Changes in Western Pacific Tropical Cyclones Associated with the El Nino-Southern Oscillation Cycle

This study examines the interannual variability of three groups of tropical cyclones (TCs)-super typhoons (STYs), typhoons (TYs), and tropical storms and tropical depressions (TSTDs)-and their relationship with the El Nino-Southern Oscillation (ENSO). Both wavelet analysis and correlation studies of upper-ocean heat content reveal significant differences for the three types of TCs. In particular, an increase (decrease) in the frequency of STYs is usually associated with the mature phase of El Nino (La Nina) events, while the converse is true for TSTDs. In contrast, the frequency of TYs increases (decreases) during the transition period from La Nina to El Nino (El Nino to La Nina) events. The results suggest that the timing with which ENSO impacts STYs, TYs, and TSTDs varies and that their corresponding changes in frequency closely follow the evolution of the ENSO cycle. Empirical orthogonal function analysis is also conducted to investigate the impact of different environmental factors influenced by ENSO on TCs. The vertical wind shear and moist static energy associated with ENSO are identified as the dominant factors that control the frequency of STYs. In comparison, the frequency of TYs is found to be closely related to the relative vorticity and vertical wind shear associated with both the transition phase of ENSO and with other types of climate variability.