:55
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EGU23-11576
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OS1.5
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ECS
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On-site presentation
Kevin Niklas Wiegand, Dagmar Kieke, Paul G. Myers, and Igor Yashayaev

Irminger Water (IW) is a prominent water mass in the subpolar North Atlantic (SPNA). It is warm and saline and originates from the North Atlantic Current and the Irminger Current. The water mass delivers anomalously large amounts of heat and salt to the Labrador Sea. Like any other water mass, IW is subject to temporal and spatial variability, which needs to be adequately identified and tracked.

To separate IW from ambient waters, previous studies identified IW at different times using static thresholds of salinity, temperature, and density (i.e., constant over time within the individual studies). However, given the tremendous variability in the region, such static definitions often do not detect IW sufficiently since these definitions do not account for shifts in the large-scale hydrographic state of the SPNA. To address this issue, this study aims to identify non-static thresholds (i.e., incorporating temporal variability) to analyze IW variability. We refer to the method of identifying IW based on non-static thresholds as the phenomenological approach. To do so, we utilize the observation-based data set ARMOR3D between 1993 and 2022. This new approach allows us to compare estimates of IW properties and volume transports to respective estimates obtained from the static approach.

In the case of the static approach being applied to the AR7W section in the eastern part of the Labrador Sea as a test region, the water column was anomalously saline in years of high IW volume transport. Hence, the static approach identified more IW and thus overestimated its volume transport. In contrast, the water column was anomalously fresh in years when the static approach reveals a low IW volume transport. Hence, applying the static approach, less IW is identified, and thus its volume transport is underestimated. In contrast, the phenomenological approach reveals less pronounced decadal variability of the IW volume transport.

Applying a static IW definition will likely create stronger gradients between IW and ambient water masses when both are fresher. In turn, these gradients may impose or modulate unrealistic changes in the IW volume transport simply because the actual boundary of IW does not coincide with a certain isohaline or isotherm. Any correlated change or shift in IW properties and, for example, Labrador Sea Water will relocate the IW boundary causing the transport to change. The phenomenological approach introduced in our study resolves this issue.

How to cite: Wiegand, K. N., Kieke, D., Myers, P. G., and Yashayaev, I.: Assessing the variability of Irminger Water at AR7W between 1993 and 2022 using time-dependent property thresholds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11576, https://doi.org/10.5194/egusphere-egu23-11576, 2023.

Orals: Wed, 26 Apr | Room L3

Chairperson: Caroline Katsman
08:30–08:40
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EGU23-1249
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OS1.5
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ECS
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solicited
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On-site presentation
Annika Reintges, Jon Robson, Rowan Sutton, and Stephen Yeager

Climate models are a valuable tool to study the interaction between ocean and atmosphere. Nevertheless, they are known to suffer from various biases and uncertainties. In the subpolar North Atlantic typical biases among models from the Coupled Model Intercomparison Project phase 6 (CMIP6) are found in the mean surface temperature and salinity, and in the mean sea ice concentration. These biases will affect the air-sea interaction.

In this study, we are investigating the diversity of CMIP6 models with respect to their response of the Atlantic Meridional Overturning Circulation (AMOC) to the North Atlantic Oscillation (NAO) in pre-industrial control experiments. This response is sensitive to the mean spiciness of the North Atlantic. Thus, we focus on two categories of models: Models that are spicy (warm-salty) and models that are minty (cold-fresh) within the subpolar gyre of the North Atlantic. Spicy models tend to have a lower sea ice cover in the Labrador Sea (LS) and larger LS heat loss during a positive NAO, compared to minty models. Also, spicy models have a weaker stratification in the LS. Sub-surface density changes 1 to 3 years after the NAO are larger in the spicy models and establish a zonal density gradient that can cause a stronger delayed AMOC response that is also more coherent across latitudes.

Although some metrics seem to be more realistic in the spicy models, other characteristics seem less realistic compared to the minty models, like the mixed layer depth relative importance between the eastern and the western subpolar North Atlantic. This could be a sign for how some mean states or processes might be right for the wrong reasons and stresses the need for model improvement.

How to cite: Reintges, A., Robson, J., Sutton, R., and Yeager, S.: Spiciness of the subpolar North Atlantic affects the response of the Atlantic Meridional Overturning Circulation to the North Atlantic Oscillation in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1249, https://doi.org/10.5194/egusphere-egu23-1249, 2023.

08:40–08:50
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EGU23-1169
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OS1.5
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ECS
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On-site presentation
Casey Patrizio, Panos Athanasiadis, Claude Frankignoul, Dorotea Iovino, Simona Masina, Luca Famooss Paolini, and Silvio Gualdi

The simulated North Atlantic atmosphere­–ocean variability is assessed in a subset of models from HighResMIP that have either low-resolution (LR) or high-resolution (HR) in their atmosphere and ocean model components. In general, the LR models overestimate the low-frequency variability of subpolar sea-surface temperature (SST) anomalies and underestimate their correlation with the NAO compared to ERA5 reanalysis. These biases are substantially reduced in the HR models, and it is shown that the improvements are related to a reduction of intrinsic (non-NAO-driven) variability of the subpolar ocean circulation.

To understand the mechanisms behind the overestimated intrinsic subpolar ocean variability in the LR models, a link is demonstrated between the biases in subpolar ocean variability and known biases in the mean state of the Labrador-Irminger seas. Supporting previous studies, the Labrador-Irminger seas are found to be too cold and too fresh in the LR models compared to observations from EN4 and the HR models. This causes upper-ocean density and hence convection anomalies in this region to be more salinity-controlled in the LR models versus more temperature-controlled in the HR models. It is hypothesized that this may cause the excessive subpolar ocean variability in the LR models by 1) promoting a positive feedback between subpolar upper-ocean salinity, convection and Atlantic Meridional Overturning Circulation (AMOC) anomalies, and 2) weakening the negative feedback between subpolar upper-ocean temperature, convection and AMOC anomalies that is apparent in the HR models. The results overall suggest that mean ocean biases play an important role in the simulation of the variability of the extratropical ocean.

How to cite: Patrizio, C., Athanasiadis, P., Frankignoul, C., Iovino, D., Masina, S., Famooss Paolini, L., and Gualdi, S.: Improved simulation of extratropical North Atlantic atmosphere-ocean variability in HighResMIP models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1169, https://doi.org/10.5194/egusphere-egu23-1169, 2023.

08:50–09:00
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EGU23-14080
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OS1.5
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ECS
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On-site presentation
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Johannes Mayer, Leopold Haimberger, and Michael Mayer

Air-sea heat fluxes play a key role for many processes in the North Atlantic Ocean, such as the lateral transport of energy or the formation of storm tracks. Thus, an accurate estimation of air-sea heat flux trends is pivotal and helps to understand implications of climate change. To do so, reanalysis products are attractive candidates due to their excellent spatial coverage over multiple decades. However, trend estimations based on reanalysis data are challenging as changes in the observing system can introduce temporal discontinuities.

In this study, we explore the reliability and temporal stability of net air-sea heat flux trends from ERA5 forecasts in the North Atlantic basin over the period 1950-2019.  The assessment is complemented with an indirect estimate of the net surface flux derived from the atmospheric energy budget. Causes of trends in latent and sensible heat fluxes are identified based on monthly analyzed state quantities from ERA5, such as wind speed, moisture, and temperature. Additionally, the impact of the North Atlantic Oscillation (NAO) and Atlantic Multi-decadal Oscillation (AMO) as well as analysis increments, as introduced by the ERA5 data assimilation, is investigated.  

