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OS1.7

Please note that this session will be linked to a special session for the presentation of the Fridtjof Nansen Medal. We also have Daniela Domeisen and Caroline Katsman as invited speakers.

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 years, several projects have been focused on the Atlantic circulation changes, for instance OVIDE, RACE, OSNAP, and ACSIS. 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
-- 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
-- response of the atmosphere to changes in the North Atlantic
-- 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

Public information:
This session will continue at 12:45 with OS1.0 (https://meetingorganizer.copernicus.org/EGU2020/session/38390)

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Co-organized by AS1
Convener: Richard Greatbatch | Co-conveners: Monika Rhein, Bablu Sinha
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| Tue, 05 May, 08:30–12:30 (CEST)

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Chat time: Tuesday, 5 May 2020, 08:30–10:15

D2704 |
EGU2020-10546
Lorenza Raimondi, Kumiko Azetsu-Scott, Toste Tanhua, Igor Yashayaev, and Doug Wallace

Over the last thirty years the Bedford Institute of Oceanography (BIO) has been maintaining the Atlantic Zone Off-Shore Monitoring Program (AZOMP), which includes annual occupation of several sections and stations in the Northwest Atlantic Ocean. Among these, the AR7W line across the Labrador Sea has one of the longest time-series where both transient tracers and dissolved inorganic carbon (DIC) have been collected since the early 1990s.

Among multiple transient tracers that have been measured along this transect (CFC-11, CFC-113, CCl4 and SF6), only measurement of CFC-12 extends over the full time-series from 1992 to 2018, overlapping with DIC observations. Measurements of CFC-12 were also available for a previous cruise in 1986, extending the time-series to three decades.

In this work we present the temporal variability of CFC-12 (1986-2016) and DIC (1992-2016) concentrations as well as their distribution in the major water masses of the region.

The CFC-12 data are used to reconstruct the time-history of the tracer’s saturation at the time of convection based on multiple regression with the atmospheric input function of CFC-12 and the annual maximum mixed layer depth. The so-modelled time-varying saturation is employed to relax the constant saturation assumption of the Transit Time Distribution (TTD) method, allowing for a better estimate of anthropogenic carbon (Cant) in the region.

We present the column inventories and storage rate of Cant in central Labrador Sea between 1986 and 2016 obtained using the TTD method with time-varying saturation. We compare these estimates with a classical TTD approach that assumes constant saturation, and we highlight the differences in trends and magnitudes obtained with the two approaches.    

Finally, our work shows the multi-decadal dataset of DIC in the Labrador Sea which enables a comparison between the TTD-based Cant estimates and the measured DIC trends, providing insights into temporal variability of natural carbon in the region.

D2705 |
EGU2020-13682
Karin Margretha Húsgarð Larsen, Bogi Hansen, Hjálmar Hátún, and Svein Østerhus

Since November 1995, we have monitored the volume transport of Faroe Bank Channel overflow (FBC-overflow) and since 2001, the bottom temperature at the sill of the channel. The FBC-overflow is the coldest and densest overflow component and contributes approximately one third of the total overflow. Together with water that it entrains en route, it is therefore one of the main sources for North Atlantic Deep Water and the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). In spite of reported AMOC weakening, the FBC-overflow has shown no signs of reduced volume transport. In contrast, a linear trend analysis indicated a weak (but non-significant) positive trend in volume transport of +5% from 1996 to 2018. The bottom water at the sill of the channel is the coldest component of the FBC-overflow and the densest overflow component overall. Since high-quality monitoring of the bottom water temperature began in summer 2001, the bottom water has warmed by approximately 0.2 °C with most of the warming occurring in two periods: 2004-2007 and 2015-2019. During the period, salinity has also been changing and the combined temperature/salinity effect on the density of the FBC-overflow will be discussed.

D2706 |
EGU2020-19663
Pascale Lherminier, Herlé Mercier, Fiz F. Perez, and Marcos Fontela

According to the subpolar AMOC index built from ARGO and altimetry, the AMOC amplitude across the OVIDE section (from Greenland to Portugal) was similar to that of the mid-1990s between 2014 and 2017, i.e. 4-5 Sv above the level of the 2000s. It then returned to average values in 2018. The same index computed independently from the biennial summer cruises over 2002-2018 confirms this statement. Interestingly, despite the concomitant cold and fresh anomaly in the subpolar Atlantic, the heat flux across OVIDE remains correlated with the AMOC amplitude. This can be explained by the paths taken by the North Atlantic Current and the transport anomalies in the subarctic front. In 2014, the OVIDE section was complemented by a section from Greenland to Newfoundland (GA01), showing how the water of the lower limb of the AMOC was densified by deep convection in the Labrador Sea. The spatial patterns of volume, heat, salt and oxygen transport anomalies after 2014 will be discussed at the light of the 2000s average.

D2707 |
EGU2020-4416
Martin Moritz, Kerstin Jochumsen, Dagmar Kieke, Birgit Klein, Holger Klein, Manuel Köllner, and Monika Rhein

Since 2016 a moored observatory is operated at the eastern extension of the “North Atlantic Changes (NOAC)” array at 47°/48°N. This observatory is installed across the shelf break at Goban Spur and consists of two deep-sea moorings that are separated by about 60 km.  

The aim of this ongoing monitoring program is to quantify the variability and trends in the properties and transport rates of water masses that are advected northwards along the North Atlantic Eastern Boundary and modify the adjacent regions, i.e. the Northwest European Shelf, North Sea, Nordic Seas and Arctic Ocean. Furthermore, the continuous long term time series are essential for a thorough understanding of the circulation system in the eastern North Atlantic and the underlying physical mechanisms that govern its variability.

Here, we present results of the analysis of temperature, salinity and current velocity time series from 2016 to 2019. These provide a descriptive view of the complex current structure and variability of water masses on daily to intra- and inter-annual time scales.

The most pronounced signal in the variability of temperature and salinity is caused by the presence of Mediterranean Outflow Water located at about 1000 m depth. During the observation period we find significant positive trends in temperature and salinity in the depth range of 500 to 1500 m. The velocity measurements of the onshore mooring show a northeastward directed mean flow following the topography with along-slope variations, while the flow at the offshore mooring position is more unstable with predominantly cross-slope variations. 

The combination of our observations with float and altimeter data indicates that the presence of eddies and the interaction with the topography seems to play a crucial role for setting the variability of the flow in this region.

Finally, we present an approach to evaluate the volume fluxes at the eastern boundary that will add toward an integrated estimate of the strength of the Atlantic Meridional Overturning Circulation at 47°/48°N.

D2708 |
EGU2020-3615
Tillys Petit, Susan Lozier, Simon A. Josey, and Stuart A. Cunningham

The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, transforms warm and salty waters from the subtropical gyre into colder and fresher waters in the subpolar gyre and Nordic Seas. To measure the mean AMOC and its variability at subpolar latitudes, the Overturning in the Subpolar North Atlantic Program (OSNAP) array was deployed in the summer of 2014. Based on observations through May 2016, the majority of the light‐to‐dense water conversion takes place north of the OSNAP East line, which runs from the southeast tip of Greenland to the Scottish shelf. In this study, we assess the transformation of dense waters in the area located between the Greenland-Scotland Ridge and the OSNAP East section. From 2014 to 2016, the mean overturning within this area is estimated at 6.9 ± 1.3 Sv across σ0 = 27.55 kg m-3, the isopycnal that separates the northward and southward flows. This mean overturning estimate is in close agreement with the value (6.5 ± 1 Sv) derived by applying water mass transformation theory to air-sea buoyancy fluxes from atmospheric reanalysis. However, the large monthly variability of the overturning (standard deviation of 4.1 Sv) cannot easily be attributed to the buoyancy forcing or to variability in the overflow through the Greenland-Scotland Ridge. We explore possible mechanisms that can account for this variability.  

D2709 |
EGU2020-7488
Steingrímur Jónsson

The temperature in the Atlantic waters south of Iceland has increased by about 1°C since 1995 with most of the rise occurring before 2000. A similar rise in air temperature in Iceland was observed simultaneously and the rise in temperature is often interpreted as being caused by global warming. Many effects of this in the ocean and on land such as changed distribution of marine species in the area as well as melting of glaciers in Iceland have been attributed to this rising temperature. However, it is unlikely that this rapid increase in temperature was solely due to global warming, especially since it was accompanied by an increase in salinity. It is more likely that there was a change in the ocean circulation in the area leading to more sub-tropical water entering the sub-polar gyre causing a shift in temperature and salinity. A similar increase in temperature and salinity was observed earlier during 1930-1964 in this area. Between the two warm periods the waters were dominated by lower temperature and salinity. These changes have been related to the Atlantic Multidecadal Oscillation. By comparing the water mass properties in the two warm periods it is possible to estimate the relative contribution from natural variability and global warming for the recent warm period. It will be shown how the retreat and advancing of glaciers in Iceland are in harmony with the changes in water mass properties in the waters south of Iceland. It is important that decisions about how to adapt to coming climate change are based on how much of the observed change is due to natural variability and global warming respectively. This is a method that can be used in other areas of the northern North Atlantic.

