Analysis of atmospheric circulation from climate model big data -Current approaches and future challenges

A large part of low-frequency variability in the climate system on sub-seasonal to decadal timescales can be described in terms of preferred atmospheric circulation patterns, often called circulation regimes. Such recurring and persistent, large-scale patterns of pressure and circulation anomalies span vast geographical area and are closely related to atmospheric teleconnection patterns like the famous North-Atlantic Oscillation (NAO). Within the conceptual framework of circulation regimes, low-frequency variability can be observed as a result of transitions between the distinct atmospheric circulation regimes. Moreover, the frequency of occurrence of preferred atmospheric circulation regimes is influenced by the external forcing factors such as other components of the climate system and anthropogenic forcing. This determines, at least partly, the time-mean response of the atmospheric flow to the external forcing. In this framework, one of our research foci is to advance the understanding of past, recent and future changes in the spatial/temporal structure of atmospheric circulation regimes and to assess the impact of internal climate dynamics versus external forcing. To tackle these questions, we exploit large global gridded data sets either from different reanalysis data sets or from model simulations with state of the art climate models mostly performed in the framework of CMIP (Coupled model intercomparison project) initiatives. We introduce and apply a hypothesis-driven approach, in particular to study the impact of sea-ice changes on atmospheric circulation patterns. The hypothesis-driven approach consists in three (iterative) steps: (i) Application of statistical methods for pattern recognition on reanalysis and climate model data, (ii) development of a hypothesis about underlying dynamical mechanisms of the impact of sea-ice changes on atmospheric circulation patterns, (iii) testing of the new hypothesis by performing new well designed climate model experiments and new model data analysis. By applying this approach, we identified tropospheric and stratospheric dynamical pathways which explain, how Arctic climate changes, in particular sea-ice changes, influence the weather and climate in mid-latitudes

The linkage between Arctic sea ice changes and mid-latitude atmospheric circulation in reanalysis data and model simulations— The role of barotropic-baroclinic interactions

bserved global warming trends have their maximum in Arctic regions, a phenomenon referred to as Arctic Amplification. Consequently, Arctic sea ice shows a strong decreasing trend. These changes imprint modifications on atmospheric flow patterns not only in Arctic regions themselves. Changes of teleconnections and planetary scale motions like Rossby wave trains affect mid-latitude climate as well. In extension to the studies by Jaiser et al. (abstract submitted) here we study the impact of sea-ice changes on changes in atmospheric synoptic and planetary waves. Therefore, we analyse the atmospheric kinetic energy spectra for ERA-Interim reanalysis and the properly designed Atmospheric General Circulation Model (AGCM) experiments with prescribed sea-ice changes (cf. abstract by Jaiser et al.). Special emphasis has been put on the the role of barotropic-baroclinic interactions and corresponding changes in the tropospheric planetary wave trains by examining the nonlinear kinetic energy and enstrophy interaction and subsequent redistribution of kinetic energy and enstrophy

Atmospheric winter response to Arctic sea ice changes in reanalysis data and model simulations - The role of troposphere-stratosphere coupling

In recent years, Arctic regions showcased the most pronounced signals of a changing climate: Sea ice is reduced by more the ten percent per decade. At the same time, global warming trends have their maximum in Arctic latitudes often labled Arctic Amplification. There is strong evidence that amplified Arctic changes feed back into mid-latitudes in winter. We identified mechanisms that link recent Arctic changes through vertically propagating planetary waves to events of a weakened stratospheric polar vortex. Related anomalies propagate downward and lead to negative AO-like situations in the troposphere. European winter climate is sensitive to negative AO situations in terms of cold air outbreaks that are likely to occur more often in that case. These results based on ERAInterim reanalysis data do not allow to dismiss other potential forcing factors leading to observed mid-latitude climate changes. Nevertheless, properly designed Atmospheric General Circulation Model (AGCM) experiments with AFES and ECHAM6 are able to reproduce observed atmospheric circulation changes if only observed sea ice changes in the Arctic are prescribed. This allows to deduce mechanisms that explain how Arctic Amplification can lead to a negative AO response via a stratospheric pathway. Further investigation of these mechanisms may feed into improved prediction systems

The linkage between Arctic sea ice changes and mid-latitude atmospheric circulation in reanalysis data and model simulations - The role of tropo-stratospheric coupling

Observed global warming trends have their maximum in Arctic regions, a phenomenon referred to as Arctic Amplification. Consequently, Arctic sea ice shows a strong decreasing trend. These changes imprint modifications on atmospheric flow patterns not only in Arctic regions themselves. Changes of teleconnections and planetary scale motions like Rossby waves affect mid-latitude climate as well. We identified mechanisms that link recent Arctic changes through vertically propagating planetary waves to weakening events of the stratospheric polar vortex. Related anomalies then propagate downward and lead to negative AO-like situations in the troposphere. These results based on ERA-Interim reanalysis data do not allow to entirely dismiss other potential forcing factors leading to observed mid-latitude climate changes. More importantly, properly designed Atmospheric General Circulation Model (AGCM) experiments with AFES and ECHAM6 are able to reproduce observed atmospheric circulation changes if only observed sea ice changes in the Arctic are prescribed. This includes the potential mechanism explaining how Arctic Amplification can lead to a negative AO response via a stratospheric pathway. A further examination of barotropic-baroclinic interactions based on nonlinear kinetic energy and enstrophy interaction will be given by Handorf et al. (abstract submitted)

