1.State Key Laboratory of Numerical Modeling of Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences;2.Collaborative Innovation Center on Forecast and Evaluation of Metrological Disasters (CIC-FEMD);3.Hubei Meteorological Service Center;4.University of Chinese Academy of Sciences
The bimodal distribution of the South Asian high (SAH) corresponds to the east–west oscillation of its center locations on a quasi-biweekly timescale of 10–20 days in summer, which has significant impact on the water vapor distribution and transport in the upper troposphere over the Tibetan Plateau (TP) area. In this study, we used the daily averaged European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERAI) datasets, and the phases composited analysis method which bases on the east–west oscillation indexes of the SAH in July–August. The results show that, for the TP mode of the SAH, the water vapor content is abnormal high (low) in the upper troposphere over the TP (Iranian Plateau, IP). For the IP mode of the SAH, the opposite occurs. In association with the centers of the SAH moving westward from the TP to the IP, the centers of positive anomaly of the water vapor content in the upper troposphere also gradually propagate westward from the eastern TP to the west of IP. Further diagnosis shows that, in the upper troposphere, except the abnormal meridional adiabatic transport plays a counteract effect in the northern and southern TP, the water vapor tendency anomalies over the two plateaus are mainly contributed by the abnormal zonal adiabatic water vapor transport and their convergence and divergence anomalies. While the abnormal upward diabatic water vapor transport due to convective over the TP are mainly counteracted by the residual term (water vapor condensation and evaporation). Therefore, when the water vapor content anomaly is positive in the upper troposphere over the TP (IP), there are abnormally weak (strong) convective over the TP. Corresponding to the center locations of the SAH and the centers of positive anomaly of the water vapor content in the upper troposphere moving from the TP to the IP, both the abnormal convective and upward diabatic water vapor transport continuously enhanced over the TP. Meanwhile, the water vapor condensation anomaly in the upper troposphere also continuously enhanced. Furthermore, with the SAH moving westward, the abnormal convergence (divergence) of the adiabatic water vapor transport in the upper troposphere mainly occur to its west (east) sides, which is the main reason for leading to the zonal propagation of the centers of water vapor content anomaly.