Cariaco basin dynamics: Study of the thermocline depth variability and its relation with open ocean conditions

The Cariaco basin (Venezuela) is a semi-enclosed trench located along the coast of Venezuela, with maximum depths of about 1400 m. It is connected to the open ocean by two shallow passages of less than 150 m depth. Limited basin ventilation, coupled with a small vertical mixing results in anoxic conditions from about 250 m to the bottom. The dynamics of the Cariaco Basin are studied by means of a three-dimensional hydrodynamic model. The numerical model has a resolution of 1/60 degree and is an implementation of the Regional Ocean Modeling System (ROMS) nested in the global HYCOM solution from the Naval Research Laboratory. Of particular interest are the mechanisms that link the basin's interior to the Caribbean Sea, which can lead to the ventilation of the basin's anoxic sub-surface waters. To assess the influence of the open ocean on the basin, the spatial and temporal evolution of the thermocline depth and width is studied, as well as its relationship with wind variability and chlorophyll-a concentration: at seasonal scales, the interior of the basin reacts to easterly winds intensification with a rising of the thermocline, resulting in a coastal upwelling response, with the consequent increase in chlorophyll-a concentration. Outside the Cariaco basin, where an open-ocean, oligotrophic regime predominates, wind intensification increases mixing of the surface layers and induces therefore a deepening of the thermocline. At shorter time scales (i.e. days), it is shown that other processes, such as the influence of the meandering Caribbean Current, can also influence the thermocline variability within the Cariaco basin.