Properties of an upper ocean front associated with water mass boundaries at the entrance to the Gulf of California, November 2004

The structure of an upper ocean front at the entrance to the Gulf of California is described. The front occurred in a region of strong cyclonic shear between fresher Pacific waters, which flowed into the Gulf along Sinaloa, and salty Gulf of California waters, which flowed out of the Gulf along Baja California. Observations included two high-resolution hydrographic sections across the entrance to the Gulf. One section used a towed CTD that profiled from near surface to 125 m. A second across-Gulf section was obtained using CTDs spaced about every 10 km. Both sections indicated a nearly vertical salinity front near 108.6°W, which extended to a depth of 120 m at the northern front. The surface salinity change across the front was ΔS~0.4 and the width of the front 10–15 km. The surface horizontal mixing ratio, −0.15 to −0.7, indicated that salinity dominated the density change. The mixed layer was strongly developed to a depth of ~45 m. Features that were observed at the front included a surface jet (10 km wide, 0.25 m s−1), subduction from the mixed layer to the top of the pycnocline, evidence of symmetric instability driven by atmospheric cooling of surface waters, and a deformation rate of −0.5×10−5 s−1. Northward ageostrophic currents in the upper 60 m extended inflow 55 km to the west of the surface front. Immediately below the bottom of the mixed layer, the seasonal pycnocline was characterized by a 15-m thick and 50-km wide salinity maximum (minimum), S=34.9–35 (S=34.3–34.4) for the outflow (inflow). These features were not distinct in the frontal zone. Temperature flux and diffusivity were estimated to be about 7.6×10−2 °C m s−1 and 8600 m2 s−1, respectively.