Rainfall Variability in the Tropical Atlantic region

Seasonal and interannual variability of rainfall in the tropical Atlantic are quantified using a 25-year (1979-2003) monthly rainfall dataset from the Global Precipitation Climatology Project (GPCP). Evident seasonal cycles exist in both the intensity and latitudinal positions of the ITCZ measured by monthly-mean rainfall between 13.75W-36.25W, and also in the basin-mean rainfall (16.25S-21.25N, 13.75W-36.25W). The ITCZ becomes strongest as it moves to its northernmost latitude (about 8N-10N) during July-September in which most basin-mean rainfall appears; it becomes weakest during January-February along with the least total rainfall in the tropical Atlantic basin. In contrast, rainfall variability on the interannual time scale shows a quite different pattern. Most intense interannual variability of the ITCZ and basin-mean rainfall appears during March-April when the ITCZ reaches its southernmost location. Furthermore, the anomalies of the ITCZ strength and the basin-mean rainfall are significantly negative-correlated with the latitude anomalies of the ITCZ during boreal spring and early summer.

Rainfall interannual variability in the tropical Atlantic are thus further accounted for by examining the relationships between rainfall and sea surface temperature (SST) anomalies in both the tropical Atlantic and Pacific oceans. The ITCZ strength and basin-mean rainfall are significantly modulated by the El Niño and the Atlantic equatorial mode particularly during boreal spring and summer; whereas the impact of the SST gradient mode in the Atlantic is much weaker. Regarding the latitudinal anomalies of the ITCZ, the influences seem to come from both the local SST forcing, i.e., the Atlantic SST gradient and equatorial modes, and remote forcing, i.e., the El Niño. The direct influence of the El Niño, however, can only be found during April-July, though the SST in the north tropical Atlantic may be modulated by the El Niño during boreal spring. Detailed impacts of the El Niño and the Atlantic equatorial mode are further explored by a composite analysis.