A severe drought event in northern China in winter 2008-2009 and the possible influences of La Nina and Tibetan Plateau

摘要

Severe drought occurred in northern China in winter 2008-09 and the La Nina event might have exerted a major influence on the drought. Both correlation analysis and case investigation indicate that severe wintertime droughts in northern China mostly occur under La Nina conditions. The La Nina event in 2008-2009 increased the differences in temperature and atmospheric pressure between the Indo- Pacific Oceans and the Asian continent and intensified the northeasterly East Asian winter monsoon flow. In winter 2008-2009, the western Pacific subtropical high was located farther south than normal, associating with a southward shift of the Intertropical Convergence Zone so that wet and warm flow could hardly reach northern China from the tropical oceans. Furthermore, the La Nina event in 2008-2009 strengthened the upstream portion of the upper tropospheric jet stream and the descending branch of the meridional cell over the subtropics, causing a negative divergence anomaly to the northern side of the jet entrance. As a result, the airflow over northern China is more convergent (divergent) at the upper (lower) levels, unfavorable for precipitation. In winter 2008-2009, the surface and tropospheric temperatures over the Tibetan Plateau were above normal. As a result, the India-Burma trough was shallower and less active, weakening the moisture transportation from the Bay of Bengal to eastern China. This feature was possibly related to the less snow over the plateau. Results also show that both the Nino3.4 SST and the Tibet Plateau temperature in the previous autumn can be considered indicators of the winter drought in northern China.