Arctic Climate Processes Linked Through the Circulation of the Atmosphere

The wintertime atmospheric circulation in the northern hemisphere mid and high latitudes is strongly governed by the phase of the North Atlantic Oscillation (NAO). The recent winters have been remarkably cold over Europe and North America, characterised by a pronounced NAO negative phase. During such a phase, the westerly jet stream that normally brings storms and mild oceanic air across the Atlantic toward Norway, is displaced further southwards, bringing bitter Arctic air to Europe. This was specially the case for the winter 2009/10, when the NAO index was at a record low for over a century. In this project, we are able to specifically estimate the sole contribution of the snowpack (the snow cover extent and depth) to the anomalous winter atmospheric circulation, thus demonstrating that the anomalous autumn Eurasian snowpack played a key role in maintaining the pronounced negative NAO observed during that winter. To do this, we carried out coupled ocean-atmosphere simulations at high spatial resolution with the state-of-the-art ensemble prediction system of European Centre for Medium-Range Weather Forecasts (ECMWF), including a realistic snow and sea-ice initialisation. We performed in parallel two nearly-identical ensembles forecasts, differing only in the treatment of the snowpack. Looking at the difference between the two sets of forecasts, it appears clearly that by imposing a realistic, thick insulating snowpack at the beginning of the forecast simulations, induces a pronounced surface cooling. We next demonstrated how the hemispheric-wide cold anomaly enhances vertical wave planetary propagation into the stratosphere, contributing to decelerate the polar stratospheric jet. The rapid tropospheric response then readily appears within 15 days and is confined to the North Atlantic sector, maintaining the north/south pattern in sea-level pressure across the Atlantic, which is characteristic of the NAO. Hence we can strongly conclude that vertical propagation into the stratosphere is a key element in the Eurasian snow influence on the sub-seasonal forecasts even on weekly time scales. We also explored atmospheric conditions and feedback mechanisms during summer months of observed anomalous sea ice melt in the Arctic. We searched for distinct patterns in atmospheric circulation, precipitation, radiation, temperature and storm tracks. Compared to summer months of anomalous low sea ice melt, high melt months are characterized by anticyclonic surface anomaly over the Arctic with a corresponding tendency for storms to track on a more zonal path. As a result, the Arctic receives less precipitation overall and about only half snowfall of the climatological mean. The storm avoids the Arctic Ocean on their more zonal path across Eurasia, where the sub-polar jet is considerably weakened. In midlatitudes, the more zonally tracking cyclones result in stormier, cloudier, wetter and cooler summers in most of northern Europe and over the Far East. Farther south, the region from the Mediterranean Sea to South Central Asia experiences significant surface warming and dry conditions, possibly linked to changes in the jet stream. The partners of this Europe-Russia collaborating project were the University of Bern, the Alfred-Wegener Institute for Polar and Marine Research, the University of Vigo and the Russian Research Institute for Hydrometeorological Information.

The climate of the Arctic is the product of interactions between a large range of physical, chemical, and radiative processes, involving ocean, sea ice, land-surface, snow cover, clouds, and aerosols. Many of the interactions operate via atmospheric circu lation. The circulation moves weather systems across the Arctic and controls surface climate, including snow cover, it transports heat, water vapour, and black carbon and other aerosol particles into the Arctic, it distributes these quantities within the Arctic, it affects sea ice through wind stress, and it is coupled with the stratosphere. At the same time, circulation itself is affected by the radiation balance, sensible and latent heat transport (and thus seaice and snow cover), and by factors outside the Arctic. In this project we will study the role of these interactions for decadal variability and trends in Arctic and subarctic climate, with a focus on Eurasia. The project consists of 6 subprojects (SP), one led by each team (with a partner team) and a common activity. The SPs will use a variety of methods (statistical analysis, numerical modelling, Lagrangian modelling) and data (observations, reanalyses, model data). While each SP is able to stand on its own and will lead to new insights, the t rue benefit of the project results from linking the different aspects to a comprehensive overview of climate processes in the Arctic and subarctic region. The project will strengthen our understanding of Arctic climate processes and their role for decadal variability and trends. The knowledge may eventually lead to a better assessment of climate models, supporting an increased accuracy of seasonal predictions, projections, and adaptation plans. The more immediate outcome of the project will be topical sci entific papers and a common review article.