CURRENTS AND WATER MASSES OF THE COASTAL TRANSITION ZONE OFF NORTHERN CALIFORNIA, JUNE TO AUGUST 1988

In summer 1988, we made repeated mesoscale surveys of a grid extending 200 km offshore between 37-degrees-N and 39-degrees-N in the coastal transition zone off northern California, obtaining continuous acoustic Doppler current profiler data and conductivity-temperature-depth data at standard stations 25 km apart on alongshore sections 40 km apart. All surveys showed a baroclinic equatorward jet, with core velocities of > 50 cm s-1 at the surface decreasing to about 10 cm s-1 at 200 m, a width of 50-75 km, and a baroclinic transport of about 4 Sv. The core of the jet lay between the 8.6 and 9.4 m2 s-2 contours of geopotential anomaly (relative to 500 dbar). Three current meter moorings, deployed at 25-km separation across the jet at the beginning of the survey sequence, provided time-series of the velocity; throughout the 37-day deployment, at least one mooring was within the core defined by the 8.6 and 9.4 m2 s-2 contours. The jet flowed southwestward across the grid from late June until mid-July 1988, when the jet axis moved offshore in the north and onshore in the southern portion of the grid. Temperature-salinity analysis shows that jet waters can be distinguished from both the freshly upwelled coastal waters and the offshore waters. Isopycnal maps indicate alongshore advection of relatively fresh, cool water from farther north, as well as small-scale patchiness not resolved by our survey grid. The baroclinic jet observed here may be continuous with the core of the California Current off central California. The later surveys clearly showed a poleward-flowing undercurrent adjacent to the continental slope, with core velocities up to 20 cm s-1 at depths of 150-250 m. Its baroclinic transport (relative to 500 dbar) increased from < 0.5 Sv to > 1.0 Sv between late June and early August 1988. Within the survey grid, there was a definite onshore gradient in the characteristics of North Pacific Intermediate Water. The subsurface waters adjacent to the continental margin were warmer and more saline than those offshore, indicating net northward advection by the California Undercurrent over the inshore 100 km and equatorward advection farther from shore.