Coupling of Indian and East Asian Monsoon Precipitation in July-August

Abstract:

Recent work suggests that summer rainfall in the Indian and East Asian monsoons results from different mechanisms. The onset of intense convection in India is mediated by Hadley Cell transitions, whereas frontal rainfall in China (most notably during Meiyu season in June) arises from forced meridional convergence and zonal heat transport in the wake of the Tibetan Plateau. However, the leading mode of July-August interannual rainfall variability for All-Asia (defined as the region within 68E-140E and 5N-45N) demonstrates a statistically significant coupling between monthly anomalies in India and China. In particular, positive anomalies along the Himalayan Foothills are associated with positive anomalies along the Yangtze River, and also with negative anomalies over central India and northern and southern China. The entire pattern reverses in dry years over the Himalayan Foothills. This coupling is not significantly correlated with ENSO, the leading mode of global interannual variability. We propose that a channel of moisture transport links the Bay of Bengal to the Yangtze River valley across the high terrain of the Yunnan Plateau, on the southeast edge of the Tibetan Plateau. This channel only activates in July, when the maximum of moist static energy (MSE) shifts to land, and weakens in September with the cooling of Bay of Bengal SST. Our mechanism is substantiated by analysis of output from the LMDZ5 model, which includes a high-resolution nested grid nudged to reanalysis, improving the simulation of the Indian Monsoon and performance near high topography. Potential changes in moisture transport across the Yunnan Plateau under 21^st century warming conditions may lead to modified interannual variability of Asian rainfall.