UP3.2

The atmospheric circulation response to the anthropogenic increase of CO₂ is known to generally involve a number of pathways of change, often leading to contrasting responses, the so called “tug of wars”. Multiple pathways of change in atmospheric dynamics are due to the development of direct and indirect dynamical responses to global warming, the latter being the thermodynamical response to the anthropogenic increase of CO₂. The most known direct dynamical response to global warming is the strengthening and poleward shift of the tropospheric eddy-driven jets, a direct consequence of the tropical upper troposphere warming. During Northern Hemisphere winter, where and when the two-way dynamical stratosphere-troposphere coupling is active, stratosphere-troposphere coupling has been shown to provide for indirect dynamical responses, which are relevant to regional circulation changes, over the North Atlantic and Eurasia, for instance affecting the European precipitation changes. However, it has been found that the dynamical response of the stratospheric vortex is highly uncertain. Changes from ensemble of models (such as the CMIP intercomparisons) show a spread in the stratospheric responses, although the stratospheric response can be significant by model. This spread in the stratospheric responses in turn leads to uncertainty in the impact on surface climate of the stratospheric change. For instance, a stratospheric vortex weakening with global warming will counteract to some extend in some regions, the strengthening and poleward shift of the tropospheric eddy-driven jets. However, a polar vortex strengthening will feedback positively on the strengthening and poleward shift of the tropospheric eddy-driven jets. Here we review our knowledge and include new selected results, based on CMIP6, on the role of dynamical stratosphere-troposphere coupling in providing for pathway of atmospheric circulation changes in response to global warming.

How to cite: Manzini, E.: Stratosphere-Troposphere Coupling and Global Warming, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-191, https://doi.org/10.5194/ems2021-191, 2021.

Many seasonal forecast systems show skill at monthly to seasonal timescales in predicting the winter North Atlantic Oscillation (NAO), the primary mode of variability in surface pressure over the North Atlantic and European sector.  This skill has practical benefit for prediction of winter conditions over Northern Europe, and arises from the representation of remote teleconnections within the prediction system, such as from the stratosphere or the tropical troposphere.  Despite skill in the NAO, most prediction systems have little skill in other patterns of North Atlantic winter circulation variability, such as East Atlantic Pattern (EAP – the second mode of regional winter surface pressure variability). This is despite the clear contribution that patterns such as the EAP make to European winter climate variability and their demonstrated role in the generation of extreme winter conditions.

We examine the role of the tropical troposphere and extra-tropical stratosphere in driving North Atlantic and European winter circulation patterns, with a focus on teleconnections to the EAP.  We use relaxation experiments, in which a set of seasonal-length hindcasts are run with the atmospheric conditions within the relaxation region constrained to be similar to reanalysis.  These are then compared with an initialised, but otherwise freely evolving, hindcast set, and with reanalysis, in regions outside the relaxation region. The aim is to assess how better prediction of the relaxation regions would influence the skill in prediction of winter atmospheric circulation in the North Atlantic-European sector.

We find that both regions play a role in influencing regional circulation. Tropical tropospheric relaxation in particular increases the reproduction of winter surface pressure anomalies. A key part of this improvement is in the EAP, which is very well reproduced. It is shown that forcing of the EAP occurs via propagating Rossby waves linked to convective anomalies in the tropical Atlantic. In addition, we find that teleconnections from either the tropics or stratosphere lead to reproduction of observed large-scale surface pressure patterns in most winters.  In contrast, the diagnosed response to tropical forcing is rarely matched in the hindcast without relaxation, despite a similar rate of matches with the response to stratospheric forcing. This suggests that while winter stratospheric influences are well represented in the prediction system, tropical influences are under-represented.  The results suggest that the improvement of tropical Atlantic predictability could lead to improvements for European winter predictability, and should be an important focus for future work.

