The climate patterns across Northwest Europe are shaped by the transportation of warm and moist air from the North Atlantic Ocean, driven by large-scale atmospheric circulation. A possible key to this system is the variability in sea surface temperatures (SST) southeast of Greenland, possibly influencing the trajectory of weather systems.

A hypothesis suggests that the melting of the Greenland Ice Sheet plays a role in altering deep ocean convection in the Labrador Sea, leading to cooling in the ocean region southeast of Greenland. Studies propose that a substantial increase in meltwater from the Greenland Ice Sheet could potentially slow down the Atlantic Meridional Overturning Circulation (AMOC), impacting the Atlantic Storm track. In a worst-case scenario, this could shift Northwest Europe's climate from mild to subarctic conditions, reminiscent of glacial periods.

However, conflicting model studies suggest a different outcome, proposing that subpolar gyre cooling induced by freshwater fluxes might intensify the North Atlantic storm track.

To establish a robust connection between Greenland Ice Sheet melt and climate fluctuations in Northwest Europe, extended time series data beyond the instrumental record is essential. Additionally, a comprehensive understanding of specific climatic modes and associated storm track paths influenced by freshwater from the Greenland Ice Sheet is needed.

Preliminary evidence suggests a link between Greenland Ice Sheet melt variations and climate fluctuations in Northwest Europe. If fully validated, this connection holds significant implications for accurate climate predictions, particularly given the anticipated rise in melt rates of the Greenland Ice Sheet in the future. Ensuring precise climate predictions is critical for comprehending and preparing for potential shifts in weather patterns that could impact the region's climate and ecosystems

How to cite: Christensen, J. H., Andresen, C., Hvidberg, C., and Van der Laan, L.: How Greenland Ice Melt Could Influence Atmospheric Variability, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13128, https://doi.org/10.5194/egusphere-egu24-13128, 2024.

Weather regimes are recurrent and quasi-stationary atmospheric circulation patterns, typically linking to surface weather and extremes. Despite their widespread use, little is known on whether or how regimes defined in different regions relate to each other and reflect long-distance teleconnection patterns. Here, we shed light on this knowledge gap, focussing on North American and Euro-Atlantic regimes. The selection of these two regions is motivated by recent evidence pointing to a systematic connection between winter weather in North America and Europe. We find that specific pairs of North American and Euro-Atlantic regimes show a close statistical correspondence and that their joint analysis can provide medium-range statistical predictability for anomalies in their occurrence frequencies. Conditioning on North American weather regimes also results in anomalies in both the large-scale circulation during specific Euro-Atlantic regimes, and the associated European surface weather. We conclude that there is a benefit in conducting joint analyses of North American and European weather regimes, as opposed to considering the two in isolation.

How to cite: Messori, G. and Dorrington, J.: Connecting North American and European Weather Regimes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14656, https://doi.org/10.5194/egusphere-egu24-14656, 2024.

Weather regimes (WRs) provide useful information about large scale variability over relatively large regions, and they can be linked to extreme events occurring at the land surface, such as heat waves and extreme precipitations. Studies in this direction have shown how e.g., Euro-Atlantic WRs modulate flow dependent variability in North America and Europe and their links to extreme precipitation events. To date, the relationship of weather regimes specific to the Mediterranean with extreme events have not been sufficiently explored. The Mediterranean region is a hotspot for climate change and for its peculiar position, at the frontier between very different systems, it is influenced by a complex mix of large-scale variability processes. Under the hypothesis that these processes are explained better by considering atmospheric fields over the Mediterranean region, we proceed by extracting EOFs over the Mediterranean domain (25 to 50 North, and −10 to 40 East), and we identify, for the first time in this region, year-round weather regimes. This allows for a systematic detection of extremes that is not limited to a specific season but throughout the year. After describing each regime pattern and corresponding average conditions (temperature, precipitation), we explore their links to extreme precipitations in the area and finally compare results with EAT-WRs to assess which WR domain is more closely related to these events.

How to cite: Giuntoli, I. and Corti, S.: Year-round Mediterranean Weather Regimes for exploring the occurrence of extreme precipitations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17290, https://doi.org/10.5194/egusphere-egu24-17290, 2024.

