The Elements of Climate Variability in the Tropical Atlantic Region

The tropical Atlantic region, unlike the tropical Pacific, is not dominated by any single mode of climate variability such as the El Niño–Southern Oscillation (ENSO). Rather, this region is subject to multiple competing influences of comparable importance. The nature and potential predictability of these various influences has been investigated by analysis of an ensemble of atmospheric GCM integrations forced with observed SST for the period December 1948–November 1993. The dominant modes of internal atmospheric and SST-forced variability are determined. Internal variability in the tropical Atlantic region is dominated by the equatorward extension of extratropical patterns, especially the North Atlantic oscillation. Three different SST-forced signals are identified. These are (a) a remote response to ENSO, (b) a response to the so-called Atlantic Dipole SST pattern, and (c) a response to equatorial Atlantic SST anomalies. The spatial structure and seasonality of these different elements of climate variability are diagnosed and feedbacks onto the ocean are assessed. The evidence presented supports the possibility of ENSO-like variability in the equatorial Atlantic, but does not support the suggestion that the Atlantic Dipole is a coupled ocean–atmosphere mode of variability. An important feature of this study is that the results include quantitative estimates of the comparative importance, in different regions and different seasons, of the various influences on tropical Atlantic climate variability. These estimates are used to assess the potential predictability of key climatic indices.