Decadal change of the spring snow depth over the Tibetan Plateau: The associated circulation and influence on the East Asian summer monsoon

The decadal change in the spring snow depth over the Tibetan Plateau and impact on the East Asian summer monsoon are investigated using station observations of snow depth data and the NCEP-NCAR reanalysis for 1962 - 93. During spring ( March - April), both the domain-averaged snow depth index ( SDI) and the first principal component of the empirical orthogonal function (EOF) analysis exhibit a sharp increase in snow depth after the late 1970s, which is accompanied by excessive precipitation and land surface cooling. The correlation between SDI and precipitation shows a coherent remote teleconnection from the Tibetan Plateau - northern India to western Asia. It is found that the increased snow depth over the plateau after the mid-1970s is concurrent with a deeper India - Burma trough, an intensified subtropical westerly jet as well as enhanced ascending motion over the Tibetan Plateau. Additional factors for the excessive snowfall include more moisture supply associated with the intensification of the southerly flow over the Bay of Bengal and an increase of humidity over the Indian Ocean. While the extensive changes of the circulation in Eurasia and the Indian Ocean are associated with a climate shift in the Northern Hemisphere after the mid-1970s, some regional factors such as the enhanced coupling between the sea surface temperature (SST) warming in the northern Indian Ocean/Maritime Continent and the tropical convective maximum (TCM), as well as local feedback of the land surface cooling due to excessive snow cover and the atmosphere may contribute to the regional circulation changes. The former enhances the western Pacific subtropical in the South China Sea - Philippine Sea through modulation of the local Hadley circulation and results in stronger pressure gradients and fronts in southeastern and eastern Asia. A close relationship exists between the interdecadal increase of snow depth over the Tibetan Plateau during March - April and a wetter summer rainfall over the Yangtze River valley and a dryer one in the southeast coast of China and the Indochina peninsula. It is proposed that the excessive snowmelt results in a surface cooling over the plateau and neighboring regions and high pressure anomalies that cause a more northwestward extension of the western Pacific subtropical high in the subsequent summer. Additionally, the increased surface moisture supply provides more energy for the development of the eastward-migrating low-level vortex over the eastern flank of the Tibetan Plateau. Both factors lead to a wetter summer in the vicinity of the Yangtze River valley.