U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications


Volume transport in the Alaska Coastal Current

James D. Schumacher, Phyllis J. Stabeno, and Andrew T. Roach

NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Continental Shelf Research, 9(12), 1071-1083 (1989)
Not subject to U.S. copyright. Published in 1990 by Pergamon Press.

Conclusions

For the first time, estimates of volume transport in the ACC are available from both moored current and bottom pressure records. Our field observations from the Shelikof sea valley and adjacent shelf region between August 1984 and July 1985 lead to the following conclusions:

1. The mean volume transport of the ACC calculated from current records was 0.85 × 10 m s. This is in good agreement with estimates of transport from CTD data (provided that the level of no motion is carefully selected to approximate the two-layered velocity field generally present over the sea valley). Approximately 75% of the mean transport was through the sea valley with the remaining flux along the Alaska Peninsula. There was a relative volume transport maximum (>1.0 × 10 m s) associated with accumulation of freshwater in autumn. The greatest monthly transport, however, occurred in winter and were associated with wind-driven perturbations.

2. Wind forcing was the primary cause of fluctuations in transport. This occurred through Ekman convergence, Ekman pumping (resulting from the curl of the wind stress), and the convergence of transport through Shelikof Strait with that over the open shelf. From estimates of coherence, about half of the transport fluctuations in the ACC were accounted for by the alongshore wind.

3. At sections 2 and 3, transport fluctuations were generally geostrophic. Geostrophy accounted for about 62% of the variance of transport at section 3 and 50% at section 2. While neglecting the terms that are a function of density seemed of little consequence at sections 2 and 3, estimates of geostrophic transport from bottom pressure across section 1 may require time series of density.

The new results further document the relatively vigorous nature of the ACC. Mean transport here is similar in magnitude and more consistent in direction than the northward flux through Bering Strait (Muench et al., 1988; Coachman and Aagaard, 1988). Volume transport in the ACC is also greater than observed values for the east coast of North America. Estimates of transport along the shelf of Nova Scotia indicate an annual mean flux of about 0.25 × 10 m s which is related to outflow from the Gulf of St. Lawrence (Drinkwater et al., 1979). Farther south along the coast at Nantucket Shoals, estimates of annual mean volume transport were approximately 0.35 × 10 m s (Ramp et al., 1988). Santa Barbara channel (off the west coast of North America) is a region with similar topography to Shelikof Strait; however, forcing for circulation here is oceanic rather than regional runoff. Estimates of transport calculated from current data (Brink and Muench, 1986) for a 2-month period indicate a mean volume flux similar to those during winter in the ACC. Clearly, the ACC is one of the largest and most consistent nearshore currents found along the North American coast.

Acknowledgments--We wish to thank the many people who assisted in field operations, data processing and discussions. In particular we thank the complements of the NOAA ships Fairweather and Discoverer and the USCG ship Firebush. Special thanks to T. Jackson and W. Parker who prepared all the equipment and deployed and recovered the moorings. L. Long and P. Proctor processed the time series with great care and patience. Discussions with R. Reed, L. Incze and R. Romea were extremely useful. Reviewers comments improved the manuscript. This publication is contribution 0071 to the Fisheries Oceanography Coordinated Investigations (FOCI) of NOAA. Contribution no. 984 from Pacific Marine Environmental Laboratory.


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