Recent Weakening in Interannual Variability of Mean Tropical Cyclogenesis Latitude over the Western North Pacific during Boreal Summer

The intensity of the interannual variability (IIV) of the mean tropical cyclone (TC) genesis latitude over the western North Pacific (WNP) has been weakening significantly since the late 1990s. It is found that the IIV of the mean TC genesis latitude depends largely on the strength of the out-of-phase relationship between TC genesis numbers in the north (north of 15°N) and south (south of 15°N) of the WNP. A weaker (stronger) north–south TC see-saw has led to a smaller (larger) IIV of the mean TC genesis latitude after (before) the late 1990s. Different configurations of sea surface temperature (SST) anomalies are found to be responsible for the decadal changes in the north–south TC see-saw and dipole structure. Before the late 1990s, the joint effect of SST anomalies over the tropical Pacific and tropical North Indian Ocean dominated, rendering the obvious north–south TC see-saw and larger IIV of the mean TC genesis latitude. After the late 1990s, however, the dominant SST anomalies associated with TC genesis shift to the tropical central Pacific (CP) and tropical North Atlantic Ocean, which have weakened the north–south TC see-saw and reduced the IIV of the mean TC genesis latitude. These observed decadal changes in the configuration of SST anomalies are considered to be closely associated with the shift of the El Niño–Southern Oscillation (ENSO) from eastern Pacific (EP) type to the CP type during the recent decades. The results suggest that the increased influences from the tropical Atlantic Ocean have become more important to the variations of TC activity in the WNP during the recent decades. These results may have important implications for assessing the latitudinal distributions of TC-induced hazards.