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

Abstract

We use a three‐dimensional, off‐line geochemical model to simulate the nitrate cycle in the equatorial Atlantic during the years 1983–1984 corresponding to the Français Océan 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 equatorial circulation and biological activity on both seasonal and interannual timescales. Our results suggest that the upwelling of nitrate in the surface layer is strongly dependent on the behavior of both the nitracline and Equatorial Undercurrent (EUC). In the western basin, the equatorial upwelling partly feeds the EUC and has a low signature on surface nitrate. On the contrary, 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 transferred to the very surface by vertical diffusion, whereas the contribution by vertical advection is negligible. While slightly cold oceanic conditions prevailed in 1983, a warm anomaly produced by a decrease in trade winds and upwelling was observed in 1984. In our simulations, the significant changes in circulation do not notably alter the seasonal cycle of new production. Consequently, variations of annual primary production between 1983 and 1984 are small (9% decrease in the 2°S–2°N band) compared to the amplitude of the seasonal cycle (twofold variations). Contrary to the Pacific Ocean, where the interannual signal dominates, our results suggests that seasonal variability is the most significant large‐scale signal on primary production in the equatorial Atlantic.