A study of the velocity structure near a cold filament from ADCP and CTD measurements

Abstract

CTD and ADCP data from the Coastal Transition Zone (CTZ) pilot cruise off Point Arena, California during June 1987, were combined to make optimal estimates of the current velocity field. The region was characterized by upwelling over the shelf, a single strong offshore geostrophic jet to the north, and a meandering equatorward flow which advected upwelled water from the coastal region up to 150 km offshore. Geostrophic velocity profiles referenced to 500 dbar were adjusted to the ADCP-measured velocity in the 190-274 m layer. Comparison of the unadjusted and adjusted profiles sets showed generally good agreement below 200 m but marked differences in several of the profiles above 200 m. Sections of geostrophic and ADCP velocity indicated that the flow in the region was highly geostrophic, but ageostrophic flow components were also present, particularly in the high velocity regions. Volume transport was computed for a portion of the survey area using the two velocity data sets and the Ekman transport, computed from the observed wind data. Transport in the chosen subregion was not balanced due to 1) rapid temporal changes in the meandering upwelling jet, and 2) the influence of high frequency variability which impacted both the CTD and the ADCP data.