Abstract. The upwelling system off the southern Caribbean coast is probably the main nutrients source that support the biological productivity in this oligotrophic sea. The Subtropical Water Mass (SUW) that forms the subsurface salinity maximum in the Caribbean is the main source of upwelled waters in this system. Salinity and temperature vs. depth profiles from 4 oceanographic cruises in the western sector of the Caribbean upwelling system, showed that the salinity of upwelled SUW waters is ~ 0.2 g kg⁻¹ lower than the SUW salinity in the central Caribbean and have a slight seasonal variation that agree with with the rainy/dry seasons in the region. Besides, the depth of these SUW waters in the continental slope (~ 100 m) is ~ 50 m shallower than the SUW depth in the rest of the Caribbean sea. The origin of these modified SUW waters was analyzed using the Mercator numerical model, which reproduces the main vertical characteristics of the subsurface salinity maximum. Modeled data showed that SUW waters upwelled off La Guajira peninsula come from the western Caribbean and arrive to the system transported by an intense Caribbean Coastal Undercurrent (CaCU, mean speed ~ 0.4 m s⁻¹). The lower salinity observed in the upwelled SUW waters may be the result of intense vertical mixing processes with diluted surface waters in the Panama-Colombia gyro that could occur when the CaCU flows below this region before reaching the upwelling zones. The mixing processes in the SUW by double diffusion and mechanical turbulence driven by vertical shear of horizontal currents, were analyzed using the Turner angle and the Thorpe scale, respectively. Double diffusion by salt fingers was found between the SUW and the Central North Atlantic Waters (NACW) with diffusivity values ~ 10⁻⁵ m² s⁻¹. But the mechanical diffusivity was two orders of magnitude higher (10⁻³ m² s⁻¹.) than the double diffusivities in the entire water column, generating salt fluxes from the SUW towards surface and towards depth over 2 g kg⁻¹ m d⁻¹. Beyond modifying the salt content in coastal SUW waters, these mixing processes may also alter the nutrient content of upwelling waters, with effects still unknown to the upwelling ecosystem.