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Interannual Long Equatorial Waves in the Tropical Atlantic from a High Resolution OGCM Experiment in 1981-2000

S. Illig, B. Dewitte, N. Ayoub, Y. du Penhoat, G. Reverdin, P. De Mey, F. Bonjean and G.S. E. Lagerloef

We investigate the vertical structure variability in the Tropical Atlantic based on the CLIPPER project high-resolution Ocean General Circulation Model (OGCM) simulation for the 1981-2000 period.
Near the equator the linear vertical modes of the OGCM climatological stratification are estimated at each grid point. The baroclinic mode contributions to surface zonal current and sea level anomalies are calculated and analyzed for interannual time scales. The second baroclinic mode is the most energetic, with a variability peak in the central basin. The first and the third modes contribute with comparable amplitude but with different spatial distribution in the equatorial wave guide. The first mode exhibits a variability peak in the western part of the basin, where the largest variability in zonal wind stress is observed, whereas the energy of the third baroclinic mode is confined in the eastern region, where the thermocline rises. The summed-up contribution of the high-order baroclinic modes variability (4 to 6) is as energetic as the gravest modes and is the largest in the east.
Kelvin and meridional Rossby components are then derived for each of the gravest baroclinic mode contributions by projecting onto the associated meridional structures. The effect of longitudinal boundaries close to the equator is taken into consideration. Equatorial Kelvin and Rossby waves propagations, with phase speed values close to the theoretical ones, are identified for the first three baroclinic modes. The results of the comparison with a multi-mode linear simulation, using the projection coefficients and phase speed values derived from the OGCM simulation, corroborates the propagating properties of the OGCM derived waves coefficients. In addition, an estimation of the meridional boundary reflection efficiency in the OGCM, along with the results of linear simulations in which the boundary reflections are cancelled, indicates that wave reflections take place at both boundaries. A reflection efficiency of ~65% is found at the eastern boundary.
Our study suggests that the zonal equatorial mode observed in the tropical Atlantic should be associated to a large extent to the low frequency waves dynamics and that it may involve ocean-atmosphere coupled mechanisms similar to the Pacific ENSO mode.

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