18th Conference on Climate Variability and Change


Dynamics of the Eastern Pacific Intertropical Convergence Zone

Violeta E. Toma, Georgia Institute of Technology, Atlanta, GA; and P. J. Webster

The location of the Inter-Tropical Convergence Zone (ITCZ) is a persistent and important feature of the tropics. Yet, despite its importance, the basic physics that determine its location and variability are not fully understood. To investigate the ITCZ more thoroughly, we examine the new ECMWF reanalysis (ERA 40) data set concentrating on the Eastern equatorial Pacific Ocean where a strong climatological ITCZ exists. We perform composite analyses to study the structure of the summer meridional circulation. In this region we find that in a climatological sense, the zone of maximum convection and precipitation is not collocated with the highest sea surface temperature (SST), or correspondingly, the lowest sea level pressure We test the Tomas and Webster (1997) hypothesis that if a strong enough cross-equatorial pressure gradient exists, the system may be inertially unstable and a secondary ameliorating meridional circulation produces cyclonic vorticity to compensate for the advection across the equator of anticyclonic vorticity. The location of convection occurs where the poleward advection of anticyclonic vorticity is balanced by the generation of cyclonic vorticity. The secondary circulation occurring in a conditionally unstable environment produces forced ascent and deep convection. The intensity of the convection appears to be determined by the magnitude of the cross-equatorial pressure gradient. The secondary meridional cell is found to have a complex structure. Besides occupying the entire troposphere, the mean cell shows a second return flow in the middle to low troposphere. We interpret the appearance of the complex structure as a reflection of the two states of the atmosphere with the deep meridional cell associated with the disturbed state of the ITCZ and the shallow cell associate with its undisturbed state.

Poster Session 2, Observed seasonal to interannual climate variability and climate applications
Wednesday, 1 February 2006, 2:30 PM-4:00 PM, Exhibit Hall A2

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