D2710 |
EGU2020-2645
Vimal Koul, Jan-Erk Tesdal, Manfred Bersch, Sebastian Brune, Hjálmar Hátún, Helmuth Haak, Leonard Borchert, Corinna Schrum, and Johanna Baehr

The North Atlantic Subpolar Gyre (SPG) has been widely implicated as the source of large-scale changes in the subpolar marine environment. However, inconsistencies between different indices of SPG strength based on Sea Surface Height (SSH) observations have raised questions about the active role SPG strength and size play in determining water properties in the eastern subpolar North Atlantic (ENA). Here, by analyzing SSH-based and various other SPG-strength indices derived from observations and a global coupled model, we show that the interpretation of SPG strength-salinity relationship is dictated by the choice of the SPG index. Our results emphasize that SPG indices should be interpreted cautiously because they represent variability in different regions of the subpolar North Atlantic. Idealized Lagrangian trajectory experiments illustrate that zonal shifts of main current pathways in the ENA and meridional shifts of the North Atlantic Current (NAC) in the western intergyre region during strong and weak SPG circulation regimes are manifestations of variability in the size and strength of the SPG. Such shifts in advective pathways modulate the proportions of subpolar and subtropical water reaching the ENA, and thus impact salinity. Inconsistency among SPG indices arises due to the inability of some indices to capture the meridional shifts of the NAC in the western intergyre region. Overall, our results imply that salinity variability in the ENA is not exclusively sourced from the subtropics, instead the establishment of a dominant subpolar pathway also points to redistribution within the SPG.

D2711 |
EGU2020-5644
Caroline Katsman, Nils Brüggemann, Sotiria Georgiou, Juan-Manuel Sayol Espana, Stefanie Ypma, Carine van der Boog, and Julie Pietrzak

In the North Atlantic Ocean, intense downward motions connect the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC). In addition, the AMOC also displays a pronounced signature in density space, with lighter waters moving northward and denser waters returning southward.

While at first glance it is appealing to associate this sinking of water masses in the North Atlantic Ocean with the occurrence of the formation of dense water masses by deep convection, this is not correct: the net vertical motion over convection areas is small. The downward flow required to connect the upper and lower branches of the AMOC thus has to occur outside the deep convection areas. Indeed, earlier studies have pointed out theoretically that strong sinking can only occur close to continental boundaries, where ageostrophic processes play a role. However, observations clearly indicate that convected water masses formed in marginals seas constitute an important component of the lower limb of the AMOC.

This apparent contradiction is explored in this presentation, by studying the overturning in the AMOC from a perspective in depth space (Eulerian downwelling) and density space (downwelling across isopycnals). Based on analyses of both a high-resolution global ocean model and dedicated process studies using idealized models we analyze the characteristics of the sinking, of diapycnal mixing, and investigate how these are linked. 

It appears that eddies play a crucial role for the overturning, both in depth space and density space. They control the characteristics of the yearly cycle of convection and restratification, the magnitude of the Eulerian sinking near continental boundaries, and steer the export of dense waters formed in the interior of the marginal seas via the boundary current system.

These studies thus reveal a complex three-dimensional view on sinking, diapycnal water mass transformation and overturning in the North Atlantic Ocean, involving the boundary current, the interior and interactions with the eddy field.  This implies that it is essential to resolve these eddies to be able to properly represent the overturning in depth and density space in the North Atlantic Ocean and its response to changing conditions in a future climate.

D2712 |
EGU2020-13859
Stefan Rahmstorf and Levke Caesar

The Atlantic Meridional Overturning Circulation (AMOC) is a major mechanism for northward heat transport on our planet and the prime reason why the Northern Hemisphere is warmer than the Southern Hemisphere (Feulner et al. 2013). The AMOC is a sensitive non-linear system dependent on subtle thermohaline density differences in ocean water, and major AMOC transitions have been implicated e.g. in millennial climate events during the last glacial (Rahmstorf 2002).

There is evidence that the AMOC is slowing down in response to modern global warming, as predicted by climate models (Caesar et al. 2018). We will review and compile proxy evidence for AMOC changes during the past 1-2 millennia, including e.g. Sherwood et al. 2011, Thibodeau et al. 2018, Thornalley et al. 2018, Rahmstorf et al. 2015, Zanna et al. 2019. We conclude that there now is substantial and consistent evidence from multiple independent sources for a modern AMOC slowdown that is unprecedented in at least a millennium.

References

Caesar, L., S. Rahmstorf, A. Robinson, G. Feulner, and V. Saba. 2018. Nature, 556: 191-96.

Feulner, G, S Rahmstorf, A Levermann, and S Volkwardt. 2013. Journal of Climate, 26: 7136-50.

Rahmstorf, S. 2002. Nature, 419: 207-14.

Rahmstorf, S., Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford, and Erik J. Schaffernicht. 2015. Nature Climate Change, 5: 475-80.

Sherwood, O. A., M. F. Lehmann, C. J. Schubert, D. B. Scott, and M. D. McCarthy. 2011. Proc Natl Acad Sci U S A, 108: 1011-5.

Thibodeau, Benoit, Christelle Not, Jiang Hu, Andreas Schmittner, David Noone, Clay Tabor, Jiaxu Zhang, and Zhengyu Liu. 2018. Geophysical Research Letters, 45: 12,376-12,85.

Thornalley, D. J. R., D. W. Oppo, P. Ortega, J. I. Robson, C. M. Brierley, R. Davis, I. R. Hall, P. Moffa-Sanchez, N. L. Rose, P. T. Spooner, I. Yashayaev, and L. D. Keigwin. 2018. Nature, 556: 227-30.

Zanna, L., S. Khatiwala, J. M. Gregory, J. Ison, and P. Heimbach. 2019. Proc Natl Acad Sci U S A, 116: 1126-31.

D2713 |
EGU2020-1384
Alex Megann, Adam Blaker, Simon Josey, Adrian New, and Bablu Sinha

The recent decline in the Atlantic meridional overturning circulation (AMOC) has attracted more than a little interest. The strongest AMOC recorded by the RAPID campaign at 26°N was at the start (2004/5), after which it declined about 3 Sv with a pronounced minimum in 2010. Proxies based on temperature and surface elevation have been used to extrapolate the AMOC strength before the RAPID era, and point reasonably reliably to a maximum strength in the mid 1990s, followed by a rise to a maximum at the start of the RAPID campaign in around 2005. Further back, less robust proxy data suggest that the AMOC gradually rose from the 1970s to the peak in 1990. This raises two questions: firstly, what drove these decadal variations in the overturning circulation (and hence of the ocean heat transport); and secondly whether there are observations that lead to useful predictive skill for changes in the AMOC. The surface-forced streamfunction, estimated from modelled/observed buoyancy fluxes, has been shown to be a reasonably good predictor of decadal changes in the overturning strength, preceding the latter with a lead time of about 5 years. although the reliability of the correlations before 2000 is limited by data sparsity, and especially so in the pre-satellite era.

To verify a causal link between surface forcing and decadal variations in the AMOC over longer timescales, numerical simulations present a powerful tool. A set of hindcast integrations of a global 0.25° NEMO ocean configuration has been carried out from 1958 until nearly the present day, with a selection of standard surface forcing datasets (CORE2, DFS5.2 and JRA55). These show an evolution of the AMOC strength from 1970 onwards which is consistent, both between forcing datasets and with that inferred from observations. The surface-forced streamfunction is evaluated for these experiments and is used to relate the time evolution of the AMOC to changes in the individual components of the buoyancy flux, and the surface heat loss from the Labrador and Irminger Seas is found to be the dominant predictor of AMOC changes.

D2714 |
EGU2020-5785
Ben Moat, David Smeed, Eleanor Frajka-Williams, Damien Desbruyeres, Claudie Beaulieu, William Johns, Darren Rayner, Alejandra Sanchez-Franks, Molly Baringer, Denis Volkov, and Harry Bryden

The strength of the Atlantic meridional overturning circulation (AMOC) at 26°N has now been continuously measured by the RAPID array over the period April 2004 - Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008-2010. In 2013-2015, a period of strong buoyancy- forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing.

Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3-9 years. At 45°N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26°N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26°N is higher than its previous low. Extending the record at 26°N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45°N by 0-2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.

