Enhanced High-Latitude Concurrent Blockings Cause the Warm Arctic-Cold Continent Pattern

Abstract

A rapidly warming Arctic and mild cold mid-latitude continents have been one of the main characteristics of the Northern Hemisphere winter during the last two decades. However, the factors contributing this warm Arctic-cold continent (WACC) pattern remain unclear, although anomalies in blocking highs and the Arctic oscillation (AO) are possible factors. This study revealed that the mode with high-latitude concurrent blockings (HCBs) is more consistent with the WACC pattern than the AO. In the HCB mode, a strong anticyclonic anomalous circulation over the Arctic Circle, excited by the strong HCBs over the Ural Mountains and the North Pacific, changes the polar atmospheric circulation and redistributes both momentum and heat to give a warmer Arctic than in the AO mode. A weak polar night jet and a poleward shift in the subtropical westerly jet result in mild cold mid-latitude continents. However, these features are not seen in the AO mode, which is characterized by cool continents and a warm Greenland. Besides, human activities may also be contributed to the interdecadal WACC because there is an interdecadal increase in the WACC pattern with HCBs in the CMIP6 simulations with human activities. These results suggest that the role of HCBs should be highly paid attention in winter mid-high latitude extreme events in recent decades.