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

Type Article
Date 2020-01
Language English
Author(s) Radenac Marie-Helene1, Jouanno Julien1, Tchamabi Christine Carine1, Awo Mesmin1, 2, 3, Bourles Bernard4, Arnault Sabine5, Aumont Olivier5
Affiliation(s) 1 : Univ Paul Sabatier, IRD, LEGOS, Observ Midi Pyrenees, F-31400 Toulouse, France.
2 : Univ Cape Town, Dept Oceanog, Nansen Tutu Ctr Marine Environm Res, Cape Town, South Africa.
3 : IRD, IRHOB, LHMC, Cotonou, Benin.
4 : IRD, US191 Instrumentat, Observ Geophys & Oceanog IMAGO, Technopole Pointe Diable,Moyens Analyt, Plouzane, France.
5 : Sorbonne Univ, MNHN, IRD, LOCEAN,CNRS, F-75005 Paris, France.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2020-01 , Vol. 17 , N. 2 , P. 529-545
DOI 10.5194/bg-17-529-2020
WOS© Times Cited 2
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.

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