Reversal of monthly East Asian winter air temperature in 2020/21 and its predictability

Abstract

In this study, the reversal of monthly East Asian winter air temperature (EAWT) in 2020/21 and its predictability were investigated. The reversal of monthly EAWT in 2020/21 was characterized by colder temperatures in early winter (December 2020 to mid-January 2021) and warmer temperatures in late winter (mid-January to February 2021). Results show that the reversal in the intensity of the Siberian high (SH) also occurred between early and late winter in 2020/21. In early winter, as the Barents–Laptev sea ice in the previous September (i.e., in 2020) reached a minimum for the period 1981–2020, the SH was strengthened via a reduction of the meridional gradient between the Arctic and East Asia. In late winter, as a sudden stratospheric warming occurred on 5 January 2021, the stratospheric polar vortex weakened, with the weakest center shifting to North America in January. Subsequently, the negative Arctic Oscillation–like structure shifted towards North America in the middle and lower troposphere, which weakened the SH in late winter. Furthermore, the predictability of the reversal in EAWT in 2020/21 was validated based on monthly and daily predictions from NCEP-CFSv2 (National Centers for Environment Prediction–Climate Forecast System, version 2). The results showed that the model was unable to reproduce the monthly reversal of EAWT. However, it was able to forecast the reversal date (18 January 2021) of EAWT at lead times of 1–20 days on the daily scale.摘要在全球变暖的背景下, 近年来东亚冬季气温存在复杂的季节内变化. 本文研究了2020/21年东亚冬季气温的月际转折及可预测性. 结果表明, 2020/21年东亚冬季气温前冬 (2020年12月–2021年1月中旬) 偏冷, 后冬 (2021年1月中旬–2月) 偏暖. 西伯利亚高压强度在前冬和后冬也出现转折变化. 在前冬, 由于2020年9月巴伦支海–拉普捷夫海海冰达到1981–2020年以来的最小值, 北极至东亚地区的温度梯度减弱, 西伯利亚高压增强. 在后冬, 由于2021年1月5日发生平流层爆发性增温, 1月平流层极涡减弱增暖, 且最弱中心向北美偏移. 因此, 对流层中低层负的类北极涛动位相也向北美偏移, 西伯利亚高压减弱.基于NCEP-CFSv2月际和逐日的预测结果显示, 该预测系统不能较好地再现此次东亚冬季气温的月际转折. 然而, 在日尺度上, 模式能提前1–20天较好地预测东亚冬季气温的转折日期.