Changes In The Extreme Weather Events And The Relationships With The Atmospheric Circulation Types In The Arid Northwest China

This article embarks from the daily time scale, with the help of Lamb-Jenkinson atmospheric circulation objective classification method combined with the meteorological station data and NCEP reanalysis data, the northwest arid areas is divided into seven areas. In order to explore the change and the possible links between the extreme events in the study area change and reveal the influence of the atmospheric circulation variation of extreme weather events. Statistical analysis is applied to the different area circulation pattern and the characteristics of extreme events and change.Firstly, combined with meteorological data, we proved the northwest arid areas of atmospheric circulation characteristics of parting season and year. Based on the climate system differences, the study area is divided into four similar unit: the northern Xinjiang subarea, southern Xinjiang subarea, western Tianshan and eastern Tianshan mountain subarea, west Gansu subarea, Hexi Corridor subarea and Ningxia subarea. Each unit has a certain similarity circulation and climate characteristics.Secondly, we counted and analyzed the distribution characteristics of different seasonsrelated to the extreme temperatures and the precipitation events leading circulation type in each area. It is found that the extreme high temperature events usually happened in cyclonic type and northerly flow type; On the probability of occurrence, The liability of westerly and northerly winds circulation types is obvious. Extreme low temperature events normally happened in the anticyclonic type, anticyclonic combined with eastern flow and eastern flow. On the probability of occurrence, it was the same circulation types that the liability was high. Extreme precipitation events easily happened in the anticyclonic type, anticyclonic combined with northerly and northeast flow type and northerly and northeast flow type. On the probability of occurrence, the liability of northerly and northeast flow and anticyclonic combined with northerly and northeast flow type are high. The cyclonic combined with northeast direction flow are significant in some parts. But it is especially different in southern and western Tianshan mountain subareas.Thirdly, we analyzed the change and variation of season with the circulation type on the time scale. For years zonal wind had westerly strengthening trend in the subregion of high latitude area, including the northern Xinjiang subarea, western Tianshan subarea, east Tianshan subarea. However,low latitude area of southern Xinjiang,western Gansu, Hexi corridor and Ningxia have weakened westerly winds for years.Meridional wind throughout the northwest arid areas basically present the process of south wind strengthen, only in individual regions and season appeared opposite trend. In terms of geostrophic vorticity index, only in winter and spring in Ningxia subarea has the tendency of anticyclone weakened, the rest are present anticyclone strengthening over the years. Compared with other subarea, the year’s circulation change of Ningxia is relatively an exception.Corresponding with extreme events and circulation index, the u component and the epsilon component of extreme high and low temperature, the v component of extreme precipitation event in northern Xinjiang and eastern Tianshan are obvious difference between with seasonal mean value. The u component of extreme high and low temperature events in Western Tianshan is different from seasonal value.The rest of the seasons of southern Xinjiang were not significant except in winter. While in west Gansu, Hexi corridor and Ningxia subareas, the mainly difference of the extreme high and low temperature events and seasonal mean value is epsilon component. Among them, the seasonal characteristics of Ningxia is significant, the u v epsilon components of extreme high temperature and u v components of extreme precipitation in spring and autumn are obvious different from seasonal value, while in summer and winter, are the u v components of extreme high temperature and epsilon of extreme precipitation.When talked about the possible links between circulation type changes and extreme events, although it is found that the regional circulation type for many years changes have certain regularity, but the dominant factor on the changes of extreme events varies. The increase of extreme precipitation events most affected by the strengthen of northeaster flow and anticyclone. The decrease of extreme low temperature events mainly by the north wind weakened or west wind strengthened. However, the dominant factor of the increase of extreme high temperature events varies in different areas and seasons. Due to the effect of monsoon climate, extreme events change in the most eastern, Hexi corridor and the Ningxia subareas, is not completely corresponding with the circulation type changes.