A Further Study of Causes of the Severe Drought in Yunnan Province during the 2009/2010 Winter

To understand the mechanisms responsible for the severe drought in Yunnan Province during the boreal winter (DJF) of 2009/2010, by using NCEP/NCAR reanalysis data and winter precipitation index in Yunnan Province which is calculated from station precipitation data in Yunnan Province, this study investigates the relationship between the North Atlantic Oscillation (NAO) and the precipitation in Yunnan Province during boreal winters. In the 49 winters of 1961/1961－2009/2010 period, the correlation coefficient between the time series of winter mean precipitation indices in Yunnan Province and the NAO indices is 0.373 (exceeds the 95% confidence level), which indicates that the winter precipitation in Yunnan Province is linked to the variability of the NAO. Regression results between the winter precipitation indices in Yunnan Province and the northern hemispheric anomalous geopotential height at 300 hPa show that the circulation systems associated with the winter precipitation in Yunnan Province have a south and a north major components: the southern branch trough and the ridge of Lake Baikal. And the results show that the NAO can impact these two components through the quasi-stationary waves propagating along the Asia－Africa subtropical jet and the wave reflections, respectively. Therefore, the variations of the NAO and the winter precipitation in Yunnan Province are linked. The results also show that the linkage between the NAO and the winter precipitation in Yunnan Province is modulated by ENSO phenomenon. The relationship between the NAO and the winter precipitation in Yunnan Province is much close in the warm ENSO winters, while, they are barely linked in the cold ENSO winters.