Unusual Late-Season Cold Surges during the 2005 Asian Winter Monsoon: Roles of Atlantic Blocking and the Central Asian Anticyclone

The highest frequency of late-winter cold-air outbreaks in East and Southeast Asia over 50 years was recorded in 2005, when three strong successive cold surges occurred in the South China Sea within a span of 30 days from mid-February to mid-March. These events also coincided with the first break of 18 consecutive warm winters over China. The strong pulsation of the surface Siberian Mongolia high (SMH) that triggered these events was found to result from the confluence of several events. To the east, a strong Pacific blocking with three pulses of westward extension intensified the stationary East Asian major trough to create a favorable condition for cold-air outbreaks. To the west, the dominance of the Atlantic blocking and an anomalous deepened trough in the Scandinavian/Barents Sea region provided the source of a succession of Rossby wave activity fluxes for the downstream development. An upper-level central Asian anticyclone that is often associated with a stronger SMH was anomalously strong and provided additional forcing. In terms of the persistence and strength, this central Asian anticyclone was correlated with the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) only when SMH is weak (warm winters). During strong SMH seasons (cold winters) the correlation vanishes. However, during late winter 2005 the central Asian anticyclone was strengthened by the Atlantic blocking through both the downstream wave activities and a circulation change that affected the Atlantic and west Asian jets. As a result, the period from mid-February to mid-March of 2005 stands out as a record-breaking period in the Asian winter monsoon.