Water mass pathways to the North Atlantic Oxygen Minimum Zone

EUR-OCEANS Hot Topics Conference - A Changing Ocean, 6-8 November 2013, Gran Canaria, Spain
Regions with extremely weak thermocline circulation are found just outside the subtropical gyres, equatorwards and in the eastern margin of all oceans. These regions, called Shadow Zones, present the lowest thermocline water masses renovation rates and thus the longest residence times. These long residence times combine with enhanced organic remineralization, due to highly productive surface waters in eastern margins, to drive the development of regions of very low oxygen concentrations, named Oxygen Minimum Zones (OMZ). The North Atlantic OMZ (naOMZ) is one of the weakest (relatively high oxygen content) OMZ. The water mass renovation of the naOMZ is typically explained by the advection of southern origin water, namely South Atlantic Central Water (SACW), from the tropical system of zonal jets south of 10N. Well ventilated North Atlantic Central Water (NACW), found north of the naOMZ in the Subtropical Gyre is supposed to not play a meaningful role on the ventilation of the naOMZ. In this study we combine field and numerical data to examine the water mass pathways to the OMZ. First, we use Argo and National Oceanic and Atmospheric Administration (NODC) CTD datasets to carry out an analysis of the water masses in the region. And second, a numerical circulation model is employed to run Lagrangian experiments that may describe the naOMZ water masses renovation patterns. Our results show an abrupt and permanent thermohaline vertical transition in the naOMZ at sigma0 26.8 kg/m3, with pure SACW in the upper layer while a 50% mixture of SACW and NACW in the lower layer. The Lagrangian experiments show that the high-‐oxygen content upper layers are directly and only renovated with SACW by a northward extension of the northern branch of the North Equatorial CounterCurrent that flows at 9N. The oxygen-‐ poor lower layers are renewed instead with a mixture of SACW and NACW but with minimum oxygen values through a basin wide zonal flow centered at 14N not connected with the tropical system of zonal jets further south. Finally, more tan 60% of the oxygen influx to the naOMZ can be associated to NACW inflow. This distinct dynamic above and below sigma0 26.8 kg/m3 explained the observed thermohalines transition. Also, the existence of this zonal source of relatively old NACW into the naOMZ possibly explains the higher oxygen content of the naOMZ as compared with other basins