The structure of the transition zone between coastal waters and the open ocean off northern California, winter and spring 1987

Abstract

Physical and biological fields in the coastal transition zone off northern California were measured during February, March, May and June 1987 in an extended alongshore region between 60 km and 150 km offshore. The spring transition, as seen in coastal sea level and winds, occurred in mid‐March. Surface variability during the two spring cruises was stronger and of larger scale than that seen during the two winter cruises. An equatorward‐tending current, flowing along the boundary between low steric sea level inshore and high steric sea level offshore, dominated both the directly‐measured (acoustic Doppler current profiler) and geostrophic current fields during spring. Current jets of comparable strength directed both offshore and onshore were seen off Cape Mendocino and Point Arena; these evolved significantly in the 3 weeks between cruises. Inshore of the current, properties associated with upwelled water were found near the surface, including low temperature and high salinity, nutrients and chlorophyll; offshore of the current, waters were warmer, less saline, lower in nutrients and more oligotrophic. Geostrophic and directly measured volume transports in the current were about 2–3 Sv. Isopycnals inshore of the spring upwelling front were displaced vertically by O(40–80 m) from their depths during the winter survey; these displacements extended deep into the water column and were largely independent of depth between 100 and 400 m. Surface mixed layers tended to be deep in winter and shallower inshore of the upwelling front in spring. A connection between the equatorward‐tending frontal jet off northern California and the more well‐studied California Current further south is suggested by the similarity of their transports and of their dynamic height values.

Role of the California Undercurrent in the export of denitrified waters from the eastern tropical North Pacific

“…The California Current system (CCS) has been studied extensively from dynamic and hydrographic perspectives [Wooster and Reid, 1963;Hickey, 1979;Chelton, 1984;Kosro and Huyer, 1986; Brink and Cowles, 1991; Kosro et al, 1991;processes as the primary candidates. Codispoti and Richards [1976] …”

Subtidal cross-shelf circulation on the Northern California shelf

“…This suggests the possibility of seasonal (or interannual) variation in along-shelf scales of near-surface u. Seasonal variability could be due to greater offshore mesoscale activity in the summer [Kosro et al, 1991] which would tend to shorten correlat ion length scales in summer. Offshore mesoscale variability in summer may or may not be related to instability of an upwelling front.…”

“…Three possible reasons include offshore mesoscale variability, a non-linear response to persistent wind forcing, and a complicated ocean response to wind relaxation events. In 1987, Kosro et al [1991] found mesoscale variability in a region extending from 60 to 150 km off the northern California coast to be greater in 2 cruises which occurred after the transition to strong upwelling than during 2 similar cruises in winter. Some of this variability may affect shelf circulation.…”

“…The same plot shows it is also surface intensified, and hence has a strong baroclinic component. Seasonal variability may also play a role in mesoscale forcing as Kosro et al [1991] Extension of the locally wind-forced cross-shelf transport model to areas beyond the northern California shelf is another natural question. The wind forcing and narrow, straight shelf off t he northern California coast are fairly typical of much of the North American Pacific coast, and I expect the model has a similar applicability over this region.…”

Diagnosing CO<sub>2</sub> fluxes in the upwelling system off the Oregon–California coast

“…1). While strong equatorward winds in spring/summer drive offshore Ekman transport at the surface over the coastal waters, the carbon-and nutrient-rich deep water is transported shoreward and upward over the shelf to compensate for the offshore transport in the surface layer (Huyer, 1983;Kosro et al, 1991;Federiuk and Allen, 1995;Gan and Allen, 2002). Outcrops of waters from depths of 150-200 m are frequently observed nearshore the Oregon-California shelf, where the surface partial pressure of CO 2 (pCO 2 ) can reach levels near 1000 µatm.…”