Circulation over the shelf and slope off northern California

Abstract

Hydrographic observations taken during the Northern California Coastal Circulation Study (NCCCS) are analyzed to examine the structure and variability of density and velocity over the shelf and slope, to a distance of 70 km offshore and a depth of 1500 m. Cross‐shore sections with closely spaced stations were occupied between the Oregon border and San Francisco seven times between March 1987 and October 1989. Historical observations from CaICOFI line 60 off Point Reyes are also analyzed to provide better resolution of seasonal and interannual variability in the region than is available from the NCCCS data. Poleward flow dominates the circulation in this easternmost part of the California Current system, much more so than the historical description would indicate. Because there is substantial poleward shear below 500‐m depth, poleward transports in the top 500 m referred to 1500‐m depth are 2 to 5 times the values referred to 500 m. Equatorward flow occurs on average over the shelf, and in an organized fashion offshore. The shelf flows appear to be the local response to wind‐driven upwelling. The offshore equatorward flow is an energetic, narrow feature that persists for several months at a time and has alongshore structure aligned to the shape of the coastline. It can be identified in one of the NCCCS surveys as the same jet described by the 1988 Coastal Transition Zone experiment observations. Empirical Orthogonal Function analysis indicates that seasonal variability in the top 500 m of the water column generally explains less variance (∼20%) than does interannual variability (∼50%). Between 500 and 1500 m, seasonal fluctuations explain a greater proportion of the variance (∼50%). The shape of the coastline, the coastal mountains, and the bathymetry of the shelf and slope have a significant influence on the circulation in this region. There is enhanced upwelling near the two major capes (Mendocino and Point Arena), as well as substantial increases in poleward transports near both. Flow is directed offshore on the north sides of both features and onshore to the south. The equatorward flowing offshore jet is found closer to shore in the embayment between the two capes and farther offshore at the latitude of the capes.