Interannual-interdecadal variability of extreme low temperature in North Asia and its driving mechanisms

To investigate the spatio-temporal characteristics and its impact of winter extreme low temperature in North Asia, the extreme cold days index (TX10p) is used in this study. The first empirical orthogonal function (EOF) mode of extreme cold days in North Asia exhibits a consistent distribution centered on Lake Baikal, primarily showing interannual variability. It is influenced by two key factors: snow cover in the previous autumn and the Arctic Oscillation (AO) in the wintertime, modulated by the land-atmosphere. Specifically, the previous autumn snow cover can affect the strength of mid-high latitude troughs and ridges by adjusting the meridional temperature gradient and the strength of the westerly wind, thereby influencing cold waves. Meanwhile, the winter AO can modulate the southward movement of polar cold air by adjusting the strength of the polar vortex, ultimately impacting the frequency of extreme cold days in North Asia.

The second EOF mode of the extreme cold days in North Asia displays a northeast-southwest dipole distribution bounded by Lake Baikal, mainly showing an interdecadal transition. Temporrally, before 2000, the pattern featured more cold days in the northeast and less in the southwest, with opposite trend in the later period. Spatially, the spatial distribution also has a corresponding interdecadal change around 2000. The northeast-southwest boundary of the dipole pattern shifted from around 50°N in the early period to approximately 60°N in the later period, reflecting an expansion of southwest region. The interdecadal change characteristics of the second mode are mainly modulated by the combined effects of the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). Under the synergistic changes of the AMO and PDO phases, the sea surface temperatures of the Pacific and Atlantic can influence the strength of the polar vortex, thereby affecting the north-south gradient in the mid to high latitudes, and the strengths of westerlies and troughs and ridges, ultimately being conducive or not conducive to the southward movement of cold air. In addition, shifts in the PDO phase impact the strength and position of the Aleutian Low, which alters the East Asian trough and contributes to the north-south displacement of the EOF2 mode's spatial pattern.