The Alternating Change of Cold and Warm Extremes Over North Asia During Winter 2020/21: Effect of the Annual Cycle Anomaly

Large-scale cold and warm spells successively attacked North Asia (NA) and resulted in a subseasonal alternation of extreme surface air temperature (SAT) before and after 15 January 2021. Our results show the synergistic effect of annual cycle and intraseasonal oscillation component (AC and ISO) of SAT and circulation anomalies caused temperature extremes. Before 15 January 2021, an eastward-propagating Rossby wave train strengthened the cold circulation regime with an enhanced Ural blocking high (UBH) and surface Siberian high (SH). Circulation anomalies of the negative ISO were superimposed on the counterpart of AC, which easily resulted in cold extremes via nonlinear interaction of meridional temperature gradient. After mid-January 2021, however, weakened UBH and SH caused warm advection and spells over NA. This synergistic effect of AC and ISO anomalies in SAT accounted for most historical temperature extremes, implying that AC anomalies have important indications for subseasonal alternation of temperature extremes.