UP3.2

Mid-latitude atmospheric teleconnection dynamics

Covariability between remote regions – often named teleconnections – are at the basis of our current knowledge of a large part of Earth’s climate variations and represent an important source of weather and climate predictability. Tropospheric and stratospheric pathways have been suggested to play a role in connecting internally-generated and radiatively-forced anomalies at mid-latitudes, as well as in settling tropical-extratropical and polar-nonpolar interactions. However, the underlying processes behind these linkages are still not properly understood, misled by different metrics and diagnostics, and/or generally poorly simulated by global climate models (GCMs). A continuous assessment of these atmospheric teleconnections is thus necessary, since advances in process understanding could translate into improving climate models and predictions.

This session aims at gathering studies on both empirical and modelling approaches, dealing with a dynamical characterization of mid-latitude atmospheric teleconnections. It invites contributions using observational datasets; coupled and uncoupled (atmosphere-only) GCM simulations; pre-industrial, present, and future climate conditions; idealised sensitivity experiments; or theoretical models.

Keynote talk:

Elisa Manzini - "Troposphere-Stratosphere Coupling and Global Warming"

Including EMS Young Scientist Conference Award winner
Convener: Javier Garcia-Serrano | Co-conveners: Paolo Davini, Yannick Peings
Lightning talks
| Tue, 07 Sep, 09:00–10:30 (CEST)

Lightning talks: Tue, 07 Sep

09:00–09:05
From the stratosphere and the tropics
09:05–09:20
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EMS2021-191
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solicited
Elisa Manzini

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.

09:20–09:25
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EMS2021-320
Anna Maidens, Jeff R Knight, and Adam A Scaife

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.

09:25–09:30
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EMS2021-117
Froila M. Palmeiro, Javier García-Serrano, Paolo Ruggieri, Lauriane Batté, and Silvio Gualdi

Using the complete ERA-Interim reanalysis and three state-of-the-art models, this study explores how El Niño-Southern Oscillation (ENSO) can influence the frequency and seasonal cycle of sudden stratospheric warmings (SSWs) by modulating the background upward wave propagation. Reanalysis data shows that in the last four decades, winters with SSWs were significantly more common than inactive winters, particularly under La Niña (LN) conditions but not during El Niño (EN), regardless of the ENSO/SSW definitions considered herein. In agreement with previous studies, our models tend to simulate a linear ENSO-SSW relationship, with more SSWs for EN, that show a peak of occurrence around January — as occurs in reanalysis —, and less SSWs for LN when compared to neutral conditions. Independently of ENSO, the main tropospheric precursor of SSWs appears to be an anomalous wave-like pattern over Eurasia, but it is dominated by wavenumber 1 (WN1) for EN and shows an enhanced wavenumber 2 (WN2) for LN. The differences in this Eurasian wave pattern, which is largely internally generated (ENSO-unforced), emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO, favouring a relative stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO-forced signal relies on preconditioning the seasonal-mean polar vortex, becoming weaker and displaced (stronger and more stable) for EN (LN), while ENSO-unforced wave activity represents the ultimate trigger of SSWs. This supports the view that ENSO and SSWs are different sources of variability of the winter atmospheric circulation and may reconcile previous findings in this context.

How to cite: Palmeiro, F. M., García-Serrano, J., Ruggieri, P., Batté, L., and Gualdi, S.: Intraseasonal modulation of Sudden Stratospheric Warmings by ENSO, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-117, https://doi.org/10.5194/ems2021-117, 2021.

09:30–09:35
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EMS2021-14
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EMS Young Scientist Conference Award winner
Bianca Mezzina, Froila M. Palmeiro, Javier García-Serrano, Ileana Bladé, Lauriane Batté, and Marianna Benassi

The impact of El Niño-Southern Oscillation (ENSO) on the late-winter extra-tropical stratosphere (January-March) is assessed in a multi-model framework. Three state-of-the-art atmospheric models are run with prescribed SST anomalies representative of a strong ENSO event, with symmetric patterns for El Niño and La Niña. The well-known temperature perturbation in the lower stratosphere during El Niño is captured by two models, in which the anomalous warming at polar latitudes is accompanied by a positive geopotential height anomaly that extends over the polar cap. In the third model, which shows a lack of temperature anomalies over the pole, the anomalous anticyclone is confined over Canada and does not expand to the polar cap. This anomalous center of action emerges from the large-scale tropospheric Rossby wave train forced by ENSO, and conservation of potential vorticity around the polar vortex is invoked to link it to the temperature response. No disagreement across models is found in the lower stratosphere for La Niña, whose teleconnection is opposite in sign but weaker. In the middle-upper stratosphere (above 50 hPa) the geopotential height anomalies project on a wavenumber-1 (WN1) pattern for both El Niño and, more weakly, La Niña, and show a westward tilt with height up to the stratopause. It is suggested that this WN1 pattern arises from the high-latitude lower-stratospheric anomalies, and that the ENSO teleconnection to the polar stratosphere can be interpreted in terms of upward propagation of the stationary Rossby wave train and quasi-geostrophic balance, instead of wave breaking.
The multi-model assessment, with 50 members for each experiment, contributes to the ERA4CS-funded MEDSCOPE project and includes: EC-EARTH/IFS (L91, 0.01hPa), CNRM/ARPEGE (L91, 0.01hPa), CMCC/CAM (L46, 0.3hPa).

