Deconstructing Atlantic ITCZ variability: influence from the
local cross-equatorial
SST gradient, and remote forcing
from the eastern equatorial Pacific
Abstract
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.
Send preprint requests to jchiang@ldeo.columbia.edu