Deconstructing Atlantic ITCZ variability: influence from the
local cross-equatorial SST gradient, and remote forcing
from the eastern equatorial Pacific


We investigate causes of interannual variability in monthly mean convection associated with the Atlantic Intertropical Convergence Zone (ITCZ) using a global precipitation dataset spanning 1979-1999. Starting from the hypothesis of two dominant influences on the Atlantic ITCZ, namely the cross-equatorial gradient in tropical Atlantic sea surface temperature (SST), and the anomalous Walker circulation due to the rearrangement of tropical Pacific convection associated with the El Niņo-Southern Oscillation, we analyze anomaly composites over the 1979-1999 period that best isolate the effects of each mechanism. Our results suggest that both the anomalous Walker circulation and SST gradient influences act substantively on Atlantic convection, and in different ways. To first order, the anomalous Walker circulation acts to suppress precipitation over the tropical Atlantic, and anomalous SST gradient positions the meridional location of maximum ITCZ convection. We also examine the processes underlying each of the two mechanisms. For the anomalous Walker circulation, we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial Pacific. For the SST gradient mechanism, our results confirm previous studies that link convection to cross-equatorial winds forced by meridional SST gradients. Based on the results of this and other studies, we propose a more complete physical picture that explains key features of Atlantic ITCZ variability, including its seasonal preference, its sensitivity to small anomalous SST gradients, and its role in the context of tropical Atlantic SST gradient variability.

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