Preliminary study on the causes and decay of severe drought in a century in the Sichuan Basin, China

In 2006, the large area of the Yangtze River (Chang Jiang) valley, especially the eastern Sichuan Basin and Chongqing City, suffered the most severe drought in the past a hundred years. This was an extraordinary climatic event with the largest area affected, most lasting high temperature, and most severe damages in the history of the region since climatic data were available. Because the precipitation in the summer of 2006 in the upstream region of the Yangtze River was 40%-50% less than the normal level, the water level of the Yangtze River still remains at the lowest level for the past a hundred years.

In this paper, we use dynamic data to analyze this rare climatic event. We suggest that the causes of this drought are related to the fact that the climate of the Sichuan Basin, China, began to experience warming since early 1990s after its 10-year lag of global warming trend. Specifically, we suggest the following four factors may be primarily responsible for the development of the drought: (1) the snow cover of the Tibetan Plateau in the winter-spring season of 2006 was about 20-30 % less than the normal level due to higher than normal temperature; (2) the sea temperature in the western Pacific in spring of 2006 exhibits about 0.5 C of positive departure; (3) the lower level cross-equatorial air flow near the southeastern China had anomalously strong intensity, strengthening the western Pacific Subtropical High; (4) there was a heavy rain in the Liangshan region of Sichuan (southwestern China) shortly before the drought, potentially facilitating the release of latent heat and strengthening the Tibetan High Pressure in the upper troposphere. Moreover, the merge of the Tibetan High Pressure in the lower troposphere with the western Pacific Subtropical High on the mainland may also contribute to the development of the severe drought. The merge formed a continental high pressure, which might have forced the cold advection near --- N to move eastward in higher latitudes, and interrupt the moisture transportation from the Bay of Bengal and the South China Sea.

The amelioration of the drought seems to benefit from the withdrawal of the continental high pressure to the southeast in early September of 2006. A strong baroclinic disturbance occurred in the northwest Tibetan Plateau at the same time, which moved eastward and brought cold air into the Sichuan Basin and Liangshan region, causing a strong precipitation process.

Keywords: Drought, Tibetan Plateau, Subtropical High, Snow Cover, Sea Temperature, Heavy Rain