Hydrography of the entrance to the Gulf of California: the upper limit of the oxygen minimum.

Abstract

We used historical data to describe the hydrograph variability at the entrance to the Gulf of California, particularly of the limit with the depth of the oxygen minimum. At the entrance to the Gulf of California three surface waters converge: California Current, Tropical Surface and Gulf of California water masses. The historical data includes observations of 10 cruises between 1991 and 2011. The Gulf of California water mass does not appear to have a significant presence at the entrance, we only found it at the northern end of the entrance. The water of the California Current is typically found at the surface but in this region it is often at about 50 m depth, below the Surface Tropical Water mass. In contrast, the Subtropical Subsurface Water is often very close to the surface and because of this, the oxygen minimum is also shallower. The confluence of water masses at the entrance of the gulf results in changes in vertical composition between oceanic and coastal regions. In the oceanic region we find two water masses (Tropical Superficial and California Current) above the oxygen minimum. Whereas near the coastal the oxygen minimum occurs immediately below the Tropical Surface water mass. However, the stratification here is more intense than in the open ocean hindering the mixing of low oxygen values ​​with the surface. At smaller scales, the density surfaces limiting with the layer of oxygen minimum are influenced by the mesoscale structures in the region. They deepen/rise below the hot/cold-core eddies and rise close to the surface below coastal currents.