Interannual rainfall variability in the tropical Atlantic region

Abstract

Rainfall variability on seasonal and interannual‐to‐interdecadal timescales in the tropical Atlantic is quantified using a 25‐year (1979–2003) monthly rainfall data set from the Global Precipitation Climatology Project. The Intertropical Convergence Zone (ITCZ), measured by monthly rainfall between 15° and 37.5°W, attains its peak as it moves to the northernmost latitude (∼8°–10°N) during July–September, during which the most total rainfall is observed in the tropical Atlantic basin (17.5°S to 22.5°N, 15°–37.5°W); the ITCZ becomes weakest during January–February, with the least total rainfall observed as it moves to the south. In contrast, rainfall variability on interannual‐to‐interdecadal timescales shows a quite different seasonal preference. The most intense interannual variability occurs during March–May, when the ITCZ tends to be near the equator and becomes weaker. The relationships between rainfall anomalies in the tropical Atlantic and three distinct sea surface temperature (SST) modes are further explored on interannual‐to‐interdecadal timescales. The ITCZ strength and total rainfall amount in the tropical Atlantic basin are significantly modulated by the Pacific El Niño and the Atlantic equatorial mode (or Atlantic Niño) particularly during boreal spring and summer, whereas the impact of the Atlantic interhemispheric mode is considerably weaker. Regarding the anomalous latitudes of the ITCZ, the influence could come from both local, i.e., the Atlantic interhemispheric and equatorial modes, and remote forcings, i.e., El Niño; however, a direct impact of El Niño on the latitudes of the ITCZ can only be found during April–July, not in winter and early spring, during which the largest SST anomalies are usually observed in the equatorial Pacific.