Subseasonal reversals of winter surface air temperature in mid-latitude Asia and the roles of westward-shift NAO

Abstract

Reversals of surface air temperature anomalies over mid-latitude Asia (SATMA) have become more frequent. Six winters have been reversed over the last decade, which had serious negative consequences for agricultural production, ecological systems, and human health. This subseasonal reversal can be observed in the second mode of season-reliant empirical orthogonal function analysis, showing a persistent opposite anomaly between early and late winter. The difference in SATMA between early and late winter exceeds 3 °C when the reversal occurs. At the time of the reversal, the North Atlantic jet stream extends eastward in early winter and recedes westward in late winter. The main influencing system changes from the Scandinavia teleconnection to the Ural blocking high, accompanied by a change from strong southerly to northerly winds. These factors jointly lead to the reversal of SATMA between early and late winter. More importantly, the North Atlantic Oscillation (NAO) has been shown to affect SATMA variability in a variety of different ways. Changes in SATMA are influenced not only by the intensity of the NAO, but also by its spatial distribution. When the southern center of the NAO shifts westward and is located over the North Atlantic, atmospheric circulation and SATMA in early and late winter show similar changes to those seen in SATMA reversal winters. In contrast, when the NAO southern center shifts eastward and is located over Western Europe, persistent anomalies are observed in atmospheric circulation and SATMA throughout the winter. Therefore, when studying the relationship between SATMA and the NAO, attention should be paid to the different responses of SATMA to the spatial distribution of the NAO at subseasonal timescales.