Local Coupled Equatorial Variability versus Remote ENSO Forcing in anIntermediate Coupled Model of the Tropical Atlantic

摘要

The relative roles played by the remote El Nino-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 six-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 the University of California, Los Angeles (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 northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Nino 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 1-3-yr 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 Nino is qualitatively a noise-driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial 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 Nino events have characteristics (location and peak phase) that depend on air-sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein 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.