Contrasting Deep and Shallow Winter Warming over the Barents–Kara Seas on the Intraseasonal Time Scale

Abstract

The vertical structure of Arctic warming is of great importance and attracts increasing attention. This study defines two types of Arctic warming events (deep versus shallow) according to their temperature profiles averaged over the Barents–Kara Seas (BKS), and thereupon compares their characteristics and examines their difference in generation through thermodynamic diagnoses. A deep Arctic warming event—characterized by significant bottom-heavy warming extending from the surface into the middle-to-upper troposphere—emanates from the east of Greenland and then moves downstream toward the BKS primarily through zonal temperature advection. The peak day of deep warming event lags that of the precipitation and resultant diabatic heating over southeast Greenland by about four days, suggesting that the middle-to-high tropospheric BKS warming is likely triggered by the enhanced upstream convection at the North Atlantic high latitudes. In contrast, a shallow warming event—manifested by warming confined within the lower troposphere—is preceded by the meridional advection of warm air from inland Eurasia. These anomalous southerlies over Eurasian lands during shallow warming events are related to the eastward extension of the deepened Icelandic low. During deep warming events, the in situ reinforcement of the Icelandic low favors abundant moisture transport interplaying with the southeast Greenland terrain, leading to intense precipitation and latent heat release there. Both deep and shallow warming events are accompanied by Eurasian cooling, but the corresponding cooling of the deep warming event is profoundly stronger. Further, intraseasonal deep Arctic warming events could explain nearly half of the winter-mean change in the warm Arctic–cold Eurasia anomaly. Divergent conclusions on whether Arctic warming is influencing the midlatitudes impede a clear understanding of the warm Arctic–cold Eurasia (WACE) phenomenon. Recent findings that on the interannual or longer time scales, Eurasian cooling tends to occur in the presence of deep rather than shallow Arctic warming have attracted increasing concern regarding the vertical structure of Arctic warming. On this basis, here we classify intraseasonal Arctic warming events into deep and shallow groups and contrast them from various aspects. Emerging near eastern Greenland and associated with upstream convection activities, intraseasonal deep Arctic warming events are accompanied by significant Eurasian cooling, largely determining the seasonal-mean WACE condition. However, caused by meridional temperature advection from Eurasian lands, shallow warming events less correlate with Eurasian cooling.