How to cite: Mezzina, B., Palmeiro, F. M., García-Serrano, J., Bladé, I., Batté, L., and Benassi, M.: Multi-model assessment of the late-winter stratospheric response to El Niño and La Niña, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-14, https://doi.org/10.5194/ems2021-14, 2021.

09:35–09:40
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EMS2021-35
Sara Ivasić, Ivana Herceg Bulić, and Martin P. King

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.

Other forcings at the extratropics
09:40–09:45
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EMS2021-70
Ralf Jaiser, Mirseid Akperov, Alexander Timazhev, Erik Romanowsky, Dörthe Handorf, and Igor Mokhov

The study addresses the question, if Arctic sea ice decline is the main driver of observed changes in terms of Arctic-midlatitude linkages during winter. We discuss, if the increase of global sea surface temperatures plays an additional role. A set of four model sensitivity experiments with different sea ice and sea surface temperature boundary conditions is analyzed and compared to observed changes in reanalysis data. A detection of atmospheric circulation regimes is performed. These regimes are evaluated for their cyclone and blocking characteristics and their changes in frequency during winter to reveal tropospheric changes induced by the change of boundary conditions. Furthermore, the impacts on the large-scale circulation up into the stratosphere are investigated. The results show that the impact from sea surface temperature changes is generally stronger than the impact of sea ice concentration changes alone. However, in particular in terms of the startospheric pathway, the combined impact of sea ice and sea surface temperature changes reproduces findings from the reanalysis best.

For early winter, the observed increase in atmospheric blocking in the region between Scandinavia and the Ural are primarily induced by the changes in sea surface temperatures. Nevertheless, the impacts on the stratospheric circulation in terms of a weakened polar vortex, are only observed if sea ice is reduced and sea surface temperatures are increased. Late winter impacts are more inconsistent in the model sensitivity study, but slightly improved when both components of forcing are changed. In this context, we further identify a discrepancy in the model to reproduce the weakening of the stratospheric polar vortex through blocking induced upward propagation of planetary waves.

How to cite: Jaiser, R., Akperov, M., Timazhev, A., Romanowsky, E., Handorf, D., and Mokhov, I.: Linkages between Arctic and Mid-Latitude Weather and Climate: Unraveling the Impact of Changing Sea Ice and Sea Surface Temperatures, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-70, https://doi.org/10.5194/ems2021-70, 2021.

09:45–09:50
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EMS2021-15
Mireia Ginesta, Javier García-Serrano, and Guillaume Gastineau

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.

09:50–09:55
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EMS2021-321
Luca Famooss Paolini, Alessio Bellucci, Paolo Ruggieri, Panos Athanasiadis, and Silvio Gualdi

Western boundary currents transport a large amount of heat from the Tropics toward higher latitudes; furthermore they are characterized by a strong sea surface temperature (SST) gradient. For such reasons they have been shown to be fundamental in influencing the climate of the Northern Hemisphere and its variability, and a  potentially relevant source of atmospheric predictability. General circulation models show deficiencies in simulating the observed atmospheric response to SST front variability. The atmospheric horizontal resolution has been recently proposed as a key element in understanding such differences. However, a multi-model analysis to systematically investigate differences between low-resolution and high-resolution atmospheric response to oceanic forcing is still lacking. The present work has the objective to fill this gap, analysing the atmospheric response to Gulf Stream SST front (GSF) shifting using data from recent High Resolution Model Intercomparison Project (HighResMIP). Ensembles of historical simulations performed with three atmospheric general circulation models (AGCMs) have been analysed, each conducted with a low-resolution (LR, about 1°) and a high-resolution (HR, about 0.25°) configuration. AGCMs have been forced with observed SSTs (HadISST2 dataset), available at daily frequency on a 0.25° grid, during 1950–2014. Results show atmospheric responses to the SST-induced diabatic heating anomalies that are strongly resolution dependent. In LR simulations a low-pressure anomaly is present downstream of the SST anomaly, while the diabatic heating anomaly is mainly balanced by meridional advection of air coming from higher latitudes, as expected for an extra-tropical shallow heat source. In contrast, HR simulations generate a high-pressure anomaly downstream of the SST anomaly, thus driving positive temperature advection from lower latitudes (not balancing diabatic heating). Along the vertical direction, both in LR and HR simulation, the diabatic heating in the interior of the atmosphere is balanced by upward motion south of GS SST front and downward motion north and further south of the Gulf Stream. Finally, LR simulations show a reduction in storm-track activity over the North Atlantic, whereas HR simulations show a meridional displacement of the storm-track considerably larger (yet in the same direction) than that of the SST front. HR simulations reproduce the atmospheric response obtained from observations, albeit weaker. This is a hint for the existence of a positive feedback between ocean and atmosphere, as proposed in previous studies. These findings are qualitatively consistent with previous results in literature and, leveraging on recent coordinated modelling efforts, shed light on the effective role of atmospheric horizontal resolution in modelling the atmospheric response to extra-tropical oceanic forcing.

How to cite: Famooss Paolini, L., Bellucci, A., Ruggieri, P., Athanasiadis, P., and Gualdi, S.: Atmospheric response to Gulf Stream front shifting:impact of horizontal resolution in an ensemble of global climate models, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-321, https://doi.org/10.5194/ems2021-321, 2021.

09:55–10:30

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