OS1.7

The North Atlantic: natural variability and global change

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 observational projects have been focused on the Atlantic circulation changes, for instance ACSIS, RACE, RAPID, OSNAP, and OVIDE. 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

Co-organized by AS1/CL4
Convener: Richard Greatbatch | Co-conveners: Damien Desbruyeres, Caroline Katsman, Bablu Sinha
Presentations
| Tue, 24 May, 08:30–11:48 (CEST), 13:20–16:34 (CEST)
 
Room L3

Presentations: Tue, 24 May | Room L3

Chairperson: Richard Greatbatch
08:30–08:36
08:36–08:42
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EGU22-6750
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ECS
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Virtual presentation
Yuta Kuniyoshi, Ayako Abe-Ouchi, Sam Sherriff-Tadano, Wing-Le Chan, and Fuyuki Saito

Using the climate model MIROC4m, we simulate self-sustained oscillations of millennial-scale periodicity in the climate and Atlantic meridional overturning circulation under glacial conditions. We show two cases of extreme climatic precession and examine the mechanism of these oscillations. When the climatic precession corresponds to strong (weak) boreal seasonality, the period of the oscillation is about 1,500 (3,000) years. During the stadial, hot (cool) summer conditions in the Northern Hemisphere contribute to thin (thick) sea ice, which covers the deep convection sites, triggering early (late) abrupt climate change. During the interstadial, as sea ice is thin (thick), cold deep-water forms and cools the subsurface quickly (slowly), which influences the stratification of the North Atlantic Ocean. We show that the oscillations are explained by the internal feedbacks of the atmosphere-sea ice-ocean system, especially subsurface ocean temperature change and salt advection feedback with a positive feedback between the subpolar gyre and deep convection.

How to cite: Kuniyoshi, Y., Abe-Ouchi, A., Sherriff-Tadano, S., Chan, W.-L., and Saito, F.: Effect of Climatic Precession on Dansgaard-Oeschger-like oscillations , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6750, https://doi.org/10.5194/egusphere-egu22-6750, 2022.

08:42–08:48
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EGU22-1023
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ECS
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On-site presentation
Katinka Bellomo, Virna Meccia, Roberta D'Agostino, Federico Fabiano, Jost von Hardenberg, and Susanna Corti

The Atlantic Meridional Overturning Circulation (AMOC) is thought to exist in multiple states of equilibria. In the present climate, the AMOC is believed to be in a relatively strong state, bringing warm waters into the North Atlantic and contributing to mild winters over Europe. However, proxy data show evidence of abrupt declines in the strength of the AMOC, often associated with the initiation of ice ages. The abrupt shifts in the strength of the AMOC are usually referred to as ‘tipping points’. Presently, state-of-the-art climate models are unable to spontaneously reproduce tipping points in the AMOC, preventing an accurate study of the climate impacts of an abrupt AMOC shutdown. Contextually, although it is deemed unlikely that the AMOC will collapse in response to climate change, it is expected to further slow down into the 21st century. The impacts of this weakening, relative to those of global warming, are poorly understood, especially on daily timescales.

            To address this question, we run water hosing experiments with the EC-Earth3 earth system model to investigate the impacts of an AMOC abrupt weakening on the winter climate variability focusing on the North Atlantic and Europe. We confirm results from previous studies showing a large decrease in temperature, precipitation, and an increase in the jet stream over Europe. However, we further investigate the moisture budget and the impacts on daily weather regimes and blocking. In contrast to previous hypotheses, we find that the reduction in precipitation over Europe is due to changes in the storm tracks rather than thermodynamic effects. Further, we find a significant increase in the frequency and persistence of NAO+ days. Finally, we show precipitation and temperature extremes that are expected in response to the AMOC weakening.

            Our results show the climate impacts on weather events that can be expected from an AMOC weakening alone, and are relevant to understanding the relative roles of greenhouse gas forcing and AMOC weakening on the European climate in simulations of future climate change.

