Features And Mechanisms Of The Subseasonal Intensity Variation Of The South Asian High

This study investigates the characteristics of the subseaonal intensity variation of the South Asian high(SAH),as well as the precipitation anomalies and integrated water transport anomalies,based on the 1979-2013 ERA-interim reanalysis data and NOAA/CPC precipitation data.The relative vorticity equation diagnosis reveals the dynamical linkage between the subseasonal SAH's intensity variation and the diabatic heating.The relationship between the SAH and persist heavy rainfall and persist heat wave are also analysed.Finally,based on the 18 CMIP5 modes,we reveal the key dynamic processes for model simulation associated with the subseasonal intensity variation.The main conclusions are as followings:1.Features of the subseasonal intensity variation of the SAHThe first mode of an empirical orthogonal function(EOF)analysis on the standardized subseasonal anomalies in 200 hPa geopotential height over(20°-35°N,35°-110°E)during summer of 1979-2013,reveals the strengthening/weakening of the SAH's main body that is complete with a period of approximately 10-36 days.The high anomaly center is located over the Iranian Plateau and its poleward arise low anomaly.From day-9 to-3,the high and low centers become more signifant.Meanwhile,an enhanced rainfall belt migrates northward from the equatorial region.The northward propagating rainfall belt occupies almost the whole Indian subcontinent,Indochina Peninsula and subtropical east Asian regions.With the process,the western North Pacific high extends westward and the water transport enhance over the Somali coastal region and the North Pacific.2.Mechanism of the subseasonal intensity variation of the SAHIn the perspective of the atmospheric internal dynamic process,both the atmosphere diabatic heating and propagation of the low-frequency signal may influence the subseasonal intensity variation of SAH.After dignosed the relative vorticity equation,we reveal the diabatic heating associated with the northward propagation rainfall belt plays a significant role in the enhancement of the SAH on subseasonal timescale.When the rainfall belt locate over the SAH regions,the vertical gradient of the diabatic heating acts to contribute the strengthening the SAH.The horizontal advection process favors the westward expansion of the SAH.Based on the 18 CMIP5 models data,The relative vorticity equation diagnosis reveals the dynamical linkage between the subseasonal SAH's intensity variation and the precipitation anomalies over the Asian monsoon region.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.3.Climate effects of the subseasonal intensity variation of the SAHThe influence of subseasonal intensity variation of the SAH on different thresholds of persist heavy rainfall and heat waves over the Asian monsoon regions are investigated.It is seen that during the strong SAH,the probability of lower threshold persist heavy rainfall is about 30%of the total persist heavy rainfall events,while during the weak SAH is about 20%.For higher threshold,the persist heavy rainfall related to the strong SAH can be reach 40%-50%,and those related to the weak SAH decrease to 10%,which show larger contrast.This means the SAH has more significant influence on the heavier persist rainfall and less significant on the lighter persist rainfall over the Asian monsoon regions.The heat waves events associated with the strong SAH is only 10%of the total heat waves events,while those associated with the weak SAH can be reach 20%.The SAH has asynchronous influences on heat waves over the Asian regions.Higher threshold or the extreme heat waves related to the weak SAH takes place over the Indian subcontinent,while the lower ones occur over the south China.