Deconstructing Atlantic Intertropical Convergence Zone variability: Influence of the local cross‐equatorial sea surface temperature gradient and remote forcing from the eastern equatorial Pacific

Abstract

We investigate causes of interannual variability in Atlantic Intertropical Convergence Zone (ITCZ) convection using a monthly mean global precipitation data set spanning 1979–1999. Starting from the hypothesis of two dominant influences on the 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 Niño–Southern Oscillation, we analyze anomaly composites over the 1979–1999 period that best isolate the effects of each mechanism. Our results suggest that to first order, a strong anomalous Walker circulation suppresses precipitation over the tropical Atlantic, whereas an anomalous warm north/cool south SST gradient shifts the meridional location of maximum ITCZ convection anomalously north. We examined 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. We find that positive surface flux feedback brought about through the cross‐equatorial winds is weak and confined to the deep tropics. On the basis of the results of this and other studies we propose an expanded 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.