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A subsurface low oxygen zone is located in the eastern tropical NorthAtlantic Ocean (ETNA) in the upper ocean with the core of the hypoxic(O2 ≦60 µmol kg−1) oxygen minimum zone (OMZ) at400 to 500 m depth. The subsurface circulation in the OMZ region is derivedfrom observations and data assimilation results. Measurements in the ETNA ofvelocity, oxygen and of a tracer (CF3SF5) that was released inApril 2008 at  ∼  8° N, 23° W (at  ∼  330 m depth)in November–December 2008, in November–December 2009 and October–November2010 show the circulation in the upper part of the OMZ with spreading to theeast in the North Equatorial Countercurrent (NECC) region and northwestwardaround the Guinea Dome. Three floats equipped with oxygen sensors deployed at ∼  8° N, 23° W with parking depths at 330, 350 and400 m depths were used to estimate velocity along the float trajectory atthe surface and at the parking depth. At the 350 m park depth north of9° N a cyclonic northwestward flow across the OMZ was observed. Thenorthward drift of a float into the upper OMZ and a stronger cyclonic flowaround the Guinea Dome seem to be connected to a strong Atlantic MeridionalMode (AMM) event in 2009. A near-surface cyclonic circulation cell east ofthe Cape Verde Islands reaches down into the OMZ layer. The circulation ofthe upper OMZ mirrors the near-surface circulation. Oxygen measurements fromthe cruises used here, as well as from other recent cruises up to the year2014, confirm the continuous deoxygenation trend in the upper OMZ since the1960s near the Guinea Dome. The three floats deployed with the tracer showspreading paths consistent with the overall observed tracer spreading. Oxygensensors on the floats remained well calibrated for more than 20 months, andso the oxygen profiles can be used to investigate mesoscale eddy signatures.Mesoscale eddies may modify the oxygen distribution in OMZs. However, ingeneral eddies are less energetic in the ETNA south of the Cape Verde Islandscompared to similar latitudes in the eastern tropical South Pacific

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