How to cite: Bellomo, K., Meccia, V., D'Agostino, R., Fabiano, F., von Hardenberg, J., and Corti, S.: The climate impacts of an abrupt AMOC weakening on the European winters , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1023, https://doi.org/10.5194/egusphere-egu22-1023, 2022.

08:48–08:54
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EGU22-3596
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ECS
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On-site presentation
Eleonora Cusinato, Angelo Rubino, and Davide Zanchettin

Dominant Euro-Atlantic climate modes such as the North Atlantic Oscillation (NAO), the Eastern Atlantic pattern (EA), the Eastern Atlantic Western Russian pattern (EAWR), and the Scandinavian pattern (SCA) significantly affect interannual-to-decadal Euro-Mediterranean climate fluctuations, especially in winter.

In this contribution, we will present and discuss results from a CMIP6 multi-model analysis performed to investigate the robustness of historical and projected state and variability of such modes under the historical and ssp585 future scenario of anthropogenic forcing (fossil-fueled development with 8.5W/m2 forcing level) simulations, focusing on the winter season.

Toward this goal, we first search for a reliable box-based index definition for each of the abovementioned observed climate modes and, then, we perform a comparative assessment of the temporal, spectral and distributional properties of the so-defined indices during the historical (1850-2014) and ssp585 future scenario (2015-2099) time periods, with a special focus on the two interdecadal periods 1960-1999 and 2060-2099.

Results show overall good skills of the historical ensemble to reproduce the observed temporal, spectral and distributional properties of all considered modes. At the end of the 21st Century the ssp585 ensemble yields non-significant distributional changes for NAO, EAWR, and SCA indices and a transition to a stronger baroclinic structure for EA, with persistent positive anomalies in the mid-troposphere enhancing globally-driven warming over the Euro-Mediterranean region. The hemispheric spatial correlation patterns with temperature and precipitation significantly change for all modes, that is, we observe a significant modulation of the teleconnections associated with each index.

 

How to cite: Cusinato, E., Rubino, A., and Zanchettin, D.: Winter Euro-Atlantic Climate Modes: Future Scenarios From a CMIP6 Multi-Model Ensemble, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3596, https://doi.org/10.5194/egusphere-egu22-3596, 2022.

08:54–09:00
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EGU22-4430
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Presentation form not yet defined
Positive and negative phases of the North Atlantic Oscillation – not just images in the mirror
(withdrawn)
Jens H. Christensen, Torben Schmith, Anna I.K. Kirchner, Shuting Yang, and Steffen M. Olsen
09:00–09:06
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EGU22-5057
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Virtual presentation
Dandan Tao, Erica Madonna, and Camille Li

The 20th century “early warming” (1910-1940) and cooling (1940-1970) of the Northern Hemisphere offer an interesting contrast of periods with opposite temperature trends, similar hemispheric temperature anomalies, yet very different temperature anomaly patterns. These contrasts are particularly clear in the North Atlantic sector, which exhibits large climate variability over a range of time scales, from short (weather regimes) to long (Atlantic Multidecadal Variability). In this study, we explore the role of the atmospheric circulation (North Atlantic jet stream) in determining the temperature anomaly patterns over the 20th century. While different jet configurations are associated with distinct synoptic temperature patterns in the North Atlantic sector, only some are found to contribute substantially to longer term temperature trends. Notably, the southern jet configuration has the strongest temperature anomalies, with a dipole signal that is opposite from the one under the tilted jet configuration. At the same time, these two jet configurations exhibit relatively large decadal variations in frequency (days of occurrence in given winter seasons), with trends that are almost the opposite. In fact, changes in the frequency of southern and tilted jet “days” alone account for much of the North Atlantic and Arctic temperature variability on decadal time scales, including the differences between the early warming and cooling periods (e.g., the flipped warming versus cooling patterns are associated with fewer southern jet days and more tilted jet days). However, the reconstruction skill of the 30-year mean temperature anomaly in the North Atlantic sector using jet frequency exhibits decadal variability, with high skill scores interestingly coinciding with the positive phases of the Atlantic Multidecadal Variability. The lower reconstruction skill especially during the global warming period from the1980s onwards is likely due to the impact from the warming hole in the North Atlantic, which dominates the temperature patterns in the North Atlantic. Overall, the evolution of Northern Hemisphere surface temperature over the 20th century is found to be influenced by North Atlantic jet variability, with lower frequency ocean effects contributing more in recent decades.

