Tropical Atlantic Variability: Patterns, Mechanisms, and Impacts

This chapter reviews the progress made in the past decade in understanding tropical Atlantic climate variability. In addition to an equatorially anti-symmetric seasonal cycle forced directly by the seasonal march of the sun, Atlantic sea surface temperature (SST) displays a pronounced annual cycle on the equator that results from continental monsoon forcing and air-sea interaction. This cycle interacts with and regulates the meridional excursions of the Atlantic intertropical convergence zone (ITCZ). On interannual timescales, there is an equatorial mode of variability that is similar to El Nino/Southern Oscillation (ENSO) in the Pacific. This Atlantic Nino is most pronounced in boreal summer coinciding with the seasonal development of the equatorial cold tongue. In boreal winter, both ENSO and the North Atlantic Oscillation exert a strong influence on the northeasterly trades and SST over the northern tropical Atlantic. In boreal spring when the equatorial Atlantic is uniformly warm, anomalies of the cross-equatorial SST gradient and the ITCZ are closely coupled, resulting in anomalous rainfall over northeastern Brazil. There is evidence for a positive air-sea feedback through windinduced surface evaporation that organizes off-equatorial SST anomalies to maximize their cross-equatorial gradient. The resultant anomalous shift of the ITCZ may affect the North Atlantic Oscillation, helping to organize ocean-atmospheric anomalies into a panAtlantic pattern.