Warmer and Wetter Winters over the high-latitude North Atlantic : an atmospheric circulation perspective

Abstract

The high-latitude North Atlantic -especially the Svalbard archipelago and its surrounding seas - has undergone remarkable changes in the last half-century. These regions are recognized as a hotspot of the Arctic Amplification, referring to the faster warming rates in the Arctic when compared to lower latitudes, manifested in the form of rapid warming and sea ice loss. The most pronounced changes occur in winter, when the sun remains below the horizon, and heat transport from lower latitudes in both atmosphere and ocean is the main energy source. The relationship between atmospheric circulation changes and the AA is not fully understood, and there are knowledge-gaps in particular on the regional scale. This thesis is motivated by the wish to quantify cyclone track changes and increase the knowledge of their role in the documented rapid temperature increases in Svalbard and the sea ice losses in north and east of this archipelago. We utilized an automated cyclone tracking algorithm to create a seasonal cyclone climatology for the high-latitude North Atlantic in the period 1979-2016 based on the mean sea-level pressure from the reanalysis product ERA-Interim from the European Centre of Medium-Range Weather Forecasts (ECMWF). When calculating the trends over this period we find an increase of cyclone densities around Svalbard and a corresponding decrease in the southeastern Barents Sea of a magnitude (+/- 3 cyclones/winter per decade). Based on composite analysis, we find that these changes are in line with a trend towards warmer and wetter winter conditions in the northern Barents Sea and over Svalbard. The opposing trends, i.e. no significant changes in cyclogenesis and an increase in local baroclinicity north of Svalbard, all speak for a shift towards a more meridional winter cyclone track in the seas around Svalbard. To advance the understanding of local 2-m air temperature and precipitation sensitivity to atmospheric circulation in the recent climate, with more winter cyclones around Svalbard, we studied the period 2013-2018 with a high-resolution numerical weather prediction model. The main finding of this study, forming a part this thesis, is that in the current climate, potentially high societal impact, rain-on-snow (ROS) events are 10 common phenomena in the recent Svalbard winters with up to 11 events per winter in the southwestern parts of the archipelago. The clear majority of events with widespread ROS (in this study defined as more than 3% of Svalbard's land grid points experiencing ROS) occur with southerly and southwesterly flow over Svalbard. These flow directions occur in situations with a cyclone in southern Fram Strait - a region with a positive cyclone trend in the 1979-2016 winter climatology. On a regional scale the warming has been largest in the northern and eastern parts of the Svalbard archipelago. We report extreme winter warming rates up to 3 K/decade in the north accompanied by significant (p <0.05) increases in both accumulated precipitation (+ 44.6 mm/decade) and the number of precipitation days (+5.2 days/decade ) over the period 1960-2017 based on the Norwegian hindcast product NORA10. The warming is significant, but only half of that in magnitude in the southern and western parts of the archipelago. We found the local warming and wettening in the north to correlate well with sea ice changes north of the archipelago and this is attributed to upwind changes of northeasterly flow. This effect is, however, further enhanced by a moderate, but significant increase in northerly winds aligning with the increased frequency of winter season meridional flow.