A simple model for the Eastern Tropical North Atlantic Oxygen Minimum Zone

The Eastern Tropical North Atlantic Oxygen Minimum Zone (ETNA OMZ) is located in a region of northward Sverdrup transport, opposite to that of the subtropical gyre. This means that its dynamics are unlikely to be associated with the shadow zone associated with the Ventilated Thermocline Theory, as has traditionally been assumed. Rather, we argue an important role for the latitudinally alternating zonal jets associated with the mesoscale eddy field. To illustrate this, we use an advection-diffusion model coupled to a simple dynamical ocean model. The advection-diffusion model carries a passive tracer with a source at the western boundary and a Newtonian damping term to mimic oxygen consumption. The dynamical model is a non-linear 1 1/2 layer reduced-gravity model. The latter is forced by an annually oscillating mass flux confined to the near-equatorial band that, in turn, leads to the generation of mesoscale eddies and latitudinally alternating zonal jets at higher latitudes. The model uses North Atlantic geometry and develops a tracer minimum zone remarkably similar in location to the ETNA OMZ. Although the model is forced only at the annual period, the model nevertheless exhibits decadal and multidecadal variability in its spun-up state. The associated trends are comparable to observed trends in oxygen within the ETNA oxygen minimum zone. Notable exceptions are the multi-decadal decrease in oxygen in the lower oxygen minimum zone, and the sharp decrease in oxygen in the upper oxygen minimum zone between 2006 and 2013.