Different influences of La Niña types on the winter sub-seasonal Eurasian cold anomalies linked to Ural blocking

Abstract

The effects of different types of La Niña on winter sub-seasonal cold anomalies or cold extremes over Eurasia and Siberia are examined through exploring changes in Ural blocking (UB). It is found that the Central Pacific (CP) La Niña can significantly influence surface air temperature over Eurasia mainly by regulating the movement, persistence, zonal scale and location of the cyclonic anomaly associated with the UB. This influence is much greater than that resulting from an Eastern Pacific (EP) La Niña. A long-lived and quasi-stationary cyclonic anomaly with a large zonal scale of UB is induced to the southeast of the Ural Mountains for CP-type La Niña conditions, which leads to a strong warm Arctic-cold Eurasia pattern with an intense sub-seasonal cold anomaly or cold event in the broad region of Eurasia due to strongly reduced downward infrared radiation over Eurasia. In contrast, for EP-type La Niña winter the cyclonic anomaly, while still present, is much weaker, short-lived and eastward propagating, resulting in only a very weak cold anomaly or cold event over a small region of Eurasia. The reduced (enhanced) eastward movement and increased (decreased) persistence of the cyclonic anomaly with large (small) zonal scale of UB are shown to be associated with reduced (enhanced) meridional potential vorticity gradient in the midlatitudes (30o-50oN) to the east of 60°E during the CP-type (EP-type) La Niña winter.