Response Of Spring Eurasian Snow Modes To North Atlantic Sea Temperature Anomalies And Its Influence On Precipitation

With the global warming,the snow cover in Eurasia is also changing significantly.Through its high reflectivity,heat exchange isolation and hydrological effects,the local energy budget and water cycle are abnormal,which interact with the general circulation of the atmosphere,affect the monsoon and precipitation in Eurasia,even the weather and climate change in the whole northern hemisphere,and it have an impact on the ecosystem and economic system significantly.Eurasia is a vast continent with complex snow spatial distribution and significant seasonal distribution differences.However,there are still insufficient discussions on snow distribution and its formation mechanism in spring.Thus,based on the daily snow depth(SD)data provided by the Japanese 55-year Reanalysis(JRA)project,the global reanalysis data from the European Centre for Medium-Range Weather Forecasts(ECMWF)and the sea temperature data from Hadley Center(Hadley),this paper analyzes the temporal and spatial characteristics of asymmetric modes of the spring SD anomaly in Eurasia.By discussing the difference of the Rossby wave energy dispersion characteristics between the two modes,the teleconnection between snow depth main modes and North Atlantic sea surface temperature(SST)is explored,and through the multiscale energy and vorticity analysis(MS-EVA)method,the kinetic energy transmission and budget of the energy source area which over the North Atlantic are analyzed.The teleconnection results are verified by CAM5.1 numerical simulation analysis.Finally,the influence of second modes of snow depth on the summer precipitation in the mid and high latitudes is discussed.The results are as follows:(1)There are significant differences between the first two modes of the spring SD in the mid-high latitudes of Eurasian continent,which are represented as two asymmetric forms in this paper: the first mode features a zonal uniform distribution,and the second displays an obvious west-east contrast distribution.In the late 1980 s and in the mid-1990 s,their time evolution showed significant interdecadal changes respectively.The March snow depth of Eurasia shows multi-scale change characteristics as a whole,and the interannual and interdecadal oscillations are significent.The circulation pattern over Eurasia which corresponds to the first SD mode shows a distribution characteristic of zonal wave trains,while the second SD mode presents a distribution pattern of the meridional wave trains.The activity fluxes corresponding to the two modes show two propagation characteristics: Eurasian teleconnection pattern(EU)and Silk Road pattern(SRP),and the both of wave energy can be traced back to the North Atlantic.(2)The North Atlantic "tri-polar" and the "saddle" SST modes have significant correlation with the first and the second mode of the SD of the corresponding period,respectively.Corresponding to the two SST modes,the wave activity fluxes in the mid-high latitudes over the northern hemisphere are also characterized by two kinds of propagation characteristics: SRP and EU,which have different effects on the position and intensity of the westerly air flow in the mid-high latitudes,and thus exert different remote influences on the SD distribution in Eurasia.The MS-EVA shows that the kinetic energy(KE)of source region in the North Atlantic has a transform process from bottom to top.In addition,the average KE conversions enhance over the exit region of the Westerly Jet,benefiting the high-level KE accumulation and divergence outward and creating a remote effect on downstream areas.The CAM5.1 model simulation is used to study the effects of the two SST modes on the propagation characteristics of the wave activities flux.The simulation results verified the observation results well.The SST modes may responsible for SRP and EU propagation characteristics of the wave activities flux,meanwhile,the changes of the climatic field elements of the two SST modes are consistent with the distribution characteristics of the corresponding snow depth modes.(3)The summer precipitation of the mid-high latitude over Eurasia has a strong response to the second mode of the March snow depth,which shows positive anomaly in the West Siberia and Eastern Europe plain while on both sides it shows negative anomalies.The distribution pattern is the same as the first mode of summer precipitation EOF analysis in mid-high latitude.By analyzing the response of atmospheric circulation field to the snow depth PC2,it is found that there is a significant anomalous convergence of low-layer flow over the positive precipitation area,and the potential height field is abnormally low.Compared with the climate mean state,the trough to the west of the West Siberian Plain develops deeper while the whole troposphere is abnormal updraft,which is consistent with the dynamics of precipitation anomalies.The abnormal cooling area of the surface temperature field corresponds to the low pressure area of the height field,where the soil humidity is high and anomalous convergence of water flux is enhanced.The temperature and humidity anomalies interact with the circulation background,provided favorable conditions for the response of summer precipitation to snow depth mode.