Interannual Variation of the Tropical Cyclone Activity over the Western North Pacific

Abstract

An effort was made to search for relationships between interannual variations of population, lifetime, genesis locations, and intensity of named typhoons and numbered tropical depressions in the western North Pacific during the 1979-2002 period. To support this research task, climatological relationships of tropical cyclone characteristics were also investigated for these cyclones. Major findings of this study are summarized as follows:1) Climatology: Measured by the intensity scale of the Japan Meteorological Agency, three groups of tropical cyclones were identified in terms of population versus intensity: Group 1 [tropical depression (TD) ϩ typhoon (TY)], Group 2 (strong ϩ very strong TY), and Group 3 (catastrophic TY). This group division coincides with that formed in terms of lifetime of tropical cyclones versus intensity. Weak cyclones (Group 1) have a larger population than strong cyclones (Group 3), while the former group has shorter lifetime than the latter group. For genesis locations, the monsoon trough is established as a favorable region of tropical cyclone genesis because it provides an environment of large vorticity. Therefore, the northward latitudinal displacement of the maximum genesis frequency in the three groups of tropical cyclones follows that of the monsoon trough. 2) Interannual variation: Any mechanism that can modulate the location and intensity of the monsoon trough affects the genesis location and frequency of tropical cyclones. In response to tropical Pacific sea surface temperature anomalies, a short wave train consisting of east-west oriented cells emanates from the Tropics and progresses along the western North Pacific rim. Population of the Group-1 tropical cyclones varies interannually in phase with the oscillation of the anomalous circulation cell northeast of Taiwan and south of Japan in this short wave train, while that of Group 3 fluctuates coherently with the tropical cell of this short wave train. Because these two anomalous circulation cells exhibit opposite polarity, the out-of-phase interannual oscillation between these two cells results in the opposite interannual variation of genesis frequency between tropical cyclones of Groups 1 and 3.

Contrasting Various Metrics for Measuring Tropical Cyclone Activity

“…Fig. 4b), however, manifests a very different picture, featuring a much more widespread TC activity pattern due to the inclusion of TC duration and intensity within and are consistent with the findings of Chen et al (2006). The ACE pattern (see Fig.…”

“…5a), positive anomalies dominate the southeastern part of the WNP; while negative anomalies dictate the northwestern part. This implies that the major birthplace of TC would be shifted southeastward during an El Niño year (Chen et al 1998;Chia and Ropelewski 2002;Chen et al 2006). For TCD, ACE, RACE, PDI and RPDI anomalies (see Figs.…”

“…In calculating TCD, each TC record accounts for a duration period of 6 hours. Since a stronger TC system also tends to last longer than a weaker one (Camargo and Sobel 2005;Chen et al 2006), the contribution from TC intensity is implicitly included in TCD as well.…”

Influence of Tropical Indian Ocean Warming and ENSO on Tropical Cyclone Activity over the Western North Pacific

“…Previous studies also noticed that the interannual changes of TC activity in the WNP are associated with a longitudinal shift in the upward and downward branches of the Walker Circulation during the warm phase of ENSO (e.g., Chan 1985;Wu and Lau 1992). Lander (1994) and Chen et al (1998) attributed the eastward shift of the genesis region to an eastward extension of the monsoon trough in the WNP since most TCs form in the shear zone between the monsoonal westerlies and the trade easterlies (Sadler 1967;Ritchie and Holland 1999). Most of the previous studies on interannual variability of the WNP TCs have focused on the El Niño developing years.…”

“…The interannual variations of tropical cyclone (TC) activity in the western North Pacific (WNP) have been extensively investigated in the past 2-3 decades (Chan 1985(Chan , 2000Lander 1994;Chen et al 1998;Wang and Chan 2002;Chia and Ropelewski 2002;Chu 2004;Camargo and Sobel 2005;Camargo et al 2007). Chan (1985) first examined the relationship between the WNP TC activity and the El Niño-Southern Oscillation (ENSO) and found that the spectra of both TC annual counts and ENSO Index peak in the 3-3.5 year frequency band.…”