The Multi-scale Characteristics And Influence Mechanism Of Summer Extreme Precipitation Variation In Central Asia

Arid and semi-arid Central Asia is short of water resources and acts as one of the most sensitive regions to recent climate change.Precipitation in Central Asia features adequate in winter and summer and in mountainous areas.In recent decades,under the influence of global climate change,precipitation and the uncertainty and intensity of extreme precipitation events have increased in Central Asia,which leads related secondary disasters and economic losses have exponentially risen.Since the 1990 s,five Central Asian countries canceled meteorological station observations which results the lack of basic extreme climate events datasets and researches.Therefore,more in-depth studies on the characteristics and physical mechanism of summer extreme precipitation in arid regions have scientific guiding significance for improving the understanding of extreme events,forecasting level of extreme precipitation,deployment of disaster prevention and rational allocation of water resources.Therefore,by combining observation and reanalysis datasets,using meteorological statistics and synoptic dynamic analysis methods as well as numerical model simulation,this study systematically analyzed the variation characteristics of summer extreme precipitation in Central Asia and the influence of multi-scale coupling.The main conclusions of this study include:1.The summer extreme precipitation in Central Asia generally increased during1979-2018,significantly in the eastern Central Asia(east of 65°E).The CPC(Climate Prediction Center,CPC)daily precipitation data has been selected as the basic data of this study,which could commendably describe the characteristics of summer precipitation in Central Asia.According to the daily CPC precipitation data: The summer precipitation and extreme precipitation in Central Asia gradually decrease from the north to south,and more precipitation occurs in mountainous areas,and both of them have quasi-3-year and quasi-10-year scale period.The contribution rate of summer extreme precipitation to precipitation in Central Asia has been up to 51.5%.In the past 40 years,the summer precipitation and extreme precipitation in Central Asia showed significant increasing trends,but the trends showed obvious regional differences.The eastern Central Asia(east of 65°E)showed more significant increasing,especially in northern Kazakhstan and Tianshan Mountains.Extreme precipitation showed significant interdecadal increasing in eastern Central Asia since 2000.The intensity and probability of extreme precipitation increased most significantly in the latest decade.In the future,summer extreme precipitation may continue to increase in eastern Central Asia,especially in Tianshan Mountains.2.The interdecadal increase of extreme precipitation in eastern Central Asia is influenced by the interdecadal strengthening of large-scale circulation and key circulation factors.The intensification of the anomalous quasi-stationary wave train near50°N over Eurasia continent affects the variations of extreme precipitation in eastern Central Asia,which is characterized by the enhanced cyclonic anomaly in the high latitude of North Atlantic and Central Asia,and the anticyclone anomaly in the low latitude of North Atlantic,Europe and East Asia.The anomalous circulations led to the deepening of the large-scale trough near Balkhash Lake.Besides,the northerly water vapor from the Barents Sea and Kara Sea and the warm and humid southwesterly water vapor were transported to the eastern Central Asia and converged in front of the trough.The strengthening of NAO(North Atlantic Oscillation,NAO)and EA/WR(East Atlantic/Western Russia,EA/WR)negative phase act as the key circulation factors which intensifies the above quasi-stationary wave.The interdecadal strengthening of anomalous circulation and key circulation factors adjust the interdecadal increase of extreme precipitation in eastern Central Asia since 2000.3.The synoptic-scale transient and large-scale circulation anomalies jointly contributed to the development of long wave trough,the synoptic-scale transient disturbances and meso and micro-scale vorticities,which increased the extreme precipitation in eastern Central Asia.During the extreme precipitation,the synoptic-scale kinetic energy transfer at lower and middle levels is one of the major contributions to the increased kinetic energy.Under the influence of anomalous large-scale circulation,two sets of synoptic-scale transient waves over Eurasia enhanced the abnormal cyclones in Central Asia and deepened the long wave trough near the Balkhash Lake by increasing the transient vorticities and the transient disturbance kinetic energy.The deepening of the long wave trough promoted the transportation of the southerly water vapor and meso and micro-scale vorticities from the southern Central Asia to the eastern Central Asia,which leads to the extreme precipitation increasing.Simultaneously,the increased extreme precipitation and meso and micro-scale vorticities induced by topography in Tianshan Mountains was transported to the northeast Kazakhstan along the southwesterly,which provides sufficient water vapor and kinetic energy conditions for this region.Under the combination effects of different scale circulations,extreme precipitation finally increased in eastern Central Asia.4.The summer SST anomalies in the North Atlantic and spring Sea Ice degradation in the Barents Sea are important external forcing factors which affects the extreme precipitation increasing in eastern Central Asia.The variation of summer SST in the North Atlantic presents a "+-+" zonal distribution,the increasing rate of SST in the high latitudes North Atlantic is faster,which strengthens the SST gradient and eastward Rossby wave energy dispersion near 50°N and enhanced the aforesaid large-scale circulation anomalies.The interdecadal warming of SST at high latitudes North Atlantic is closely related to the interdecadal increase of circulation anomalies and extreme precipitation in eastern Central Asia.The positive SST anomaly in the middle and low latitudes North Atlantic caused the strengthening of the anticyclone over the Indian Peninsula and East Asia which promote the southwesterly water vapor transport to eastern Central Asia by enhancing the quasi-stationary wave near 30°N.Besides,it influenced the interannual increase of extreme precipitation in the eastern Central Asia by NAO and EA/WR negative phases.The melting of sea ice in the Barents Sea in spring,on the one hand,strengthens the cyclone anomaly from the Barents Sea to the Kara Sea,which deepens the long wave trough near the Balkhash Lake and affects the extreme precipitation increasing in northern Kazakhstan.On the other hand,it warms the summer SST in the Barents Sea to Kara Sea and the high latitudes North Atlantic,which further influence the interannual and interdecadal increase of extreme precipitation in eastern Central Asia by strengthening the Polar-Eurasian teleconnection pattern,EA/WR and NAO negative phases,respectively.