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Pacific Coastal & Marine Science Center

This is an abstract from the 1992 Eastern Pacific Ocean Conference at Timberline Lodge, Mt. Hood, Oregon.

Observations of Currents Over the Continental Slope near the Farallon Islands, CA

Marlene A. Noble and Steven R. Ramp

Six current meter moorings were deployed from February 1991 through March 1992 along two across-shore transects over the continental shelf and slope near the Farallon Islands, CA. One line had four moorings (A-D) between 100 and 1400 m depth, and the second line, 28 km to the north, had two moorings (F and E) at 400 and 2000 m depth. All the moorings were instrumented between 75 m depth and 15 m off the bottom. The goal of the program was to study the circulation patterns over the slope and their influence on the dispersal of dumped dredged materials in the water column and the transport of sediment over the shelf and slope.

The most prominent feature of the subtidal slope circulation was a wedge-shaped region of coherent flow between 75 and 800 m in the water column. This region was characterized by strong (>30 cm/s) poleward flow from mid-April through August, which is assumed to be the California Undercurrent. The current was at least 30 km wide during this time, but extended beyond the moored array and the offshore boundary was not well delineated. The flow weakened and became variable in direction during the fall, then strengthened again during the winter. The flow at mooring F (400 m depth) was anomalously weak relative to other observations over the California continental slope, and did not show any evidence of the undercurrent. The bottom instrument at mooring E (1987 m) showed strong, persistent onshore flow up a small submarine canyon.

The diurnal and semi-diurnal tidal currents at the six moorings showed strong bottom trapping at many sites where currents near the bottom had 2-3 times the amplitude of currents higher up in the water column. These enhanced bottom flows at tidal frequencies may be indirectly responsible for the up-canyon flow at mooring E due to rectification processes.

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