Distribution and Transport of the Upper Tropospheric Water Vapor over the Tibetan Plateau Area during the East–West Oscillation of the South Asian High in Summer

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 during summer, significantly influencing the water vapor distribution and upper troposphere transport 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 phases composited analysis method based on the east–west oscillation indexes of the SAH in July–August. The results indicated that, for the TP mode of the SAH, the water vapor content is abnormally high (low) in the upper troposphere over the TP (IP, Iranian Plateau) . However, the opposite is true over the TP and IP respective, for the IP mode of the SAH. In association with the centers of the SAH moving westward from the TP to the IP, the positive anomaly corresponding to the water vapor content in the upper troposphere gradually propagates westward from the eastern TP to the west of the IP. Furthermore, in the upper troposphere, except where the abnormal meridional adiabatic transport has a counteracting effect on the northern and southern TP, the water vapor tendency anomalies over the two plateaus are primarily driven by the abnormal zonal adiabatic water vapor transport as well as their convergence and divergence anomalies. However, the abnormal upward diabatic water vapor transport due to convection over the TP is 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), an abnormally weak (strong) convective occurs. Corresponding to the center locations of the SAH and centers of the positive anomaly of the water vapor content in the upper troposphere as it moves from the TP to the IP, the abnormal convection and upward diabatic water vapor transport continue to increase over the TP. Meanwhile, the water vapor condensation anomaly in the upper troposphere increased steadily. Furthermore, as the SAH moves westward, the abnormal convergence (divergence) of the adiabatic water vapor transport in the upper troposphere occurs on its west (east) sides, which is the primary reason for the zonal propagation of the centers of water vapor content anomaly.