Abstract. 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.
<p><strong>Abstract.</strong> 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.</p>
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