Interdecadal Change of Tropical Cyclone Translation Speed during Peak Season in South China Sea: Observed Evidence, Model Results, and Possible Mechanism

Long-term variations in the translation speed of tropical cyclones (TCs) in the South China Sea (SCS) are examined based on five TC datasets from different institutions. TC translation speed during the TC peak season in the SCS shows an evident rhythm of interdecadal change throughout 1977-2020. This interdecadal change in TC translation speed in the SCS can be well reproduced by a newly developed trajectory model. The model results indicate that the interdecadal change in TC translation speed is primarily due to an interdecadal change in the steering flow in the SCS. Such an interde-cadal change in the steering flow is closely related to an east-west shift of the subtropical high in the western North Pacific (WNP) ocean basin, which may be driven by the zonal sea surface temperature (SST) gradient between the north Indian (NI) and WNP ocean basins. A new index of the zonal SST gradient is proposed, which is shown to be effective for indicat-ing the interdecadal change in east-west shift of subtropical high, and thus, the TC translation speed in the SCS.