The 6-year (1998-2003) high-quality rainfall products from the Tropical Rainfall Measuring Mission (TRMM) are applied to quantify the InterTropical Convergence Zone (ITCZ) in the eastern Pacific (about 90W - 120W) during the boreal spring. Except in 1998 and 2003, the double ITCZs regularly appear during March-April; more interestingly, the southern ITCZ in March is somewhat stronger than its northern counterpart. Major convective events on the weekly time scale may entirely shift into the Southern Hemisphere during a certain time period. In contrast, only one convective zone occurs right across the equator in 1998 and around 5N in 2003, manifesting the impact of the El Nino event and the abnormally cold equatorial SST, respectively. The double ITCZs appear to be closely related to the spatial and temporal variation of SST, more specifically to its related meridional gradient and then the forced surface convergence. The magnitudes of the SST gradients south and north of the equator seem to be the dominant factors regulating the relative intensity of the two corresponding ITCZs. However, the ITCZ-related convection and rainfall do not constantly follow surface convergence field. Two convergence zones derived from the QuickScat wind field south and north of the equator are far more frequently observed than their corresponding rainfall peaks. Water vapor is thus shown to be another critical player for the double ITCZs defined by convection and rainfall.

Based on a 24-year rainfall dataset from the Global Precipitation Climatology Project (GPCP), three ITCZ types in March are clarified, corresponding to three distinct SST meridional profiles. These ITCZ types are consistent with the meridional distribution of the large-scale vertical motion and seem to have an impact on the regional meridional circulations.