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Pacific Coastal & Marine Science Center

Monterey Bay Studies

Mooring deployment and recovery in Monterey Bay August 15 - August 18, 1996

USGS Guard Bouys on Deck

Overview: On August 15, 1996 the research vessel William A. McGaw sailed from Redwood City, California with 6 USGS scientists from Woods Hole, Massachusetts and Menlo Park, California on board. The purpose of the cruise was to deploy 5 current meter moorings. Three moorings were deployed in a line across the shelf south of Davenport, a single mooring was deployed inside Monterey Bay, south of Santa Cruz and one other mooring was deployed in Monterey Canyon. All 3 areas are part of the Monterey Bay National Marine Sanctuary.

The 4 shallow moorings (deployed in water less than 120 meters) were designed to measure near-surface, mid-depth and near-bed currents, water temperature and salinity. These moorings were also designed to monitor near-bed water clarity, collect sediments resuspended near the bed in sediment traps and record pressure, in addition to wave statistics at 2 of the sites. A surface guard buoy is attached to the top of each shallow mooring. The surface buoy has lights and radar reflectors to mark the mooring site. Instruments are usually hung 5 meters below the surface buoy to monitor near-surface currents and other water properties. Other instruments are hung further below the buoy to monitor processes that occur much deeper in the water column. The data collected will be used to learn more about the winter and summer transport patterns in this area.

The moorings will be in the water until April/May of 1997 at which time each mooring will be recovered and another mooring will be deployed at the same site. These moorings will stay in place until October, 1997.

In addition to the 4 moorings deployed near the shore another mooring was recovered and replaced at a site farther offshore, in Monterey Canyon in about 3223 meters of water. The USGS has maintained a mooring at this site since August 1993 to collect information about circulation patterns and sediment transport in Monterey Canyon.

Current meters in a rack

Purpose of the study: It is generally thought that the waters and resuspended materials on the Farallon shelf, the region south of Point Reyes and north of Davenport move in a single circulation unit. In late spring and early summer, the cold, nutrient-rich, upwelled waters that make this area productive join fresher water from San Francisco Bay and flow generally southeastward towards Monterey Bay. It is not known if these waters and material directly enter Monterey Bay because Ano Nuevo is a strong upwelling center. Satellite imagery suggests that the cold, nutrient-rich surface waters in this region either moves offshore in an jet, smaller but similar to the offshore jet that forms north of Point Reyes, or moves off the shelf and across the mouth of Monterey Bay. A tongue of this cold surface water can usually be seen fairly far offshore extending across the rim of Monterey Canyon. The surface water inside Monterey Bay itself tends to flow in a counter-clockwise gyre around the Bay in the summer. Hence at the northern end, there is a convergence of westward flowing surface water from Monterey Bay and southeastward flowing water from the Farallones shelf. Exactly what happens at this convergence zone is not really known. Water from the Farallon shelf may be forced offshore, where it could disappear or interact with another semi-permanent eddy that can bring the water back into the Bay over Monterey Canyon, or it could override the outflow from Monterey Bay and enter the Bay directly. The fate of either mid-depth or near-bed waters and suspended materials is also unknown because there are no observation in the region of flow beneath the surface layers.

In winter, water on the Farallon shelf tends to flow toward the northwest. The circulation in Monterey Bay is less well-known. Limited surface observations suggest that the counter-clockwise gyre is not as persistent as it is in summer - water in the northern portion of the Bay often flows toward the southeast. Hence, it is not known if the circulation patterns on the Farallon shelf and inside the Bay interact in either season.

It is hoped that the knowledge gained in this study will help us to understand:

current meter being readied for deployment

For additional information on this study, contact Marlene Noble

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