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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 interannual variability, of the large-scale convection anomalies in the Asian summer monsoon, using outgoing longwave radiation (OLR), SST, and NCEP/NCAR reanalysis data. This asymmetric pattern of the convection anomalies is particularly dominated in boreal autumn. Some recent studies have noted that these anomalies, based on the atmosphere-ocean coupling phenomenon, can be developed and maintained by itself. The time evolution shows that the eastern part of the zonally asymmetric anomalies over the Indian Ocean lead the western part of those. In July, the negative SST anomalies and positive OLR anomalies first appeared off the Sumatra coast, and southeasterly wind anomaly accelerated the climatological southeasterly wind along the west coast of Sumatra. This southeasterly wind acceleration provide a SST cooling over the southeastern Indian Ocean, and play a role in triggering of the zonally asymmetric anomalies in the following autumn. It is suggested that this southeasterly wind acceleration over the southeastern 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 intensification of the local Hadley circulation over the western Pacific associated with the enhanced convection over the SCS/PS, and suppressed convection over the maritime continent, is found to be a clear precursory signal of the zonally asymmetric anomalies over the Indian Ocean. It has also been noted that the convection anomalies over the southern and northern parts of the meridionally asymmetric anomalies over the western Pacific are not always the opposite sign, and seem to have different interannual variability 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 intraseasonal variation of the Asian summer monsoon. The seasonality of the zonally asymmetric anomalies is also suggested from the occurrence of the intensification of the local Hadley circulation in boreal summer.

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