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Pacific Coastal & Marine Science Center

This is an abstract from the 1991 Eastern Pacific Ocean Conference at Lake Arrowhead, California.

Enhanced Diurnal tidal currents in the Gulf of the Farallones

Marlene A. Noble

The Gulf of the Farallones is a complex region of the northern California shelf that lies west of the entrance to San Francisco Bay. The shelf at the northern and southern boundaries of the Gulf is narrow, with widths less than 12 km. The shelf is nearly 4 times broader off San Francisco Bay. The depth of the mid-shelf portion of the Gulf has considerable variation, with depths ranging from 100 m to the north, 65 m in the middle and 90 m to the south. The Farallon Island chain is located at the seaward edge of this wide portion of the shelf. In April 1988, 2 current-meter moorings were deployed for a year in water depths of 42 and 87 m. Subsequently, in May 1989, 2 other moorings were deployed for 5 months in 60 m of water, at sites separated by 15 km in the central portion of the Gulf.

The tidal currents accounted for 30 to 58% of the current variance. Surprisingly enough, in a region where the semi-diurnal tides were thought dominant, the K1 diurnal tide was the largest. The K1 tidal ellipse was oriented alongshelf and had amplitudes between 7.8 and 8.8 cm/s along the major axes at all sites. The current in the ellipses rotated in a clockwise sense. Both the phase and amplitudes of the diurnal tides were fairly constant over the year and a half deployment periods. The diurnal tides were not the strongest tidal component in sea level. Water level measurements at the coast and bottom pressure measurements at a site in the middle of the Gulf showed that the semi-diurnal component was dominant.

The observations indicate that the diurnal currents were not driven by the diurnal component of the wind stress. The diurnal currents had a constant amplitude over the year, while the diurnal wind stress did not. In addition, the diurnal frequencies of the currents matched the astronomical forcing frequencies at K1, O1, and P1. The diurnal winds had a 24 hour period. We suspect that the diurnal currents are topographic waves that are strongly influenced by the geometry of the Gulf of the Farallones.


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