Decadal Change of Extreme Cold Days over South Korea for Early Winter

Extreme climate events such as heat wave, cold surge, drought, heavy rainfall are increasing worldwide in recent decade. Especially, despite under the global warming, the extreme cold days(ECDs) in the northern hemisphere for wintertime are more frequent than in past. In this study, we found the decadal change of ECDs frequency, which is defined by number of days than 10 percentile of observational daily surface air temperature anomalies, over South Korea in early winter(Nov.-Dec.) for 1973-2016. The ECDs frequencies are clearly distinguished in three decades of 1973-1985(P1), 1986-2001(P2) and 2002-2016(P3). The ECDs occur more frequently in P1 and P3 decade than in P2 decade. Moreover, the ECDs in P1 and P3 is longer-lasting than in P2 decades. This decadal fluctuation is closely related to the decadal change of atmospheric mean circulation over Eurasia. The mean circulations in the P1 and P3 decade provide more favorable condition for ECDs occurring in South Korea than in the P2 decade. In P1 decade, East Asia is dominated by upper-level cyclonic anomaly, which indicates deepening of East Asian trough and surface cold condition, by the wave type circulation from northern Europe toward East Asia. In contrast, the upper-level circulation over East Asia in P2 decade shows the opposite phase to the wave train through the Eurasian continent in P1 decade. The opposite circulation in P2 decade may restrain the ECDs occurrence. In P3 decade, the upper-level cyclonic anomalies over East Asia are similar to the P1 period. Whereas, unlike the P1 period, anti-cyclonic circulations are quite strong over northern Europe and northern Siberia. The difference of decadal mean state between P1 and P3 period is clearly reflected in the sub-seasonal evolution of ECDs. The ECDs occurring in P1 decade are related to the cyclonic anomaly caused by the upper-level circulation of the wave-train type propagating from Siberia to East Asia, while the cyclonic anomaly dominating over East Asia in ECDs of P3 period are developed by the dipole type circulation in northern Siberia and East Asia.

The possible cause of the decadal change of ECDs frequency over South Korea is also examined in this study. Compared to P1 decade, the decrease of ECDs frequency in South Korea in early winter of P2 decade is likely to stem from the weakening of East Asian winter monsoon(EAWM) and Siberian high under the global warming. Moreover, the reemerge of frequent ECDs in P3 decade is connected with the development of anti-cyclonic anomalies in Arctic and northern Eurasia, which plays an important role for deepening of climatological East Asian trough by the teleconnection. This intensification of anti-cyclonic circulation is likely to be related drastic Arctic warming and sea ice loss since the 2000s.