Modulation of coupled Ural-Okhotsk circulation field on winter extreme cold events over East Asia

Extreme cold events have a great impact on environment and human health. The occurrence of winter extreme cold events (WECEs) in the Northern Hemisphere is usually modulated by circulations over different specific regions. However, how to detect and quantify the effects of these coupled circulations over different regions on WECEs is still unclear. In this study, the dynamic system method is extended with lags to investigate the modulation of coupled Ural (UR) and Okhotsk (OK) circulation fields on WECEs over East Asia. Taking the dynamical lead-lag relationship into account of the interaction between the circulations over these two regions, contributions of the modulation of coupled Ural (UR) and Okhotsk (OK) circulation fields to WECEs over East Asia are mainly classified into two distinct categories. When the circulation fields over OK dynamically lag behind those over UR, WECEs are mainly controlled by Ural blocking and polar vortex splitting. The development and decay of the upstream Ural Ridge promote the movement of the downstream trough to lead to the circulation turning meridional-oriented, together with the polar vortex from strong to weak and splitting southwards. Then cold air is guided to move southwards to form winter cold events over a larger domain beyond East Asia. Whereas, when the circulation fields over UR dynamically lag behind those over OK, there exist North and South branches of the circulation, with the North one mainly controlled by UR and OK blockings and the South one mainly controlled by a cyclone over the sea surface. Two blockings move towards each other and create a larger blocking which results in enhanced meridional flow, cyclone moves eastwards by transporting water vapor to a cool surface and forms weaker winter cold events over a confined domain within East Asia.