Impact of winter blocking on surface air temperature in East Asia: Ural versus Okhotsk blocking

Abstract

Atmospheric blockings and their impacts on wintertime surface air temperature in East Asia are investigated by using three blocking indices. Blockings in two specific regions that indirectly or directly modulate East Asian climate variability are analyzed: Ural (UR) blocking over East Asia's northwest and Okhotsk (OK) blocking over East Asia's northeast. Both lead to cold anomalies in East Asia but through slightly different mechanisms. In the typical case of UR blocking, cold air associated with a mid-level trough develops to the east of the quasi-stationary blocking and slowly moves southeastward as the Siberian high intensifies. Four days after the blocking onset, East Asia experiences significant cooling due to the temperature advection associated with anomalous temperature gradient and adiabatic cooling. During the typical OK blocking event, a preexisting cold anomaly over northern Eurasia is pushed southeastward by the westward expansion of the blocking. The anomalous meridional flow due to blocking-induced dipolar circulation over the North Pacific further reinforces the cold advection over East Asia, with a minor contribution of vertical processes. In both types of blocking, the cold anomaly persists even in the decaying phase, due to an offset between the temperature advection and the diabatic heating.