Variation Characteristics Of Eurasian Snowmelt In Spring And Its Impact And Mechanism On Summer Precipitation In Eastern China

Long-term trend and variability of Eurasian spring snowmelt(SSD)are analyzed using the monthly global snow water equivalent data from the fifth generation ECMWF reanalysis(ERA5)datasets,provided by European Centre for Medium-Range Weather Forecast(ECMWF).Meanwhile,the influence of SSD on Eastern China Summer precipitation(ECSP)respectively on interannual and interdecadal scales and the possible mechanisms are also discussed using the monthly surface precipitation in China,provided by National Meteorological Information Center(NMIC,CMA).Long-term trend of Eurasian SSD characterizes as north-south opposite changes,with decreasing SSD at middle latitude and the increasing SSD at high latitude.With the linear trend removed,the first EOF mode of Eurasian SSD,featured as a north-south dipole pattern,displays obviously interannual oscillation while the second EOF mode,featured as a west-east dipole pattern,displays two interdecadal changes near late-1970 s and mid-2000 s respectively.On the interannual scale,an independent positive relationship between SSD variability over northern Eurasia and the averaged June-July summer precipitation over Yangtze-Huai River(YHRV)is observed.Anomalies of atmospheric circulation and surface heat fluxes are further investigated to explaining the related mechanisms using synthetic analysis.Results show that wetter soil moisture related to more SSD and observed stronger snow albedo in summer could collaborate together and lead to the local cooling of atmosphere accompanied by the anomalous lower geopotential height.With Rossby spreading downstream,reflected by the spread WAFs,atmospheric circulations over China are changed with higher pressure located from Baikal River to northeastern China and the opposite near YHRV.With the anomalous ascending motion and the associated water vapor transport,more precipitation over YHRV could be produced.The linkage between ECSP and the second mode of Eurasian SSD is observed closer on the interdecadal scale.Increased spring snowmelt over Western Siberia(WSI)and the opposite over Eastern Europe(EEU)are significantly linked to a meridional quadrupole summertime rainfall pattern,with excessive rainfall over southern China(SC)and the regions of Huang-Huai River(HHR)and insufficient rainfall over the middle and lower reaches of Yangtze River Valley(YRV)and Inner Mongolia-northeastern China(IMNC).The possible mechanisms could be explained through hydrological effect: the dipole pattern of anomalous soil moisture related to SSD could memory the snow effect to summer and lead to the opposite quasi-barotropic structures of local atmospheric circulations over WSI and EEU.The eastward wave activity flux(WAF)could be strengthened over WSI and EEU in upper-middle troposphere and propagate downstream,leading to an anomalous anticyclonic circulation near Lake Baikal and weaker cyclonic circulation over Nanhai-southern China.Associated with the upper-to-lower convergence and divergence activities,the quadrupole secondary circulation could be found and lead to the anomalous precipitation activities along with the associated water vapor transport.