A Diagnostic Study of the Role of Remote Forcing in Tropical Atlantic Variability

Abstract

This observational study focuses on remote forcing of the dominant pattern of north tropical Atlantic sea surface temperature (SST) anomalies by ENSO and NAO. Based on a spring SST index of the north tropical Atlantic (NTA) SST (5°–25°N), it is shown that almost all NTA–SST extreme events from 1950 to the present can be related to either ENSO or NAO. Since the SST NTA events lag NAO and ENSO events, NTA variability is interpreted as being largely a response to remote NAO or ENSO forcing. The local response of the tropical Atlantic to these external sources—whether it be ENSO or the NAO—is observed to be rather similar: changes in surface winds induce changes in latent heating that, in turn, generate SST anomalies. Once generated, the latter are damped through local air–sea interaction, at a rate estimated to be 10 W m−2 K−1. Experiments with simple models, but driven by observations, strongly suggests that variability on interannual to interdecadal timescales—both time series and spectral signatures—can be largely explained as a result of direct atmospheric forcing, without the need to invoke a significant role for local unstable air–sea interactions or ocean circulation.