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Local Coupled Equatorial Variability Versus Remote ENSO forcing in an Intermediate Coupled Model of the Tropical Atlantic

Séréna Illig and Boris Dewitte

The relatives roles played by the remote El Niño Southern Oscillation (ENSO) forcing and the local air-sea interactions in the Tropical Atlantic are investigated using an Intermediate Coupled Model (ICM) of the Tropical Atlantic. The oceanic component of the ICM consists of a 6-baroclinic-mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global Atmospheric General Circulation Model developed at UCLA. In a forced context, the ICM realistically simulates both the Sea Surface Temperature Anomaly (SSTA) variability in the equatorial band and the relaxation of the Atlantic North East trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows 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 time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA 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. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise-driven stable system. Such a system can produce coherent inter-decadal variability that is not forced by the Pacific or extra-equatorial variability. It is shown that when a simple slab mixed-layer model is embedded into the system to simulate the Northern Tropical Atlantic (NTA) SST variability, the warming over NTA following EL Niño events have characteristics (location and peak phase) that depends on air-sea interaction in the Equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipole-like structure of the SSTA variability that evolves at decadal timescale. Our results illustrate the complexity of the Tropical Atlantic ocean-atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.

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