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Pacific Coastal & Marine Science Center

This is an abstract from the 1993 Eastern Pacific Ocean Conference at Stanford Sierra Camp, Fallen Leaf Lake, California.

The Subtidal Current Field Observed 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 had a variety of instruments spaced from 75 m below the surface to 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 dredged materials disposed of at proposed sites on the shelf and slope. The mean currents flowed northwestward along the slope, with the strongest currents found above 400 m. The spatial pattern of the subtidal flow field reflected the pattern of the mean currents. These oscillatory currents also flowed primarily parallel to the alongslope isobaths. Again, the strongest currents (with speeds in excess of 20 cm/s) were observed above 400 m in water depths between 400 and 1400 m. About 40% of the entire variability in the subtidal current field moved together in this well-defined spatial pattern. Subtidal currents measured deeper in the water column were weaker and showed temporal patterns that were more characteristic of a specific measurement site. Most of the variability in the subtidal current field had fluctuations with periods larger than 10 days. The annual fluctuation was particularly prominent. The strongest poleward flows were observed from March through August. Subsequently, the speed of the dominant flow pattern decreased in amplitude and became more restricted in its spatial extent. We assume that the flow dynamics for this portion of the current field was controlled by and is representative of the dynamics associated with the California Undercurrent. The wind does not directly affect the slope currents on these long time scales, for the wind is not correlated with the current field. However, on shorter timescales, periods of 3 to 8 days, the wind does drive currents over the slope down to depths of 200 m.


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