Simulation of the nitrate seasonal cycle in the equatorial Atlantic Ocean during 1983 and 1984

We use a three-dimensional, off-line geochemical model to simulate the nitr ate cycle in the equatorial Atlantic during the years 1983-1984 correspondi ng to the Francais Ocean et Climat dans l'Atlantique Equatorial (FOCAL) and Seasonal Response of the Equatorial Atlantic programs. After comparing our simulations with FOCAL data, we investigate interactions between equatoria l circulation and biological activity on both seasonal and interannual time scales. Our results suggest that; the upwelling of nitrate in the surface l ayer is strongly dependent on the behavior of both the nitracline and Equat orial Undercurrent (EUC). In the western basin, the equatorial upwelling pa rtly feeds the EUC and has a Low signature on surface nitrate. On the contr ary, in the eastern basin, where the upwelling core and the nitracline are closer to the surface, vertical advection is the driving mechanism causing seasonal variations of nitrate concentration. Above the EUC, nitrate is tra nsferred to the very surface by vertical diffusion, whereas the contributio n by vertical advection is negligible. While slightly cold oceanic conditio ns prevailed in 1983, a warm anomaly produced by a decrease in trade winds and upwelling was observed in 1984. In our simulations, the significant cha nges in circulation do not notably alter the seasonal cycle of new producti on. Consequently, variations of annual primary production between 1983 and 1984 are small (9% decrease in the 2 degrees S-2 degrees N band) compared t o the amplitude of the seasonal cycle (twofold variations). Contrary to the Pacific Ocean, where the interannual signal dominates, our results suggest s that seasonal variability is the most significant large-scale signal on p rimary production in the equatorial Atlantic.