Interannual Variabilities Of The Meridional Circulation Of East Asian Summer Monsoon And The Atmospheric Response To The SSTA In Indian Ocean

Based on the NCEP/NCAR reanalysis data, the Hadley center Global sea-surfacetemperature (SST), and rainfall data at 160 stations in China from 1979 through to 2006, we haveinvestigated the meridional circulation (MC) changes in the East Asian summer monsoon regionon the interannual timescales. Its association with SSTA and precipitation anomalies over themid-and lower reaches of the Changjiang (MiLC) have also been studied. The comparison of theEast Asian summer monsoon MC with the Indian summer monsoon MC has been made forunderstanding the differences between variabilities of ISM and of EASM. Furthermore, theatmospheric response to IOD was simulated with Gill model. The conclusions are as follow:(1)The East Asian summer monsoon MC establishing in May, maturating in August, anddisappearing in October, emerges together with the other monsoon MC that ascends in20°N-30°N and descends around 40°N. This pair of the MCs shifting north-southward withperiodicities 2-4 years, vary with periodicities 2-3 years and 4-6 years in intensity in borealsummer.(2)The more northward (southward) monsoon MC in East Asia, which has someassociations with the IOD and ENSO, is more closely related to the positive (negative) IOD inpreceding autumn and winter and in turn contributes to negative (positive) IOD. As far as thestrength is concerned, the monsoon MC in East Asia is significantly correlated with the monopolemode of the Indian Ocean SST and the SST in the middle and eastern Pacific in the equatorialregion.(3)The anomalous monsoon MC is found to be a significant signal of occurrences of thedrought/flood events during the Meiyu period over the MiLC, more rainfall in the MiLC seems tobe related to stronger MC, while less rainfall to weaker MC (i.e. Hadley circulation) with itsdescending branch at around 30°N.(4)During past 3 decades, the MCs of East Asian and Indian summer monsoons tend to shift southward before mid-1990s and then shift northward after middle of 1990s. The shifts ofthese two circulations are apparently correlated with ENSO and IOD. Besides, there aresignificant differences between the two circulations in their structures, the Indian summermonsoon MC that establishes in May has a bi-cell structure while that of East Asian summermonsoon which set up one month earlier has a single cell structure.(5) The positive IOD SSTA as adiabatic heating forces the tropic atmosphere to change,which induces two low-level cyclones on the north-west and south-west flanks of the heatingregion respectively and an anticyclone in the cooling region that generates the easterlies over theequatorial Indian ocean along with the southwesterly flows near the west edge of Bay of Bengaland tropical western Pacific, and westerlies in the tropical Pacific in favorable for E1 nino.Negative IOD SST forcing induces the circulation changes just with the opposite signature ascompared to the above. This atmospheric response changes when the locations and intensities ofSSTA forcing vary.