Variation Of Extreme Snowfall In Central Asia And Its Association With The Enhancement Of Two Types Of SST Modes In The North Atlantic

Arid Central Asia(CA)is the largest non-zonal arid zone in the world.It has complex terrain,shortage of water resources and fragile ecological environment.Therefore,studying the changes of extreme snowfall in CA is of great significance to the local ecological environment and the sustainable development of the “One Belt One Road” strategy.Based on the daily precipitation data from 1979 to 2018 provided by the America Climate Prediction Center(CPC),the global reanalysis data from the European Center for Medium-Range Weather Forecast(ECMWF)and the sea surface temperature(SST)data from the National Oceanic and Atmospheric Administration(NOAA),this paper mainly investigates the temporal and spatial changing characteristics of extreme snowfall in CA as well as its relationship with the North Atlantic SST.Diagnostic analysis methods and numerical simulation are used during analyzing these questions.The main conclusions are as follows:(1)The spatial distribution of extreme snowfall in winter of CA is extremely uneven.Extreme snowfall mainly occurs in Kazakhstan,Kyrgyzstan,Tajikistan and northern Xinjiang,and there is less extreme snowfall in the desert regions of Uzbekistan,Turkmenistan and southern Xinjiang.The six extreme snowfall indices show a significant increasing trend in CA and all climate subregions.The increase in frequency and intensity of extreme snowfall together lead to the increase in total amount of extreme snowfall in CA.Among them,the southern key subregion represented by the Tienshan mountain has the most extreme snowfall and the largest inter-annual variability,and its abrupt change occures around the end of the20 th century.(2)The increased frequency of extreme snowfall in the key southern subregion of CA corresponds to the enhancement of the two wave trains along the North Atlantic jet and the Asian-African jet: in the mid-to-high latitude,the abnormal geopotential height from the North Atlantic to the Balkhash Lake is “positive-negative-positive-negative”,in which the Balkhash Lake area is an abnormal cyclonic circulation;in the low latitude area,the northern Arabian Sea is an abnormal anticyclonic circulation.(3)The wave sources of the Eurasian circulation anomaly locate in the subtropical North Atlantic and the tropical North Atlantic,and correspond to the enhanced North Atlantic SST “uniform” and “tri-polar” modes.Since the end of the 20 th century,the increased tropical(subtropical)SST and the first two modes indices of the North Atlantic SST are helpful for the westward extension of the Asian-African jet,and docking with the North Atlantic jet,which strengthen the energy dispersion downstream along the two jets,as a result,strengths the two wave trains along the North Atlantic jet and the Asian-African jet,which in turn leads to the strengthening of the abnormal cyclonic circulation around the Balkhash Lake and the abnormal anticyclonic circulation in the northern Arabian Sea.Both of them enhance the transport of warm and humid water vapor from the Arabian Sea,etc.,as well as the convergent upward movement in southern region of CA,which result the increase of extreme snowfall in key southern subregion.With the climate warming,the impact of this water vapor transport on snowfall and extreme snowfall in CA will be more significant.(4)Using the Community Atmosphere Model CAM5.1 of the Community Earth System Model CESM1.0.4,the North Atlantic SST “uniform” and “tri-polar” modes are numerically simulated.The experimental results show that the two SST modes can reproduce the enhanced wave trains along the North Atlantic jet and the Asian-African jet,and the wave source positions are consistent with the observed results.The results verfity the effect of SST in the subtropical North Atlantic and the tropical North Atlantic on extreme snowfall through wave trains and vortex anomalies.