Title: Interannual and intraseasonal variability of tropical cyclones in the Western North Pacific
Authors: Li, Cheuk Yin (李卓然)
Abstract: This study investigates the variabilitiy of tropical cyclones (TCs) over the Western North Pacific (WNP) at interannual and intraseasonal time scales. Interannually, the El Niño-Southern Oscillation (ENSO) plays a dominant role in affecting the genesis frequency of different groups of TCs in the WNP. An increase (decrease) in the frequency of super typhoons (STYs) is usually associated with the mature phase of El Niño (La Niña) events, while the converse is true for tropical storms and depressions (TSTDs). In contrast, the frequency of typhoon (TYs) increases (decreases) during the transition period from La Niña to El Niño (El Niño to La Niña) 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. By examining different environmental parameters, it is found that the vertical wind shear and moist static energy associated with ENSO are the key factors in controlling 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 climatic variability.
At an intraseasonal time scale, TC activities in the WNP are found to be significantly modulated by the two major components of the ISO, which comprise of both the 30-60 day Madden-Julian Oscillation (MJO) and the 10-20 day quasi-biweekly oscillation (QBWO). These two ISO modes exhibit different origins, spatial scales, and propagation characteristics, which result in distinctive TC modulation in the WNP. The northeastward-propagating MJO predominantly controls the basin-wide 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 (120°-150°E, 0°-30°N) and WNP2 (150°-180°E, 0°-30°N) 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. Apart from cyclogenesis, the prevailing tracks and landfalls of TCs are also modulated distinctively by these two types of ISO.
Besides, the coupled influences of ENSO and MJO on TCs are also studied. Results reveal a stronger modulation of cyclogenesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other. The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation under different ENSO conditions. During El Niño years, MJO activity is intensified and extends farther eastward, instead of being confined west of 150°E in neutral and La Niña periods. Thus, the influence of MJO is stronger and more widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. On the contrary, the westward-confined MJO signals in neutral and La Niña years lead to changes in TC-related parameters mainly in the western region, which contribute to the comparatively weaker TC modulations.
Apart from ENSO and ISO, the Pacific-Japan (PJ) teleconnection pattern also affects TC and synoptic-scale variability (SSV) in the WNP. With dominant peaks at 10-20 days and 20-50 days, the positive (negative) PJ pattern results in strengthened (weakened) SSV and TC activity over the WNP. Both observational and numerical
studies suggest that thermodynamic parameters of the planetary boundary layer (PBL) play a determining role in controlling the development of synoptic disturbances in the WNP. The increase (decrease) in PBL moisture during positive (negative) PJ phases enhance (suppress) perturbation moisture convergence and thus convective heating associated with SSV, leading to strengthened (weakened) synoptic-scale activity and a significant increase (decrease) in TC frequency in the WNP. Besides, it is further noticed that the synoptic eddies can in turn regulate the intraseasonal PJ-related precipitation by exerting an upscale feedback through interacting with the background mean flow, contributing to about 10-15% of the total intraseasonal PJ-related precipitation anomalies.
As for the Okhotsk high (OH), it exhibits a dominant intraseasonal peak at 10-50 day. An increase (decrease) in intraseasonal OH activity induces an anomalous cyclone (anticyclone) over southeast Japan through a Eurasia-Japan Rossby wave train. The weakened (strengthened) geopotential height, together with strengthened (weakened) PBL moisture, vorticity and rising motion, in the subtropical region lead to an overall increase (decrease) in SSV during positive (negative) OH phases. Compared to the PJ pattern, it is found that the changes in cyclogenesis frequency are not as obvious, which can be attributed to the locations of the OH-induced circulation anomalies. Nonetheless, the prevailing TC tracks do vary significantly due to the alternations in Western North Pacific subtropical high (WNPSH) during different OH phases. The weakened (strengthened) and westward-retreating (eastward-extending) WNPSH leads to an increase (decrease) in recurving TC activity over East Asia, Korea and Japan during positive (negative) OH events.
Notes: CityU Call Number: QC959.N67 L45 2013; xxi, 194 p. : col. ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2013.; Includes bibliographical references (p. 181-192)
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