Changes in Western Pacific tropical cyclones associated with the El Nino-Southern Oscillation cycle

The present study examines the interannual variability of the three groups of TC – the intense super typhoon (STY), moderate strong typhoon (TY) and weak tropical storm and tropical depression (TSTD) and their relationship with the El Niño-Southern Oscillation (ENSO). Both the wavelet analysis and correlation studies with the upper ocean heat content (OHC) reveal significant differences for the three groups of TC. In particular, an increase (decrease) in STY frequency is usually associated with the mature phase of El Niño (La Niña) events while the opposite is true for TSTD. The TY frequency, on the other hand, shows an increase (decrease) during the transition period from La Niña to El Niño (El Niño to La Niña) events. The results suggest that the timing at which ENSO impacts on STY, TY and TSTD is indeed different and the corresponding change in frequency closely follows the evolution of an ENSO cycle.

Empirical orthogonal function (EOF) analysis is also conducted to investigate the different impact of environmental factors induced by ENSO on TC. The vertical wind shear and moist static energy induced by the ENSO are identified as the dominant factors in controlling the STY frequency. On the other hand, the TY number is found to be related mostly to relative vorticity and vertical wind shear associating with both the ENSO transition phase as well as with other climate variability.