Abstract
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 µmol kg−1) oxygen minimum zone (OMZ) at 400 to 500 m 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 ∼ 8° N, 23° W (at ∼ 330 m 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 ∼ 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 parking depth. At the 350 m 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.
Highlights
In the eastern tropical North Atlantic (ETNA) a subsurface low-oxygen zone exists with a pronounced minimum in oxygen at about 400 to 500 m depth
Measurements in the eastern tropical North Atlantic Ocean (ETNA) of velocity, oxygen and of a tracer (CF3SF5) that was released in April 2008 at ∼ 8◦ N, 23◦ W in November–December 2008, in November–December 2009 and October–November 2010 show the circulation in the upper part of the oxygen minimum zone (OMZ) with spreading to the east in the North Equatorial Countercurrent (NECC) region and northwestward around the Guinea Dome
There is an inclined boundary between North Atlantic Central Water (NACW) and South Atlantic Central Water (SACW) rising from south to north, i.e., SACW lying on top of NACW (Tomczak, 1984); near the Cape Verde Islands the lower OMZ is more influenced by NACW than in the upper OMZ layers
Summary
In the eastern tropical North Atlantic (ETNA) a subsurface low-oxygen zone exists with a pronounced minimum in oxygen at about 400 to 500 m depth. There is an inclined boundary between NACW and SACW rising from south to north, i.e., SACW lying on top of NACW (Tomczak, 1984); near the Cape Verde Islands the lower OMZ is more influenced by NACW than in the upper OMZ layers Based on this Central Water distribution, Peña-Izquierdo et al (2015) proposed different flow regimes for the upper and intermediate Central Water layer separated by the isopycnal σθ = 26.8 kg m−3 at about 300 m depth. The major supply paths of oxygen above and in the uppermost reaches of the OMZ in the eastern tropical North Atlantic are from the eastward flowing and latitudinally stacked zonal jets at and near the Equator (Brandt et al, 2015) and from the North Equatorial Countercurrent (NECC). While previous studies focused on the 23◦ W section or on hydrographic and current meter measurements from single cruises, here we combine different measurements including float and tracer measurements to investigate the flow field and oxygen distribution of the OMZ in the ETNA varying on intraseasonal and seasonal timescales
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