2015: Impacts of Arctic sea ice and continental snow cover changes on atmospheric winter teleconnections. Geophys

Abstract Extreme winters in Northern Hemisphere midlatitudes in recent years have been connected to declining Arctic sea ice and continental snow cover changes in autumn following modified planetary waves in the coupled troposphere-stratosphere system. Through analyses of reanalysis data and model simulations with a state-of-the-art atmospheric general circulation model, we investigate the mechanisms between Arctic Ocean sea ice and Northern Hemisphere land snow cover changes in autumn and atmospheric teleconnections in the following winter. The observed negative Arctic Oscillation in response to sea ice cover changes is too weakly reproduced by the model. The planetary wave train structures over the Pacific and North America regions are well simulated. The strengthening and westward shift of the Siberian high-pressure system in response to sea ice and snow cover changes is underestimated compared to ERA-Interim data due to deficits in the simulated changes in planetary wave propagation characteristics

Evaluating Causal Arctic‐Midlatitude Teleconnections in CMIP6

To analyze links among key processes that contribute to Arctic-midlatitude teleconnections we apply causal discovery based on graphical models known as causal graphs. First, we calculate the causal dependencies from observations during 1980–2021. Observations show several robust connections from early to late winter, such as atmospheric blocking within central Asia via the Ural blocking and Siberian High, the North Atlantic Oscillation phase and the polar vortex (PV). The PV is affected by poleward eddy heat flux at 100 hPa, which is also directly connected with the Aleutian Low. We then evaluate climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) by comparing their causal graphs with those derived from observations. Compared to observations, CMIP6 historical and future simulations do not robustly capture Arctic-midlatitude teleconnections arising from Arctic sea ice variability. This highlights the role of atmospheric internal variability in modulating the Arctic-midlatitude teleconnections. However, we find several distinct patterns that are simulated by most of the analyzed climate models. For example, both historical and future model simulations robustly capture observed atmospheric blocking in central Asia. But contrary to observations, model simulations show a robust link between the Arctic temperature and sea ice cover over Barents and Kara seas. The analysis of future changes also reveals that the connection between the Aleutian Low and the poleward eddy heat flux at 100 hPa is expected to become more robust toward the end of the 21st century than in the analyzed past

The role of stratospheric ozone for Arctic-midlatitude linkages

Arctic warming was more pronounced than warming in midlatitudes in the last decades making this region a hotspot of climate change. Associated with this, a rapid decline of sea-ice extent and a decrease of its thickness has been observed. Sea-ice retreat allows for an increased transport of heat and momentum from the ocean up to the tropo- and stratosphere by enhanced upward propagation of planetary-scale atmospheric waves. In the upper atmosphere, these waves deposit the momentum transported, disturbing the stratospheric polar vortex, which can lead to a breakdown of this circulation with the potential to also significantly impact the troposphere in mid- to late-winter and early spring. Therefore, an accurate representation of stratospheric processes in climate models is necessary to improve the understanding of the impact of retreating sea ice on the atmospheric circulation. By modeling the atmospheric response to a prescribed decline in Arctic sea ice, we show that including interactive stratospheric ozone chemistry in atmospheric model calculations leads to an improvement in tropo-stratospheric interactions compared to simulations without interactive chemistry. This suggests that stratospheric ozone chemistry is important for the understanding of sea ice related impacts on atmospheric dynamics

Review article: A European perspective on wind and storm damage – from the meteorological background to index-based approaches to assess impacts

Wind and windstorms cause severe damage to natural and human-made environments. Thus, wind-related risk assessment is vital for the preparation and mitigation of calamities. However, the cascade of events leading to damage depends on many factors that are environment-specific and the available methods to address wind-related damage often require sophisticated analysis and specialization. Fortunately, simple indices and thresholds are as effective as complex mechanistic models for many applications. Nonetheless, the multitude of indices and thresholds available requires a careful selection process according to the target sector. Here, we first provide a basic background on wind and storm formation and characteristics, followed by a comprehensive collection of both indices and thresholds that can be used to predict the occurrence and magnitude of wind and storm damage. We focused on five key sectors: forests, urban areas, transport, agriculture and wind-based energy production. For each sector we described indices and thresholds relating to physical properties such as topography and land cover but also to economic aspects (e.g. disruptions in transportation or energy production). In the face of increased climatic variability, the promotion of more effective analysis of wind and storm damage could reduce the impact on society and the environment