Upper-ocean water mass characteristics of the California current, Summer 1993

Abstract

Two large, high-resolution upper-ocean surveys of the California Current region were conducted in the summer of 1993. Temperature and salinity were measured from a Seasoar vehicle, and velocity was measured by a ship-borne acoustic Doppler current profiler. Both surveys extended from the continental margin to 128°W, and consisted of zonal sections 28 km apart. The first survey (7–28 June) extended from 39.5°N to 36.25°N, and the second (16 August to 1 September) extended from 39.0°N to 37.0°N. The current field was fairly simple in June, showing an equatorward surface jet with only one gentle meander, a nearly continuous poleward California Undercurrent near the continental margin, and two anticyclonic subsurface eddies offshore of the jet. By August meanders in the surface jet had strengthened, and eddies dominated the flow field. Large-scale trends in the water-mass characteristics (“spiciness”) along three isopycnal surfaces (26.4, 26.0 and 25.6 kg m -3) show that temperature and salinity tend to increase towards the equator and towards shore, consistent with poleward advection along the continental margin. Local minima in spiciness tend to coincide with the equatorward surface jet. Local maxima in spiciness are associated with subsurface anticyclonic eddies which have a core depth of about 150 m, and seem to originate in the California Undercurrent over the continental slope at local latitudes. We identified several different anticyclonic eddies, each with different water-mass characteristics; all of the eddies we observed had diameters larger than the baroclinic radius of deformation. Water-mass characteristics and migration rates are consistent with formation over the continental margin within the preceding 6–8 months.