Deoxygenation in the oxygen minimum zone of the eastern tropical North Atlantic

Observations and model results both indicate increasing oxygen minimum zones (OMZ) in the tropical oceans. Here we report on record low dissolved oxygen minimum concentrations in the eastern tropical North Atlantic in fall of 2008, with less than 40 μ mol kg −1 in the core of the OMZ. There we find a deoxygenation rate of ∼0.5 μ mol kg −1 a −1 during the last decades on two repeat sections at 7.5 and 11°N. The potential temperature and salinity in the surface and central water layers increased on both sections compared to previous observations. However, in contrast to the oxygen decrease in the core of the OMZ, increasing oxygen concentrations were observed in the central water layer above the OMZ. The observed deoxygenation was thus restricted to the core of the oxygen minimum layer. It remains unclear whether the vertical expansion of the oxygen minimum represents a long time trend or decadal variations.