On the role of cross-equatorial pressure gradients in the development of tropical convection over the Eastern Pacific Ocean: A Modeling Study

Analysis of the ECMWF ERA 40- reanalysis data set show that in the Eastern Pacific Ocean and Atlantic Ocean, the climatological location of ITCZ is permanently in northern latitudes. Moreover for the Eastern Pacific Ocean the zone of maximum convection and precipitation is not collocated with the highest sea surface temperature (SST) or the lowest sea level pressure. In this study, the NCAR MM5 mesoscale model (nonhydrostatic version 3.5.2) is used to investigate the effect of the cross-equatorial pressure gradient on off-equatorial convection in the Eastern Pacific. The model simulates the atmospheric circulation over the eastern Pacific, surface winds in the equatorial regions, and the poleward location of ITCZ precipitation fairly well. A series of experiments are performed to examine the evolution of precipitation in the ITCZ. Various oceanic-atmospheric feedback mechanisms involved in the formation of the ITCZ are examined. The model response to the variations of SST and SST gradient is tested. The distribution of the SST plays a fundamental role in determining the latitudinal location of the ITCZ but indirectly by setting the cross-equatorial pressure gradient. The experiments show that the magnitude of the cross-equatorial pressure difference, so set up, plays an important role in the intensity of convection, and in the absolute vorticity advection equatorward of the į=0 line. Significant SST gradients which result in stronger pressure gradients, generate greater advection of negative absolute vorticity across the equator, with enhancement of convection, north of the į=0 line. Finally we pose a theory for the genesis of easterly waves in the Pacific Ocean.