Our results show a robust increase of latent heat fluxes in the tropical North Atlantic over the past seven decades, which is likely caused by the intensification of the Hadley cell favouring subsidence and advection of drier air masses. In the Norwegian Sea, positive net air-sea heat flux trends (increased ocean heat uptake) are largely dominated by changes in sensible heat fluxes, which are driven by a trend towards more southerly winds and the advection of warmer air. In the Gulf Stream region, the AMO likely drives the multi-decadal variability of net air-sea heat fluxes, while long-term trends over the 1950-2019 period remain insignificantly small. Furthermore, we find significant changes over the North Atlantic Warming Hole and western North Atlantic associated with more frequent positive NAO phases during the past 30 years. From our analysis, we conclude that analysis increments most likely influence the magnitude of these trends, especially at low latitudes where the impact can be as large as ~2 W m-2 dec-1, while the basin-wide trend pattern remains unaffected. The net effect of the found regional changes in fluxes is assessed by the spatial average trend over the whole North Atlantic north of 26°N, which yields a positive but statistically insignificant trend of 0.5 W m-2 dec-1 over the past 70 years. Potential implications for trends in the AMOC are discussed. 

How to cite: Mayer, J., Haimberger, L., and Mayer, M.: Changes in air-sea fluxes over the North Atlantic during 1950-2019 as derived from ERA5 data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14080, https://doi.org/10.5194/egusphere-egu23-14080, 2023.

09:00–09:10
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EGU23-1796
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OS1.5
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On-site presentation
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Alessio Bellucci, Denis Mattei, Paolo Ruggieri, and Luca Famooss Paolini

The connection between the Atlantic meridional overturning circulation (AMOC) and the Atlantic multidecadal variability (AMV) is inspected in a suite of pre-industrial integrations from the 6th phase of the Coupled Model Inter-comparison Project (CMIP6), using a change-point detection method to identify different AMOC-AMV co-variability regimes. A key finding of this study is that models robustly simulate multi-decadal windows where the AMV and the AMOC are essentially uncorrelated. These regimes coexist with longer periods with relatively high correlation. Drops and recoveries of correlation are found to be often abrupt and confined in a temporal window of the order of 10 years. Phenomenological evidence suggests that the no-correlation regimes may be explained by drops in the variance of the AMOC: a less variable meridional heat transport leads to a suppressed co-variability of the AMV, leaving a larger role for non-AMOC drivers, consistent with a non-stationary AMOC-stationary noise interpretative framework.

How to cite: Bellucci, A., Mattei, D., Ruggieri, P., and Famooss Paolini, L.: Intermittent Behavior in the AMOC-AMV Relationship , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1796, https://doi.org/10.5194/egusphere-egu23-1796, 2023.

09:10–09:20
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EGU23-11588
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OS1.5
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Virtual presentation
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Jon Robson

Over the observed period, North Atlantic Sea surface temperatures have gone through cycles of anomalous warming and cooling relative the global mean. This variability has become known as the Atlantic Multidecadal Variability (AMV), and it has been associated with important regional climate impacts. However, in recent years there has been considerable controversy over the origins of AMV. In particular, there is debate over whether AMV is a natural phenomenon (e.g., an expression of internal variability or natural external forcings), or whether it was caused by human activity through the impact of anthropogenic aerosol forcing.

Here, an analysis of CMIP6 multi-model historical simulations is presented which isolates the internal and externally forced AMV. The analysis shows that, although there is substantial externally forced AMV in the CMIP6 historical simulations, the forced variability is part of a wider hemispheric signal and is not specific to the North Atlantic like in observations. Therefore, the magnitude of the externally forced variability is highly dependent on the definition of the AMV index used. Ocean circulation changes consistently lead the internal AMV across models, but there is no-clear relationship for the external AMV. AMV is also associated with broader changes than just sea surface temperatures, but this multivariate fingerprint of AMV is significantly different between the internal and external components. For example, internal AMV is associated with salinity anomalies and increased turbulent heat loss across the subpolar North Atlantic that agree broadly with observations. However, in contrast, the externally forced AMV is associated with freshening and reduced heat loss across the subpolar North Atlantic and especially in models with the strongest aerosol forcing. Overall, the analysis suggests that internal variability remains a likely hypothesis to explain AMV, but questions remain on whether models adequately simulate the forced response.

How to cite: Robson, J.: Contrasting the internal and external components of Atlantic Multidecadal Variability in CMIP6 historical simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11588, https://doi.org/10.5194/egusphere-egu23-11588, 2023.

09:20–09:30
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EGU23-5739
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OS1.5
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ECS
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On-site presentation
Kunpeng Yang

Multicentennial North Atlantic climate variability revealed by paleoclimate reconstruction has been linked to the Atlantic meridional overturning circulation (AMOC) variability. However, mechanisms of multicentennial AMOC variability in coupled models have yet to reach a consensus, reflecting a necessity of more fundamental theoretical studies. To this end, we propose an ocean-only North Atlantic 4-box theoretical model. A self-sustained AMOC oscillation with a typical period of 300-400 years exhibits. The timescale is largely set by rate of AMOC advection but also modulated by thermal processes, while the self-sustained oscillation mechanism can be generalized as a combination of a linear growing oscillation and a nonlinear restraining. The linear growing oscillation is energized by the salinity advection feedback and stabilized by the temperature advection feedback, while the latter is hampered by surface temperature restoring. Nonlinear restraining processes restrict the runaway tendency of the linear growing oscillation and finally turn it into a self-sustained one. We further identify a 300-400-year AMOC oscillation in a CESM1 control simulation, which can be well explained by the self-sustained oscillation mechanism of the theoretical model. Our work demonstrates that internal variability plays a vital role in multicentennial AMOC variability, while the dominating processes primarily lie in the North Atlantic.

How to cite: Yang, K.: A theory for self-sustained multicentennial AMOC oscillation and its evidence in CESM1, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5739, https://doi.org/10.5194/egusphere-egu23-5739, 2023.

09:30–09:40
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EGU23-14759
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OS1.5
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On-site presentation
Joakim Kjellsson and Wonsun Park

We explore the amplitude and frequency of Atlantic Multi-decadal Variability (AMV) in a 2,000-year pre-industrial control simulation with the FOCI-OpenIFS coupled climate model. We find a statistically significant AMV-like mode on the 20-year and 80-year time scales. We also find a mode of multi-centennial variability where the North Atlantic Ocean shifts a regime of a warm period to/from a cold period of ~400 years. The warm period is characterised by mean states of a stronger and deeper Atlantic Meridional Overturning Circulation (AMOC), less Arctic sea ice, and more deep convection in the Labrador Sea than the cold period. 

 

We find that the AMV has a much higher amplitude in the cold period compared to the warm period, and also that the lead-lag relationship between the AMOC and the AMV is different between the two periods. In the warm period, AMOC leads the AMV; a strong AMOC enhances the oceanic poleward heat transport which warms the North Atlantic Ocean both at the surface and deeper down, producing a positive AMV. In the cold period, however, AMV leads AMOC; a warm surface anomaly reduces the sea ice in the Labrador Sea which enhances local air-sea interactions and deep convection, and later a stronger AMOC. In the cold period, the warm anomaly associated with the AMV does not extend below the mixed layer, suggesting that it is driven by the atmosphere and not ocean dynamics.

How to cite: Kjellsson, J. and Park, W.: Multi-centennial modulation of Atlantic multi-decadal variability in a 2000-year climate integration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14759, https://doi.org/10.5194/egusphere-egu23-14759, 2023.

09:40–09:50
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EGU23-4340
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OS1.5
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On-site presentation
René van Westen and Henk Dijkstra

The Atlantic Meridional Overturning Circulation (AMOC) is considered to be a multi-stable system with a northward overturning and a southward overturning circulation state. It has been proposed that the stability of the AMOC system can be represented through the net freshwater transport at 34°S (the Atlantic's southern boundary), the so-called Fov index. For example when AMOC transports net freshwater out of the Atlantic sector at 34°S (Fov < 0), freshwater (i.e., salinity) perturbations may grow over time through the salt-advection feedback which eventually can induce a state transition. Present-day observations indicate that Fov is negative and  hence the present-day AMOC is in its multi-stable regime.

