Generation of equatorial Atlantic warm and cold events in a coupled general circulation model simulation

Warm and cold events in the Gulf of Guinea, characteristic of the tropical interannual variability, can be generated in several ways. This emerges from a statistical analysis of 200 years of interannual variability simulated by a coupled ocean–atmosphere General Circulation Model. The application of a clustering technique to the anomalies of the thermal energy stored in theupper oceanic layers leads to the separation of the events, either warm or cold, into a numberof classes, each of them distinguished by a particular generation scheme. The physical mechanisms involved are identified by examining the contributions of the various terms in the mixed layer tendency equation.

Basically, those few classes can be sorted into two larger groups. In one of them, the onsetstage is characterized by an eastward propagation of the anomalies. Atmospheric flows play aleading part in the generation of events within this group. In the second group, thermal energy anomalies are generated in situ in the Gulf of Guinea, and it is mixing that gives the most important contribution to the development of the events.

The different classes are related to different seasonal signatures and also to differences in the influence of the El Nino-Southern Oscillation (ENSO). While some of the classes are strongly influenced by ENSO, while in others this influence is not significant. This can explain the barely significant correlations between ENSO and the Atlantic warm events, a feature that the simulation analyzed here shares with the observations.