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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-338
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-338
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Sep 2019

Submitted as: research article | 18 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

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

Marie-Hélène Radenac1, Julien Jouanno1, Christine Carine Tchamabi1,†, Mesmin Awo1,2,3, Bernard Bourlès4, Sabine Arnault5, and Olivier Aumont5 Marie-Hélène Radenac et al.
  • 1LEGOS, IRD-Université Paul Sabatier-Observatoire Midi-Pyrénées, Toulouse, 31400, France
  • 2Nansen-Tutu Centre for Marine Environmental Research, Department of Oceanography, University of Cape Town, Cape Town, South Africa
  • 3LHMC, IRHOB, IRD, Cotonou, Bénin
  • 4IRD, US191 "Instrumentation, Moyens Analytiques, Observatoires en Géophysique et Océanographie" (IMAGO), Technopole Pointe du Diable, Plouzané, France
  • 5LOCEAN, CNRS, IRD, Sorbonne Universités, MNHN, Paris, 75005, France
  • deceased

Abstract. 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.

Marie-Hélène Radenac et al.
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Marie-Hélène Radenac et al.
Marie-Hélène Radenac et al.
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Short summary
Elevated chlorophyll concentrations are observed in the equatorial cold tongue of the Atlantic Ocean. Satellite data and a remarkable set of repeated in situ measurements show a main bloom of chlorophyll in summer and a secondary bloom in December. A coupled physical-biogeochemical simulation is used to investigate the processes driving this seasonality. Semiannual variations of horizontal and vertical nitrate supplies and losses explain the surface chlorophyll seasonality.
Elevated chlorophyll concentrations are observed in the equatorial cold tongue of the Atlantic...
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