AMOC state transitions have regional and global impacts and it is therefore important to study the AMOC stability under climate change. However, most climate models have a tendency of simulating a positive Fov index, implying that the AMOC is too stable in these climate model simulations. Here we analyse Fov-related biases using a high-resolution and a low-resolution model version of the Community Earth System Model (CESM). Under constant pre-industrial conditions, the Fov index drifts from negative values to positive values over a 300-year simulation period. The Fov biases are related to biases in the E-P fluxes, freshwater runoff from Greenland, Agulhas leakage, Southern Ocean deep convection and the (meridional) location of the Antarctic Circumpolar Current front. These numerous processes contributing to Fov are responsible the difficulty in simulating realistic AMOC behaviour in climate model simulations. The implication is that climate models with an inconsistent Fov index are not fit for purpose in making AMOC projections.

How to cite: van Westen, R. and Dijkstra, H.: Model Biases in the AMOC Stability Indicator, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4340, https://doi.org/10.5194/egusphere-egu23-4340, 2023.

09:50–10:00
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EGU23-14096
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OS1.5
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ECS
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On-site presentation
The weakening AMOC under extreme climate change 
(withdrawn)
Gaurav Madan, Ada Gjermundsen, Silje C. Iversen, and Joseph H. LaCasce
10:00–10:10
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EGU23-13393
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OS1.5
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ECS
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On-site presentation
Levke Caesar and Gerard McCarthy

It has been suggested previously that the long-term warming hole in the subpolar North Atlantic, that is the relative cooling in this region compared to the rest of the globe, is an indicator of a slowdown of the Atlantic Meridional Overturning Circulation (Caesar et al., 2018; Drijfhout et al., 2012; Rahmstorf et al., 2015), yet other drivers like aerosols or a change in the local atmospheric forcing (e.g., the wind stress curl) have been proposed (Li et al., 2021; Piecuch et al., 2017). The still not fully answered question of the driver(s) of the warming hole also raises the question of whether or not ocean temperatures in the subpolar North Atlantic can be used as an indicator for AMOC strength. While several studies suggest that AMOC strength and temperatures in the subpolar North Atlantic are dynamically linked through the AMOC’s northward heart transport (Dima et al., 2022; Latif et al., 2022; Zhang, 2008), a recent model-based study suggests that the correlation between temperature-based AMOC index (Caesar et al., 2018) and AMOC strength depends largely on the subtraction of the global warming signal (Little et al., 2020).

Based on the knowledge that the AMOC transports both heat into and freshwater out of the North Atlantic, we apply a lead-lag correlation analysis to both the North Atlantic’s heat and freshwater content to identify the region and the time lag that give the strongest correlation with the strength of the AMOC (to make use of the available observational data we consider the AMOC strength at 26˚N). We find that an AMOC weakening (strengthening) leads to cooling (warming) and simultaneous freshening (salinification) in the eastern subpolar North Atlantic with the upper ocean (200-1000m) contents showing a higher correlation with AMOC strength than the surface (0-200m) contents. The temporal evolution of heat and freshwater content in the eastern subpolar gyre region are furthermore strongly anticorrelated, with a correlation value of -0.82 (for the annual values) as expected for an AMOC (or otherwise advective) driven signal. On longer time scales this anticorrelation decreases unless the heat content is corrected for a large scale warming signal. This could suggest that it is indeed necessary to look at the relative not the absolute temperature evolution in the subpolar North Atlantic to extract the AMOC signal.

Both the absolute freshening in the eastern subpolar North Atlantic as well as the relative (compared to the rest of the North Atlantic) cooling in this region suggest a linear AMOC trend of about -2 Sv from 1957-2013.

References

Caesar, L., et al. (2018).  https://doi.org/10.1038/s41586-018-0006-5

Dima, M., et al. (2022).  https://doi.org/10.1007/s00382-022-06156-w

Drijfhout, S., et al.  (2012). https://doi.org/10.1175/jcli-d-12-00490.1

Latif, M., et al.  (2022). https://doi.org/10.1038/s41558-022-01342-4

Li, L., et al.  (2021). https://doi.org/10.1007/s00382-021-06003-4

Little, et al.  (2020).  https://doi.org/10.1029/2020gl090888

Piecuch, C. G., et al.  (2017).  https://doi.org/https://doi.org/10.1002/2017JC012845

Rahmstorf, S., et al.  (2015). https://doi.org/10.1038/nclimate2554

Zhang, R. (2008). https://doi.org/10.1029/2008GL035463

How to cite: Caesar, L. and McCarthy, G.: AMOC changed derived from simultaneous (absolute) freshening and (relative) cooling in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13393, https://doi.org/10.5194/egusphere-egu23-13393, 2023.

Coffee break
Chairperson: Bablu Sinha
10:45–10:55
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EGU23-10569
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OS1.5
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On-site presentation
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Matthew H. England, Bryam Orihuela-Pinto, and Andréa Taschetto

The Atlantic Meridional Overturning Circulation (AMOC) has a profound impact on both global and regional climate, yet our understanding of the mechanisms controlling remote teleconnections remains limited. In addition, it is unclear how remote processes impact the North Atlantic and alter the strength of the AMOC.  In this presentation I will show how a slowdown in the AMOC can drive an acceleration of the Pacific trade winds and Walker circulation by leaving an excess of heat in the tropical South Atlantic. This tropical Atlantic warming drives anomalous atmospheric convection, resulting in enhanced subsidence over the east Pacific, and a strengthened Walker circulation and trade winds. Further teleconnections include a shift in the ITCZ, enhanced zonal SST gradients across the tropical Pacific, strengthened convection over the West Pacific Warm Pool, and a deepening of the Amundsen Sea Low off Antarctica.  Teleconnections back to the North Atlantic can in turn be triggered by Southern Hemisphere wind anomalies on a relatively rapid time-scale via propagating planetary waves in the ocean.  There is also evidence that tropical Pacific cooling can feedback and influence the strength of the AMOC.  These findings have implications for understanding both intrinsic decadal climate variability as well as longer-term climate change.

How to cite: England, M. H., Orihuela-Pinto, B., and Taschetto, A.: Global climate teleconnections into and out of the North Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10569, https://doi.org/10.5194/egusphere-egu23-10569, 2023.

10:55–11:05
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EGU23-2777
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OS1.5
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ECS
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On-site presentation
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Matthew Patterson and Tim Woollings

The large amplitude of low-frequency sea surface temperature (SST) variability in the North Atlantic, often known as Atlantic Multi-decadal Variability (AMV), raises the question of what impact this phenomenon has on atmospheric circulation. However, the coupled nature of AMV, makes disentangling the influence of the ocean on the atmosphere and that of the atmosphere on the ocean, challenging. This problem is further confounded by the relatively short observational record, when considering decadal to multi-decadal timescales.

To address this, we utilize information from both SSTs and ocean-atmosphere turbulent heat fluxes,  in a single index, to separate the influences that the ocean and atmosphere have on one another. This technique is then applied to both free-running coupled simulations and observations. This methodology will help further our understanding of North Atlantic variability on long timescales.

How to cite: Patterson, M. and Woollings, T.: Impacts of Atlantic Multi-decadal Variability on the mid-latitude atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2777, https://doi.org/10.5194/egusphere-egu23-2777, 2023.

11:05–11:15
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EGU23-3850
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OS1.5
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On-site presentation
Jacek Piskozub

Winter (December-March) temperatures in most Europe is strongly correlated with the zonal circulation index of the Atlantic sector, North Atlantic Oscillation (NAO), regardless of which from its several available definitions to choose. However, the variability of this index also has a distinct multi-decade component, which makes it difficult to study trends of several decades. In addition, the NAO index itself is also significantly positively correlated with the global anthropogenic forcing and with the global temperature itself. Therefore, using purely statistical methods, it is not easy to distinguish the influence of zonal circulation variability from the trend resulting from the increasing forcing of greenhouse gases when examining the variability of winter temperatures in Europe.

