The eastern Pacific intertropical convergence zone (ITCZ) generally prefers to stay north of the equator. On the other hand, its southern-hemispheric counterpart is often observed in March and April. While the origins of the meridional asymmetry of the eastern Pacific ITCZ have been extensively studied, it is less understood why the double ITCZ appears in austral fall and is absent in the other equinoctial season.

This study investigates the eastern Pacific double ITCZ based on an 8-year (2000-2007) record of satellite data. Tropical Rainfall Measuring Mission (TRMM) measurements are used to analyze convective variability, SST, and column water vapor. QuikSCAT SeaWinds measurements are employed to investigate near-surface wind and Ekman upwelling velocity. The analysis result indicates that a warm spot emerges off the Ecuadorian coast (near 90W and 5S) in December, followed by the continuous westward growth of a warm band eventually leading to the southern ITCZ. Upwelling and near-surface wind show no apparent sign that accounts for the initial warm spot. Column water vapor, in contrast, exhibits a sharp increase off the Ecuadorian coast even earlier than the warm spot starts to develop. This moistening results from the southward bulge of the northern ITCZ at its eastern end. The eastern Pacific double ITCZ is visible in February in the shallow cumulus distribution, almost a month earlier than the double ITCZ becomes recognizable in deep convection. A possible mechanism exlaining the present findings will be also discussed.