How to cite: Tao, D., Madonna, E., and Li, C.: Using atmospheric variability to understand the wintertime regional warming and cooling patterns in the North Atlantic Sector, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5057, https://doi.org/10.5194/egusphere-egu22-5057, 2022.

09:06–09:12
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EGU22-6401
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On-site presentation
Tom Bracegirdle, Hua Lu, and Jon Robson

Climate model biases in the North Atlantic (NA) low-level tropospheric westerly jet are a major impediment to reliably representing variability of the NA climate system and its wider influence, in particular over western Europe. We highlight an early-winter equatorward jet bias in Coupled Model Inter-comparison Project (CMIP) models and assess whether this bias is reduced in the CMIP6 models in comparison to the CMIP5 models. Historical simulations from the CMIP5 and CMIP6  are further compared against reanalysis data over the period 1862-2005.  

The results show that an equatorward bias remains significant in CMIP6 models in early winter. Almost all CMIP5 and CMIP6 model realizations exhibit equatorward climatological jet latitude biases with ensemble mean biases of 3.0° (November) and 3.0° (December) for CMIP5 and 2.5° and 2.2° for CMIP6. This represents an approximately one-fifth reduction for CMIP6 compared to CMIP5. The equatorward jet latitude bias is mainly associated with a weaker-than-observed frequency of poleward daily-weekly excursions of the jet to its northern position. A potential explanation is provided.  Our results indicate a strong link between NA jet latitude bias and systematically too-weak model-simulated low-level baroclinicity over eastern North America in early-winter.  

Implications for model representation of NA atmosphere-ocean linkages will be presented. In particular CMIP models with larger equatorward jet biases tend to exhibit weaker correlations between temporal variability in jet speed and sea surface conditions over the NA sub-polar gyre (SPG). This has implications for the ability of climate models to represent key aspects of atmospheric variability and predictability that are associated with atmosphere-ocean interactions in the SPG region.  

How to cite: Bracegirdle, T., Lu, H., and Robson, J.: Equatorward North Atlantic jet biases in CMIP models and implications for simulated regional atmosphere-ocean linkages, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6401, https://doi.org/10.5194/egusphere-egu22-6401, 2022.

09:12–09:18
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EGU22-10571
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ECS
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On-site presentation
Amar Halifa-Marín, Enrique Pravia-Sarabia, Sergio M. Vicente-Serrano, Pedro Jiménez-Guerrero, and Juan Pedro Montávez

