Different processes of occurrence of cold events over East Asia in El Niño and La Niña winters

The occurrence of cold events over East Asia is modulated by the background mean state associated with the El Niño-Southern Oscillation (ENSO). The present study compares the processes of occurrence of cold events over East Asia between El Niño and La Niña winters. It is found that the difference in atmospheric mean state in El Niño and La Niña winters leads to different processes of occurrence of cold events over East Asia during boreal winter. The cold events during El Niño winters are closely related to negative Arctic Oscillation (AO) and those during La Niña winters are contributed by the Ural blocking. Both negative AO in El Niño winters and the Ural blocking in La Niña winters are followed by a Rossby wave train over Eurasia that strengthens the Siberian high and the East Asian trough, leading to cold anomalies over East Asia. The negative AO during El Niño winters is related to a tropospheric teleconnection that weakens the polar vortex, which is confirmed by numerical experiment with a linear baroclinic model. The Ural blocking is associated with the Arctic sea ice loss in the Barents Sea during La Niña winters through the reduced meridional temperature gradient and decreased zonal mean winds. The Arctic sea ice loss over the Barents Sea during La Niña winters is contributed by the development of anticyclonic anomalies induced by the enhanced stationary waves.