Advanced onset of South Asian summer monsoon increases pre-monsoon precipitation over Tibetan Plateau

Abstract

The South Asian summer monsoon (SASM) exerts a profound influence on precipitation over Tibetan Plateau (TP), yet the impact of its recently advanced onset of the SASM (SASMO) on pre-monsoon TP precipitation remains unclear. Using observational, satellite, and reanalysis datasets, this study investigates how interannual variability in the SASMO affects May precipitation over the TP, based on moisture and dynamical budget analyses. Results show that the SASMO explains 45 % of interannual variability in May precipitation over the TP, with an advanced (delayed) onset corresponding to increased (decreased) precipitation. This relationship is linked to an anomalous Rossby wave train, characterized by an anticyclone over the western TP, a cyclone over the Indian subcontinent, and another cyclone over the eastern TP. These circulation patterns promote the northwestward expansion of the South Asian High and the intensification of the India-Burma trough in May. As a result, lower-level westerlies over the monsoon region during advanced SASMO years intensify to twice the climatological mean. Accordingly, net water vapor transport, moisture convergence, and ascending motion over the TP are enhanced by 123 %, 28 %, and 22 %, respectively, ultimately leading to a 14 % increase in May precipitation over the TP. Dynamic diagnostics indicate that vertical motion over the TP is primarily driven by vorticity advection processes. Moisture budget analysis suggests that the increase in May precipitation over the TP is dominated by dynamic processes induced by anomalous wind convergence associated with SASMO anomalies. These findings improve our understanding of the monsoon–precipitation relationship over the TP.