Physical drivers of the nitrate seasonal variability in the Atlantic cold tongue

Abstract. Ocean color observations show semiannual variations of chlorophyll in the Atlantic cold tongue with a main bloomin boreal summer and a secondary bloomin December. In this study, ocean color and in situ measurements, and a coupled physical-biogeochemical model are used to investigate the processes that drive this variability. Results show that the main phytoplankton bloomin July-August is driven by a strong vertical supply of nitrate in May-July and the secondary bloomin December is driven by a shorter and moderate supply in November. The upper ocean nitrate balance is analyzed and shows that vertical advectioncontrols the nitrate input in the equatorial euphotic layer and that vertical diffusion and meridional advectionare key in extending and shaping the bloomoff equator. Horizontal advectionmostly acts to bring nitrate low water below the mixed layer. Our results also give insights on the influence of intraseasonal processes in these exchanges. Observations and model show that the Equatorial Undercurrent brings low-nitrate water (relatively to off-equatorial surrounding waters) but still rich enough to enhance the cold tongue productivity.