Variability of Tibetan spring snow and its associations with the hemispheric extratropical circulation and East Asian summer monsoon rainfall: An observational investigation

Using station observations of the number of days covered by snow (SCD) and snowfall over the Tibetan Plateau (TP), the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, and precipitation from rain gauge stations in China for the period of 1973-2001, temporal/spatial variations of SCD over the TP and its associations with the hemispheric extratropical atmospheric circulation and East Asian summer monsoon rainfall are investigated. An increase of spring (April-May) SCD over the TP is associated with decreases of local tropospheric temperature and geopotential height in the spring and early summer (June). The anomalies in the tropospheric temperature and geopotential height show a westward propagation from the TP to western Asia as a result of the westward propagation of the anomalous wave energy. These tropospheric anomalies over the TP are connected with changes in the hemispheric extratropical atmospheric circulation along the westerly jet stream that acts as a waveguide. The increase of the spring SCD is also associated with the variation of the East Asian summer monsoon rainfall. Soil moisture in May-June might act as a bridge linking the spring snow anomaly and the subsequent summer monsoon. Corresponding to the increase of SCD, there is a significant decrease of the June 500-mb geopotential height from the TP to the western North Pacific. Meanwhile, the anomalous north-easterlies extend from Japan, through the east coast of China, to central-eastern China, which weaken the East Asian summer monsoon, leading to a decrease of surface air temperature and rainfall in the Yangtze and Hwai Rivers and an increase of rainfall in southeastern China. Additionally, the spring SCD anomaly is likely due to a variation of local synchronous snowfall, rather than previous winter SCD conditions. The spring SCD is not related to previous winter El Nino/La Nina events, but is associated with the equatorial central and eastern Pacific sea surface temperature from the subsequent summer through winter. The climatic implications for this relationship are not clear.