Weather types and their frequencies over Central Asia –an ERA-Interim based analysis of monthly climate variability and change for the boreal cold season

Climate research in Central Asia is usually based on the analysis of meteorological observations. However, data scarcity in mountain regions causes uncertainties and, thus, the magnitude of climate change and variability in Central Asia is still under debate. Furthermore, the investigation of observations does only allow an assessment of the near surface climate. Since the meteorological conditions in the upper troposphere are generally unknown, the atmospheric mechanisms leading to observed climate changes remain unexamined. Here, the authors present a study of climate change and variability in Central Asia based on the ERA-Interim reanalysis, which provides gridded data-sets of various meteorological parameters for 60 atmospheric levels. In order to investigate the climatic conditions during the boreal cold season, the authors apply an objective weather type classification to 500hPa geopotential height fields. The results show that warm and wet conditions in Central Asia are associated with an anticyclonic anomaly over South Asia or a southward shift of the westerly jet stream. Dry conditions are accompanied by a cyclonic anomaly over South Asia. The authors show that the WT composition strongly affects the monthly and seasonal temperature and precipitation characteristics and that prevailing climatic trends are partially triggered by changing WT frequencies. About 50% of the seasonal temperature trend and 60% of the trend in March can be explained by WT frequency changes. While the observed seasonal precipitation trends cannot be explained by WT frequency changes, a positive trend in November seems to be accompanied by a decreasing frequency of high pressure over Central Asia.