An intermediate-complexity moist general circulation model is used to disentangle changes in the large-scale zonally asymmetric circulation due to rising GHGs. We run multiple idealized experiments in order to isolate, and subsequently synthesize, the physical processes driving these changes. In particular, we examine stationary wave changes forced by land–sea contrast, horizontal heat fluxes in the ocean, and orography, in response to a quadrupling of CO2 concentrations. A particular focus is on the anomalous ridge in the Mediterranean region associated with the decline in precipitation in this heavily populated region. 

 Our results suggest a combination of two mechanisms is responsible for future Mediterranean drying. The first is a global phenomena, a lengthening of intermediate-scale stationary waves due to strengthening of subtropical upper-tropospheric zonal mean zonal winds, shown previously to account for hydroclimatic changes in the western US. We find this mechanism to be dominated by change in waves forced by ocean horizontal heat fluxes. The second mechanism is a regional one, a strengthening of large-scale stationary wave modes over Europe and the north Atlantic, dominated by changes in stationary waves forced by land-sea contrast. This second mechanism is strongly tied to an altered temperature gradient between the North Atlantic and Europe, in response to rising GHGs. Our work demonstrates how large-scale upper-tropospheric circulation changes are directly tied to regional hydroclimate.

How to cite: Keller, B. and Garfinkel, C.: Disentangling projected stationary wave changes: implications for future drying of the Mediterranean region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9099, https://doi.org/10.5194/egusphere-egu24-9099, 2024.

Large-scale atmospheric circulation is the major driver of near surface climatic variability and extremes, with teleconnection patterns being a significant component by connecting climates in remote locations. The recently developed powerful concept of causal effect networks (CENs) enables the detection of causal relationships among a set of actors by removing the confounding effects of autocorrelation, indirect links, and common drivers, retaining eventually only the actual causal links.

In this study, we apply the causal discovery algorithm to analyze the causal links among teleconnection patterns and other circulation features of the North Hemisphere and their influence on Mediterranean winter climate variability. We employ different sets of actors and multiple-scale temporal resolution reanalysis datasets to examine the consistency of the CENs across different timescales and to uncover the underlying mechanisms of their links. By investigating the strength of the different remote drivers compared to local drivers, this analysis contributes to a better understanding of the mechanisms controlling the intraseasonal variations in boreal winter circulation over the Mediterranean. In addition, we find that the strength of the causal links is affected by interannual and multidecadal variability, suggesting the potential involvement of external physical mechanisms.

How to cite: Hatzaki, M., Di Capua, G., Chaniotis, I., Patlakas, P., Donner, R., and Flocas, H. A.: Causal networks for quantifying the links of boreal winter atmospheric variability with Mediterranean climate on multiple temporal scales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15725, https://doi.org/10.5194/egusphere-egu24-15725, 2024.

In recent decades, the Northern Hemisphere (NH) exhibits a long-term regional cooling (central Eurasia) and warming (Arctic and Northern America) trend caused by human-induced anthropogenic forcing and internal decadal variability. In this study, we quantify the contribution of internal decadal variability to recent NH temperature trend using an atmosphere general circulation model (OpenIFS) by designing some sensitivity experiments for the period 1950-2014. In the reanalysis dataset (ERA5), we find a significant teleconnection between Interdecadal Pacific Variability (IPV) and surface air temperature (SAT) over Eastern Eurasia, the Barents Sea, and the Kara Sea for the periods 1950-2014 and 1993-2014, whereas we have not seen such a significant teleconnection with Atlantic Multidecadal Variability (AMV). The model simulates temperature anomalies associated with the IPV consistent with the ERA5, except for northern Eurasia where the sign of the temperature anomaly is reversed compared to ERA5. The model simulates AMV teleconnection with SAT is positive and significant over most of the places during 1950-2014, and it is significant over central Asia during 1993-2014. By analyzing the sensitivity experiments, in which we removed the decadal variability associated with IPV and AMV, we find that Eurasian cooling significantly increases without IPV and there is not much change without AMV. This indicates that some of the recent cooling over the Eurasian region is not driven by the IPV at least in the OpenIFS model, which shows IPV contributes to warm the Eurasian region. The preliminary results of this study suggest the potential importance of the internal variability of the Pacific Ocean is not only crucial on a regional scale but also crucial on a on a hemispheric scale (high latitudes).