The North Atlantic Oscillation (NAO) represents an essential large-scale pattern of utmost importance in the understanding of the wintertime climate variability over North America and Eurasia. Despite a very large number of papers have disentangled the response of regional climate to its temporal changes, only recent works suggest that the role of spatial variability of NAO (NAO flavors) also demands attention (e.g. Rousi et al., 2020). These flavors are defined as the range of positions detected for the NAO action centers, which commonly locate over Iceland (Low) and Azores (High). This work analyses 1) the behaviour of NAO flavors (based on the first empirical orthogonal function -EOF- of Sea Level Pressure field, framed in -90W/40E/20N/80N and computed for chain 30-years periods) in the NOAA-CIRES Reanalysis, and 2) precipitation observations registered in Western Europe (Vicente-Serrano et al., 2021), across the period 1851-2015. One of the main objectives of this contribution is to assess the potential links between NAO flavors and regional wet/dry cycles in the recent past. Results reveal a physically coherent response between this spatial variability of NAO and European precipitation records. Significant positive/negative anomalies of precipitation are distinguished during different NAO flavors, ranged from -40% to +30% compared to the full period average. Likewise, the changes of mean wind direction/speed at mid/low levels have been identified as a potential physical cause. Also, the complex orography contributes to the spatial differences between wet/dry regimes. It should be highlighted that those changes of precipitation have affected European societies and ecosystems. In the case of the Iberian Peninsula, the drastic/strong reduction of winter precipitation and run-off records since 1980s (Halifa-Marín et al., 2021) is attributed to an abrupt shift eastward of NAO low action center. This work thus sheds some light on the lack of knowledge about how NAO flavors contribute to the European climate variability, meanwhile it might help understanding the abrupt shifts on regional precipitation regimes.

Acknowledgments

The authors acknowledge the ECCE project (PID2020-115693RB-I00) of Ministerio de Ciencia e Innovación (MCIN/AEI/10.13039/501100011033/) and the European Regional Development Fund (ERDF/ FEDER Una manera de hacer Europa). A.H-M thanks his predoctoral contract FPU18/00824 to the Ministerio de Ciencia, Innovación y Universidades of Spain. 

References

Halifa-Marín, A., Torres-Vázquez, M. Á., Pravia-Sarabia, E., Lemus-Cánovas, M., Montávez, J. P., and Jiménez-Guerrero, P.: Disentangling the scarcity of near-natural Iberian hydrological resources since 1980s: a multivariate-driven approach, Hydrol. Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/hess-2021-565, in review, 2021.

Rousi, E., Rust, H. W., Ulbrich, U., & Anagnostopoulou, C.: Implications of winter NAO flavors on present and future European climate. Climate, 8(1), 13, https://doi.org/10.3390/cli8010013, 2020.

Vicente-Serrano, S. M., Domínguez-Castro, F., Murphy, C., Hannaford, J., Reig, F., Peña-Angulo, D., ... & El Kenawy, A.: Long‐term variability and trends in meteorological droughts in Western Europe (1851–2018), International journal of climatology, 41, E690-E717, https://doi.org/10.1002/joc.6719, 2021.

How to cite: Halifa-Marín, A., Pravia-Sarabia, E., Vicente-Serrano, S. M., Jiménez-Guerrero, P., and Montávez, J. P.: Assessing the wintertime NAO flavors contribution to wet/dry cycles over Western Europe across the recent past, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10571, https://doi.org/10.5194/egusphere-egu22-10571, 2022.

09:18–09:24
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EGU22-13094
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ECS
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Virtual presentation
Rei Chemke, Laura Zanna, Clara Orbe, Lori Zentman, and Lorenzo Polvani

Climate models project an intensification of the wintertime North Atlantic storm track, over its downstream region, by the end of this century. Previous studies have suggested that ocean-atmosphere coupling plays a key role in this intensification, but the precise role of the different components of the coupling has not been explored and quantified. Here, using a hierarchy of ocean coupling experiments, we isolate and quantify the respective roles of thermodynamic (changes in surface heat fluxes) and dynamic (changes in ocean heat flux convergence) ocean coupling in the projected intensification of North Atlantic storm track. We show that dynamic coupling accounts for nearly all of the future strengthening of the storm track as it overcomes the much smaller effect of surface heat flux changes to weaken the storm track. We further show that by reducing the Arctic amplification in the North Atlantic, ocean heat flux convergence increases the meridional temperature gradient aloft, causing a larger eddy growth rate, and resulting in the strengthening of the North Atlantic storm track. Our results stress the importance of better monitoring and investigating the changes in ocean heat transport, for improving climate change adaptation strategies.