How to cite: Maidens, A., Knight, J. R., and Scaife, A. A.: The remote influence of the extra-tropical stratosphere and tropical troposphere on North Atlantic-European winter circulation, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-320, https://doi.org/10.5194/ems2021-320, 2021.

New observational evidence for variability of the atmospheric response to wintertime El Niño-Southern Oscillation (ENSO) is found. A weakening in the recent ENSO teleconnection over the North Atlantic-European (NAE) region is demonstrated by using various methods (e.g. composite analysis, running correlation, regression maps) applied onto different observational datasets and reanalyses (HadSLP, NOAA 20th Century reanalysis). Changes in both the spatial pattern and strength of the ENSO teleconnection indicate a turning point in the 1970s, with a shift from a response resembling the North Atlantic Oscillation (NAO) in late winter to an anomaly pattern with very weak or statistically non-significant values; and to nearly non-existent teleconnection in the most recent decades. Weakening of the ENSO signal is found at the surface (sea level pressure), but also at higher levels for different variables (geopotential height, temperature, zonal wind). To offer a possible reason behind the observed change, we have investigated the potential role of sea-ice and SST climatology in modulating the ENSO-NAE teleconnection. Sensitivity experiments made with a GCM of indermediate complexity (ICTP AGCM) using different combinations of sea-ice and sea surface temperature (SST) climatology conditions enabled the investigation of their respective roles. As indicated by the targeted simulations, recent change in the SST climatology in the Atlantic and Arctic has contributed to the weakening of the ENSO effect. Results highlight the importance of the background SST state and sea-ice climatology having opposite effects in modulating the ENSO-NAE teleconnection over the area of interest. The findings of this study could further our understanding of modulations of ENSO teleconnections and the role of ENSO as a source of predictability in the NAE sector.

How to cite: Ivasić, S., Herceg Bulić, I., and King, M. P.: Recent weakening in the winter ENSO teleconnection over the North Atlantic-European region, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-35, https://doi.org/10.5194/ems2021-35, 2021.

The accelerated warming linked to climate change has become a topic of great interest due to its projected impact on ecosystems. In this work, we assess the causes and impacts of the anthropogenic radiative forcing on the North Atlantic-European atmospheric circulation in boreal winter (DJF). To isolate the response to radiative forcing, we have used two approaches, whose simulations follow the historical/scenario concentrations from CMIP6. The first approach consists of three 240-year simulations with the European Consortium – Earth System model version 3.3 (EC-EARTH v3.3) keeping fixed the radiative forcing at 1950, characterizing the Past climate, at 2000, representative of Present-day conditions, and at 2050, projecting the near-Future climate. The second approach makes use of the Large Ensemble (i.e. 24 members) of transient simulations performed with the Institut Pierre-Simon Laplace Coupled Model version 6 (IPSL-CM6), where three 10-year periods have been considered, namely 1949-1959, 1999-2009, and 2049-2059, assuming that the radiative forcing remains relatively constant in each of them. Results show that both approaches yield a consistent forced response, and that it scales linearly with radiative forcing, increasing in amplitude from Present-minus-Past to Future-minus-Present. At low latitudes, in the tropical Atlantic, the forced atmospheric response is characterized by a Gill-type baroclinic structure, where the anomalous anticyclonic circulation at upper levels reinforces the westerly wind at the equatorward flank of the North Atlantic jet. At high latitudes, the forced response is reminiscent of the ‘Arctic Amplification’ linked to sea-ice reduction, and the thermally-driven baroclinic structure can be seen over the Labrador Sea-Hudson Bay region. At mid-latitudes, the forced response shows a barotropic pattern, with a cyclonic (anticyclonic) circulation in the North Atlantic (Euro-Mediterranean) sector, pointing out a role for non-radiative, eddy-related effects.  

How to cite: Ginesta, M., García-Serrano, J., and Gastineau, G.: Impact of the radiative forcing on the winter North Atlantic-European atmospheric circulation, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-15, https://doi.org/10.5194/ems2021-15, 2021.