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Volume 12, issue 1 | Copyright
Ocean Sci., 12, 153-167, 2016
https://doi.org/10.5194/os-12-153-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 18 Jan 2016

Research article | 18 Jan 2016

The flow field of the upper hypoxic eastern tropical North Atlantic oxygen minimum zone

L. Stramma1, R. Czeschel1, T. Tanhua1, P. Brandt1,2, M. Visbeck1,2, and B. S. Giese3 L. Stramma et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
  • 2Kiel University, Christian-Albrechts-Platz 4, 24118 Kiel, Germany
  • 3Department of Oceanography, Texas A&M University, College Station, Texas, USA

Abstract. A subsurface low oxygen zone is located in the eastern tropical North Atlantic Ocean (ETNA) in the upper ocean with the core of the hypoxic (O2≦60µmolkg−1) oxygen minimum zone (OMZ) at 400 to 500m depth. The subsurface circulation in the OMZ region is derived from observations and data assimilation results. Measurements in the ETNA of velocity, oxygen and of a tracer (CF3SF5) that was released in April 2008 at  ∼ N, 23°W (at  ∼ 330m depth) in November–December 2008, in November–December 2009 and October–November 2010 show the circulation in the upper part of the OMZ with spreading to the east in the North Equatorial Countercurrent (NECC) region and northwestward around the Guinea Dome. Three floats equipped with oxygen sensors deployed at  ∼ N, 23°W with parking depths at 330, 350 and 400m depths were used to estimate velocity along the float trajectory at the surface and at the parking depth. At the 350m park depth north of 9°N a cyclonic northwestward flow across the OMZ was observed. The northward drift of a float into the upper OMZ and a stronger cyclonic flow around the Guinea Dome seem to be connected to a strong Atlantic Meridional Mode (AMM) event in 2009. A near-surface cyclonic circulation cell east of the Cape Verde Islands reaches down into the OMZ layer. The circulation of the upper OMZ mirrors the near-surface circulation. Oxygen measurements from the cruises used here, as well as from other recent cruises up to the year 2014, confirm the continuous deoxygenation trend in the upper OMZ since the 1960s near the Guinea Dome. The three floats deployed with the tracer show spreading paths consistent with the overall observed tracer spreading. Oxygen sensors on the floats remained well calibrated for more than 20 months, and so the oxygen profiles can be used to investigate mesoscale eddy signatures. Mesoscale eddies may modify the oxygen distribution in OMZs. However, in general eddies are less energetic in the ETNA south of the Cape Verde Islands compared to similar latitudes in the eastern tropical South Pacific.

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The subsurface circulation in the eastern tropical North Atlantic OMZ is derived from velocity, float and tracer data and data assimilation results, and shows a cyclonic flow around the Guinea Dome reaching into the oxygen minimum zone. The stronger cyclonic flow around the Guinea Dome in 2009 seem to be connected to a strong Atlantic Meridional Mode (AMM) event. A continuous deoxygenation trend of the low oxygen layer was confirmed. Eddy influence is weak south of the Cape Verde Islands.
The subsurface circulation in the eastern tropical North Atlantic OMZ is derived from velocity,...
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