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 Nino-Southern Osc illation (ENSO). Rather, this region is subject to multiple competing influ ences of comparable importance. The nature and potential predictability of these various influences has been investigated by analysis of an ensemble o f atmospheric GCM integrations forced with observed SST for the period Dece mber 1948-November 1993. The dominant modes of internal atmospheric and SST-forced variability are d etermined. Internal variability in the tropical Atlantic region is dominate d by the equatorward extension of extratropical patterns, especially the No rth 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 element s of climate variability are diagnosed and feedbacks onto the ocean are ass essed. The evidence presented supports the possibility of ENSO-like variabi lity in the equatorial Atlantic, bur does not support the suggestion that t he 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 differen t seasons, of the various influences on tropical Atlantic climate variabili ty. These estimates are used to assess the potential predictability of key climatic indices.