D2715 |
EGU2020-8758
Yao Fu, Feili Li, Johannes Karstensen, N. Penny Holliday, and Chunzai Wang

The Atlantic Meridional Overturning Circulation (AMOC) is crucially important in the global climate system due to its role in the meridional heat and freshwater distribution. Model simulations and constructed AMOC indices suggest that the AMOC may have been weakening for decades. However, direct AMOC observations, introduced in 2004 in the subtropics (the RAPID program) and in 2014 in the subpolar North Atlantic (the OSNAP program), are not sufficiently long to capture changes dating back to previous periods. Here we use repeated hydrographic sections in the subtropical and subpolar North Atlantic through the early 1990s to the mid-2010s, combined with a box inverse model that is constrained using satellite altimetry, to analyze hydrographic changes and the AMOC. In combination with a state-of-the-art ocean state estimate, GECCO2, we show that despite dramatic hydrographic changes in the subtropical and subpolar North Atlantic over the past two and half decades, the AMOC has not significantly weakened over the same period. Our hydrography-based estimates also illustrate a remarkably stable partition of the subpolar overturning between the Labrador basin and the eastern subpolar basins on decadal timescales since the 1990s.

D2716 |
EGU2020-5894
Yavor Kostov, Helen L. Johnson, David P. Marshall, Gael Forget, Patrick Heimbach, N. Penny Holliday, Feili Li, M. Susan Lozier, Helen R. Pillar, and Timothy Smith

The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat, carbon and other tracers. Establishing the causes of historical variability in the AMOC can tell us how the circulation responds to natural and anthropogenic changes at the ocean surface. However, attributing observed AMOC variability and inferring causal relationships is challenging because the circulation is influenced by multiple factors which co-vary and whose overlapping impacts can persist for years.  Here we reconstruct and unambiguously attribute variability in the AMOC at the latitudes of two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While on inter-annual timescales, AMOC variability at 26°N is overwhelmingly dominated by a linear response to local wind stress, in contrast, AMOC variability at subpolar latitudes is generated by both wind stress and surface temperature and salinity anomalies. Our analysis allows us to obtain the first-ever reconstruction of subpolar AMOC from forcing anomalies at the ocean surface.

D2717 |
EGU2020-11549
Alejandra Sanchez-Franks, Eleanor Frajka-Williams, and Ben Moat

The Atlantic meridional overturning circulation (AMOC) is a large-scale oceanic circulation comprising a 2-layer flow: the net northward flow in the upper 1000 m of the Atlantic and net southward flow below. Variations in the AMOC have significant repercussions for the climate system hence there is a need for proxies that can measure changes in the AMOC on larger spatial scales. Here we show a direct calculation of ocean circulation at 26°N from satellites compares well with transport estimates from the RAPID mooring array. In the surface layer (1000 m), transport is estimated from satellite altimetry and has a correlation of r=0.79 (significant at 95% level) with the MOC transport estimates from RAPID. We find that the relationship between sea level anomaly and dynamic height from the western boundary RAPID moorings is robust in the surface layer, with poor agreement occurring largely below 1000 m. Below 1000 m, the return flow of the AMOC is estimated using ocean bottom pressure from satellite gravimetry. This has a correlation of r=0.75 (significant at the 95% level) when compared to the deeper (1000-5000 m) RAPID transports. Combining the results from satellite altimetry and gravimetry, estimates of full-depth 2-layer circulation at 26°N are demonstrated. Finally, empirical orthogonal function analysis reveals that the barotropic and baroclinic streamfunctions are linked to wind stress curl and buoyancy forcing, respectively.

D2718 |
EGU2020-7815
Daniela Domeisen

The atmosphere over the North Atlantic sector exhibits significant interannual and interdecadal variability, as well as long-term trends due to global change. This variability is accompanied by changes in predictability. The origins of North Atlantic variability can to a large extent be traced back to the ocean and the land surface, the upper atmosphere, the tropics, as well as circum-global patterns. In particular, the tropical Pacific and the upper atmosphere have a strong influence on interannual and decadal variability in the North Atlantic region. As an example, the tropical Pacific affects the North Atlantic both through a tropospheric pathway across North America and through an indirect pathway through the stratosphere. Hence, due to the large number of factors influencing the North Atlantic region, their inter-dependence and their non-stationarity, the influence of these different factors is difficult to disentangle. Furthermore, models are often not able to capture the inter-dependence and superposition of these factors, which affects to what extent models are able to predict the North Atlantic region. This submission will explore the contribution to variability and predictability for several of these remote influences.

 

D2719 |
EGU2020-11083
Yochanan Kushnir, Dog Run (Donna) Lee, and Mingfang Ting

This study focuses on the decadal time scale variability of the North Atlantic Ocean-Atmosphere system. This time scale is relevant to preparedness and adaptation as society becomes increasingly threatened by the adverse impact of anthropogenic climate change. North Atlantic decadal climate variability has been related to interaction between the subpolar and subtropical gyre and manifested in persistent multi-year SST and heat content anomalies and shifts in the latitude of the Gulf Stream/North Atlantic Current (GS/NAC). We apply a space-time analysis to annual, North Atlantic, upper ocean heat content (OHC) anomalies from the National Center for Atmospheric Research (NCAR), Community Earth System Model (CESM) long pre-industrial control run. The analysis reveals decadal anomalies associated with two patterns: a dipole centered on the GS/NAC, in the western side of the Basin that oscillates quasi-regularly, reversing its sign every of 6 to 7 years. The second pattern is centered in the eastern side of the basin and lags the first by about 5 years, implying that heat is transported between the subtropical and subpolar gyres. Analysis of surface windstress anomalies connected with these OHC fluctuations implies that the latter are forced by stochastic atmospheric variability. Further analysis compares the model patterns with observations to determine their relevance and predictability and assesses their response to climate change.

D2720 |
EGU2020-18650
Helen Dacre, Simon Josey, and Alan Grant

The 2013/14 winter averaged sea surface temperature (SST) was anomalously cool in the mid-North Atlantic region.  This season was also unusually stormy with extratropical cyclones passing over the mid-North Atlantic every 3 days.  However, the processes by which cyclones contribute towards seasonal SST anomalies are not fully quantified. In this paper a cyclone identification and tracking method is combined with ECMWF atmosphere and ocean reanalysis fields to calculate cyclone-relative net surface heat flux anomalies and resulting SST changes.  Anomalously large negative heat flux is located behind the cyclones cold front resulting in anomalous cooling up to 0.2K/day when the cyclones are at maximum intensity.  This extratropical cyclone induced 'cold wake' extends along the cyclones cold front but is small compared to climatological variability in the SST's.  To investigate the potential cumulative effect of the passage of multiple cyclone induced SST cooling in the same location we calculate Earth-relative net surface heat flux anomalies and resulting SST changes for the 2013/2014 winter period.  Anomalously large winter averaged negative heat flux occurs in a zonally orientated band extending across the North Atlantic between 40-60 oN. The 2013/2014 winter SST cooling anomaly associated with air-sea interactions (anomalous heat flux, mixed layer depth and entrainment at the base of the ocean mixed layer) is estimated to be -0.67 K in the mid-North Atlantic (68% of the total cooling anomaly).  The role of cyclones is estimated using a cyclone masking technique which encompasses each cyclone centre and its trailing cold front. The environmental flow anomaly in 2013/2014 sets the overall tripole pattern of heat flux anomalies over the North Atlantic.  However, the presence of cyclones doubles the magnitude of the negative heat flux anomaly in the mid-North Atlantic.  Similarly, the environmental flow anomaly determines the location of the SST cooling anomaly but the presence of cyclones enhances the SST cooling anomaly.  Thus air-sea interactions play a major part in determining the extreme 2013/2014 winter season SST cooling anomaly. The environmental flow anomaly determines where anomalous heat flux and associated SST changes occur and the presence of cyclones influences the magnitude of those anomalies.

D2721 |
EGU2020-6802
Sybren Drijfhout, Jenny Mecking, Joel Hirschi, and Alex Megann

Leading up to and during the summer of 2015 sea surface temperatures (SSTs) in the eastern North Atlantic Subpolar Gyre reached anomalously low values while in the subtropical gyre just to the SSTs were anomalously warm. Recent observation and modelling studies have found evidence showing that these SST anomalies can be linked to the heat wave experienced over Europe that summer.  The latest observation based data still shows anomalously cold temperatures, as well as the anomalously fresh conditions that went along the 2015 cold blob in the upper layers of the eastern North Atlantic Subpolar gyre.  A second heat wave over Europe occurred in the summer of 2018 where the SSTs reached another minimum value.  Therefore, being able to predict the development, enhancement and persistence of such an anomaly is essential for good seasonal and longer predictions.  At present several modelling systems have had difficulties in simulating/maintaining the 2015 cold blob. In this work we apply a novel initialization technique using anomalous initialization from a forced ocean simulation to simulate the 2015 cold blob.  Initial results show that the model is able to maintain the cold blob as well as have a strengthening of the cold blob, however, it has difficulties capturing the timing of this strengthening.