Because of the prevailing western circulation, wintertime temperature in Europe should dpend on the intensity of the western circulation (NAO) as well as the sea surface temperature (SST) of the ocean (indexed by AMO – Atlantic Multidecadal Oscillation). However winter is the only season when there is no statistically significant correlation of AMO and temperatures in Europe. This surprising result had no explanation until the recent discovery of the northern shift of synoptic systems correlated with AMO. This could offer a “Bjerknes compensation” type of effect where the ocean circulation modifies the atmospheric one, making the air masses arriving in winter to Europe sourced in areas of the same SST, regardless of AMO (or general warming of the North Atlantic).

This study uses the data on SST and pressure fields as well as the NAO and AMO, together with an index if temperatures of Poland (as a proxy of Central Europe) in order to throw light on the relationship. The results confirm the existence of a “Bjerknes compensation” mechanism as well as suggest a dependence of wintertime NAO on the greenhouse forcing (visible in their significant correlation), caused most probably by the recently discovered strengthening of wintertime jet stream over the North Atlantic. This relationship can have important impact on future winter temperatures in a large part of Europe and therefore its possible mechanisms should be the point of further research.

 

This work been performed as a part of the SURETY project , funded by Polish National Science Centre (NCN), contract 2021/41/B/ST10/00946.

 

How to cite: Piskozub, J.: Is the North Atlantic modulating wintertime influence of NAO on Europe temperatures?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3850, https://doi.org/10.5194/egusphere-egu23-3850, 2023.

11:15–11:25
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EGU23-14285
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OS1.5
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ECS
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On-site presentation
Amar Halifa-Marín, Miguel A. Torres-Vázquez, Enrique Pravia-Sarabia, Ricardo Trigo, Sergio M. Vicente-Serrano, Marco Turco, Sonia Jerez, Pedro Jiménez-Guerrero, and Juan Pedro Montavez

This study assesses how the CMIP6 simulations capture the non-stationarity of the main source of winter climate variability in the Euro-Atlantic region, the North Atlantic Oscillation (NAO), observed in the recent past.

For that purpose, we characterise the NAO long-term variability in climate reanalysis, analysing their features in several 30-year periods since 1851; and we evaluate whether CMIP6 historical simulations capture all the observed NAO “types”. Although the literature sometimes assumes that the NAO pattern is stationary, three groups of NAO pattern have been proved in the reanalyses depend on the location of their Action Centres (ACs): 1) the north AC locates over Iceland and the south AC in Azores, 2) the north AC locates over Southern Greenland and the south AC in the Western Mediterranean, and 3) the north AC locates over Northern Scandinavia and the south AC in the Azores.

Our main finding is that the NAO long-term variability is not accurately captured by all CMIP6 models. In particular, the overestimation of the NAO group 3 is remarkable in most simulations. This NAO group mainly represents the last decades, which the literature has addressed with much interest for its exceptional features (e.g. NAO+ strengthening and northeastward shift of its north AC), and which has been generally associated with the anthropogenic warmer climate. We also found underestimation of NAO group 2.

We have also found that each NAO group could be associated with precipitation anomalies in Europe. For example, the NAO group 3 implies drier(wet) conditions in the south(north). While group 2 implies the opposite pattern of anomalies. Therefore, we have reason to suggest that the lack of accuracy of models reproducing the non-stationarity of NAO may explain some of the bias in the expected changes of winter precipitation in Europe for future scenarios.

How to cite: Halifa-Marín, A., Torres-Vázquez, M. A., Pravia-Sarabia, E., Trigo, R., Vicente-Serrano, S. M., Turco, M., Jerez, S., Jiménez-Guerrero, P., and Montavez, J. P.: On the inaccuracy of CMIP6 models in capturing the observed long-term variability of the NAO, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14285, https://doi.org/10.5194/egusphere-egu23-14285, 2023.

11:25–11:35
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EGU23-5909
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OS1.5
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On-site presentation
Torge Martin and Arne Biastoch

Both Greenland Ice Sheet mass loss and Atlantic Meridional Overturning Circulation weakening are considered tipping elements of the climate system under global warming. Ocean and climate models of varying complexity are widely applied to understand and project the future evolution of the two processes and their connection. The results are prone to model uncertainty however. Especially the role of regional mesoscale processes in the subpolar North Atlantic is still being investigated. We ran a systematic set of eight dedicated 60 to 100-year long model experiments with and without atmospheric coupling, with eddy processes parameterized and explicitly simulated, with regular and significantly enlarged Greenland runoff to reconcile findings of the regional ocean and global climate modeling communities.

The most prominent result is a major impact by an interactive atmosphere for limiting the AMOC weakening through enabling a compensating temperature feedback. Coupled experiments yield an AMOC decline of <2Sv to a freshwater perturbation of 0.05Sv whereas the AMOC weakens by >4Sv in the ocean-only runs. In addition to this large-scale effect, we find that the Labrador Sea and the Northwest Corner (off Flemish Cap) are critical regions for the role of mesoscale eddies in redistributing Greenland meltwater and affecting the timing of its impact. We show that an ocean grid at 1/10˚–1/12˚, which is currently used in global high-resolution climate simulations, can already significantly improve the path of the meltwater along the North American coast and into the wider North Atlantic. But the same resolution still falls short in providing sufficient dynamical exchange between the boundary current and the interior Labrador Sea and especially lacks capability in restratifying the Labrador Sea after deep convection. Our experiments demonstrate where an eddy parameterization works quite successfully and where only high resolution (>1/12˚) yields a realistic ocean response. This underlines the necessity to advance scale-aware eddy parameterizations for next-generation climate models.

How to cite: Martin, T. and Biastoch, A.: Ocean response to Greenland melting in a hierarchy of model configurations: Relevance of eddies and an interactive atmosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5909, https://doi.org/10.5194/egusphere-egu23-5909, 2023.

11:35–11:45
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EGU23-5941
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OS1.5
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Virtual presentation
Marion Devilliers, Shuting Yang, Steffen Olsen, and Annika Drews
This study summarizes the findings of several realistic freshwater forcing experiments around Greenland and surrounding regions, which were conducted using climate and ocean models over the historical period. The results of the experiments are discussed in terms of their impact on the Atlantic Meridional Overturning Circulation (AMOC) as well as on temperature and salinity changes in the North Atlantic and in the Arctic. It was determined that the addition of freshwater led to a decrease of the AMOC and a reduction of the temperature and salinity biases in the North Atlantic. These results will be of particular interest to researchers interested in the effects of current and future Greenland melting on local and global ocean.

 

How to cite: Devilliers, M., Yang, S., Olsen, S., and Drews, A.: Realistic freshwater forcing around Greenland in climate models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5941, https://doi.org/10.5194/egusphere-egu23-5941, 2023.

11:45–11:55
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EGU23-7844
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OS1.5
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ECS
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On-site presentation
Tillys Petit, Jon Robson, David Ferreira, and D. Gwyn Evans

The surface forced water mass transformation (SFWMT) is known to be the main contributor of the Atlantic Meridional Overturning Circulation (AMOC) over the subpolar gyre. Over the eastern part of the subpolar gyre, a recent study revealed the dominant role of surface density changes in driving the SFWMT as opposed to the direct influence of air-sea fluxes. Indeed, the distribution at surface of the isopycnal associated with the maximum overturning streamfunction, Smoc, modulates the area of dense water formation induced by the air-sea fluxes.

The Overturning in the Subpolar North Atlantic Program (OSNAP) showed that the density of Smoc is highly variable in time along each section of the array. However, the drivers of Smoc remain unclear. In our work, we use a combination of atmospheric reanalysis and coupled simulations of HadGEM3-GC3.1 to evaluate the Smoc variability over the subpolar gyre as well as its connection with the overturning strength. At interannual timescale, the variability of Smoc at OSNAP East is strongly related to those at OSNAP West and at 45°N. However, its connection with the overturning strength is more complex. Although Smoc is not well related to the overturning at OSNAP, it is associated with a shift in density of the overturning stream function. The Irminger Sea is identified as being the centre of action driving this variability.

