Relations of Enhanced High‐Latitude Concurrent Blockings With Recent Warm Arctic‐Cold Continent Patterns.

A warm Arctic‐cold continent (WACC) pattern has been one of the main characteristics of the Northern Hemisphere (NH) climate variability in winter during the last two decades. However, the factors contributing this pattern remain unclear. We compared the two leading modes of surface temperature in the NH winter on an interdecadal timescale and explored the role of high‐latitude concurrent blockings (HCBs) and the Arctic oscillation (AO) in these two modes. Results show that the first mode resembling the WACC pattern is more related to enhanced HCBs over the Ural Mountains and the North Pacific than the AO. The HCBs induce a warmer Arctic by simultaneously transporting large amounts of moisture and heat. The HCBs excite a tropospheric anticyclonic anomaly near the Arctic Circle, which, in turn, triggers anomalies in the propagation of planetary waves and the meridional circulation, with a subsequent redistribution of momentum and heat. This is accompanied by a weak polar night jet and a poleward shift in the subtropical westerly jet, resulting in mild cold mid‐latitude continents. The second mode represents the AO pattern, which has a different mechanism from WACC. The high‐pressure anomaly over the North Pacific has played an increasingly important part in the WACC mode in recent decades and the WACC pattern with HCBs is more easily reproduced in simulations with human activity. HCBs therefore require further consideration under the current and future conditions of global warming. Plain Language Summary: 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. 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. Key Points: A warm Arctic‐cold continent (WACC) pattern is associated with enhanced high‐latitude concurrent blockings (HCBs) over the Ural Mountains and North PacificThe HCBs excite a tropospheric anticyclonic anomaly near the Arctic Circle, which leads to more warm moist air into the ArcticThe WACC pattern with HCBs is more easily reproduced in CMIP6 simulations with human activity than in simulations without human activity [ABSTRACT FROM AUTHOR]