D2722 |
EGU2020-19025
Melissa Gervais, Jeffrey Shaman, and Yochanan Kushnir

In future climate projections there is a notable lack of warming in the North Atlantic subpolar gyre, known as the North Atlantic warming hole (NAWH). The NAWH has been previously shown to contribute to a poleward shift and eastward elongation of the North Atlantic jet that constitutes an additional important driver of future changes in the North Atlantic jet using a set of large-ensemble atmosphere simulations with the Community Earth System model.  The current study investigates the impact of the warming hole on sensible weather, particularly over Europe using the same simulations. North Atlantic jet regimes are classified within the model simulations by applying self-organizing maps to winter daily wind speeds on the dynamic tropopause. The NAWH is found to increase the prevalence of jet regimes with stronger and more poleward jets.  A previously identified transient eddy-mean response to the NAWH that leads to downstream enhancements of wind speeds is found to be dependent on the jet regimes. These localized regime-specific changes vary by latitude and strength, combining to form the broad increase in seasonal mean wind speeds over Eurasia. Impacts on surface temperature and precipitation within the various North Atlantic jet regimes are also investigated. A large decrease in surface temperature over Eurasia is found to be associated with the NAWH in regimes where air masses are advected over the subpolar gyre.  Precipitation is found to be locally suppressed over the warming hole region and increased directly downstream. The impact of this downstream response on coastal European precipitation is dependent on the strength of the NAWH.

D2723 |
EGU2020-18861
Thomas Bracegirdle

Research to date has shown strong multi-decadal variability of the North Atlantic Oscillation (NAO) in late winter, particularly in March when correlations to North Atlantic (NA) ocean variability (Atlantic multi-decadal variability (AMV)) are particularly strong. This late-winter low-frequency atmospheric variability appears too weak in the majority of climate models across a range of indices of North Atlantic large-scale atmospheric circulation. It appears that models do not successfully reproduce responses to either (or both) proximal sea-surface temperature (SST) variability at mid-latitudes or teleconnections to SST variability in the sub tropics. 

Here, an in-depth analysis of the winter evolution of multiple indices of North Atlantic mid-latitude atmospheric circulation will be presented based on both re-analysis data and historical simulations of coupled climate models (CMIP5 and CMIP6). The atmospheric indices assessed will include the NAO, speed and latitude of the NA eddy driven jet and lower-tropospheric westerly wind strength in a region of maximum variability to the west of the UK. Results so far indicate that the CMIP6 models do not exhibit a clear change from CMIP5 in terms of the representation of low-frequency late-winter atmospheric variability. To diagnose in more detail possible origins of differences between observed and simulated variability, a detailed evaluation of early- to late-winter evolution in variability of the above indices will be presented, with an initial focus on observations (re-analysis and SST re-constructions) and incorporating the following questions:  
- Are there significant differences in the relative strength of linkages to tropical and extra-tropical SST variability across the different atmospheric indices? 
- Is the observed late-winter maximum in correlations between NA atmospheric indices and North Atlantic SSTs still apparent at sub-decadal timescales?
Initial results indicate that there are stronger tropical linkages for jet speed and that at sub-decadal timescales late winter is does not dominate in terms of correlations between atmospheric and SST variability. Updates on these early results will be presented along with implications of the results for differences between observed and simulated variability. 

D2724 |
EGU2020-2414
Satyaban Bishoyi Ratna, Timothy Osborn, Manoj Joshi, and Juerg Luterbacher

We simulate the response of Asian summer climate to AMO-like (Atlantic Multidecadal Oscillation) sea surface temperature (SST) anomalies using the Intermediate General Circulation Model version 4 (IGCM4). Separate AMO SST patterns are obtained from seven Coupled Model Intercomparison Project phase 5 (CMIP5)/Paleoclimate Model Intercomparison Project phase 3 (PMIP3) global climate models, to explore the sensitivity of the atmospheric response to the SST pattern. Experiments are performed with seven individual and composited AMO SST anomalies globally, and over the North Atlantic Ocean only, for both the positive and negative phases of the AMO. During the positive AMO phase, a Rossby wave train propagates eastward, causing a high pressure anomaly over eastern China/Japan region, associated with a low level anomalous anticyclonic circulation along with warm and dry anomalies. In contrast, the mid-latitude Rossby wave train is less robust in response to the cold phase of the AMO. The circulation response and the associated temperature and precipitation anomalies are sensitive to the AMO SST anomaly patterns. The comparison between global SST and N Atlantic SST experiments indicates that midlatitude East Asian climate anomalies are forced from the North Atlantic region. However, global SST anomaly experiments show that the SST anomalies outside the North Atlantic region, but still associated with AMO, strongly influence South Asian climate as they either strengthen or reduce the precipitation. Experiments with different amplitudes of negative and positive AMO anomalies test the linearity of the response. Over a large region of South and East Asia, temperature has a linear response to the amplitude of North Atlantic SST anomaly associated with both positive and negative AMO conditions, but the precipitation response is nonlinear.

D2725 |
EGU2020-594
Diana Iakovleva and Igor Bashmachnikov

Interannual variations in the upper ocean heat and freshwater contents in the subpolar North Atlantic has important climatic effect. It affects the intensity of deep convection, which, in turn, forms the link between upper and deep ocean circulation of the global ocean Conveyor Belt.

The upper ocean heat content is primarily affected by two main process: by the ocean-atmosphere heat exchange and by oceanic heat advection. The intensity of both fluxes in the subpolar gyre is linked to the character of atmospheric circulation, largely determined by the phase of the North Atlantic Oscillation (NAO).

To study the interannual variability of the oceanic heat advection (in the upper 500th meters layer) we compare the results from four different data-sets: ARMOR-3D (1993-2018), SODA3.4.2 and SODA3.12.2 (1980-2017), and ORAS5 (1958-2017). The ocean-atmosphere heat exchange is accessed as the sum of the latent and the sensible heat fluxes, obtained from OAFlux data-set (1958-2016).

The oceanic heat advection to the Labrador and to the Irminger seas has high negative correlation (-0.79) with that into the Nordic Seas. During the years with high winter NAO Index (NAOI) the oceanic heat advection into the Subpolar Gyre decreases, while to the Nordic Seas – increases. These variations go in parallel with the intensification of the Norwegian, the West Spitsbergen and the slope East Greenland currents and weakening of the West Greenland and the Irminger Currents. During the years with high NAOI, the ocean heat release (both sensible and latent) over the Labrador and Irminger seas increases, but over the Norwegian Sea it decreases.

In summary, the results show that, during the positive NAO phase, the observed decrease of the heat content in the upper Labrador and Irminger seas is linked to both, a higher oceanic het release and a lower intensity of advection of warm water from the south. In the Norwegian Sea, the opposite sign of variations of the fluxes above leads to a simultaneous warming of the upper ocean.

The investigation is supported by the Russian Scientific Foundation (RSF), number of project 17-17-01151.

 

 

D2726 |
EGU2020-614
Hyojeong Kim and Soon-Il An

Previous studies showed that both AMOC and AMO work in different ways in interdecadal and multidecadal timescales. Although their relationship has also been covered in many studies, the possibility that overlapping between multiple timescales may have diluted their inherent relation has not been considered. To understand their physical relation correctly, it is necessary to consider interdecadal and multidecadal timescales, separately.

Here, we apply a band-pass filter to the AMO and AMOC indices obtained from a present-day climate simulation, to separate interdecadal and multidecadal variability. The results show that strong AMOC induces a warm phase of AMO by the northward heat transport in both timescales, but with a different time lag. This is because, in the interdecadal timescale, the southward propagation of AMOC anomaly gradually warms up the Atlantic basin from the high to low latitudes, resulting in a lag of seven years. As the delayed AMO peak provides negative feedback to AMOC by surface density modulation, the AMOC-AMO relationship can be described as an oscillatory system. On the other hand, AMOC in the multidecadal timescale matures at once in the entire basin, simultaneously warming the surface. The synchronous maturity of AMOC and AMO indicates that AMO-related density changes cannot account for the AMOC phase transition, and AMO remains a relatively passive component in their relationship. This study implies that overlooking timescale-dependency in physical processes may obscure our understanding of interactions between climate components.

