3.5
The equatorial Atlantic variability in an Intermediate Coupled Model: Air-sea coupling versus remote ENSO forcing
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The oceanic component of ICM consists in a 6-baroclinic-mode ocean model, in which a mixed layer model is embedded. The atmospheric component is the global general circulation atmospheric model developed at UCLA. This model is shown to simulate reasonably well the relaxation of the North East trades winds and the intensification of the equatorial westerlies in the boreal spring usually consecutive to an El Niño event.
The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air-sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the whole frequency spectrum of the variability cannot be explained solely by ENSO. It is demonstrated that the peak of SST variability in the 1 to 3 year band as observed in the equatorial Atlantic is due to the local air-sea interactions and is not a linear response to ENSO. The analysis of the inter-annual variability in the model indicates that the Atlantic equatorial mode is stable and phase-locked with the seasonal cycle (most active in boreal summer). It is demonstrated that this mode of variability can only be sustained in the presence of noise or remote forcing.
These results are further investigated with a focus on the inter-annual warm event that occurred in the equatorial Atlantic in boreal spring-summer 1996. We demonstrate that the 1995 oceanic conditions in the Tropical Pacific triggers the growth of local air-sea interactions within the Equatorial Atlantic basin that led to the observed Atlantic event in 1996.