SHELFBREAK CIRCULATION, FRONTS AND PHYSICAL OCEANOGRAPHY: EAST AND WEST COAST PERSPECTIVES

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Abstract
A survey of fundamental physical oceanographic processes that may affect sediment distribution along shelfbreak regions is presented, emphasizing the Atlantic and Pacific coasts of the USA. The processes encompass the entire spectrum of known motions and are thus generic to all shelfbreak interfacial zones. These shelfbreak strips couple the bounded coastal oceans to the open seas, but there is no systematic pattern to this coupling. In the South Atlantic Bight, the Gulf Stream acts like a vibrating, permeable wall which can variously entrain shelf waters, flood the shelf with North Atlantic Central Water and violently mix shelf waters by towing whirling vortices through the outer shelf. Middle Atlantic Bight, New York Bight and Gulf of Maine shelfbreak processes contain many of the dynamic elements of their southeastern counterpart, but the relative importances of various random surface and offshore driving forces change. Pacific coast shelfbreak processes tend to be less energetic than those on the Atlantic coast since the Pacific coast is missing a Western Boundary Current and because the shelf is narrow and deep. Subinertial frequency shelfbreak motions on the west coast are typically manifested across the entire shelf, while those on the east coast tend to be confined to a loosely defined band, which brackets the break. Principal Pacific coast circulation elements include forms of continental shelf waves and thermohaline driven and mechanically wind forced currents, as well as the California Current System. While high frequency edge waves and inertial currents are indigenous in similar fashion to all coasts, east and west coast tides are shown to be quite disparate, given tradeoffs between dominance of diurnal and semidiurnal constituents as a function of topographic constraint and strength of density stratification. All of the shelfbreak zones are graced by thermohaline fronts. The fronts are progradational on the west and south-eastern coasts and retrogradational on the northeastern shelf. These fronts are an integral ingredient of all aspects of physical processes at the shelfbreak strip. The interplay of bottom topography with the physics of the outer continental margin is significant. Bottom features such as shoals, bumps, ridges and canyons are shown to be regions of sediment erosion, deposition and draping. Moreover, these features are shown to be causally related to upwelling and down-welling phenomena and to the deflection and scattering of waves and currents. Both subtidal and supertidal frequency events are shown to be capable of initiating sediment motion and of suspending sediments, but lower frequency events are shown to be responsible for the buLk of sediment migration on the outer shelf and upper slope environs.

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