D2727 |
EGU2020-754
Daria Kuznetcova and Anna Mamadzhanian

Atlantic Meridional Overturning Circulation (AMOC) contribute to long-term climate variability of Northern Hemisphere. The North Atlantic Ocean carries 25% of global heat transferred tropics to polar latitudes of the Northern Hemisphere (Srokosz, 2012). In the subpolar seas of the North Atlantic water goes down in few localized areas to deep convection, where all Atlantic deep water masses are formed. This process pumps a huge amount of CO2 to the deep ocean, which have strong consequence for global climate (Buckley and Marshall, 2016; Kuhlbrodt, 2007). The water comes back to the surface mainly in upwelling regions of the Southern Ocean (Toggweiler and Samuels 1998, Delworth and Zeng, 2008), as well as in the tropics due to vertical mixing.

In this study we try to link the long-term variability of the AMOC to it’s main driving mechanisms: the deep ocean convection in the Greenland, the Labrador and the Irminger seas, and to wind driven upwelling in the Southern Ocean.

As a reference for the AMOC intensity on the decadal and longer time scales, we use AMOC indexes from several studies (Caesar, 2018; Chen and Tung, 2018), which extend the time series back to the 1950s. The intensity of deep convection (IC) over the same time period is computed using convection index (Bashmacnikov et al., 2019). Wind-driving upwelling in the Southern Ocean is computed through evaluation of the divergence of Ekman fluxes (ED), using the wind velocity from atmospheric reanalysis (ERA 40 1957-1979 and ERA-Interim 1980-2016).

To estimate contribution of each of the forcing factors to the temporal variability of the AMOC, were used cross-correlation and regression analyses with varying time lags. The biggest cross-correlation coefficient was found with the IC in the Greenland Sea, the negative lags indicate that it is the AMOC, which affects the variability of convection intensity. The second largest cross-correlation coefficient was found with the IC in the Labrador Sea (0.7) with the lag of 13 years. The maximum cross-correlation with the IC in the Irminger Sea was 0.6 on a narrow interval of the time lags. The ED in Southern ocean demonstrate a significant correlation with the AMOC, with the correlation coefficient of 0.5 at the time lag of 15 years.

The contributions of each of the control mechanisms to temporal variability of the AMOC were investigated by the regression analysis for the time lags at which the maximum cross-correlations of each of the parameters are obtained. As a result the maximum regression coefficient was obtained for the IC in the Irminger Sea (0.65), the second one for the ED (0.35) using the time lags of 9 and 25 years, respectively. The regression coefficient for the IC in the Labrador Sea did not exceed 0.2 for all tested time lags. The physical mechanism, connecting the variability of the AMOC intensity to these two control mechanisms is a subject of our further research.

The work was supported by a grant from the Russian science Foundation (project No. 17-17-01151)

Chat time: Tuesday, 5 May 2020, 10:45–12:30

D2728 |
EGU2020-844
Weimin Jiang, Guillaume Gastineau, and Francis Codron

The centennial to multi-centennial variability of the Atlantic Meridional Overturning Circulation (AMOC) is studied in a 1200-yr pre-industrial control simulation of the IPSL-CM6-LR atmosphere-ocean coupled model. In this run, a spectrum analysis finds a periodicity of the low-frequency variability of AMOC, with a period of about 200-year. This variability alters the Northern Hemisphere climate over the land and modulates the Arctic sea ice extent and volume. The associated density variations show large positive (negative) salinity-driven density anomalies in the Nordic Seas and subpolar gyre associated with a strong (week) AMOC state. The positive salinity anomalies in the Greenland Sea are found to be generated by anomalous southward salinity transport in the Fram Straits. The gradual AMOC increase and the associated oceanic northward heat transport melt the sea ice in the Arctic and build shallow negative salinity anomalies in the central Arctic. In parallel, the AMOC is also associated with a northward salt transport into the Eastern Arctic, by an inflow of Atlantic water from the Barents Sea to the East Siberian Ocean. The accumulated surface freshwater in the central Arctic is ultimately exported into the Atlantic mainly through the Fram Strait via intensified East Greenland Current, lowering the upper ocean density and enhancing the stratification at the regions where the cold deep limb of AMOC is formed. The positive salinity anomalies at subsurface then slowly reach the surface though diffusion, increasing the surface salinity. The oscillation then turns into the opposite phase.

D2729 |
EGU2020-2379
Matthew Menary, Jon Robson, Richard Allan, Ben Booth, Christophe Cassou, Guillaume Gastineau, Jonathan Gregory, Dan Hodson, Colin Jones, Juliette Mignot, Mark Ringer, Rowan Sutton, Laura Wilcox, and Rong Zhang

The Atlantic Meridional Overturning Circulation (AMOC) has been, and will continue to be, a key factor in the modulation of climate change both locally and globally. Reliable simulations of its decadal to century-timescale evolution are key to providing skilful predictions of future regional climate, and to understanding the likelihood of a potential AMOC collapse. However, there remains considerable uncertainty even in past AMOC evolution. Here, we show that the multi-model mean AMOC strengthened by approximately 10% to 1985 in new historical simulations for the 6th Coupled Model Inter-comparison Project (CMIP6), contrary to results obtained from CMIP5. The simulated strengthening is due to a stronger anthropogenic aerosol forcing, in particular due to aerosol-cloud interactions. However, comparison with an observed sea surface temperature fingerprint of AMOC evolution during 1850-1985, and the shortwave forcing during 1985-2014, suggest that anthropogenic forcing and the subsequent AMOC response may be overestimated in some CMIP6 models.

D2730 |
EGU2020-2981
Susan Lozier, Matthew Menary, and Laura Jackson

The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate change and variability. Since continuous, direct measurements of the overturning strength in the North Atlantic subpolar gyre (SPG) have been unavailable until recently, the understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence of the credibility of the AMOC representation in climate models. Here, we reconcile these viewpoints by comparing the OSNAP data with a new, high-resolution coupled climate model: HadGEM3-GC3.1-MM. Unlike many previous models, we find our model compares well to the OSNAP overturning observations. Furthermore, overturning variability across the eastern OSNAP section (OSNAP-E), and not in the Labrador Sea region, appears linked to AMOC variability further south. Labrador Sea densities are shown to be an important indicator of downstream AMOC variability, but these densities are driven by upstream variability across OSNAP-E rather than local processes in the Labrador Sea.

D2731 |
EGU2020-3036
Aleksandr Fedorov and Belonenko Tatyana

The Lofoten basin (the LB) contains relatively warm and salty waters regarding border basins such as Greenland and Barents Seas. Variability of the processes inside occurring in the basin reflects on the climate as on the mesoscales as on the interannual scales. We use a term mixed layer depth (MLD) as a border of the pycnocline in the water column, this parameter lets us evaluate the intensity of the convection in the region. Several methods of MLD calculations are tested in the current study: Kara, Montegut, and Dukhovskoy. The convection in the basin destructs stratification and forms massive intermediate water mass. The MITgcm data for 1993-2012 and over 5000 in-situ Argo T, S profiles for 2001-2017 were used in the calculations of the MLD.

We consider the maximum MLD (mMLD) in the region and its spatial distribution. The mMLD is higher in the central part of the LB and corresponds to the location of the Lofoten basin eddy (the LBE). Here the mMLD reaches 1000 meters, the medium maximum is 400 meters based both on the in-situ and model data. The maximum mixed layer depth ​​varies in the range of 400-1000 meters according to both datasets were used. The MLD over 400 meters is observed from January to May every year.

Acknowledgments: The authors acknowledge the support of the Russian Science Foundation (project No. 18-17-00027). The results of the MITgcm were provided by D.L. Volkov, Cooperative Institute for Marine and Atmospheric Studies, University of Miami, USA.

D2732 |
EGU2020-4992
Antoine Hochet, Thierry Huck, Olivier Arzel, Florian Sevellec, Alain Colin de Verdiere, Matthew Mazloff, and Bruce Cornuelle

The North Atlantic is characterized by basin-scale multidecadal fluctuations of the sea surface temperature with periods ranging from 20 to 70 years.
One candidate for such a variability is a large-scale baroclinic instability of the North Atlantic Current. Because of the long time scales involved, most of the studies devoted to this problem are based on low resolution numerical models leaving aside the effect of explicit meso-scale eddies.   
How high-frequency motions associated with the meso-scale eddy field affect the basin-scale low-frequency variabiliy is the central question of this study.

This issue is addressed using an idealized configuration of an Ocean General Circulation Model at eddy-permitting resolution (20 km). A new diagnostic allowing the calculation of nonlinear fluxes of temperature variance in frequency space is presented. Using this diagnostic, we show that the primary effect of meso-scale eddies is to damp low frequency  temperature variance and to transfer it to high frequencies.

D2733 |
EGU2020-5498
Laura Jackson and Richard Wood

We conduct idealised experiments with HadGEM3-GC2, which is a pre-CMIP6 eddy-permitting GCM, to test for the presence of thresholds in the AMOC. We add fresh water to the North Atlantic for different rates and lengths of time, and then examine the AMOC recovery. In some cases the AMOC recovers to its original strength, however if the AMOC weakens sufficiently it does not recover and stays in a weak state for up to 300 years.

