Can the Southern Hemisphere annular mode affect China winter monsoon?

Abstract

Many previous studies suggested that the anomalous Northern Hemisphere annular mode (NAM) and associated low boundary forcing (e.g., snow cover) greatly influence the China winter monsoon (CWM) variability on interannual to inter‐decadal timescales. In this article, it is found that the Southern Hemisphere annular mode (SAM) also well correlates with the two observed CWM major modes, which has not been revealed before. Note that the two CWM major modes are obtained by performing Empirical Orthogonal Function (EOF) analysis on winter surface air temperature at 160 gauge stations across China for the 1951–2006 period. They explain around 70% of the total CWM variances. The first EOF mode exhibits a homogeneous spatial pattern with the corresponding principal component displaying a significant inter‐decadal variation, which reflects the warming trend in China during the past 56 years. The second EOF mode shows a meridional seesaw pattern and is basically associated with significant interannual variations. Both of the leading modes are intimately associated with the simultaneous SAM‐like hemispheric circulation anomalies. Moreover, the SAM‐like anomalies signal precursory conditions for the first CWM mode in boreal autumn. The relevant physical mechanisms by which anomalous autumn SAM may affect the CWM are investigated with NCAR Community Atmospheric Model version 3 (CAM3). When SAM is in a strong phase during boreal autumn, the circum‐Antarctic upper level jet stream displaces poleward and the corresponding surface wind speeds reduce in the region between 45°S and 30°S, inducing a hemispheric‐scale warm sea surface temperature (SST) belt beneath. Such an anomalously warm SST belt persists through boreal winter and weakens the Hadley cell. The weakened Hadley cell in boreal winter corresponds to anomalous southerlies prevailing in the lower troposphere over China, which favor a weak CWM. The intimate linkage between the autumn SAM and CWM may be instrumental for understanding interactions between the Northern and Southern Hemisphere and can provide a way to predict the CWM variations.