Impact of the East-West Phase of South Asia High on Water Vapor Distribution near Tropopause over the Asian Monsoon Region

Based on the ERA-40 reanalysis data provided by the European Center for Medium-Range Weather Forecasts (ECMWF) from 1958 to 2002 and the NCEP/NCAR reanalysis data provided by the National Centers for Environmental Prediction and the National Center for Atmospheric Research, we study the relationship between the east-west phase of the South Asia High (SAH) and water vapor distribution near the tropopause over Asian monsoon region. The results show that the east-west phase of the SAH is correlated with the intensity of high water vapor center at 200 hPa and has an obvious impact on the intensity and location of high water vapor center at 100 hPa, but it has almost no effect on the intensity and location of water vapor at 70 hPa. During the east phase of the SAH, the high value centers of water vapor at 200 hPa and 100 hPa both are located over the Tibetan Plateau, which are related to strong ascending motion and the subtropical westerly jet that transport water vapor to near tropopause over the Tibetan Plateau. At 100 hPa, the strong northerly and easterly winds play an important role for horizontal transport of water vapor, which explains why the spatial distribution of water vapor and its high value center are similar to that of pressure. During the west phase of the SAH, the center of high water vapor shifts westward to Iranian Plateau because of the strong ascending motion resulted from the terrain lifting effect along the western Tibetan Plateau and the northward displacement of the westerly jet. The distribution of temperature anomaly is consistent with that of water vapor, and the warm center is favorable for the generation of high moisture at 200 hPa in SAH. The distribution of temperature anomaly is contrary to that of water vapor and the warm center corresponds to low water vapor at 100 hPa, indicating that the distribution of water vapor in the stratosphere in SAH is influenced by the interaction of the circulate and temperature fields. The present study is helpful for understanding the mechanism of the east-west phase of the SAH and improving the quality of climate forecast.