Arctic oscillation and southeast China winter temperature anomalies

Abstract

Since the identification of the Arctic Oscillation (AO) in the 1998 by Thompson and Wallace, numerous studies on the teleconnection between the AO and the East Asian Winter Monsoon (EAWM) have been undertaken. Generally, the theory is robustly developed that when AO is at the positive phase, warmer winter would be experienced by the Eurasia due to trapping of the cold Polar air mass at the Arctic region by a strong Polar Jet Stream; whereas cold winters are expected during negative AO phases, in which the cold Polar air mass is able to invade southwards. Yet, whether AO could be attributed as the whole reason for the exceptional cold period during some winter - identified as the “cold surges” is not well understood. In this study, more than 50 years of meteorological data from 4 representative stations along the coastal East Asia, namely Tokyo, Shanghai, Hangzhou and Hong Kong, together with the AO and ENSO indices are extracted to investigate the trend of cold surges in the past 50 years and moreover, the actual correlation between the AO, ENSO and the occurrence of cold surges. Correlation coefficients determined in this study strongly suggest that a positive spring AO would motivate the occurrence of cold surges in the succeeding winter, especially before 1985, however, winter AO seems only imposing a minor effect to the occurrence of the cold surges. After 1985, the influence from the spring AO on the occurrence of cold surges weakened while winter AO may play a more important role in developing cold surges at stations of higher latitude. ENSO also serves as an important factor in affecting the occurrence of cold surges before 1985 but its importance fades after 1985. According to these observations, it is suggested that before 1985, a positive spring AO would facilitate the accumulation and development of cold Polar air mass that eventually outflows in the succeeding winter, occasionally due to the transition to a negative winter AO phase, and results in the cold surges through either direct southwards invasion or strengthening the Siberian High. After 1985, the effect of global warming progressively strengthens, especially by causing thinning and melting of the Arctic sea ice cover. This disfavors the development of the cold air mass over the Arctic region and therefore, weakens the strength of the EAWM. Consequently, occurrence of cold surges decreases after 1985 and shows fundamentally no correlation with the spring AO. This study also highlights the role of ENSO, especially during the La Nina year, in motivating strong EAWM and occurrence of cold surges.