Causes Analysis On The Abnormal Summer Wet And Drought Events Over The Eastern Part Of The Northwestern China

By using of the daily precipitation, temperature and compound index (CI) of meteorological drought provided by China National Climate Center (NCC), the monthly reanalysis data from the National Centers for Environmental Prediction-the National Center for Atmospheric Research (NCEP/NCAR), and the extended reconstructed sea surface temperature (SST) from the National Oceanic and Atmospheric Administration-Climate Prediction Center (NOAA/CPC) during1951-2012, and on the basis of the rotated empirical orthogonal function (REOF) method, correlation analysis and composite analysis, investigations were conducted in this paper on the variability of the abnormal wet and drought events over the eastern part of the Northwestern China (ENWC), as well as its possible mechanism especially in summer as revealed by the atmospheric circulation anomalies and SST anomalies. The major results are summarized as shows:(1) During the warmer months (May-June-July-August-September) the rainfall over the ENWC shows a decrease trend since1990s, which is mainly resulting from the reduced rainfall to some extent in every single month. After the middle of1990s, the temperature for the warmer months continuously maintains with a positive anomaly, and there is an obvious increase trend in the frequency, persisting days and areas of the drought events. Evidence shows that a transition trend from warmer and dryer to warmer and wetter climate seems to be happening at present for the summer and other warmer months.(2) The main causes for the abnormal drought (wet) events over the ENWC occurred in summer are as follows:Responses to the tilt (flat) of the upper-level subtropical jet’s axis and its weak (strong) intensity, a wide range of convergence (divergence) in circulation may appear there, which will accordingly cause the abnormal divergence (convergence) in circulation at the low-level and thus a deficit (surplus) of water vapor budget throughout the whole troposphere, in addition, the eastern Asian summer monsoon is weak (strong), therefore, the combination of the above higher and lower systems and the abnormal atmospheric circulation is unfavorable (favorable) for precipitation then abnormal drought (wet) events. It is shown that one of the important reasons for the increase of drought events over the ENWC may be resulted from the reduce of summer zonal water vapor transport since after the mid-1990s.(3) The abnormal summer wet and drought events over the ENWC are closely related to the ENSO events in the same period, and especially have a significant negative correlation with the SSTA over the kuroshio area. The SSTAs over the westerly drift area of the north Pacific ocean, the central and eastern equatorial Pacific ocean, and the northern Indian ocean are the key affecting factors in the early winter, in particular, the SSTA over the westerly drift area of the north Pacific ocean may bring about geopotential height anomaly at500hPa from early winter to later summer, and then lead to abnormal drought and wet events over the ENWC.