How to cite: Savita, A., Joakim Kjellsson, J., Latif, M., Nnamchi, H., and Wahl, S.: Impact of multidecadal climate’ modes variability on the Northern Hemisphere temperature trend in the recent decades, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11748, https://doi.org/10.5194/egusphere-egu24-11748, 2024.

The planet is warming due to the burning of fossil fuels, but the geographical pattern of the observed temperature change over the Pacific Ocean over the past ~40 years is profoundly different from our expectations based on the CMIP5/6 climate model simulations of both historical and future warming. Here we will present an argument that the observed pattern of warming is consistent with a forced response to increasing atmospheric carbon dioxide shaped by two-way atmospheric teleconnections between the Southern Ocean and tropical Pacific Ocean. The same two-way atmospheric teleconnections might also be capable of yielding low-frequency natural variability in sea surface temperature and sea level pressure anomalies resembling the observed trend patterns. We will offer reasons why the observed pattern of warming is not simulated by the climate models. The observed pattern of warming in the Pacific has first-order implications for climate sensitivity as well as for the projected changes in global-scale precipitation.

How to cite: Battisti, D., Armour, K., and Dong, Y.: The perplexing warming trend over the Pacific Ocean and the key role of two-way teleconnections between the Southern Ocean and the tropical Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14452, https://doi.org/10.5194/egusphere-egu24-14452, 2024.

Changes in the sea surface temperature (SST) pattern in the tropical Pacific modulate radiative feedbacks to greenhouse gas forcing, the pace of global warming, and regional climate impacts.  Therefore, elucidating the drivers of the pattern is critically important for reducing uncertainties in future projections.  However, the attribution of observed changes over recent decades, an enhancement of the zonal SST contrast coupled with a strengthening of the Walker circulation, has not been successful.  Here, we review existing mechanisms of the forced response, categorized as either an energy perspective that adopts global/hemispheric energy budget constraints or a dynamical perspective that examines the tropical atmosphere-ocean coupled processes. We then collectively discuss the relative contributions to the past and future SST pattern changes to propose a narrative that reconciles them. Despite uncertainties, the balance of evidence suggests that the mechanisms leading to strengthening the zonal SST contrast have been efficient in the past and those leading to a weakening were less efficient but will become dominant in a future climate. We particularly focus on the role of Southern Ocean SST changes in shifting the tropical Pacific warming pattern. Finally, we present opportunities to resolve the model-observation discrepancy regarding the recent trend.

How to cite: Kang, S. M., Watanabe, M., Collins, M., Hwang, Y.-T., McGregor, S., and Stuecker, M. F.: Remote mechanisms for shifting the tropical Pacific warming pattern, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3098, https://doi.org/10.5194/egusphere-egu24-3098, 2024.

Most of the coarse resolution Coupled Model Intercomparison Project (CMIP) climate models simulate a weakening of the equatorial Pacific east-west sea surface temperature (SST) gradient, contrary to the observation since the mid-1970s. Proposed reasons for this model-observation discrepancy are the equatorial Pacific cold tongue bias and the underestimation of equatorial trade wind strength in enhancing the upwelling in the central to eastern Pacific Ocean. Higher oceanic resolution has been known to improve eddy-induced heat transport, equatorial Pacific mean state SSTs, and precipitation. Here, we assess SST mean state biases and trend responses in a multi-model and multi-resolution ensemble from the High-Resolution Model Intercomparison Project (HighResMIP). Some models show an alleviation of the cold tongue bias in simulations of higher resolution compared to their respective low-resolution simulations, however, there is no consistent improvement across models in the trend response of the equatorial eastern Pacific SSTs at a higher resolution. Models are deficient in simulating the synchrony of trends and the causal relationships between surface zonal wind, SSTs, and thermocline structure in the eastern equatorial Pacific Ocean on multidecadal timescales. An underestimated ocean thermostat mechanism might explain climate models’ inability to simulate equatorial Pacific SST patterns since the mid-1970s. Simulating air-sea coupling correctly on multidecadal timescales in models might reduce the uncertainty of the projected tropical Pacific SST gradient.