How to cite: Petit, T., Robson, J., Ferreira, D., and Evans, D. G.: Linkage between overturning and density anomaly over the subpolar gyre, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7844, https://doi.org/10.5194/egusphere-egu23-7844, 2023.

11:55–12:05
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EGU23-17084
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OS1.5
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On-site presentation
Denis Volkov, Claudia Schmid, Leah Chomiak, Cyril Germineaud, Shenfu Dong, and Marlos Goes

The gyre-scale, dynamic sea surface height (SSH) variability signifies the spatial redistribution of heat and freshwater in the ocean, influencing the ocean circulation, weather, climate, sea level, and ecosystems. It is known that the first empirical orthogonal function (EOF) mode of the interannual SSH variability in the North Atlantic exhibits a tripole gyre pattern, with the subtropical gyre varying out of phase with both the subpolar gyre and the tropics, influenced by the low-frequency North Atlantic Oscillation. We show that the first EOF mode explains the majority (60 %–90 %) of the interannual SSH variance in the Labrador and Irminger Sea, whereas the second EOF mode is more influential in the northeastern part of the subpolar North Atlantic (SPNA), explaining up to 60 %–80% of the regional interannual SSH variability. We find that the two leading modes do not represent physically independent phenomena. On the contrary, they evolve as a quadrature pair associated with a propagation of SSH anomalies from the eastern to the western SPNA. This is confirmed by the complex EOF analysis, which can detect propagating (as opposed to stationary) signals. The analysis shows that it takes about 2 years for sea level signals to propagate from the Iceland Basin to the Labrador Sea, and it takes 7–10 years for the entire cycle of the North Atlantic SSH tripole to complete. We demonstrate that the observed interannual-to-decadal variability of SSH, including the westward propagation of SSH anomalies, is the result of a complex interplay between the local wind and surface buoyancy forcing, and the advection of properties by mean ocean currents. The relative contribution of each forcing term to the variability is space and time dependent. We show that the most recent cooling and freshening observed in the SPNA since about 2010 were mostly driven by advection associated with the North Atlantic Current. The results of this study indicate that signal propagation is an important component of the North Atlantic SSH tripole, as it applies to the SPNA.

 

How to cite: Volkov, D., Schmid, C., Chomiak, L., Germineaud, C., Dong, S., and Goes, M.: The role of propagating signals in the gyre-scale interannual to decadal sea level variability in the subpolar North Atlantic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17084, https://doi.org/10.5194/egusphere-egu23-17084, 2023.

12:05–12:15
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EGU23-14337
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OS1.5
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ECS
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On-site presentation
Olivier Narinc, Thierry Penduff, Guillaume Maze, Stéphanie Leroux, and Jean-Marc Molines

Using ensemble ocean simulations, recent studies have shown that non-linear intrinsic oceanic processes are a source of chaotic intrinsic oceanic variability (CIOV). It was found that in eddy-active regions and at interannual timescales, this CIOV can be a significant fraction of total variability, and that as model resolution increases small-scale non-linearities can generate variability at large scales. The Eighteen Degree Water (EDW) is a mode water formed in the winter mixed layer within and south of the Gulf Stream. It is the most abundant T,S class of water in the surface North Atlantic and has been shown to be an important contributor to air-sea exchanges over the entire North Atlantic basin. Observational studies have shown that a significant part of the interannual variability of EDW cannot be explained by atmospheric variability. This motivates the present investigation of the importance of interannual CIOV in the total interannual EDW variability. The present study uses a NEMO-core, 1/4°, 50-member ensemble hindcast of the North Atlantic ocean with a realistic atmospheric forcing. This ensemble simulation is assessed using ARMOR3D, a 3-dimensional gridded observational product obtained using satellite altimetry and ARGO floats. In both datasets, the 3-dimensional structure of EDW is identified using physical criteria. This spatial structure is used to compute timeseries of the EDW’s total volume and average temperature, in each ensemble member and in the observational product. It is found that the ensemble simulation produces a realistic EDW, with a comparable total variability. In the ensemble simulation, the CIOV of integrated EDW properties is estimated from their time-averaged ensemble standard deviation, and is compared to the total variability estimated from the ensemble mean of the temporal standard deviations of all members. In the ensemble, CIOV accounts for 13% of the total interannual variability of EDW volume, and 44% of the total interannual variability of EDW temperature. Notably, this means that CIOV is a source of unquantifiable uncertainty in single-member ocean simulations. This suggests that a significant part of observed interannual variability may also be chaotic intrinsic in nature. This calls for a better parametrisation of chaotic variability in ocean simulations.

How to cite: Narinc, O., Penduff, T., Maze, G., Leroux, S., and Molines, J.-M.: Impact of interannual chaotic variability on the total interannual variability of the North Atlantic Eighteen Degree Water, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14337, https://doi.org/10.5194/egusphere-egu23-14337, 2023.

12:15–12:25
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EGU23-3476
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OS1.5
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On-site presentation
Robert Marsh, Dipanjan Dey, and Sybren Drijfhout

Subpolar and high-latitude regions of the North Atlantic are subject to changing buoyancy and mechanical forcing, alongside changing heat and freshwater exchanges with subtropical and polar regions. Associated changes in water mass formation and circulation are accompanied by changes in upper ocean stratification, of consequence for the large-scale ocean circulation, air-sea interaction, and ocean biogeochemistry. Changes in water mass volumes, and the associated overturning circulation, have been extensively evaluated with the water mass transformation (WMT) framework. Changes in stratification may be quantified with the Potential Energy Anomaly (PEA) framework, which has been extensively applied to seasonally stratified shelf sea environments. The WMT and PEA frameworks in combination provide complementary and holistic insights, for understanding hydrographic changes in relation to selected drivers. These frameworks are used with high-resolution model ocean datasets, obtained from hindcast and coupled simulations, the latter in control mode and forced by rising greenhouse gas concentrations through the 20th and 21st centuries. For selected sub-regions of the subpolar North Atlantic, bound by OSNAP and neighbouring hydrographic sections, mapped stratification (PEA) anomalies are related to respective changes in surface heat and freshwater fluxes. Residual differences between buoyancy-forced and full PEA tendencies are attributed to vertical mixing and divergences of heat and freshwater transports. Changes in regional stratification are evaluated alongside corresponding rates of water mass transformation and associated volumetric variability, for selected water masses. Eulerian perspectives provided by the WMT and PEA frameworks further complement Lagrangian perspectives provided by particle tracking. In full combination, these diagnostics elucidate multiple drivers of change in the North Atlantic that have potentially far-reaching consequences for the wider Earth System.

How to cite: Marsh, R., Dey, D., and Drijfhout, S.: Evaluating recent and future changes in North Atlantic stratification with complementary energetics and water mass frameworks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3476, https://doi.org/10.5194/egusphere-egu23-3476, 2023.

Posters on site: Wed, 26 Apr, 14:00–15:45 | Hall X5

Chairperson: Richard Greatbatch
X5.348
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EGU23-1639
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OS1.5
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ECS
Andrea Vito Vacca, Katinka Bellomo, and Jost von Hardenberg

The Atlantic meridional overturning circulation (AMOC) is a vital component of the global climate system regulating heat, carbon, and freshwater distribution. Most models predict a weakening of the AMOC throughout the 21st century, although there is significant uncertainty about its magnitude and the related regional climate impacts. In particular, the response of large-scale atmospheric circulation to the AMOC slowdown is still largely unknown, with implications for weather extremes and associated societal risks. The purpose of this study is to enhance our understanding of the impacts of an AMOC slowdown on atmospheric patterns with a focus on the Euro-Atlantic region, where the influence of AMOC is particularly relevant.

