Surface Circulation and Vertical Structure of Upper Ocean Variability Around Fernando de Noronha Archipelago and Rocas Atoll During Spring 2015 and Fall 2017

Type Article
Date 2021-04
Language English
Author(s) Costa Da Silva Alex1, Chaigneau Alexis2, 3, 4, Dossa Alina N.1, Eldin Gerard2, Araujo Moacyr1, 5, Bertrand Arnaud6
Affiliation(s) 1 : Departamento de Oceanografia da Universidade Federal de Pernambuco – DOCEAN/UFPE, Recife, Brazil
2 : Laboratoire d’Études en Géophysique et Océanographie Spatiale (LEGOS), Université de Toulouse, CNES, CNRD, IRD, UPS, Toulouse, France
3 : Institut de Recherches Halieutiques et Ocèanologiques du Beìnin (IRHOB), Cotonou, Benin
4 : International Chair in Mathematical Physics and Applications (ICMPA–UNESCO Chair), Cotonou, Benin
5 : Brazilian Research Network on Global Climate Change – Rede CLIMA, São José dos Campos, Brazil
6 : Institut de Recherche pour le Développement (IRD), MARBEC, Univ. Montpellier, CNRS, IFREMER, IRD, Sète, France
Source Frontiers In Marine Science (2296-7745) (Frontiers Media SA), 2021-04 , Vol. 8 , N. 598101 , P. 16p.
DOI 10.3389/fmars.2021.598101
Keyword(s) mesoscale activity, satellite data, western tropical Atlantic, shipboard measurements, island wake, central South Equatorial Current, South Equatorial Undercurrent
Abstract

Using current, hydrographic and satellite observations collected off Northeast Brazil around the Fernando de Noronha Archipelago and Rocas Atoll during two oceanographic cruises (spring 2015 and fall 2017), we investigated the general oceanic circulation and its modifications induced by the islands. In spring 2015, the area was characterized by lower SST (26.6°C) and deep mixed-layer (∼90 m). At this depth, a strong current shear was observed between the central branch of the eastward flowing near-surface South Equatorial Current and the westward flowing South Equatorial Undercurrent. In contrast, in fall 2017, SST was higher (∼28.8°C) and the mixed-layer shallower (∼50 m). The shear between the central South Equatorial Current and the South Equatorial Undercurrent was weaker during this period. Interestingly, no oxygen-rich water from the south (retroflection of the North Brazil undercurrent) was observed in the region in fall 2017. In contrast, we revealed the presence of an oxygen-rich water entrained by the South Equatorial Undercurrent reaching Rocas Atoll in spring 2015. Beside these global patterns, island wake effects were noted. The presence of islands, in particular Fernando de Noronha, strongly perturbs central South Equatorial Current and South Equatorial Undercurrent features, with an upstream core splitting and a reorganization of single current core structures downstream of the islands. Near islands, flow disturbances impact the thermohaline structure and biogeochemistry, with a negative anomaly in temperature (−1.3°C) and salinity (−0.15) between 200 and 400 m depth in the southeast side of Fernando Noronha (station 5), where the fluorescence peak (>1.0 mg m–3) was shallower than at other stations located around Fernando de Noronha, reinforcing the influence of flow-topography. Satellite maps of sea-surface temperature and chlorophyll-a confirmed the presence of several submesoscale features in the study region. Altimetry data suggested the presence of a cyclonic mesoscale eddy around Rocas Atoll in spring 2015. A cyclonic vortex (radius of 28 km) was actually observed in subsurface (150–350 m depth) southeast of Rocas Atoll. This vortex was associated with topographically induced South Equatorial Undercurrent flow separation. These features are likely key processes providing an enrichment from the subsurface to the euphotic layer near islands, supplying local productivity.

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Publisher's official version 16 9 MB Open access
Supplementary Figure 1 | Sea surface temperature, Sea surface Chl-a concentration, Surface winds stress, and Sea surface height/surface geostrophic velocities, obtained from OSTIA product in the south 2 MB Open access
Supplementary Figure 2 | Vertical distributions of temperature, salinity, fluorescence, dissolved oxygen concentrations and Brunt-Väisälä frequency (N) near FN, during spring 2015 (left panels) and... 1 MB Open access
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How to cite 

Costa Da Silva Alex, Chaigneau Alexis, Dossa Alina N., Eldin Gerard, Araujo Moacyr, Bertrand Arnaud (2021). Surface Circulation and Vertical Structure of Upper Ocean Variability Around Fernando de Noronha Archipelago and Rocas Atoll During Spring 2015 and Fall 2017. Frontiers In Marine Science, 8(598101), 16p. Publisher's official version : https://doi.org/10.3389/fmars.2021.598101 , Open Access version : https://archimer.ifremer.fr/doc/00691/80290/