Interdecadal change in the relationship between the Pacific Meridional Mode and landfalling tropical cyclone frequency in China

Interannual relationship between the Pacific Meridional Mode (PMM) and landfalling tropical cyclone (TC) frequency in China (LTCFC) exhibits an interdecadal enhancement since the late-1980s, Epochs I (E1, 1951-1985) and II (E2, 1987-2019) are therefore defined to unravel possible mechanisms. Strengthened low-level vorticity and weakened vertical wind shear always favor TC genesis in the western North Pacific (WNP) main development region (MDR) in positive PMM (PPMM) years of two epochs. During PPMM years of E1, many TCs recurve northeastward due to anomalous westerly steering flow over the MDR while only handful TCs move northwestward and land over China due to climatological mean southeasterly steering flow. In PPMM years of E2, prevailing easterly steering flow anomalies on the MDR's northern edge cause more TCs to move westward/northwestward and make landfall over China. Therefore, LTCFC is insignificantly (significantly) correlated with PMM during E1 (E2) due to the interdecadal change of steering flow, which is linked to different sea surface temperature (SST) patterns of PMM during two epochs. An anomalous WNP cyclone can be induced during PPMM years of both epochs. A northwestward shift of PPMM-associated positive SST anomalies causes a large zonal SST gradient in the off-equatorial WNP which induces westerlies during E1 while the zonal SST gradient is weak during E2. Thus, westerlies on the southern edge of the anomalous cyclone extend northward, leading to a northward shift of the anomalous cyclone during E1. The distinct responses of two PMM patterns can be well reproduced by the Coupled Model Intercomparison Project Phase 6 models.