We test various indictors that have been proposed for monitoring the AMOC with this ensemble of experiments (and other scenarios). In particular we ask whether fingerprints can provide early warning or faster detection of weakening or recovery, or indications of crossing the threshold. We find metrics that perform best are the temperature metrics based on large scale differences, the large scale meridional density gradient, and the vertical density difference in the Labrador Sea. Mixed layer depth is also useful for indicating whether the AMOC recovers after weakening. 

D2734 |
EGU2020-6915
Damien Desbruyeres, Bablu Sinha, Elaine McDonagh, Simon Josey, Alexis Megann, David Smeed, Penny Holliday, Adrian New, and Ben Moat

The decadal to multi-decadal temperature variability of the intermediate (700 – 2000 m) North Atlantic Subpolar Gyre (SPG) significantly imprints the global pattern of ocean heat uptake. Here, the origins and dominant pathways of this variability are investigating with an ocean analysis product (EN4), an ocean state estimate (ECCOv4), and idealized modeling approaches. Sustained increases and decreases of intermediate temperature in the SPG correlate with long-lasting warm and cold states of the upper ocean – the Atlantic Multidecadal Variability – with the largest anomalous vertical heat exchanges found in the vicinity of continental boundaries and strong ocean currents. In particular, vertical diffusion along the boundaries of the Labrador and Irminger Seas and advection in the region surrounding Flemish Cap stand as important drivers of the recent warming trend observed during 1996-2014. The impact of those processes is well captured by a 1-dimensional diffusive model with appropriate boundary-like parametrization and illustrated through the continuous downward propagation of a passive tracer in an eddy-permitting numerical simulation. Our results imply that the slow and quasi-periodic variability of intermediate thermohaline properties in the SPG are not strictly driven by the well-known convection-restratification events in the open seas but also receives a key contribution from boundary sinking and mixing. Increased skill for modelling and predicting intermediate-depth ocean properties in the North Atlantic will hence require the appropriate representation of surface-deep dynamical connections within the boundary currents encircling Greenland and Newfoundland.

D2735 |
EGU2020-7755
Extreme sea levels in the context of climate change
(withdrawn)
Lucia Pineau-Guillou, Pascal Lazure, and Guy Wöppelmann
D2736 |
EGU2020-8399
Yuan Wang, Richard Greatbatch, Martin Claus, and Jinyu Sheng

Temporal variability of the annual mean barotropic streamfunction in a high-resolution model configuration (VIKING20) for the northern North Atlantic is analyzed using a decomposition technique based on the vertically-averaged momentum equation. The method is illustrated by examining how the Gulf Stream transport in the recirculation region responds to the winter North Atlantic Oscillation (NAO). While no significant response is found in the year overlapping with the winter NAO index, a tendency is found for the Gulf Stream transport to increase as the NAO becomes more positive, starting in lead years 1 and 2 when the mean flow advection (MFA) and eddy momentum flux (EMF) terms associated with the nonlinear terms dominate in the momentum equations. Only after 2 years, the potential energy (PE) term, associated with the density field, starts to play a role and it is only after 5 years that the transport dependence on the NAO ceases to be significant. The PE contribution to the transport streamfunction has significant memory of up to 5 years in the Labrador and Irminger Seas. However, it is only around the northern rim of these seas that VIKING20 and the transport reconstruction exhibit similar memory. This is due to masking by the nonlinear MFA and EMF contributions.

D2737 |
EGU2020-8602
Patricia Handmann, Martin Visbeck, and Arne Biastoch

Water mass formation in the Subpolar North Atlantic and successive southward export, connects high latitudes with lower latitudes, as a part of the lower Atlantic meridional overturning (AMOC) limb. The role of regional importance, in particular the respective roles of the Labrador and Irminger Sea, in this process are in debate. 

This study analyses pathways connecting the Labrador and Irminger Sea in detail, using simulated Lagrangian particle trajectories. To give further insight on interconnectivity and flow patterns we used two setups with different velocity fields, a high-resolution ocean model (VIKING20X) and a gridded Argo float displacement climatology. Both setups indicate two distinct pathways with interconnectivity on the order of 20% of the total amount of seeded particles between the Labrador Sea and Irminger Sea. One pathway is following the recirculation in the Labrador Sea along the Greenland shelf break; the other is along the Newfoundland shelf break turning to the north/northwest at the Orphan-Knoll region towards the central Irminger Sea. For the Argo based advective-diffusive particle trajectory integration a 2.5–3.5 year travel time scale was derived between the Labrador and the Irminger Sea, while the experiments with the temporarily varying high-resolution model output revealed significantly shorter spreading times of about 1.5–2 years. While both pathways are represented in either setup, the pathway following the Newfoundland shelf break is populated stronger in the model-based experiments. In general we found that connectivity between the two regions is weaker in the experiments based on the climatological mean velocity output of the model than in those based on the Argo derived fields, first results indicate that this is due to stronger boundary currents and a weaker recirculation in the Labrador Sea.

D2738 |
EGU2020-8889
Simon Wett, Monika Rhein, Arne Biastoch, Claus W. Böning, and Klaus Getzlaff

The Atlantic Meridional Overturning Circulation (AMOC) is the main driver of northward oceanic volume and heat transport in the Atlantic. Due to its definition via the streamfunction the exact calculation of the AMOC requires knowledge of the full velocity field. Since the early 2000s, observations of the AMOC are available at 47° North in the form of hydrographic sections across the Atlantic and continuous current measurements from moored instruments at specific locations. However, the spatial resolution of current measurements is coarse and shipbased hydrographic sections are mostly done only once a year. Also the observational timeseries still remain too short to come to conclusions about decadal trends in the AMOC variability. Thus, today our knowledge about the role of the AMOC in the global climate system is mainly based on model simulations. Comparing these model simulations against observations remains an important task to accurately predict the future of the AMOC and adapt to changes.

We present first results of a model observations comparison in the subpolar North Atlantic between observations at 47° North and the high resolution ocean model VIKING20X. The model has a 1/20° nest in the Atlantic embedded in a global 1/4° model. It covers the years from 1980 to 2018 and thus overlaps with the whole observational period. This comparison will help assessing different methods of estimating the AMOC strength from observations.

D2739 |
EGU2020-3503
Hannah Nowitzki, Monika Rhein, Achim Roessler, Christian Mertens, and Dagmar Kieke

It is well known that ocean circulation impacts climate and weather patterns. One of the key regions influencing Europe is the subpolar North Atlantic, Here warm and saline water from the subtropics is imported with the North Atlantic Current (NAC), meeting cold and fresh water masses intruding from the north. To quantify the strength of the NAC, Uni Bremen and BSH Hamburg started in 2006 to continuously deploy instruments to quantify the volume transport. In a first step, the crossing of the NAC from the western into the eastern basin at the western flank oft the Midatalantic Ridge was covered, followed by boundary current moorings and PIES at a nominally zonal section at 47N in the western basin. In the last years, PIES and moorings have also been deployed in the eastern basin. Here we report about the recent results, focusing on the eastern basin.

D2740 |
EGU2020-9130
Adrian New, David Smeed, Adam Blaker, and Jenny Mecking

Labrador Slope Water is known to exist in the Slope Sea off the US eastern shelf as a relatively fresh and cool water mass deriving from the Labrador Current further north, and is present between the upper layer US shelf-derived water masses and the deeper Deep Western Boundary current waters, typically near 400-600m. This LSLW  is investigated in the EN4 database and shown to penetrate as far south as Cape Hatteras (74-75°W), having previously only been described as far west as the Gulf of Maine (66°W). We then examine, using both EN4 and Line W observations, the changes of this water mass between 2005-2008, when the strength of Atlantic Meridional Overturning Circulation (AMOC) measured by the RAPID array at 26°N, was high, and 2009-2015, when the AMOC was low. We show that in the AMOC high period, there was a larger volume of the LSSW present on the northern side of the Gulf Stream system which resulted in an increased meridional slope of the isopycnals near these depths, commensurate with increased geostrophic transport, and also in a more southerly position, of the Gulf Stream after separation at Cape Hatteras. The LSLW could therefore play an important role in decadal timescale variations in the North Atlantic climate system through its impact on the Gulf Stream and AMOC.

