Continuity of the poleward undercurrent along the eastern boundary of the mid-latitude north Pacific

Abstract

Several recent data sets improve our view of the poleward undercurrent of the California Current System. As part of a triennial National Marine Fisheries Service (NMFS) survey of Pacific whiting, a series of 105 shipboard acoustic Doppler current profiler (ADCP) velocity sections across the shelf break from 33 to 51°N at about 18 km meridional spacing were collected from July to August 1995. Significant (>0.05 m s −1) subsurface poleward flow occurred in 91% of the sections. A mean cross-shelf section using the entire data set has statistical significance, revealing an undercurrent core >0.1 m s −1 from 200–275 m depth 20–25 km off the shelf break. The mean poleward volume transport in a 125–325 m layer is 0.8±0.2×10 6 m 3 s −1 . We focus particular attention on the Cape Blanco to Cape Mendocino region, and the NMFS results are compared with shipboard ADCP three weeks later from a study of coastal upwelling processes near Cape Blanco. ADCP streamfunction maps are derived and strongly suggest that one portion of flow is continuous over the 440 km meridional extent of the analysis region. Other portions of the flow show evidence of offshore turning, separation, and the formation of anti-cyclonic eddies. We also note that isopycnic potential vorticity from alongslope CTD stations during the NMFS survey appears to be a tracer for the poleward flow.