The flow field of the upper hypoxic Eastern Tropical North Atlantic oxygen minimum zone
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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 ≤ 60 mol kg1) oxygen minimum zone (OMZ) at 400 to 500 m depth. The poorly known subsurface circulation in the OMZ region is derived from observations and data assimilation results. Measurements in the eastern tropical North Atlantic in November/December 2008, in November/December 2009 and October/November 2010 of velocity, oxygen and of a tracer (CF3SF5) that was released in April 2008 at 8 N, 23 W (at 330 m depth) show 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 8 N, 23 W with parking depths at 330, 350 and 400 m depths were used to estimate velocity along the float trajectory at the surface and at the park depth. South of 9 N, the zonal surface velocity estimate from float data alternate seasonally. At the 350 m park depth north of 9 N a cyclonic northwestward flow across the OMZ was observed. The northward shift into the upper OMZ and the 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 expands 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 other recent cruises up to the year 2014 confirm the continuous deoxygenation trend in the upper OMZ since the 1960's near the Guinea Dome. The three floats deployed with the tracer show spreading paths consistent with the overall observed tracer spreading. Mesoscale eddies may modify the oxygen distribution in the OMZs. 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. 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.
