Drought and wetness variability in the Tarim River basin and possible associations with large scale circulation

Drought and wetness indices are important elements since they closely associated with water and energy balance over a geographical area and extending along a certain period of time. Studying of possible cause of drought and wetness variability is of great importance to understand the hydrological cycle and disaster reduction. Based on daily observations of 39 meteorological stations in the Tarim River basin, the spatial and temporal variability of drought and wetness has been analyzed using the standardized precipitation evapotranspiration index (SPEI). The result shows an increasing trend in annual mean SPEI with a significant change point in 1986. Although the frequency of moderate and severe drought decreased after 1986, the frequency of extreme drought events increased slightly. But different categories of wetness show a consistent increase in frequency. Furthermore, we also investigated the corresponding atmospheric circulation anomalies of drought and wetness changes over the Tarim River Basin using NCEP-NCAR reanalysis datasets. Composition analysis of geopotential height and wind field at 500 hPa are performed for typical extreme dry and wetness month as well as for the warm seasons (MJJASO)of the periods 1961 to 1986 and 1987 to 2010, the result shows obvious difference in large scale circulation pattern can be found in typical wet and dry months, the intensified water vapor transportation and unstable atmospheric stratification are the main reasons of the wetter condition in warm seasons after the 1980s.