Subseasonal intensity variation of the South Asian high in relationship to diabatic heating: observation and CMIP5 models

This study investigates the subseasonal intensity variation of South Asian high (SAH) and the dynamic linkage with precipitation induced diabatic heating during the summer of 1979-2005, based on the observation and 18 models from the Coupled Model Intercomparison Project (CMIP5). The SAH's intensity variation with a period of approximately 10-36days is identified both in the observation and 18 models, by performing an empirical orthogonal function (EOF) analysis on the standardized subseasonal anomalies in geopotential height at 100hPa over the SAH's main body region. With the observed strengthening of the SAH, an enhanced rainfall belt between 70 degrees-120 degrees E migrates northward from the equatorial region. The northward propagating rainfall belt occupies almost the whole Indian subcontinent, Indochina Peninsula and subtropical east Asian regions between 20 degrees-40 degrees N when the SAH reaches the strongest. The relative vorticity diagnosis reveals the dynamical linkage between the subseasonal SAH's intensity variation and the precipitation anomalies over the Asian monsoon region: (1) The observed northward propagating rainfall band forms a horizontal gradient of diabatic heating, favoring an enhancement of the SAH over its southern part; (2) When the rainfall band approaches the regions of South and East Asia between 20 degrees-40 degrees N, the increased vertical gradient of diabatic heating in the upper level contributes significantly to the intensification of the SAH; (3) The horizontal advection process favors the westward expansion of the SAH. The observation-to-model comparisons indicate that the model's capacity to reproduce the SAH's intensity variation is highly associated with simulations of the anomalous rainfall band and its northward propagation. A realistic reproduction of precipitation anomalies is fundamental to simulating the dynamical processes related to diabatic heating, and thus the simulation of the SAH's subseasonal intensity variation.