The Intertropical Convergence Zone in Tropical Atlantic Climate Variability
John C H Chiang (jchiang@atmos.washington.edu)

The dominant interannual variability in tropical Atlantic precipitation is the variation in the southward excursion of the Intertropical Convergence Zone (ITCZ) during boreal spring. This variability is intimately related to variations in the meridional gradient of tropical Atlantic near-equatorial sea surface temperature (SST), and to the El Niņo-Southern Oscillation (ENSO). Understanding how the Atlantic ITCZ responds to these influences is central to Tropical Atlantic climate variability, and with practical implications for predicting precipitation variability over neighboring land regions. I focus on specific questions relating to the Atlantic ITCZ within this larger context.

I investigate near-surface winds over tropical oceans that are important for understanding how the atmosphere responds to, and influences, the tropical ocean. A data and simple model study of frictional characteristics in near-surface wind shows that its anisotropy in the zonal and meridional directions is related to wind conditions at the top of the planetary boundary layer (PBL), as well as variations in PBL depth. A diagnostic model study of the relative roles of elevated heating and SST gradients in driving anomalous tropical ocean near-surface winds reveals that both forcings play significant and different roles, especially near the equator.

I investigate the response of the Atlantic ITCZ to local and remote forcing. Evidence is shown to support the hypothesis of ENSO influence on the Atlantic ITCZ through an anomalous Walker circulation resulting from re-organization of tropical Pacific convection. This mechanism is able to explain an apparent interdecadal modulation in ENSO's influence on northeast Brazil rainfall. Atlantic ITCZ interannual variability is further investigated using a global precipitation dataset, assuming the anomalous Walker circulation and the meridional gradient in tropical Atlantic SST as the two dominant influences. To first order, the Walker mechanism is found to suppress near-equatorial Atlantic rainfall, while the SST gradient is found to meridionally position the maximum ITCZ precipitation. A mechanistic picture of Atlantic ITCZ variability is proposed that explains its key features of seasonal preference, sensitivity to small anomalous SST gradients, and relationship to tropical Atlantic SST gradient variability.