How to cite: Chemke, R., Zanna, L., Orbe, C., Zentman, L., and Polvani, L.: The future intensification of the North Atlantic winter storm track: the key role of dynamic ocean coupling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13094, https://doi.org/10.5194/egusphere-egu22-13094, 2022.

09:24–09:30
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EGU22-7402
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ECS
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On-site presentation
Annika Reintges, Jon Robson, Rowan Sutton, and Stephen Yeager

The variations of the winter climate in Europe are influenced by the North Atlantic Oscillation (NAO). Therefore, the ability to predict the NAO is of great value. Predictability of the NAO can be enabled through oceanic processes that are characterized by relatively long time scales, for example interannual to decadal. An important variable for the interannual to (multi-)decadal variability in the North Atlantic is the Atlantic Meridional Overturning Circulation (AMOC). The NAO and the AMOC are known to interact, but observational records of the AMOC are short and the details of this interaction are unknown. Thus, our understanding largely relies on climate model simulations. However, the interaction of NAO and AMOC is very model dependent.

Here, we present the diversity across CMIP6 models in pre-industrial control experiments. The focus lies on simulations of the NAO, the AMOC, their interaction, and related variables on interannual to decadal timescales. Regarding the NAO-AMOC interaction, there are large differences in the strength of their relationship, in the location (like the latitude of the AMOC), its periodicity and in the time-lag between both variables.

Furthermore, we propose hypotheses of the causes for this diversity in the models. Specific processes involved in NAO-AMOC interaction might be of varying relative importance from model to model, for example, NAO-related buoyancy versus wind-forcing affecting the AMOC. Also, mean state difference like in the North Atlantic sea surface temperature might play an important role for causing differences in the variability across models.

How to cite: Reintges, A., Robson, J., Sutton, R., and Yeager, S.: Diversity in NAO-AMOC interaction on interannual to decadal timescales across CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7402, https://doi.org/10.5194/egusphere-egu22-7402, 2022.

09:30–09:36
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EGU22-13394
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On-site presentation
Simon Josey and Bablu Sinha

The eastern North Atlantic subpolar gyre has become a focus of research in recent years, partly in response to the extreme cold anomaly (the 2015CA) that developed in winter 2013-14, peaked in 2015 and persisted in a weakened state for several years. The anomaly was evident both in sea surface temperature which exceeded 1.0 oC of cooling averaged over 2015 as a whole and in reduced temperatures at depth to of order 500 m. Here, we place it in a longer-term context by considering other anomalies in the observational record since 1980 and discuss its subsequent evolution through to 2022. We also explore the role played by large scale atmospheric modes of variability, particularly the East Atlantic Pattern (EAP) and North Atlantic Oscillation (NAO), in generating such anomalies. Furthermore, we draw a connection between the combined influence of these modes on both the eastern subpolar gyre and intense heat loss in the Irminger Sea which potentially leads to a coupling of mode and dense water formation processes in these two key North Atlantic regions.

How to cite: Josey, S. and Sinha, B.: Evolution of Cold Subpolar North Atlantic Conditions in the Past Decade, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13394, https://doi.org/10.5194/egusphere-egu22-13394, 2022.

09:36–09:42
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EGU22-1682
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ECS
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On-site presentation
Rachael Sanders, Daniel Jones, Simon Josey, Bablu Sinha, and Gael Forget

Record low surface temperatures were observed in the subpolar North Atlantic during 2015, despite the majority of the global ocean experiencing higher than average surface temperatures. We compute mixed layer temperature budgets in the ECCO Version 4 state estimate to further understand the processes responsible for the North Atlantic cold anomaly. We show that surface forcing was the cause of approximately 75% of the initial cooling in the winter of 2013/14, after which the cold anomaly was sequestered beneath the deep winter mixed layer. Re-emergence of the cold anomaly during the summer/autumn of 2014 was primarily driven by a strong temperature gradient across the base of the mixed layer. Vertical diffusion resulted in approximately 70% of the re-emergence, with entrainment of deeper water driving the remaining 30%. In the summer of 2015, surface warming of the mixed layer was then anomalously low, resulting in the most negative temperature anomalies. Spatial patterns in the budgets show that the initial surface cooling was strongest in the south of the region, due to strong westerly winds related to the positive phase of the East Atlantic Pattern. Subsequent anomalies in surface fluxes associated with the North Atlantic Oscillation were stronger in the north, but the impact on the average temperature of the mixed layer was largely masked by anomalously high winter mixed layer depths.

