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

Ocean color observations show semiannual variations in chlorophyll in the Atlantic cold tongue with a main bloom in boreal summer and a secondary bloom in 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 bloom in July–August is driven by a strong vertical supply of nitrate in May–July, and the secondary bloom in December is driven by a shorter and moderate supply in November. The upper ocean nitrate balance is analyzed and shows that vertical advection controls the nitrate input in the equatorial euphotic layer and that vertical diffusion and meridional advection are key in extending and shaping the bloom off Equator. Below the mixed layer, observations and modeling show that the Equatorial Undercurrent brings low-nitrate water (relative to off-equatorial surrounding waters) but still rich enough to enhance the cold tongue productivity. Our results also give insights into the influence of intraseasonal processes in these exchanges. The submonthly meridional advection significantly contributes to the nitrate decrease below the mixed layer.

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

Ocean color observations show semiannual variations of chlorophyll in the Atlantic cold tongue with a main bloom in boreal summer and a secondary bloom in 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 bloom in July-August is driven by a strong vertical supply of nitrate in May-July and the secondary bloom in December is driven by a shorter and moderate supply in November. The upper ocean nitrate balance is analyzed and shows that vertical advection controls the nitrate input in the equatorial euphotic layer and that vertical diffusion and meridional advection are key in extending and shaping the bloom off equator. Horizontal advection mostly 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.

Origin of Warm SST Bias over the Atlantic Cold Tongue in the Coupled Climate Model FGOALS-g2

Most of the coupled models contain a strong warm bias in sea surface temperature (SST) over the Atlantic Cold Tongue (ACT) region (10° S–3° N, 20° W–10° E) during June–August (JJA) and September–November (SON). In this study, the origins of the ACT SST bias and their relative contributions to the bias are explored by conducting a set of sensitivity experiments, which are based on an ocean-ice model, and by ignoring the nonlinear effects of each origin. The origins for the warm bias over the ACT in the coupled climate model during JJA are estimated as follows: westerly wind bias along the equator (5° S–5° N) during March–May (MAM; contributes approximately 32.6% of the warm bias), northerly bias over the southern tropical Atlantic (25° S–3° N, 40° W–20° E) during MAM and JJA (21.4%), bias in the surface specific humidity and surface air temperature (11.9%), and downward shortwave radiation bias (6.5%). The origins of the ACT bias during SON are as follows: northerly bias over the southern tropical Atlantic during SON (31.2%), bias in the surface specific humidity and surface air temperature (27.9%), downward shortwave radiation bias (17.4%), and zonal wind bias (13.4%). Note that these contribution ratios of these origins may be model-dependent. In addition, the local and non-local effects of the zonal wind bias are explored explicitly, while those of all the other biases are examined implicitly. Therefore, a better-performing atmospheric component is crucial when simulating zonal winds during MAM along the equator (5° S–5° N) and meridional winds during MAM, JJA, and SON over the southern tropical Atlantic, which will alleviate the warm bias over the ACT region in the coupled climate model.