We analyse changes in an ensemble of idealised abrupt-4xCO2 climate model simulations from the CMIP archives with respect to the preindustrial climate. We split the models into groups according to their AMOC response to the 4xCO2. Through rigorous statistical testing, we attribute the differences in the simulated climate impacts to the difference in the AMOC response. Specifically, we find that models that simulate a larger AMOC decline feature minimum warming in the subpolar North Atlantic (North Atlantic Warming Hole or NAWH), a southward shift of the ITCZ, and a poleward strengthening of the mid-latitude jet stream. Instead, models that simulate a smaller AMOC decline feature enhanced North Atlantic warming, an intensification of the hydrological cycle but no southward shift in the ITCZ, and smaller displacements of the mid-latitude jet.  

To better characterize the large-scale atmospheric response at daily timescales, we use k-means clustering and self-organising maps to assess the changes in weather regimes over the Euro-Atlanic sector, including the NAO.  We further compare weather regimes’ frequency of occurrence and persistence between the two groups, attributing the differences to the AMOC decline. 

Our results indicate that the AMOC is a key source of large inter-model uncertainty in the simulation of future climate change impacts. Further observational campaigns may thus help us alleviate model biases and provide constraints on a number of societally relevant climate change impacts.

How to cite: Vacca, A. V., Bellomo, K., and von Hardenberg, J.: Impacts of AMOC slowdown on European circulation patterns, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1639, https://doi.org/10.5194/egusphere-egu23-1639, 2023.

X5.349
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EGU23-3118
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OS1.5
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ECS
Weimin Jiang, Guillaume Gastineau, and Francis Codron

The climate impacts of fluctuations in the Atlantic meridional overturning circulation (AMOC) are studied using an atmosphere-ocean general circulation model (AOGCM). In two experiments, the baroclinic component of the North Atlantic Ocean currents is modified online to reproduce typical strong and weak AMOC conditions found in a preindustrial control simulation using the same model. These experiments are compared with slab ocean model (SOM) experiments that use heat flux corrections from the coupled model in the Atlantic Ocean. The main impacts of a strong AMOC include widespread warming in the Northern Hemisphere and a northward shift of the intertropical convergence zone (ITCZ). SOM experiments show similar atmospheric responses to AMOC-related heat flux anomalies, but with much larger impacts in the tropics. 
The atmospheric changes are driven by an anomalous cross-equatorial Hadley circulation transporting energy southward and moisture and heat northward. In the AOGCM, changes in the Indo-Pacific Ocean circulation and heat transport, driven by the wind stress associated with the abnormal Hadley cell, damp the atmospheric response. In the SOM simulations, the lack of Indo-Pacific transport and of ocean heat storage leads to larger atmospheric changes, that are further amplified by a positive tropical low cloud feedback. 

How to cite: Jiang, W., Gastineau, G., and Codron, F.: Climate response to Atlantic meridional energy transport variations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3118, https://doi.org/10.5194/egusphere-egu23-3118, 2023.

X5.350
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EGU23-5422
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OS1.5
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ECS
Rémy Asselot, Lidia I. Carracedo, Virginie Thierry, Herlé Mercier, Anton Velo, Raphaël Bajon, and Fiz F. Pérez

The ocean is a net sink for a quater of the carbon dioxide emitted to the atmosphere by human industrial activities and land-use change (Cant). The North Atlantic Ocean encompasses the highest ocean storage capacity of Cant per unit area. In particular, the Labrador and Irminger Seas are two basins storing a high amount of Cant due to the deep convection activity taking place there. The temporal evolution of Cant concentration in these two basins and their Cant inventories in the 0-1800 depth layer are estimated over the period 2011-2021. The Cant values are estimated from Argo floats equipped with oxygen sensors, predictive neural networks (ESPER_NN and CONTENT) and a carbon-based back-calculation method (φCOT method). On average, Cant inventories are similar in the two basins and amount to 75.3 and 75.6 mol/m2 in the Irminger and Labrador seas, respectively. Over the study period, Cant inventories increase in the two basins at a storage rate of 1.01±0.14 mol/m2/yr in the Irminger Sea and 0.94±0.2 mol/m2/yr in the Labrador Sea. The processes involved in Cant evolution in the two basins are then investigated.

How to cite: Asselot, R., Carracedo, L. I., Thierry, V., Mercier, H., Velo, A., Bajon, R., and Pérez, F. F.: Argo-based anthropogenic carbon concentration and inventory in the Labrador and Irminger Seas over 2011-2021, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5422, https://doi.org/10.5194/egusphere-egu23-5422, 2023.

X5.351
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EGU23-6219
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OS1.5
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ECS
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Ilana Schiller-Weiss, Torge Martin, Johannes Karstensen, and Arne Biastoch

Accelerated melting of the Greenland Ice Sheet has been identified as a tipping element in the freshwater balance of the subpolar North Atlantic (SPNA), where the East Greenland Current is a primary pathway for transporting Arctic-sourced freshwater and Greenland glacial meltwater. Understanding the freshwater variability of the East Greenland Current (EGC) and Coastal Current (EGCC) and their interaction is of high importance, as these gather the imprint of ice melt once the meltwater leaves the fjords and enters the open ocean. Using a high-resolution model (VIKING20X, 1/20°) and gridded, observational assimilated reanalysis (GLORYS12, 1/12°), we find the freshest water remains close to the shelf with interannual extremes in freshwater content attributable to the imprint of Greenland melt only in years 2010 and 2012. This signal is only found in the VIKING20X simulation, which in contrast to GLORYS12 uses realistic, interannually varying runoff forcing including estimates of the Greenland Ice Sheet mass balance. We further discuss the role of wind forcing, sea ice melt, and Greenland runoff, which all contribute to variability in freshwater content along the boundary current.

Our results show that slackened alongshore winds reduce onshore Ekman transport allowing for freshwater to spread laterally in the EGC, while stronger alongshore winds constrain freshwater closer to the shelf with saline intrusions from the interior basin into the outer EGC. South of ~65°N sea ice melts year round and retreats northward with melting occurring only in summer. Associated salinity and thus freshwater content anomalies are of similar magnitude as those associated with meltwater runoff and overlap in both seasonal timing and advective time scales. This could explain the challenges to identify freshening events originating from extreme melt events on the Greenland Ice Sheet at currently observed magnitudes. Their detection is critically dependent on synoptic and interannually varying processes. Our findings also suggest that ocean models or model-based reanalysis products aiming to illustrate the processes of meltwater redistribution should feature grid resolutions preferably exceeding 1/12° in order to represent coastal dynamics and fjord-shelf-open ocean exchange. With more observations on the Greenland shelf hopefully becoming available in the future, we anticipate the GLORYS12 assimilation product to show similar variability as higher resolution models.

How to cite: Schiller-Weiss, I., Martin, T., Karstensen, J., and Biastoch, A.: Does freshwater content of the East Greenland Current show imprints of increasing meltwater runoff?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6219, https://doi.org/10.5194/egusphere-egu23-6219, 2023.

X5.352
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EGU23-6169
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OS1.5
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ECS
Aleksandr M. Fedorov, M. Femke de Jong, Claudia E. Wieners, and Henk A. Dijkstra

The Atlantic Meridional Overturning Circulation (AMOC) is a three-dimensional system of ocean currents contributing to a relatively mild climate in northern Europe. AMOC transports vary on a range of time scales, from centennial to daily. Despite a wide history of research on AMOC variability from both measurements and modeling perspectives, the role of atmospheric noise in subseasonal and intra-annual AMOC variations remains unclear. In the current study, we describe the modeling experiment planned to reveal the importance of mesoscale winds around the southern tip of Greenland, named Tip Jets, in  AMOC variability. Tip Jet wind events have no regularity in frequency and intensity, they mostly depend on the Icelandic Low location and are partly associated with a positive NAO phase in winter. The experiment design is based on implying the perturbations in the momentum, heat, and freshwater forcing associated with Tip Jet events. Therefore, we constructed the composite Tip Jet forcing using daily ERA5 (25 km, December-March, 1969-2019) wind, surface fluxes, and precipitation/evaporation rates. These composite fields of anomalies are planned to be added to the CESM-derived climatology to describe the possible response of the AMOC system to these types of noise forcing. Setting up the experiment included the model verification based on the comparison between the monthly output from the ~0.1 ° CESM Parallel Ocean Product (POP) simulations and the observational OSNAP array that combines measurements along the line between Labrador, Greenland, and the European shelf. The mean state of the atmosphere from CESM (50 km, monthly) was compared to the ERA5 (25 km, monthly). Generally, the CESM model reproduces the AMOC at OSNAP well. In conclusion, this preliminary research shows that AMOC is well simulated by the Community Earth System Model in the Subpolar North Atlantic. Also, the current research proposes patterns of noise forcing over the North Atlantic Subpolar Gyre that will be used in further modeling experiments.