D2741 |
EGU2020-9328
Carolina Ernani da Silva, Miren Vizcaino, and Caroline Katsman

Coupled climate models predict a weakening of the Atlantic Meridional Overturning (AMOC) circulation in the future. However, it is not clear what is the cause of the AMOC weakening. Studies have suggested that the freshwater (FW) is an important factor in the AMOC reduction. There are different sources of FW that may play a role, such as, river discharge, sea ice melt, and precipitation. Currently, due to global warming, the Greenland Ice Sheet (GrIS) melt rate is rising, which increases the amount of freshwater (ice discharge) into the ocean. Thus, it is possible that this input of freshwater would affect the ocean circulation on a regional and global scale. Hence, the GrIS freshwater cannot be neglected. The goal of this study is to understand the impact of the freshwater from GrIS on the North AMOC (NAMOC) strength in the future. We used the Community Earth System Model (CESM) version 2.1, which contains a fully coupled and an active ice sheet, to simulate an idealized greenhouse gas scenario (1% CO2). The CO2 concentration is 1140 ppm at the end of the simulation. The results show that GrIS delivers, on average, about 0.062 Sv/yr of FW to the Subpolar North Atlantic Ocean. The bulk of the total freshwater input comes from the southeastern and southwestern parts of the ice sheet:  the regions where some fast-flowing marine-terminating glaciers are located (e.g. Helheim and Kangerlussuaq). The NAMOC index (maximum barotropic stream function from above 28°N and from 500 m to 5500 m depth) was calculated. It displays a fast weakening, approximately 16.7 Sv (0.11 Sv/yr), during the first 150 yrs. After that, the NAMOC reaches a stable state where the index is around 5.7 Sv (year 350). When the NAMOC index was compared to the FW from GrIS time series, we observed that change in AMOC occurs before the FW starts to increase (from year 200). Our results thus suggest that the FW input from GrIS does not cause significant changes in the AMOC strength. It is necessary to further investigate other possible causes for the strong NAMOC decline in this model.

D2742 |
EGU2020-9395
Lidia I. Carracedo, Elaine McDonagh, Richard Sanders, Edward Mawji, Sinhué Torres-Valdés, Molly Baringer, Herlé Mercier, and Virginie Thierry

The Florida Current (FC), upstream extension of the Gulf Stream, is a very intense current (~32 Sv) confined to a 800-m depth narrow passage off the east coast of Florida. Associated with an intense poleward (subsurface maximum) transport of nutrients, this current has earned the name of North Atlantic ‘nutrient stream’. Since the biological sequestration of carbon (namely the Biological Carbon pump, BCP) is limited by the presence of nutrients in the upper ocean (euphotic zone), the FC can be seen as precursor of the nutrient induction process downstream feeding the subtropical gyre productivity. However, the relevance of this current is not only limited to its interplay with the subtropical gyre. The FC also comprises the bulk of the warm upper limb of the meridional overturning circulation reaching subpolar latitudes. Therefore, disentangling the range of intra-annual variability of the nutrient transport by the FC is crucial to better understand its linkage to and influence on the BCP magnitude and efficiency at higher latitudes. Based on a high-quality nutrient and velocity dataset from repeated hydrography between May 2015 and Oct 2018, we present an analysis of the nutrient transport by the FC in its 3 main water masses (the surface water, upper thermocline water and lower thermocline water). Our results show that the transport of inorganic nutrients is dominated by the upper and lower thermocline waters, whose transport-weighted nutrient concentration reaches a maximum in winter. Conversely, the transport of organic nutrients is dominated by the surface and upper thermocline waters, peaking in spring and autumn. Inorganic transport-weighted nutrient concentrations strongly correlate with volume transport. The correlation is positive in the surface water and the lower thermocline water (of South Atlantic origin), whereas it correlates negatively in the lower thermocline water (North Atlantic recirculated water). This indicates a southern remote source for the inorganic nutrient supply, which is mainly driven by advection. Organic nutrients, however, do not show a clear correlation with volume transport. Only the upper thermocline water shows a certain positive correlation, confirming the North Atlantic subtropical gyre as a main source of organic nutrients, ultimately driven not only -nor mainly- by circulation, but also by biological activity upstream.

D2743 |
EGU2020-9721
Fumi Hayashi, Arnaud Czaja, and Benoit Vanniere

Western Boundary Currents (WBC), such as the Gulf Stream, leave a strong imprint on the ocean-atmosphere boundary in the form of strong gradients and high variability of Sea Surface Temperature (SST). Recent studies have shown that midlatitude oceanic fronts have an influence throughout the depth of the troposphere by means of synoptic systems such as weather fronts. An understanding of how the midlatitude ocean influences the synoptic system is crucial for better climate projection, however, this has been challenging. For example, in model simulations the sensitivity of the atmosphere to SST anomalies are dependent on its resolution, with low resolution models unable to capture the air-sea interactions occurring over warm sectors of midlatitude cyclones, possibly leading to underestimations of the oceanic influence on the atmosphere. A novel modelling technique is developed in which an interactive “mask” is used to systematically isolate and study the air-sea interaction over different synoptic regimes (warm and cold sector). Here, simulations using an idealised aqua-planet atmospheric general circulation model (AGCM) are used to study the atmospheric response to a tightening of SST gradient (comparable to that of the Gulf Stream) over the cold sector (“cold path”) and the warm sector (“warm path”) separately. Same experiments will also be performed on models with higher resolution to investigate the difference in atmospheric response between the high and low resolution models and what physical processes are responsible for such change in response.

D2744 |
EGU2020-11095
Siren Rühs, Arne Biastoch, Claus W. Böning, Michael Dowd, Klaus Getzlaff, Paul G. Myers, and Eric Oliver

Deep convection in the subpolar North Atlantic has been suggested to be a key process impacting the strength and variability of the Atlantic Meridional Overturning Circulation as well as the ocean’s uptake and deep storage of heat and anthropogenic CO2. However, the spatial pattern and strength of deep convection are subject to variability on interannual-to-decadal timescales and despite intense research in the field the nature of this variability is not fully understood. In this work, we employ a hindcast simulation with the eddy-rich (1/20°) ocean/sea-ice model configuration VIKING20X to analyze the variability of deep convection in the subpolar North Atlantic over the last decades (1980-2018). A special focus is set on mixed layer depth (MLD) pattern and deep water formation characteristics in the Labrador versus Irminger Sea. We show that, in agreement with observations, the VIKING20X hindcast captures strong convection events with particularly deep MLDs in the winters of the early 1980s, late 1980s and early 1990s, as well as in recent years. Yet, there are large differences in the spatial pattern of the deep convection events, as well as in the volume and thermohaline properties of the newly formed deep water. Most notably, in recent years deep convection intensity, and in particular its spatial extent, increased in the Irminger Sea and decreased in the Labrador Sea compared to the late 1980s and early 1990s. We finally discuss potential drivers of the simulated changes, thereby contrasting the relative importance of wintertime atmosphere-ocean buoyancy fluxes and oceanic preconditioning.

D2745 |
EGU2020-13803
Norbert Frank, Markus Miltner, Steffen Therre, Marleen Lausecker, Nadine Tisnerat-Laborde, Paolo Montagna, and Ronny Friedrich

The Northeast Atlantic is crucial regarding the northward heat and carbon export into the Arctic Ocean. At the surface and at mid-depth (0 -1000 m), however, most of the water re-circulates through two basin scale gyres, with warm and salty waters in the sub-tropical gyre (STG) and significantly fresher and colder waters in the sub-polar gyre (SPG). In addition, the Azores Front (AF) separates the northeastern branch from the southeastern branch of the STG, which is today positioned around 34.5°N in the east Atlantic. Underneath both gyres newly formed deep waters from the Arctic Ocean and Labrador Sea are spread into the Atlantic basin. Here we investigate, whether these water masses and recirculation patterns reveal distinct histories of carbon uptake and advection in the eastern North Atlantic. Therefore, water samples spanning the entire water column have been collected at 6 stations along a north-south transect at 25°W spanning from 42° to 61°N in 2012 (N/O Thalassa ICE-CTD cruise). In 2018 (RV Meteor M151 ATHENA cruise) samples further south were collected spanning until 29.5°N thus including the present day AF.

The radiocarbon content of Dissolved Inorganic Carbon (DIC) from 8 profiles (N>60, 30° to 61°N, 50-3000m depth) were measured at the AMS facility at CEZA facility in Mannheim following CO2 extraction from seawater at Heidelberg University. Δ14C values range from +50 ‰ in the upper layers of the subtropical Atlantic to -100 ‰ in 3000 m depth also in the subtropical Atlantic. Three main feature appear in the radiocarbon distribution. The surface shows a moderate difference between SPG and STG Δ14C values of <15‰ with a decreasing trend towards the North, hence indicating equilibration with the atmosphere. Underneath, between 100-1000 m depth SPG (46° – 61°N) Δ14C values of nearly 0‰ are found identical to the modern Northern Hemisphere atmosphere. In contrast, the STG (30°-43°N) reveals up to 50‰ enriched water reflecting limited carbon uptake from the atmosphere. Thus this layer will act as a source of radiocarbon to the polar seas and atmosphere in the near future. Below, between 1000 and 2000 m water masses north of the AF reveal a nearly homogeneous Δ14C value of -10‰ with a moderate decreasing trend with depth. South of the AF Δ14C values show a strong decrease with depth from 0 to -75‰, hence water masses remain still little affected by the advection of bomb radiocarbon (thus anthropogenic carbon). Thus, as expected the AF and the mid-depth gyre play a crucial role in distributing carbon throughout the east Atlantic.