How to cite: Sanders, R., Jones, D., Josey, S., Sinha, B., and Forget, G.: Using mixed layer heat budgets to determine the drivers of the 2015 North Atlantic cold anomaly in ocean state estimates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1682, https://doi.org/10.5194/egusphere-egu22-1682, 2022.

09:42–09:48
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EGU22-5829
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Presentation form not yet defined
Jennifer Mecking, Bablu Sinha, Ben Harvey, Jon Robson, and Tom Bracegirdle

The North Atlantic Jet Stream is well known to leave an imprint on the North Atlantic SST in the form of a tri-polar pattern.  The majority of the existing research has focused on the winter jet stream position or strength of the jet stream.  Here we look at the response of the North Atlantic SSTs to the strength and position of the North Atlantic Jet Stream across all seasons in the CMIP6 piControl simulations.  For the case of both the strength and position of the jet stream the multi-model mean response is a tripolar SST pattern, with the response to the changes in strength showing a slight horseshoe pattern with the northern and southern most anomalies connected on the east and most evident in the summer.  The SST response to winter and spring jet stream changes persist the longest with the northern most imprint on the SSTs lasting up to 2 years.  The response to changes in the jet stream in the summer and fall leave an imprint on the SSTs lasting atmost into the following year.   Furthermore, we investigate at how these responses vary among the CMIP6 models and potential mechanisms leading to the persistence.

How to cite: Mecking, J., Sinha, B., Harvey, B., Robson, J., and Bracegirdle, T.: Seasonal differences in the persistence of SST’s Response to the North Atlantic Jet Stream, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5829, https://doi.org/10.5194/egusphere-egu22-5829, 2022.

09:48–09:54
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EGU22-8447
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ECS
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Presentation form not yet defined
Mixing and air-sea buoyancy fluxes drive the overturning circulation in the subpolar North Atlantic
(withdrawn)
D. Gwyn Evans, N. Penny Holliday, Sheldon Bacon, and Marilena Oltmanns
09:54–10:00
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EGU22-5694
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ECS
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On-site presentation
Lara Hellmich, Daniela Matei, Laura Suarez-Gutierrez, and Wolfgang A. Müller
Mechanisms explaining the internal variability of mean summer temperatures have been
found on seasonal to sub- and multi-decadal timescales, but their contribution to variability
in extreme temperatures is not fully established. Here, we investigate the sub-decadal (5-
10yr) variability of European summer heat extremes and their potential drivers. By using
reanalyses (ERA5/ORA-20C) and the Max Planck Institute Grand Ensemble (MPI-GE), we
identify dominant timescales of temperature extremes variability over Europe. We are able
to link heat extremes over Central Europe with a southward development of a meridional
ocean heat transport anomaly over the North Atlantic (NA), starting about 6 years prior an
extreme event. This connection is reinforced by other variables such as ocean heat content
and atmospheric sea level pressure and jet stream displacement. The results indicate the
important role of the inertia of the NA for the occurrence of heat extremes over Europe, and
possibly help to improve their predictability several years ahead.

How to cite: Hellmich, L., Matei, D., Suarez-Gutierrez, L., and Müller, W. A.: Contribution of the Atlantic Ocean to European Heat Extremes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5694, https://doi.org/10.5194/egusphere-egu22-5694, 2022.

