Modulation of Western North Pacific Tropical Cyclone Activity by the ISO. Part I: Genesis and Intensity

This study investigates the intraseasonal variability of tropical cyclones (TCs) by systematically examining the two major components of the intraseasonal oscillation (ISO), the 30-60-day Madden-Julian oscillation (MJO) and the 10-20-day quasi-biweekly oscillation (QBWO). Results suggest that these two ISO modes exhibit different origins, spatial scales, and propagation characteristics, which result in distinctive TC modulation in the western North Pacific Ocean (WNP). The northeastward-propagating MJO predominantly controls the basinwide TC frequency. The significant increase (reduction) in cyclogenesis in the convective (nonconvective) phase is found to be associated with the concomitant strengthening (weakening) of the monsoon trough. In addition, the large contrast in TC frequency also results in a significant difference in daily accumulated cyclone energy (ACE) between the convective and nonconvective MJO phases. The northwestward-propagating QBWO, in contrast, is characterized by alternating signals of positive and negative convection. It leads to the opposite TC modulation in the WNP1 (0 degrees-308 degrees N, 120 degrees-150 degrees E) and WNP2 (0 degrees-30 degrees N, 150 degrees E-180 degrees) regions and results in a northwestward shift in TC genesis locations, which in turn causes substantial differences in intensity distribution and daily ACE for different QBWO phases. Finally, a brief examination of the dual mode situation suggests that the QBWO generally exerts modulation upon the background MJO, and the modulation seems to vary under different MJO conditions.