D2746 |
EGU2020-16250
Andrea Dittus, Ed Hawkins, Laura Wilcox, Dan Hodson, Jon Robson, and Rowan Sutton

The respective roles of aerosol and greenhouse-gas forcing in modulating the phasing and amplitude of large-scale modes of multi-decadal variability remain poorly understood, despite the attention that has been devoted to trying to separate the influence of forcing from internal variability in modes such as the Atlantic Multidecadal Variability and the Pacific Decadal Oscillation, for instance. However, understanding what drives multidecadal variability in these basins is imperative for improving near-term climate projections.

Here, we show how aerosol and greenhouse-gas forcing interact with internal climate variability to generate indices of multi-decadal variability in the Atlantic, using a large ensemble of historical simulations with HadGEM3-GC3.1 for the period 1850-2014, where anthropogenic aerosol emissions are scaled to sample a wide range in historical aerosol forcing. These results are complemented by early results from new stabilised warming simulations with the same climate model and analysis of future projections from models partaking in the Sixth Phase of the Coupled Model Intercomparison Project (CMIP6).

D2747 |
EGU2020-16753
Dmitry Sidorenko, Sergey Danilov, Nikolay Koldunov, and Patrick Scholz

The Atlantic meridional overturning circulation (AMOC) is the most common diagnostics of numerical simulations. Generally it is computed as a streamfunction of zonally averaged flow along the constant depth. More rarely it is computed as zonally averaged along constant isopycnals. The latter computation, however, allows one to better distinguish between water masses and physical processes contributing to the meridional transport. We analyze the AMOC in global simulations based on the Finite-volumE Sea ice–Ocean Model (FESOM 2.0) using eddy permitting to eddy resolving configurations in the North Atlantic. We (1) split the AMOC computed in density space into the constitutes induced by surface buoyancy fluxes and cross isopycnal transformations, (2) identify the water masses which contribute to the formation of the North Atlantic Deep Water and (3) study the AMOC response to the permitting or resolving eddies in the North Atlantic ocean.

D2748 |
EGU2020-17986
Kristin Burmeister, Mark Inall, and Clare Johnson

The Atlantic Ocean is influenced by large-scale physical variability like changes in the Subpolar Gyre (SPG), the Atlantic Multidecadal Variability (AMV), the Atlantic Meridional Mode (AMM) or changes in the South Atlantic Anticyclone (SAA). Associated changes in temperature and salinity may severely impact open-ocean and deep-sea ecosystems. We study the variability of potential temperature and salinity profiles associated with large-scale physical variations focusing on 12 marine regions across both the North and South Atlantic Ocean (subpolar Mid-Atlantic Ridge off Iceland; Rockall Trough to Porcupine Abyssal Plain; central Mid-Atlantic Ridge; northwest Atlantic; Sargasso Sea; eastern tropical North Atlantic; central equatorial Atlantic; ecosystems from Angola to the Congo Lobe; the Benguela Current region; ecosystems off Brazil; the Vitória-Trindade Seamount Chain off Brazil; Malvinas Upwelling Current off Argentina). These regions were selected within the framework of the EU Horizon 2020 iAtlantic project. They are in proximity to major ocean circulation pathways as well as ocean monitoring arrays and are important for international conservation, Blue Growth and Blue Economy attempts.

Our methodology builds on recent work (Johnson et al., accepted, Frontiers in Marine Science) that shows that climate indices are associated with statistically-significant and spatially-coherent changes in bottom conditions across the northern North Atlantic. We use the same composite approach to investigate the relationship between indices of physical variability and potential temperature and salinity but extend the analysis to include additional indices (e.g. AMM, SAA) and to cover the entire Atlantic basin. Additionally, we use profile data instead of a gridded data product and investigate the full water column by density class, rather than focusing on bottom conditions. This enables physical mechanisms of any observed signals across the Atlantic Ocean as a whole to be explored.

D2749 |
EGU2020-19096
Didier Swingedouw, Marion Devilliers, Juliette Mignot, Julie Deshayes, Gilles Garric, and Mohamed Ayache

Greenland experienced intensive melting over the last century, especially in the 1920s and over the last decades. The supplementary input into the ocean is influencing the freshwater budget of the North Atlantic. Simultaneously, some signs of a recent weakening of the Atlantic meridional overturning circulation (AMOC) have been reported. In order to better understand the possible impact of the melting on the North Atlantic circulation, salinity and temperature trends, we construct an observation-based estimate of the freshwater fluxes from 1840 to 2014 associated to the runoff fluxes from Greenland ice sheet and surrounding glaciers and ice caps. Input from iceberg melting is also included and spatially distributed over the North Atlantic following an observed climatology. We force historical simulations of the IPSL-CM6A-LR coupled climate model with this reconstruction from 1920 to 2014. The 10-member ensemble mean displays freshened and cooled waters around Greenland, which spread in the subpolar gyre, and then towards the subtropical gyre and the Nordic Seas. Over the whole period, the convection is reduced in the Labrador and Nordic Seas, while it is slightly enhanced in the Irminger Sea, and the AMOC is reduced by 0.32±0.35 Sv at 26°N. This highlights that the AMOC decrease due to Greenland melting remains modest in these simulations and can only explain a very moderate amount of the 3±1 Sv weakening suggested in a recent study. The multi-decadal trend of the North Atlantic surface temperature obtained with the additional freshwater forcing is more in line with observations than in standard historical simulations. We also show a clear improvement of the representation of the 1995 abrupt warming in the subpolar gyre in the melting ensemble, which may thus be partly forced by Greenland ice sheet melting. Mechanisms at play imply changes in the variability of the AMOC in the melting ensemble as compared to the historical one. Such an impact on forced decadal variability has crucial consequences for decadal prediction systems that may gain skill by including observed Greenland ice sheet melting.

D2750 |
EGU2020-22348
Caroline Katsman, Sotiria Georgiou, Juan-Manuel Sayol, Stefanie Ypma, Nils Brüggemann, Henk Dijkstra, and Julie Pietrzak

The water masses exiting the Labrador Sea, and in particular the dense water mass formed by convection (i.e. Labrador Sea Water, LSW), are important components of the Atlantic Meridional Overturning Circulation (AMOC). Several studies have suggested that the eddy activity within the Labrador Sea is of high importance for the properties of the LSW and the export routes. In this study, the pathways and the associated timescales of the water masses exiting the Labrador Sea are investigated by using a Lagrangian particle tracking tool. This method is applied to two different model simulations: to an eddy- permitting idealized model able to reproduce the essential features of the Labrador Sea, and to a high-resolution global ocean model simulation under a repeated annual climatological forcing.

In both model configurations, the Lagrangian trajectories reveal that the water masses that exit the Labrador Sea have followed either a fast route within the boundary current or a slow route that involves extensive boundary current-interior exchanges. Regions characterized by enhanced eddy activity play a significant role in determining the properties and the timescales of the water masses exiting the marginal sea, as the interior-boundary current exchange is governed by eddy activity.

Analysis of the properties of the water masses along the different pathways shows that the water masses that pass through the interior experience stronger densification than those that follow the boundary current.

This study highlights the importance of the exchanges between the boundary current and the convection area in the interior in setting the properties of the water masses that leave the Labrador Sea and the associated timescales.

D2751 |
EGU2020-22545
Ioana Colfescu and Edwin Schneider

The Atlantic Multidecadal Variability (AMV) modulates various climate features worldwide with enormous societal and economic implications, including variations in hurricane activity in the Atlantic, sea-level changes, West African and Indian monsoon rainfall, European climate, and hemispheric‐ scale surface temperature. Leading hypotheses regarding the nature and origin of AMV focus primarily on its links with oceanic and coupled ocean-atmosphere internal variability, and on its response to external forcing. The role of another possible process, that of atmospheric noise forcing of the ocean, has received less attention. This is addressed here by means of historical coupled simulations and diagnostic experiments, which isolate the influences of external and atmospheric noise forcings. Our findings show that external forcing is an important driver of the simulated AMV. They also demonstrate that weather noise is key in driving the simulated internal AMV in the southern part of the (0o-60oN) AMV region, and that weather noise forcing is responsible for up to 10%-20% of the multidecadal internal SST variability in some isolated areas of the sub-polar gyre region. Ocean dynamics independent from the weather noise forcing is found to be the dominant cause of multidecadal SST in the northern part of the AMV region.