How to cite: Dhame, S., Olonscheck, D., and Rugenstein, M.: No consistent improvement in tropical Pacific sea surface temperature pattern in high-resolution climate models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21134, https://doi.org/10.5194/egusphere-egu24-21134, 2024.

How to cite: Rauch, M., Bliefernicht, J., and Kunstmann, H.: Prediction of the Interannual Rainfall Variability in the Sahel: Insights from Atmospheric Circulation Patterns , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1280, https://doi.org/10.5194/egusphere-egu24-1280, 2024.

The long-distance linkage between weather and climate conditions in different regions, i.e., teleconnection patterns, is crucial for understanding and predicting climate variability. In South America (SA), atmospheric Rossby waves originating in the maritime continent play a key role in triggering the South American Monsoon System across different timescales. Current studies have mainly focused on the long-term variability, mostly associated with seasonal, interannual and interdecadal temporal scales. On the contrary, intraseasonal variability has remained underexplored, especially the higher frequency relevant for the under two-week weather prediction. In this research, we investigate the high-frequency intraseasonal variability (HFISV, 8 – 20 days) of precipitation in SA by performing an empirical orthogonal function (EOF) analysis. For this, we use the CPC precipitation data for the summer period between 1979 – 2018. We also track its origin on teleconnection patterns in the Southern Hemisphere (SH) and local processes in SA by using lead and lag regression techniques based on ERA5 reanalysis data and NOAA outgoing longwave radiation. For this analysis, we give particular emphasis on describing active and break rainfall phases over SA. Our results show that HFISV significantly contributes to the total precipitation variability in the region (∼28%). We also found that extreme precipitation events in SA, which can lead to floods and droughts, are closely linked to anomalous high and low-pressure systems over the SH, demonstrating strong connections with Rossby waves in the mid-latitudes originated in the South Pacific Convergence Zone. At a local scale, spatial maps and cross-sectional analysis provided further insights, confirming that local processes feed back and enhance the extreme event, where low-level winds play a critical role in transporting moisture across the region. Local processes are afterwards able to reverse the winds and redistribute the moisture leading to a change in the monsoon phase. Our work highlights that predicting teleconnections, which modulate circulation anomalies and weather patterns, is a potential tool for precipitation subseasonal predictability. This is particularly relevant in arid areas where water is primarily available in the form of seasonal convective storms (e.g., in the Altiplano region), but also in a wider range as continents like Africa, which depend on the SA monsoon.

How to cite: Aguirre Correa, F., Suárez, F., and Bollasina, M.: High-frequency Intraseasonal Variability of Precipitation in South America and its link with Southern Hemisphere Teleconnection Patterns, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12323, https://doi.org/10.5194/egusphere-egu24-12323, 2024.

Different North Atlantic winter climate regimes force different circulation patterns in the Baltic Sea. Furthermore, as the atmospheric circulation, to a large extent, controls patterns of water circulation and biophysical aspects relevant for biological production, such as the vertical distribution of temperature, salinity and oxygen, alterations in weather regimes may severely impact the trophic structure and functioning of marine food webs (Hinrichsen et al. 2007). To understand the processes linking changes in the marine environment and climate variability of the Baltic Sea, it is essential to investigate all components of the climate system which of course include also the large-scale atmospheric circulation variability. Here we focus on the link between changes/shifts in the large-scale atmospheric conditions and their impact on the regional scale variability over the Baltic Sea area for the period 1950-2022. This work is mostly an extension of previous studies which focused on the response of the Baltic Sea circulation to climate variability for the period 1958-2008 (Lehmann et al. 2011, Lehmann et al. 2014). Now extended time series ECMWF ERA 5 reanalysis for 7 decades are available, highlighting recent changes in atmospheric conditions over the Baltic Sea area. The main focus of this work is to identify predominant large scale atmospheric circulation patterns (North Atlantic winter climate regimes) on a monthly/seasonal time scale controlling the development of regional atmospheric weather types over the Baltic Sea area, which in turn can be associated with different Baltic Sea circulation patterns and water mass exchange with the North Sea.

