Increasing Warm-Season Precipitation in Asian Drylands and Response to Reducing Spring Snow Cover over the Tibetan Plateau

摘要

Precipitation is crucial for life and the ecological environment in Asian drylands. This study investigated precipitation trends in Asian drylands in the previous four decades and simulated their possible linkage with snow cover reduction over the Tibetan Plateau. The results show that precipitation has been increasing and contributing to wetter conditions in Asian drylands. The increasing trends can be attributed to the deepened quasi-stationary wave trough around Lake Balkhash and the meridional water vapor flux originating from the Arabian Sea and the Bay of Bengal. The midlatitude waves and eddy disturbances correspond to the northward upper-level Tibetan Plateau (TP) mode of the South Asian high (TP-SAH) and the Afro-Asian jet with cyclonic rotation. Both SAH and Afro-Asian jet anomalies strengthen the ascending motion and northward water vapor convergence in Asian drylands, and those are favorable for summer precipitation. The anomalous circulations are linked to the following factors. First, the reduced snow cover (SC) over the west TP in the late spring results in decreasing soil moisture and increasing diabatic heating in summer and favors northward extension of TP-SAH and the Afro-Asian jet. Second, the reduced TP SC increases surface temperature over the TP and northeast Asia, which decreases the temperature gradient between the TP and the Indian Ocean, between northeast Asia and East Asia. Decreased temperature gradients are beneficial to the southwest-northeast cyclonic rotation of the Afro-Asian jet and consequently strengthen the southerly wind and northward water vapor flux over the TP and surrounding regions. This study emphasizes important effects of the reducing TP SC on intensifying summer precipitation in Asian drylands.