Interdecadal increase of tropical cyclone genesis frequency over the western north Pacific in May

The interdecadal changes in the frequency of tropical cyclone (TC) genesis in the western North Pacific (WNP) were examined for the epoch 1979-2013. There was an interdecadal increase in the frequency of TC genesis in the WNP in May during 1999-2013 than during 1979-1998, and such changes differed from those occurring between June and December. This interdecadal change is attributed to the early onset of the South China Sea summer monsoon (SCSSM) during 1999-2013, when the southwesterly winds spread eastwards and the monsoon trough (MT) moved into the WNP earlier. The MT favoured enhanced convection and anomalous low-level cyclonic vorticity, facilitating more TC genesis. During 1979-1998, in contrast, the onset of the SCSSM was delayed and the MT was confined to the west of the South China Sea. As a result, weakened convection and anomalous anticyclonic low-level vorticity are observed over the WNP, facilitating less TC genesis.