| Low Latitude Highlands(LLH)indicates the regions located between 30°S-30°N,with average height over 1500 m.This study investigated the relationship between snow cover over Eurasian and precipitation over LLH and the associated mechanisms.The methods of Singular Value Decomposition(SVD)analysis,Monte-Carlo significance test,composite analysis and student's t-test were employed to investigate the relationship between Eurasian snow cover and precipitation of LLH and its relevant dynamical process.The climatological declining trend of snow cover had been eliminated so that the impact of global warming was excluded.The Linear Baroclinic Model(LBM)used in this research was built up in order to examine a linear dynamic mechanism in the atmosphere.The major conclusions are listed below:(1)The temporal and spatial variation of snow cover in Northern Hemisphere has been investigated.The snow cover would establish and decline along with the seasons' changes in boreal autumn and spring.The major variations of snow cover exist in the southern edges of stable snow cover area in boreal winter,around 35°N,and in some places e.g.Tibetan Plateau,20°N.The variation of snow cover in these places could also be linked to subsequent atmospheric changes in downstream area and might be key region of snow-atmosphere coupling.There is a long-term downward trend in snow cover in the northern hemisphere.Eurasian snow cover mainly has two main modes: north-south inversion and east-west dipole.In addition to the long-term downward trend of snow cover,there were significant interdecadal changes in snow cover around 2000 s.(2)There is a significant relationship between Eurasian snow cover and precipitation over LLH.As the leading SVD mode(variance contribution rate 45.7%)showed,when the snow cover in Europe and western Asia increased,the LLH area in China suffered a large reduction of precipitation in May(the starting month of its rainy season),and vice versa.This relationship accounts for the variation of precipitation over LLH in May,and is independent from ENSO modulation.The Eurasian snow cover in this case would mainly affect LLH precipitation through its cooling effect.The snow cover has led to abnormal negative surface temperature over the excessive snow cover region,accompanied by enormous negative upward surface longwave radiation which was resulted by the cooling effect from snow cover to the atmosphere above.In the given period,snow cover mainly affected energy exchange processes between land and atmosphere through snow-albedo effect,as shown in the increased amount of upward surface shortwave radiation,while snow-hydrology effect which was related to snowmelt processes and latent heat flux had existed but limited influences.(3)The cold temperature resulted by cooling effect from excessive snow cover could strengthen the meridional temperature gradient,intensify the wind speed of westerly jet and generate a northward displacement and wind speed enhancement of subtropical jet stream.These abnormal patterns could persist till May,triggering a strong low-pressure cyclone in the north and anticyclone circulation in the south,blocking the cold air mass from the north and the water vapor from the south to reach LLH area,resulting a decline in precipitation.In addition,the abnormal downdraft over LLH also contributes to the stability of the atmosphere and to the disadvantage of rainfall.(4)The key physical process mentioned above could be confirmed through a Linear Baroclinic Model(LBM).By applying the composite abnormal temperature vertical profile over the key snow cover region as steady cooling forcing,the model results could reproduce the relationship between cold air temperature and the strength and meridional displacement of jet stream,along with the abnormal cyclone and anticyclonic circulation.Besides,the regression model built by SVD timeseries could successfully project the variation of rainfall over LLH. |
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