How to cite: Fedorov, A. M., de Jong, M. F., Wieners, C. E., and Dijkstra, H. A.: Subpolar Atlantic Meridional Overturning in Community Earth System Model (CESM): setting up the further experiment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6169, https://doi.org/10.5194/egusphere-egu23-6169, 2023.

X5.353
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EGU23-7195
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OS1.5
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ECS
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Charlotte Marris and Robert Marsh

In the Subpolar North Atlantic (SPNA), interannual to multidecadal variability in the Atlantic Meridional Overturning Circulation (AMOC) is primarily attributed to surface buoyancy forcing. Here, warm surface waters arriving via the Gulf Stream and North Atlantic Current undergo an intense loss of heat and freshwater to the atmosphere and are thus transformed to cold and dense waters which subsequently sink and are returned southward at depth. Quantifying the contribution of surface buoyancy forcing to AMOC variability is essential for modelling how the AMOC will respond to predicted warming and freshening at high latitudes. In a water mass transformation framework, fields of surface density and surface density flux from the GODAS ocean reanalysis are used to construct the surface-forced overturning circulation (SFOC) streamfunction for the SPNA (48-65°N) in an operational assimilation over 1980-2020. Computed and plotted in latitude-density space, the SFOC reconstruction compares favourably with the corresponding AMOC, computed from GODAS currents. We thus conclude that subpolar AMOC variability is largely explained by changing air-sea heat and freshwater fluxes controlling water mass transformation across the region. We further highlight the changing relative influences of water mass transformation in the eastern and western subpolar gyre, by partitioning SFOC longitudinally into an East component (5-43 °W) comprising the Irminger and Iceland basins, and a West component (43-60 °W) comprising the Labrador Sea. Our analysis demonstrates that interannual to multidecadal SFOC variability is dominated by changing water mass transformation in the western subpolar gyre. This challenges a shifting consensus that highlights the eastern subpolar gyre as dominant in driving the AMOC across subpolar latitudes.

How to cite: Marris, C. and Marsh, R.: Quantifying the Contribution of Surface Buoyancy Forcing to Recent Subpolar AMOC Variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7195, https://doi.org/10.5194/egusphere-egu23-7195, 2023.

X5.354
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EGU23-10227
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OS1.5
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ECS
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María Jesús Rapanague, Dian Putrasahan, and Jochem Marotzke

The Atlantic Meridional Overturning Circulation (AMOC) is fundamental for the northward transport of heat and the vertical transport of carbon from the surface to the deep ocean in the North Atlantic Ocean, influencing the climate at both local and global scales. However, the mechanisms underlying the AMOC variability are still poorly understood, because of the lack of long-term observations and the challenge of representing key processes in standard climate models. Furthermore, assumptions widely accepted for several decades have re-entered the debate in recent years, such as the AMOC meridional coherence and the role of deep convection in the Labrador Sea in driving the AMOC variability and deepwater formation. New modeling and observational studies suggest that the overturning variability is not coherent between subtropical and subpolar latitudes on interannual to decadal scales and that climate models systematically exaggerate the importance of the Labrador Sea, pointing toward other regions like the Irminger Sea and the Nordic Sea as better candidates for deepwater formation.

In this study, we aim to critically assess the long-held notion of meridional coherence in the AMOC, using output from high- and very-high-resolution model simulations. Specifically, we investigate how the meridional coherence of the AMOC changes when increasing model resolution, via spectral analysis of the MPI-ESM1.2 control simulations with resolutions of 1°, 0.4°, and 0.1°. Preliminary analysis using lead-lag time correlations indicates a high correlation and meridional coherence between the AMOC strength and mixed layer depth variability in the Labrador Sea for the coarsest resolution. However, when increasing the resolution this relationship disappears, and the AMOC is instead better related to overflow changes in the Denmark Strait and in the Nordic Seas. Additionally, the meridional coherence of the AMOC becomes unclear.

How to cite: Rapanague, M. J., Putrasahan, D., and Marotzke, J.: Is the AMOC connected across all latitudes?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10227, https://doi.org/10.5194/egusphere-egu23-10227, 2023.

X5.355
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EGU23-11459
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OS1.5
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ECS
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Lisa Gerlinde Thekla Leist, Maxi Castrillejo, John N. Smith, Marcus Christl, and Núria Casacuberta

The Subpolar North Atlantic (SPNA) and Labrador Sea are key regions for deep and intermediate water mass formation and contribute to the southward return flow of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC).

The origin and circulation pathways of these water masses can now be studied using the artificial radionuclides 129I and 236U. These tracers are mainly released to the Nordic seas by the European nuclear reprocessing plants of La Hague and Sellafield since the 1960s. This point like source provides a unique fingerprint for Atlantic waters entering the Arctic Ocean and recirculation to the western SPNA.

Here we will present results of the distribution of 129I and 236U in the Labrador Sea (AR7W Line) and the SPNA (OVIDE Line). The 129I concentrations and its temporal evolution is studied at 11 stations on a time series that started in 2014. In addition, first results of 236U  will also be  presented along the AR7W line.

At the timeseries the 129I concentration shows a general increase with time and from east to west, reaching its highest concentration in the deep overflow waters and along the Eastern and Western Greenland current.

The combination of the well-known tracer 129I with 236U allows to study the origin and mixing of different water masses in the SPNA.

How to cite: Leist, L. G. T., Castrillejo, M., Smith, J. N., Christl, M., and Casacuberta, N.: Tracing Ocean circulation at the AR7W and OVIDE lines using artificial radionuclides, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11459, https://doi.org/10.5194/egusphere-egu23-11459, 2023.

X5.356
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EGU23-12040
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OS1.5
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ECS
Oliver Mehling, Katinka Bellomo, and Jost von Hardenberg

It has been hypothesized that climate variability on centennial timescales – in the North Atlantic region and beyond – is linked to unforced variability of the Atlantic Meridional Overturning Circulation (AMOC). Because of the presence of external forcings, uncertainties in proxy reconstructions of the AMOC and the short observational record, coupled climate models represent a key tool in assessing low-frequency AMOC variability. However, sufficiently long pre-industrial control (piControl) simulations with state-of-the-art climate models have only become widely available during the past decade. While significant centennial-scale AMOC variability has been identified in several single-model studies, proposed physical mechanisms differ considerably.

Here, we assess mechanisms of AMOC variability on centennial timescales in the CMIP6 multi-model piControl ensemble. We find that a relatively large number of models – 11 out of the 15 analyzed – exhibit a statistically significant mode of centennial-scale MOC variability in the Atlantic. We review previously proposed mechanisms for centennial-scale AMOC variability and test whether their key elements are present in the CMIP6 ensemble.

We find that salinity exchanges between the Arctic and North Atlantic basins, which have previously been proposed as drivers of multi-centennial AMOC variability in two CMIP6 models (IPSL-CM6A-LR and EC-Earth3), can also be identified in other CMIP6 models using the same ocean component (NEMO). However, we find only a weak or no signature of this mechanism in models that do not include NEMO. Even among NEMO models, the amplitude and timescale of centennial-scale AMOC variability is model-dependent, and we assess the relative role of deep-water formation sites in shaping these differences. Because AMOC fluctuations are linked to surface temperature anomalies and related impacts over land, our results motivate the need for more paleoclimate evidence at sub-centennial resolution, which would help constrain the CMIP6 inter-model spread in centennial-scale AMOC variability.

