The Possible Influence of a Nonconventional El Nino on the Severe Autumn Drought of 2009 in Southwest China

A severe drought struck southwest China during autumn 2009, which had a huge impact on productivity and the lives of the affected population. A nonconventional El Nino, the so-called warm pool (WP) El Nino, was supposed to be a principal factor of this strong autumn drought. In sharp contrast to a conventional El Nino, in the 2009 WP El Nino year the maximum sea surface temperature (SST) anomalies are confined to the central equatorial Pacific Ocean. Moreover, this WP El Nino was characterized by the relatively farther westward location and the strongest intensity among the WP El Nino events in the past 60 years. Observations and modeling studies both indicate that the rainfall deficits over southwest China are significantly influenced by the combined effects of the location and intensity of the WP El Nino. That is, the drought over southwest China tends to be more severe when the warming SST anomalies associated with the WP El Nino are located farther westward and are stronger. Therefore, the strong autumn drought over southwest China in 2009 can be largely attributed to the concurrent distinctive WP El Nino, which generates a strongly anomalous cyclone over the west North Pacific and leads to a serious reduction in rainfall over southwest China. The influence of the Indian Ocean warming on autumn rainfall over southwest China was also examined but seems to have little contribution to this drought.