The effects of asymmetric potential vorticity forcing on the instability of South Asia High and Indian summer monsoon onset  

作　　者：ZHANG YaNi WU GuoXiong LIU YiMin GUAN Yue 

机构地区：[1]National Meteorological Center [2]State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics,Institute of Atmospheric Physics,Chinese Academy of Sciences

出　　处：《Science China Earth Sciences》2014年第2期337-350,共14页中国科学（地球科学英文版）

基　　金：supported jointly by the National Basic Research Program of China(Grant Nos.2010CB950403,2012CB417203);National Natural Science Foundation of China(Grant Nos.40875034,40925015 and 41275088)

摘　　要：Based on the theory of potential vorticity(PV),the unstable development of the South Asia High(SAH)due to diabatic heating and its impacts on the Indian Summer Monsoon(ISM)onset are studied via a case diagnosis of 1998.The Indian Summer Monsoon onset in 1998 is related to the rapidly strengthening and northward moving of a tropical cyclone originally located in the south of Arabian Sea.It is demonstrated that the rapid enhancement of the cyclone is a consequence of a baroclinic development characterized by the phase-lock of high PV systems in the upper and lower troposphere.Both the intensification of the SAH and the development of the zonal asymmetric PV forcing are forced by the rapidly increasing latent heat released from the heavy rainfall in East Asia and South East Asia after the onsets of the Bay of Bengal(BOB)monsoon and the South China Sea(SCS)monsoon.High PV moves southwards along the intensified northerlies on the eastern side of the SAH and travels westwards on its south side,which can reach its northwest.Such a series of high PV eddies are transported to the west of the SAH continuously,which is the main source of PV anomalies in the upper troposphere over the Arabian Sea from late spring to early summer.A cyclonic curvature on the southwest of the SAH associated with increasing divergence,which forms a strong upper tropospheric pumping,is generated by the anomalous positive PV over the Arabian Sea on 355 K.The cyclone in the lower troposphere moves northwards from low latitudes of the Arabian Sea,and the upper-layer high PV extends downwards and southwards.Baroclinic development thus occurs and the tropical low-pressure system develops into an explosive vortex of the ISM,which leads to the onset of the ISM.In addition,evolution of subtropical anticyclone over the Arabian Peninsula is another important factor contributing to the onset of the ISM.Before the onset,the surface sensible heating on the Arabian Peninsula is very strong.Consequently the subtropical anticyclone which dominated the Arabian SeBased on the theory of potential vorticity （PV）, the unstable development of the South Asia High （SAH） due to diabatic heat ing and its impacts on the Indian Summer Monsoon （ISM） onset are studied via a case diagnosis of 1998. The Indian Summer Monsoon onset in 1998 is related to the rapidly strengthening and northward moving of a tropical cyclone originally located in the south of Arabian Sea. It is demonstrated that the rapid enhancement of the cyclone is a consequence of a baroclinic devel opment characterized by the phaselock of high PV systems in the upper and lower troposphere. Both the intensification of the SAH and the development of the zonal asymmetric PV forcing are forced by the rapidly increasing latent heat released from the heavy rainfall in East Asia and South East Asia after the onsets of the Bay of Bengal （BOB） monsoon and the South China Sea （SCS） monsoon. High PV moves southwards along the intensified northerlies on the eastern side of the SAH and travels westwards on its south side, which can reach its northwest. Such a series of high PV eddies are transported to the west of the SAH continuously, which is the main source of PV anomalies in the upper troposphere over the Arabian Sea from late spring to early summer. A cyclonic curvature on the southwest of the SAH associated with increasing divergence, which forms a strong upper tropospheric pumping, is generated by the anomalous positive PV over the Arabian Sea on 355 K. The cyclone in the lower troposphere moves northwards from low latitudes of the Arabian Sea, and the upperlayer high PV extends down wards and southwards. Baroclinic development thus occurs and the tropical lowpressure system develops into an explosive vortex of the ISM, which leads to the onset of the ISM. In addition, evolution of subtropical anticyclone over the Arabian Pen insula is another important factor contributing to the onset of the ISM. Before the onset, the surface sensible heating on the Arabian Peninsula is very strong. Consequently the subt

关 键 词：potential vorticity South Asian High diabatic heating zonal asymmetric instability summer monsoon onset 

分 类 号：P425.42[天文地球—大气科学及气象学] P532