Joint Impacts of the North African and the Western Pacific Subtropical Highs on Summer Precipitation over the Tibetan Plateau
Authors: Zhou Fang , Siseho Siseho Christonette , Liu Minghong , Zhang Dapeng , Zhang Haoxin 
Published: April 2024

Abstract
Focusing on summer precipitation over the Tibetan Plateau (TP), this study mainly investigates the joint impacts of the North African subtropical high (NASH) and the western Pacific subtropical high (WPSH) by examining circulation and moisture anomalies. Results show that there are several boundary combination types of the two subtropical highs. The anomalous vertical motion with sufficient moisture transport under different boundary types plays a dominant role in TP precipitation anomaly. When the WPSH strengthens westward approaching the TP, it can transport water vapor northward from the northwest Pacific and north Indian Oceans to the south edge of the TP and induce ascending motion over the southeastern TP, contributing to more precipitation there. When the NASH enhances and extends eastward, it can transport water vapor eastward from the North Atlantic Ocean to the southwest eastern TP and give rise to ascending motion there, inducing positive precipitation anomaly over the southwest eastern TP. When the two subtropical highs simultaneously intensify and extend to the TP, water vapor can be transported to the TP widely from the North Atlantic Ocean, the north Indian Ocean, and the northwest Pacific Ocean with the strengthening of the westerly, resulting in the location of the ascending motion and rain belt shifting obviously northward. Further analyses indicate that the prewinter El Niño–Southern Oscillation (ENSO) and summer North Atlantic air–sea interaction are two indispensable possible modulation factors for the joint impact of the two subtropical highs on TP precipitation.

Significance Statement
Under the thermal and topographic effects of the Tibetan Plateau (TP), the subtropical high belt breaks over the TP, forming the western Pacific subtropical high (WPSH) on its east and the North African subtropical high (NASH) on its west. As a permanent anticyclonic circulation over the northwest Pacific, the formation, impact, and prediction of WPSH have been fully revealed. However, few studies have been focused on the NASH. In this study, we examine the joint impacts of the two subtropical highs on TP summer precipitation based on various reanalysis datasets and phase 6 of Coupled Model Intercomparison Project (CMIP6) historical simulations. The results show distinct precipitation features with several boundary combination types of the two subtropical highs. Besides, controlling water vapor transport and vertical motion is an important way for the two subtropical highs to affect TP precipitation in summer. Further study shows that the effect of the two subtropical highs on TP summer precipitation is generally modulated by the prewinter El Niño–Southern Oscillation (ENSO) and summer North Atlantic air–sea interaction. The results from CMIP6 support the conclusions drawn from reanalysis. These findings could help deepen our understanding of the joint influencing factors on TP summer precipitation.