The Southern Caribbean upwelling system off Colombia: Water masses and mixing processes

Abstract The upwelling system off the southern Caribbean coast is probably the main source of the nutrients that support biological productivity in the oligotrophic Caribbean Sea. Subtropical underwater (SUW) that forms the subsurface salinity maximum in the Caribbean Sea is the main source of upwelled waters in this system. Profiles of salinity and temperature with depth derived from four oceanographic cruises and Argo floats showed that upwelled waters have a salinity that is ~0.11 g kg−1 lower than the SUW in the central Caribbean Sea and a seasonal variation of approximately 0.09 g kg−1 that reflects the rainy/dry seasons. In addition, the SUW is ~50 m shallower on the continental shelf slope (~100 m) compared to the depth of the SUW in the central Caribbean Sea. The origin of these modified SUW was analyzed using the Mercator numerical model, which reproduces the main vertical characteristics of the subsurface salinity maximum. The modeled data showed that SUW upwelling off of the La Guajira Peninsula and Venezuela arrive into the system via an intense Caribbean Coastal Undercurrent (CaCU, mean speed ~0.28 m s−1). This current is formed in front of the Nicaragua platform from the divergence of subsurface water flow at the salinity maximum depth. The lower salinity observed in the upwelled waters may be the result of intense vertical mixing processes that could occur when the SUW are transported by the CaCU below the Panama-Colombia Gyre (PCG) region before reaching the upwelling zones. The mixing processes—involving double diffusion and mechanical turbulence driven by vertical shear of horizontal currents—were analyzed using the Turner Angle and the Thorpe scale, respectively. Below a depth of 200 m, double diffusion by salt fingers (diffusivities > 5 x 10−5 m2 s−1) was the main process of salt diffusion, generating a downward salt flux of >2 x 10−2 g kg−1 m d−1 between the SUW and the North Atlantic Central Waters (NACW). Above a depth of 100 m, mechanical turbulent diffusion generates a salt flux towards the surface ranging 0.5–4 x 10−2 g kg−1 m d−1, where double diffusion by salt finger is not possible. The diluted SUW is subsequently transported by the CaCU, connecting—at the subsurface level—the PCG region with the upwelling zones off of Colombia and Venezuela. As well as modifying the salt content of the coastal SUW, these mixing processes may also alter the nutrient content of upwelling waters, the ecosystem effects of which are still unknown.