Water mass pathways to the North Atlantic Oxygen Minimum Zone

Abstract

Oxygen Minimum Zones (OMZ's) are considered in the classical theory of wind-driven circulation isolated from the subtropical gyres ventilation, i.e. no direct supply of water occurs from the gyres. Hence, the ventilation of the OMZ's is traditionally explained within the circulation of the tropical system of zonal jets. However, historical observations show an abrupt thermohaline transition at 300m of depth (σ0= 26.8 kg/m3) in the North Atlantic OMZ. We show that there are 2 well-differentiated water mass strata within Central Waters (CW). The upper layer (above s26.8) is dominated by South Atlantic CW which extends throughout the whole Tropical Atlantic. The lower layer instead shows a marked increase of North Atlantic CW. Indeed, the contribution of northern gyre CW is maximal (50%) at the core of the OMZ (at s27.1). According with these observations, a Lagrangian numerical simulation shows remarkably distinct pathways for both layers. In the upper layer and in agreement with previous studies, the northward branching from the North Equatorial CounterCurrent at 9N is the main water supply of the OMZ mostly with southern origin lagrangian particles. However this pathway is notably reduced in the lower layer. Below s26.8, most of the water supply occurs north of 10N, mainly via eastward jets at 14N and 18N with an increased contribution of northern origin particles. The water mass exchange from the northern subtropical gyre emerges as an essential part in the ventilation of the North Atlantic OMZ