The Influence of Sea Surface Temperature Anomaly on the East Asian Summer Monsoon Strength and Its Precursor

The strength of the East Asian Summer Monsoon (EASM) is closely connected to the summer main rainfall belt in China. The precursor index defined by the difference of zonal wind anomaly at 200 hPa between the middle latitude in Asia and eastern Pacific in February is well indicative to the strength of EASM, which is an important predicting factors in flood season. The potential mechanism of precursor signal influencing the EASM is proposed by changing the surface characteristics in South Asia continent, but it is unclear whether the atmospheric circulation anomaly in February persists in the following seasons over middle latitude region.In addition, a further investigation is needed about the surface anomaly variation over South Asia in winter-spring seasons. ERA-Interim reanalysis data, NOAA sea surface data, gridded CMAP precipitation data and precipitation observations over China are used. By composite, correlation and regression analysis approaches, the difference of wind in middle latitude over Eurasia, sea surface temperature (SST) in tropics and thermal condition in South Asia continent in previous winter-spring seasons in various strength EASM years are analyzed. Results indicate that tropical SST is the physical connection of accordant variations between the strength of EASM and its precursor.Results show that the precursor signal of EASM captures the primary feature for the first mode of empirical orthogonal function (EOF1) of zonal wind anomaly at 200 hPa over the Asia and Pacific in February. The EOF1 mode is related to SST in the central and eastern Pacific. In the previous winter, the SST is below (above) normal in the central and eastern Pacific, which is conducive to a northward (southward) shift of westerly jet over the Asia in February. The zonal wind anomaly at 200 hPa exhibits meridional positive-negative-positive (negative-positive-negative) pattern, and the precursor index is stronger (weaker) than normal. In summer, the negative (positive) SST anomaly occurs in the vicinity of Indian Ocean and South China Sea, which results in an increasing (decreasing) difference between ocean and land and stronger (weaker) Indian Summer Monsoon. Meanwhile, the western Pacific subtropical high (WPSH) is weaker (stronger) than normal, and the EASM is stronger (weaker) than normal.The anomalous feature of zonal wind in the middle latitude of the Asia in February is hardly to persist in spring. The physical connection between EASM and its precursor mainly derives from the tropical ocean.