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. Plain Language Summary: Large‐scale cold and warm spells successively occurred in North Asia (NA) and resulted in the intraseasonal oscillation (ISO) of surface air temperature (SAT) extremes during 2020/21 winter. Here, we find the annual cycle (AC) of SAT and circulation anomalies provided a favorable background in regulating the subseasonal alternation of cold and warm extremes. The negative (positive) anomalies of AC (denoted PACN (PACP)) are characterized by enhanced (weakened) Ural blocking high (UBH) and Siberian high (SH), corresponding to cold (warm) NA circulation regimes before (after) 15 January 2021. During PACN (negative anomalies of ISO, PISON), a quasi‐barotropic Rossby wave train in the midlatitudes of Eurasia enhanced AC (ISO) of the UBH and SH, causing cold extremes in NA. While during the PACP and positive anomalies of ISO (PISOP), circulation anomalies like negative Arctic oscillation (AO) weakened UBH and SH, jointly resulting in warm extremes in NA. The subseasonal temperature change process in NA was mainly attributed to the conveyance of AC anomalies of meridional temperature gradient by subseasonal winds. Historical temperature extremes were almost attributed to the synergistic effect of AC and ISO of SAT anomalies, implying that the AC anomalies have important indications for subseasonal alternation of cold and warm extremes. Key Points: The successive cold and warm spells caused a subseasonal alternation of extreme surface air temperature (SAT) in North Asia during the 2020/21 winterThe annual cycle (AC) of meridional temperature gradient dominated by the nonlinear interaction of subseasonal advection in this winterCold and warm extremes in historical winters were verified by the synergistic effect of AC and intraseasonal oscillation in SAT anomalies [ABSTRACT FROM AUTHOR]