Role of polar vortex weakening in cold events in central Asia during late winter

Abstract

The cause of cold events over central Asia was investigated. Since 1958, surface air temperature (SAT) has gradually increased over central Asia, but SAT has shown very strong multidecadal fluctuations, with cooling dominant in the 1960s-1970s and recent decades but warming dominant in the 1990s. SAT in February in central Asia has decreased by more than 7 degrees C compared to that in normal years. Analysis indicates that cold events over central Asia are related to the weakening of the polar vortex, which is indicated by the increase in polar cap height (PCH) and weaker zonal-mean zonal winds. The increase in PCH begins in January in the stratosphere and propagates down to the troposphere in February; it is well reflected in the weakening of zonal-mean zonal winds in the stratosphere in January, which extends to the troposphere in February. The January increase in PCH anomaly is associated with surface conditions in the Arctic region, especially the Barents-Kara seas, where sea level pressure increases substantially in January; high pressure then expands to the southeastern (downstream) branch of the Siberian high in February, bringing cold eastern Siberian air to central Asia.