The Role of the Local Hadley Circulation over the Western Pacificon the Zonally Asymmetric Anomalies over the Indian Ocean

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The onset processes of the zonally asymmetric anomalies of convection and sea surface temperature(SST) over the tropical Indian Ocean are investigated with considering seasonal evolution, and interannualvariability, of the large-scale convection anomalies in the Asian summer monsoon, using outgoinglongwave radiation (OLR), SST, and NCEP/NCAR reanalysis data. This asymmetric pattern ofthe convection anomalies is particularly dominated in boreal autumn. Some recent studies have notedthat these anomalies, based on the atmosphere-ocean coupling phenomenon, can be developed and maintainedby itself.
The time evolution shows that the eastern part of the zonally asymmetric anomalies over the IndianOcean lead the western part of those. In July, the negative SST anomalies and positive OLR anomaliesfirst appeared off the Sumatra coast, and southeasterly wind anomaly accelerated the climatologicalsoutheasterly wind along the west coast of Sumatra. This southeasterly wind acceleration provide a SSTcooling over the southeastern Indian Ocean, and play a role in triggering of the zonally asymmetricanomalies in the following autumn. It is suggested that this southeasterly wind acceleration over thesoutheastern Indian Ocean is closely linked to the meridionally asymmetric anomalies of convection,between the maritime continent and the South China Sea/ Philippine Sea (SCS/PS). That is, the intensificationof the local Hadley circulation over the western Pacific associated with the enhanced convectionover the SCS/PS, and suppressed convection over the maritime continent, is found to be a clearprecursory signal of the zonally asymmetric anomalies over the Indian Ocean. It has also been notedthat the convection anomalies over the southern and northern parts of the meridionally asymmetricanomalies over the western Pacific are not always the opposite sign, and seem to have different interannualvariability respectively. It is likely that the former might be strongly influenced by the ENSO,through the Walker circulation anomalies and the latter might be affected by the modulation of the intraseasonalvariation of the Asian summer monsoon. The seasonality of the zonally asymmetric anomaliesis also suggested from the occurrence of the intensification of the local Hadley circulation in borealsummer.