How to cite: Lehmann, A. and Post, P.: Changing impact of large-scale atmospheric circulation variability on the water mass exchange and circulation of the Baltic Sea for the period 1950-2022, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9354, https://doi.org/10.5194/egusphere-egu24-9354, 2024.

Classification of atmospheric circulation patterns has been a major powerful tool in synoptic climatology for a long time. Recently, its generalization has been introduced, which consists in developing geographically sliding classifications, i.e., independent classifications centred over individual gridpoints of a regular grid.

We employ such a tool in an attempt to explain the asymmetry of day-to-day temperature differences (DTDs). DTD is defined simply as a difference of daily temperature between two consecutive days. DTD in Europe is asymmetric: its skewness is negative over most of Europe in summer, while in winter, there is a tendency for a positive skewness to occur in the north and for negative skewness to occur in the south and over the British Isles.

We employ the ERA5 reanalysis as a major data source of both circulation and temperature data and the ECA&D station database for verification of temperature skewness in ERA5. Daily maximum temperature in summer and daily minimum temperature in winter are analyzed. Atmospheric circulation is characterized by sea level pressure, which is subject to classification by the Jenkinson-Collison (JC) method at all gridpoints over the European continent with spatial resolution of 2.5° x 2.5°. The JC method is based on types pre-defined by the strength, direction, and vorticity of geostrophic flow. We utilize its versions with 27 types (full version) and 11 types (with 8 directional types, two vorticity-based types and one undetermined for a weak flow). Under each type and at every gridpoint, we count small negative and small positive DTDs (small DTDs defined approximately as central 50% of its distribution). Types with the largest difference between small positive and small negative DTDs (i.e., with the largest asymmetry in small DTDs) are then identified.

A general behaviour, characteristic for the majority of European landmass, can be summarized as follows: Anticyclonic types, types with weak flow, and types with warm advection from south to southwest directions contribute to the asymmetry of Tmax DTD in summer, while anticyclonic types and types with cold northerly to northeasterly advection contribute to the Tmin DTD asymmetry in winter. Nevertheless, under specific conditions (upwind or leeward side of mountains, seashore, valley), any of the 11 JC types can occur among the three that most support the DTD asymmetry.

How to cite: Huth, R., Stryhal, J., and Krauskopf, T.: Classifications of atmospheric circulation patterns as a tool for explaining asymmetry of day-to-day temperature difference, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10222, https://doi.org/10.5194/egusphere-egu24-10222, 2024.

The Arctic Sea plays a pivotal role in Earth's climate system by modulating ocean-atmospheric interactions, influencing heat exchange, with potential implications for regional phenomena such as Indian Summer Monsoon Rainfall (ISMR). We assess the performance of 26 Coupled Model Intercomparison Project Phase 6 (CMIP6) models in simulating Arctic Sea Ice based on historical data and climatology. The evaluation of selected models focus on their fidelity in accurately representing the intricate dynamics of Arctic Sea Ice by employing various statistical skill metric parameters. Additionally, we will investigate the potential connections between Arctic Sea Ice variability and ISMR using CMIP6 models. A multi-model ensemble mean of the top-performing models will be conducted to illuminate these associations. Further, the study will provide teleconnections and potential correlations through sea-level pressure (SLP) anomalies and velocity potential during ISMR. Overall, the research aims to evaluate the tropical-polar teleconnection and its predictive capacity for ISMR through CMIP6 models, thereby ensuring a better understanding of the critical climate dynamics.

How to cite: Sardana, D. and Agarwal, A.: Investigating CMIP6 Models for Arctic Sea Ice Dynamics and Predictive Links with Monsoon Precipitation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14093, https://doi.org/10.5194/egusphere-egu24-14093, 2024.