Preliminary Study On Moisture Sources And Physical Mechanism Of Extreme Precipitation Events In The Arid Central Asia

The arid Central Asia(ACA;35.25-52.75°N,46.25-80°E)is the largest non-zonal arid region in the world.In the last 100 years,the precipitation in ACA represents significant regional differences,but annual and seasonal precipitation has a slightly increasing trend,especially for winter precipitation.Additionally,as a typical inland arid zone,the annual precipitation in ACA is dominated by a few extreme precipitation events.Therefore,investigating the moisture sources and underlying physical mechanisms of extreme precipitation in ACA can improve our understanding of related scientific issues and have implications for policymakers to make sustainable development strategies in the arid regions.In this study,the gridded daily precipitation from Global Precipitation Climatology Project(GPCC)during 1988-2013 and NCEP/NCAR reanalysis data are used to analyze the atmospheric circulation and moisture flux associated with extreme precipitation in different regions of ACA.Additionally,the monthly precipitation from GPCC during 1960-2013,NCEP/NCAR,and Sea Surface Temperature(SST)from NOAA are used to analyze the impacts of atmospheric circulations and SST on precipitation,revealing the possible physical mechanisms of precipitation in ACA.The main results are as follows:(1)The water vapor of summer extreme precipitation events in the north ACA is mainly transported from the Atlantic and the Arctic Ocean.The lower-and upper-level circulation anomalies provide the dynamic support for precipitation.At 200 hPa,the height anomalies represent a zonal "-+-+-" wave train pattern over Eurasia,strengthening the meridional circulations.At 500 hPa,the northerly wind anomalies on the east side of the anticyclonic circulation in Western Europe,couple with the northeasterly wind anomalies on the northwest side of the cyclonic circulation in ACA,strengthen moisture transported from the Arctic Ocean.At 850 hPa,the north ACA locate in the areas downstream of troughs,which is conducive to the southwesterly airflow transport warm humid air masses.The interactions of atmospheric circulation at different levels provide necessary dynamic conditions for extreme precipitation.(2)For the extreme fall precipitation in the north ACA,at 200 hpa,there is a meridional "+-+" wave train pattern of height anomalies extended from high latitude to low latitude over mid-latitude Eurasia.At 500 hPa,the westerly wind anomalies on the south(northwest)side of the cyclonic(anticyclonic)circulation from Western Europe to western Siberia(the Mediterranean Sea)strengthen the westerly moisture transportation from the Atlantic Ocean.At 850 hPa,the moisture from the Atlantic Ocean significantly increases because the precipitation anomalies are mainly controlled by westerlies.(3)For the extreme spring precipitation in the south ACA,the height anomalies at 200 hPa represents a strip-shaped feature.There is a “+-+-” wave train pattern from northwest to southeast with prominent centers over Scandinavia,Caspian,Central Asia,and India to most parts of China.The enhancement of meridional circulations is good for the southward movement of cold air masses from high latitudes.At 500 hPa,the southwesterly wind anomalies on the southeast(northwest)side of the cyclonic(anticyclonic)circulation near the Caspian Sea(North Indian Ocean)strengthen moisture transport from the northern Indian Ocean.At 850 hPa,the atmosphere condition is unstable.The cold air from the high latitudes encounter the warm and moisture from the North Indian Ocean in the south ACA,resulting in water vapor convergence and precipitation.(4)For the extreme winter precipitation in the south ACA,the height anomalies represent a "+-+-” wave train pattern at 200 hPa in Eurasia.At 500 hPa,the blocking high in Western Europe and the strong trough from West Siberia to Central Asia enhanced the meridional circulation,which is beneficial for the cold and humid air masses from high-latitude to move southward.At 850 hPa,the south ACA locate in the areas downstream of troughs,which is conducive to the southwesterly airflow transport water vapor to the south ACA and develop into cyclones.The cyclones can trap more water vapor in the south ACA.The moisture transport also play an important role.The anomalous anti-cyclone circulation in the Arabian Sea and the strong southwesterly wind enhance the water vapor transport from the Arabian Sea/northern Indian Ocean.(5)Summer precipitation in the north ACA is strongly linked to atmospheric circulations and SST.During the wetter years,the SST is mainly characterized by negative anomalies,except the Greenland Sea and part of the Pacific Ocean.The atmospheric circulations are changed due to the SST anomalies in the North Atlantic Ocean and Greenland Sea.The northeasterly wind anomalies in the Ural Mountains transport the water vapor from the Arctic Ocean into ACA,couple with the cyclonic circulation over central Asia,increasing precipitation in the north ACA.(6)Winter precipitation in the south ACA is strongly linked to atmospheric circulations and SST.During the wetter years,there are positive SST anomalies in the Arabian Sea,Indian Ocean,and the central and eastern equatorial Pacific.Negative anomalies mainly appear in the north Atlantic,Mediterranean Sea,and northwest Pacific.The SST anomalies in Indian Ocean and the central and eastern equatorial Pacific cause the changes in atmospheric circulation.Due to the positive height anomalies in the North Indian Ocean and Iran and the negative height anomalies from the West Siberia to Central Asia,the water vapor is transported from the northern Indian Ocean to the south ACA,increasing the winter precipitation.