Drought and wetness variability in the Tarim River Basin and connection to large-scale atmospheric circulation

On the basis of daily observations of 39 meteorological stations in the Tarim River Basin, the variability of drought and wetness has been analysed using the standardized precipitation-evapotranspiration index (SPEI, 1961-2010). The result shows an increasing trend in annual mean SPEI with a significant change in 1986. Although the frequency of moderate and severe droughts decreased after the change, the frequency of extreme drought events increased slightly. But different categories of wetness show a consistent increase in frequency. The return periods of drought and wetness prolonged and shortened, respectively, after 1986. Furthermore, composites of geopotential height and water vapour flux fields at 500 hPa are analyzed for extreme wet and dry months of the warm season (May to October) as well as for the warm seasons of the periods 1961 to 1986 and 1987 to 2010. The difference between composites of extreme wet and dry shows that the water vapour supply during wet events can be related to transports from the Arabian Sea and the Bay of Bengal. The midlatitude atmospheric circulation plays an important role by transporting moisture from the east into the Tarim River Basin; this is the main reason of the wetter condition in warm seasons after 1986 in the study region.