| The structure and variability of the Intertropical Convergence Zone (ITCZ) of the western North Pacific were examined for May–December 1989–1993. To document the climatology of the ITCZ structure, monthly composites centered on the locations of the monsoon trough (MT) and trade wind trough (TWT) axes were constructed. The mean ITCZ was located near the northern edge of the warmest water rather than on the axis of maximum sea-surface temperature (SST). Low-level relative vorticity was at least three times greater in the MT than the TWT. Greater convective activity and lower mean sea level pressure (MSLP) also occurred in the MT. The MT axis corresponded well with the minimum MSLP line, while the TWT was located on the southern edge of the meridional MSLP gradient. Areas of convective activity, low-level convergence, upper-level divergence, and upward motion were generally larger and less organized for the MT than for the TWT.; The higher-frequency variations of the ITCZ were examined through a time series analysis. Expected relationships between SST, convection and its associated circulations appeared in lagged cross-correlations. Links between convection/MSLP and low-level relative vorticity were significant and consistent in the MT and entire ITCZ, while they were weak and inconsistent in the TWT. Hypotheses concerning these differences between the MT and TWT as well as the various cycles of wind, vorticity, MSLP, and convection were proposed.; Relationships between time series for each variable and tropical cyclogenesis were examined. For the whole ITCZ, genesis was associated with increases in low-level relative vorticity, decreases in upper-level relative vorticity, increases in upper-level divergence, enhanced convective activity and increases in mid-level upward motion. It was hypothesized that decreases in MSLP associated with genesis occurred on a smaller scale than the ITCZ. For the MT, genesis was associated with increases in low-level vorticity and divergence, increases in upper-level divergence, decreases in MSLP, enhanced convection, and increases in mid-level upward motion. Genesis was less likely to occur if the upper-level divergence decreased, the MSLP increased, or the mid-level upward motion decreased. It is important to note that these changes occurred on a scale much larger than a tropical cyclone. |