Coffee break
Chairperson: Monika Rhein
10:20–10:26
10:26–10:36
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EGU22-2778
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solicited
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Virtual presentation
Laura Jackson, Eduardo Alastrue-De-Asenjo, Katinka Bellomo, Gokhan Danabasoglu, Aixue Hu, Johann Jungclaus, Virna Meccia, Oleg Saenko, Andrew Shao, and Didier Swingedouw

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system, which keeps the North Atlantic relatively warm. It is predicted to weaken under climate change. The AMOC may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models.

We present initial results from the North Atlantic hosing model intercomparison project, where we applied an idealised forcing of a freshwater flux over the North Atlantic in 9 CMIP6 models. The AMOC weakens in all models from the freshening, but once the freshening ceases, the AMOC recovers in some models, and in others it stays in a weakened state. We discuss how differences in feedbacks affect the AMOC response.  

How to cite: Jackson, L., Alastrue-De-Asenjo, E., Bellomo, K., Danabasoglu, G., Hu, A., Jungclaus, J., Meccia, V., Saenko, O., Shao, A., and Swingedouw, D.: AMOC thresholds in CMIP6 models: NAHosMIP, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2778, https://doi.org/10.5194/egusphere-egu22-2778, 2022.

10:36–10:42
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EGU22-4440
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ECS
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Virtual presentation
Jiao Chen, Xidong Wang, and Xuezhu Wang

Global warming since the industrial revolution has led to a series of changes in the atmosphere and ocean. As a key indicator of global ocean circulation, AMOC has shown a weakening in recent decades from both the observed and simulated results. This process which is not only affected by the local variation of the Arctic, but also by the ocean and atmosphere circulation changes in the middle and lower latitudes, might have important implications for future global climate changes. We employ the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR) and a method of perturbing coupled models to quantify and understand the impact of anthropogenic warming on the slowdown of AMOC. Conducted one control (CTRL) experiment and three sensitivity experiments (60N, 60NS, and GLOB) in which CO2 concentration were abruptly quadrupled either regionally (60N-north of 60°N, 60NS-south of 60°N) or globally (GLOB). The goal of our research is to identify the response of AMOC weakening to the quadrupling of CO2 concentration in different regions and provide future insight into ocean circulation changes in the context of climate warming. Our results show that CO2 forcing outside the Arctic dominates the weakening of AMOC. In a warming climate, the poleward heat transport increased due to the extra-Arctic CO2 forcing, which enhanced the upper ocean average stratification within the mixed-layer depth over Nordic Seas and Labrador Sea and thus weakens the AMOC to a large extent. The warming in upper-layer also lead to the dominant role of temperature contribution to stratification. However, in both the deep convection regions, the mechanism resulting in the strengthening of stratification might be quite different.

How to cite: Chen, J., Wang, X., and Wang, X.: The weakening of AMOC highly linked to climate warming outside the Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4440, https://doi.org/10.5194/egusphere-egu22-4440, 2022.

10:42–10:48
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EGU22-5908
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On-site presentation
Chris W. Hughes

Even in models with vertical sidewalls, bottom pressure torques balance the wind stress curl in a zonal integral, with local modification from nonlinear terms. This can be seen explicitly in Stommel's classic 1948 solution in which, unusually, the sea level was calculated as well as the barotropic streamfunction. Here, I explore what this and other idealised solutions tell us about how coastal sea level relates to gyre circulations, western boundary currents, and simple overturning circulations. I show that the coastal sea level signal related to the gyre (or, particularly, to changes in the gyre) need not be stronger at the western boundary. I also show that, although details of where dissipation occurs can be very important for coastal sea level when sloping sidewalls are accounted for, they are much less important for the boundary bottom pressure torque (in the vertical sidewall case, sea level and torque are closely related, so the influence of dissipation on sea level is diminished). Although the real ocean will inevitably be more complex than these ideal cases, consideration of them does alter common assumptions about how coastal sea level is likely to respond to changing circulation patterns, in response to changing climatic forcing.