Possible Linkage Between Tropical Indian Ocean SST Anomalies and the Date of First and Last Tropical Cyclones Landfalling in the Chinese Mainland

Bases on the NCEP / NCAR reanalysis products, HadISST dataset, and data of tropical cyclone (TC) landfalling in the Chinese mainland during 1960-2019, the possible impacts of Indian Ocean Dipole (IOD) mode and Indian Ocean basin (IOB) mode on the last-TC-landfall date (LLD) and first-TC-landfall date (FLD), respectively, are investigated in this study. The LLD is in significantly negative correlation with autumn IOD on the interannual time-scale and their association is independent of El Nino-Southern Oscillation (ENSO). The LLD tends to be earlier when the IOD is positive while becomes later when the IOD is negative. An anomalous lower-level anticyclone is located around the Philippines during October-November, resulting from the change of Walker circulation over the tropical Indo-west Pacific Ocean forced by sea surface temperature (SST) anomalies related to a positive IOD event. The Philippines anticyclone anomaly suppresses TC formation there and prevents TCs from landfalling in the Chinese mainland due to the anomalous westerly steering flows over southeast China during October-November, agreeing well with the earlier LLD. However, the robust connection between spring IOB and FLD depends on ENSO episodes in the preceding winter. There is an anticyclonic anomaly around the Philippines caused by the tropical SST anomalies through modulating the Walker circulation during May-June when the IOB is warming in the El Nino decaying phase. Correspondingly, the TC genesis is less frequent near the Philippines and the mid-level steering flows associated with the expanded western Pacific subtropical high are disadvantageous for TCs moving towards southeast China and making landfall during May-June, in accordance with the later FLD. By contrast, cooling IOB condition in spring of a La Nina decaying year and negative IOD cases during autumn could produce a completely reversed atmospheric circulation response, leading to an earlier FLD and a later LLD over the Chinese mainland, respectively.