How to cite: Mehling, O., Bellomo, K., and von Hardenberg, J.: Common mechanisms of centennial-scale AMOC variability in CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12040, https://doi.org/10.5194/egusphere-egu23-12040, 2023.

X5.357
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EGU23-12097
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OS1.5
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ECS
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Duncan Dale, Marcus Christl, Andreas Macrander, Sólveig Ólafsdóttir, Rob Middag, and Núria Casacuberta

Iceland stands at an important gateway where Arctic and Atlantic waters interact. Atlantic waters pass northward and circulate in the Arctic before returning southward in the East Greenland Current (EGC). Zones of deep water formation in the Nordic Seas contribute to overflows of the Iceland-Scotland Ridge such as Denmark Strait Overflow Water (DSOW). These are key processes in Arctic warming and deep ocean ventilation.

This system has been tagged with anthropogenic radionuclides 129I and 236U by bomb tests in the 1950-60s and point-source nuclear reprocessing plants (NRPs) at Sellafield (UK) and La Hague (FR) since the 1960s providing an opportunity to trace the origins of water masses in the region and their transit timescales. Here we present the results of measurements on samples taken during two cruises around Iceland in 2021 by the Marine and Freshwater Research Institute (MFRI) of Iceland (winter) and the NIOZ MetalGate Cruise of the GEOTRACES program (summer). Models for the origin of waters transiting Denmark Strait and of the evolution of Iceland Scotland Overflow Water (ISOW) are presented that provide a tracer-based perspective for comparison with models based on physical oceanographic tools. This forms a baseline for tracking changes to circulation in the Subpolar North Atlantic using the transient nature of the tracer signals.

How to cite: Dale, D., Christl, M., Macrander, A., Ólafsdóttir, S., Middag, R., and Casacuberta, N.: Treasure from trash: Using nuclear waste to trace ocean circulation around Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12097, https://doi.org/10.5194/egusphere-egu23-12097, 2023.

X5.358
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EGU23-15366
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OS1.5
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ECS
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Lauren Moseley, Galen McKinley, Dustin Carroll, Raphael Dussin, Dimitris Menemenlis, and An Nguyen

The subpolar North Atlantic (SPNA) transports surface-ocean properties deep into the interior via deep convection and is one of the most intense regions of air-sea gas exchange globally. Deep convection in the SPNA exports highly-oxygenated water masses to depth, which subsequently ventilate intermediate and deep waters throughout the North Atlantic. The SPNA thus plays a critical role in setting the oxygen inventory of the global ocean. Due to intensifying ocean warming, many climate models predict substantial global-ocean oxygen loss — albeit at magnitudes which vary widely by model. Therefore, there is a need to better understand the impacts of SPNA convective variability on oxygen saturation in intermediate and deep water masses. Here we use a physical-biogeochemical model, ASTE-BGC, which couples the Arctic Subpolar gyre sTate Estimate (ASTE) with the Biogeochemistry with Light, Iron, Nutrients, and Gas (BLING) model to quantify oxygen cycling and deep ventilation in the SPNA. ASTE utilizes the MIT General Circulation Model (MITgcm) and assimilates physical in-situ and satellite data using tools developed by the Estimating the Circulation and Climate of the Ocean (ECCO) consortium. We use a Green’s Functions approach to optimize ASTE-BGC biogeochemistry using BGC-Argo and GLODAPv2 ship-based profiles of O2 and NO3. The Green’s Functions approach allows us to adjust the biogeochemical parameters of the BLING ecosystem towards O(106) in-situ data constraints over the 2002–2017 model period. We then evaluate the optimized simulation against independent data and construct an oxygen budget for the central Labrador Sea to assess the interannual variability of SPNA oxygen.

How to cite: Moseley, L., McKinley, G., Carroll, D., Dussin, R., Menemenlis, D., and Nguyen, A.: Optimizing simulated oxygen variability, circulation, and export in the subpolar North Atlantic Ocean using BGC-Argo & ship-based observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15366, https://doi.org/10.5194/egusphere-egu23-15366, 2023.

Posters virtual: Wed, 26 Apr, 14:00–15:45 | vHall CR/OS

Chairperson: Richard Greatbatch
vCO.1
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EGU23-3197
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OS1.5
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Femke de Jong and Miriam Sterl

The Irminger Sea is one of the few regions in the ocean where deep (>1,000 m) convection
occurs. Convection is followed by restratification during summer, when the stratification of the water column
is reestablished and the convectively formed water is exported at depth. There are currently no descriptions
of interannual variability and physical drivers of restratification in the Irminger Sea. We investigate
restratification in the upper 600 m of the central Irminger Sea using reanalysis data for the years 1993–2019.
We find distinctly different restratification processes in the upper 100 m (the upper layer) and the water below
it (the lower layer). In the upper layer, the stratification is dominated by a seasonal cycle that matches the cycle
of the surface heat flux. In 2010 and 2019, there were peaks in upper layer restratification, which could partly
be related to strong atmospheric heat and freshwater fluxes. Greenland runoff likely also contributed to the
high restratification, although this contribution could not be quantified in the present study. In the lower layer
there is strong interannual variability in stratification, caused by variability both in the convection and the
restratification strength. The restratification strength is strongly correlated with the eddy kinetic energy in the
eastern Irminger Sea, suggesting that lower layer restratification is driven by lateral advection of warm, saline
waters through Irminger Current eddies. In the future, surface warming and freshening of the Irminger Sea
due to anthropogenic climate change are expected to increase upper layer stratification, potentially inhibiting
convection.

How to cite: de Jong, F. and Sterl, M.: Restratification Structure and Processes in the Irminger Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3197, https://doi.org/10.5194/egusphere-egu23-3197, 2023.

vCO.2
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EGU23-12244
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OS1.5
Robin Fraudeau and the 4DAtlantic-OHC Team

Given the major role of the Atlantic Ocean meridional heat transport in the climate system, it is essential to characterize its temporal variations at different locations. The 4DATLANTIC-OHC Project (https://eo4society.esa.int/projects/4datlantic-ohc/) aims at developing and testing space geodetic methods based on satellite altimetry and space gravimetry to estimate the local ocean heat content (OHC) changes over the Atlantic Ocean. Combined with independent estimates of the surface heat fluxes this approach holds promise to estimate the Atlantic Meridional Heat Transport (MHT) at any section across the Atlantic basin. 

The official version 1.0 of the 4DAtlantic-OHC product has been released, it provides estimates of local changes in OHC with their uncertainties. This product is accessible with DOI https://doi.org/10.24400/527896/A01-2022.012  and can be downloaded on AVISO portal. At two test sites, OHC changes derived from in situ data (RAPID and OVIDE) are used to evaluate the accuracy and reliability of the new space geodetic based OHC change estimate. Combined with ERA5 estimate of the surface heat fluxes, the Atlantic OHC product will be used to derive an energy budget of the North Atlantic basin and estimate the associated divergence in ocean heat transport. From the divergence field we will derive at the end of the project new estimates of the Atlantic meridional heat transport at different sections in the North Atlantic basin (RAPID and OSNAP sections) and compare it with in situ estimates. 

The V1.0 of 4DAtlantic-OHC products over the Atlantic Ocean, the evaluation results of the OHC against in situ data and preliminary results of MHT estimation will be presented.

How to cite: Fraudeau, R. and the 4DAtlantic-OHC Team: Monitoring local Ocean Heat Content changes with satellite altimetry and space gravimetry to assess the variability of the Meridional Heat Transport in the North Atlantic: the 4DATLANTIC-OHC Project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12244, https://doi.org/10.5194/egusphere-egu23-12244, 2023.