Wintertime Extreme Warming Events in the High Arctic: Characteristics, Drivers, Trends, and the Role of Atmospheric Rivers

Abstract. An extreme warming event near the North Pole, with temperature rising above 0 ℃, was observed in late December 2015. This specific event has been attributed to cyclones and their associated moisture intrusions. However, little is known about the characteristics and drivers of similar events in the historical record. Here, using data from ERA5, we study these winter extreme warming events with temperature above 0 ℃ over the high Arctic (poleward of 80° N) occurred during 1980–2021. In ERA5, such extreme events can only be found over the Atlantic sector. They occur rarely, with a seasonal occurrence frequency less than one over most of the regions. Furthermore, even when occurring, they tend to be short-lived, with the majority of the events lasting for less than a day. By examining their surface energy budget, we found that these events transition with increasing latitude from a regime dominated by turbulent heat flux into the one dominated by downward longwave radiation. Blockings over the northern Eurasia are identified as a key ingredient in driving these events, as they can effectively deflect the eastward propagating cyclones poleward, leading to intense moisture and heat intrusions into the high Arctic. Using an atmospheric river (AR) detection algorithm, the roles of ARs in driving these events are explicitly quantified. The importance of ARs in inducing these events increases with latitude. Poleward of about 83° N, ARs are the direct driver for 100 % of these events, corroborating the indispensable roles ARs played in driving these events. Over the past four decades, both the frequency, duration, and magnitude of these events have been increasing significantly. As the Arctic continues to warm, these events are likely to increase in both frequency, duration and magnitude, with great implications for the local sea ice, hydrological cycle and ecosystem.