Interannual long equatorial waves in the tropical Atlantic from a high-resolution ocean general circulation model experiment in 1981-2000

[1] We investigate the tropical Atlantic vertical structure variability ( 1981-2000) based on the CLIPPER ocean general circulation model (OGCM). We aim at determining to what extent the observed interannual variability can be explained by the low-frequency wave dynamics. The linear vertical modes of the OGCM climatological stratification are estimated along the equator. The baroclinic mode contributions to surface zonal current and sea level anomalies are calculated and analyzed at interannual timescales. The second baroclinic mode is the most energetic. The first ( third) mode exhibits a variability peak in the west ( east). The summed-up contribution of the high-order baroclinic modes ( 4-6) is as energetic as the gravest modes and is largest in the east. Wave components are then derived by projection onto the associated meridional structures. The effect of longitudinal boundaries near the equator is taken into consideration. Equatorial Kelvin and Rossby waves propagations, with phases speed close to the theory, are identified for the first three baroclinic modes. The comparison with a multimode linear simulation corroborates the propagating properties of the OGCM waves coefficients. An estimation of the meridional boundary reflection efficiency indicates that wave reflections take place at both boundaries. A 65% reflection efficiency is found at the eastern boundary. Our study suggests that low-frequency wave dynamics is to a large extent at work in the tropical Atlantic. On the basis of what is known on the Pacific El Nino-Southern Oscillation mode this may provide a guidance for investigating ocean-atmosphere mechanisms that can lead to the Atlantic zonal equatorial mode.