Currents and water masses of the Coastal Transition Zone off northern California, June to August 1988

Abstract

In summer 1988, we made repeated mesoscale surveys of a grid extending 200 km offshore between 37°N and 39°N in the coastal transition zone off northern California, obtaining continuous acoustic Doppler current profiler data and conductivity‐temperature‐depth data at standard stations 25 km apart on alongshore sections 40 km apart. All surveys showed a baroclinic equatorward jet, with core velocities of >50 cm s−1 at the surface decreasing to about 10 cm s−1 at 200 m, a width of 50–75 km, and a baroclinic transport of about 4 Sv. The core of the jet lay between the 8.6 and 9.4 m2 s−2 contours of geopotential anomaly (relative to 500 dbar). Three current meter moorings, deployed at 25‐km separation across the jet at the beginning of the survey sequence, provided time‐series of the velocity; throughout the 37‐day deployment, at least one mooring was within the core defined by the 8.6 and 9.4 m2 s−2 contours. The jet flowed southwestward across the grid from late June until mid‐July 1988, when the jet axis moved offshore in the north and onshore in the southern portion of the grid. Temperature‐salinity analysis shows that jet waters can be distinguished from both the freshly upwelled coastal waters and the offshore waters. Isopycnal maps indicate alongshore advection of relatively fresh, cool water from farther north, as well as small‐scale patchiness not resolved by our survey grid. The baroclinic jet observed here may be continuous with the core of the California Current off central California. The later surveys clearly showed a poleward‐flowing undercurrent adjacent to the continental slope, with core velocities up to 20 cm s−1 at depths of 150–250 m. Its baroclinic transport (relative to 500 dbar) increased from <0.5 Sv to >1.0 Sv between late June and early August 1988. Within the survey grid, there was a definite onshore gradient in the characteristics of North Pacific Intermediate Water. The subsurface waters adjacent to the continental margin were warmer and more saline than those offshore, indicating net northward advection by the California Undercurrent over the inshore 100 km and equatorward advection farther from shore.

Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems

“…These sites have been previously recognized as hot spots for eddy generation [Chaigneau et al, 2009;Kurian et al, 2011]. The observed negative S 0 (Figure 8c) is probably due to the local eddy-induced downwelling of the surface layers composed of fresh Pacific Subarctic Water [Simpson et al, 1984;Emery and Meincke, 1986;Huyer et al, 1991;Checkley and Barth, 2009;Dong et al, 2012] that can be clearly identified in Figure 9a (color shading). The temporal evolution of the corresponding composite anomalies is consistent with their mean profiles and their intensity does not present strong variations between the growth, mature and decay phases (Figures 10a-10c).…”

“…by the California Current [Simpson et al, 1984;Emery and Meincke, 1986;Huyer et al, 1991;Checkley and Barth, 2009;Dong et al, 2012]. Both the shoaling of the isopycnals associated with these surface-intensified eddies and the advection of saltier near-coastal water across fresher offshore waters can lead to this observed positive S 0 .…”

Marine mammal occurrence and ocean climate off central California, 1986 to 1994 and 1997 to 1999

“…The onset of upwelling (spring through late summer) differs annually, and is marked by the 'spring transition' (Huyer et al 1990), when sea-surface temperatures drop abruptly in association with a sudden strengthening of the northwest wind (Breaker & Mooers 1986). This equatorward wind stress, in combination with Earth's rotation, leads to the offshore transport of surface water (Ekman transport).…”

Leg 167 Introduction

“…The southward jets separate nutrient-rich upwelled waters from the relatively barren offshore waters (e.g., Huyer et al, 1991). The core of the offshore California Current flow is located approximately 250−350 km from the coast at the border of Oregon and California and is about 300 km from the coast at Point Conception (~35°N; Hickey, 1979;Lynn and Simpson, 1987).…”