Circulation near a submarine canyon is analyzed with a numerical model. Previous theoretical work indicated that stratification controlled the interaction of coastal flow with canyons, specifically, the ratio of canyon width to the internal radius of deformation. A wide canyon was thought to merely steer the flow, while a narrow canyon would create substantia• cross-shelf exchange. Four cases are analyzed considering two directions of alongshore flow and two choices of initial stratification. The weakly stratified case has an internal radius about equal to the canyon width, while the strongly stratified case has one about 3 times the canyon width. The direction of the alongshore flow is shown in this study to be the more important of the two factors. In particular, right-bounded flow (flow with the coast on the right, looking downstream in the northern hemisphere) leads to shallow downwelling in the c•nyon and we•k exchange •cross the shelf bre•k, while left-bounded flow creates upwelhng at the head of the canyon and strong exchange between the ocean and shelf. In left-bounded flow (upwelling), dense water is pumped onto the shelf, even for strong stratification. However, the stratification hmits the vertical extent of the topographic influence so that the alongshore flow above the canyon is only weakly affected in the strongly stratified case. With any level of stratification the surface temperature (density) is not modified at all by the flow interaction with the sub•narine canyon. The important dynamics involve pressure gradients and Coriolis acceleration and how they interact with the bathymetric gradients but not advection of momentum. Advection of density is clearly important in the upwelling cases. Finally, continued upwelling onto the shelf acts as a drag mechanism and retards the alongshore coastal flow. A study of circulation and sediment transfer in Quinault Canyon off the U.S. northwest coast [Hickey ½l al., 1986] found that subtidal variations in the circulation in the canyon and vertical excursion of turbidity within the canyon were correlated with variations in the alongshore flow. A similar study in Astoria Canyon (B. Hickey, Response of a narrow submarine canyon to strong wind forcing, submitted to Journal of Physical Oceanography, 1995] (hereinafter referred to as submitted manuscript, 1995), also located on the U.S. north-1211 1212 KLINCK: CIRCULATION NEAR SUBMARINE CANYONS west coast, analyzed the response to wind-driven reversal of the alongshore flow. Left-bounded flow was shown to create strong upwelling and cyclonic circulation within the canyon. Weak alongshore flow seemed to result in a closed circulation above the canyon. For stronger flow there was only a weak disturbance over the canyon. Baltimore canyon is typical of submarine canyons on the U.S. northeastern coast which are 10 km in width and about 3 times as long as they are wide. Hunkins [1988] demonstrated several peculiarities of canyon circulation. The mean alongshore flow above the level of the surrounding shelf is unaffected ...

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Oceanic exchanges across the continental shelves of Antarctica play an important role in biological systems and the mass balance of ice sheets. The focus of this study is on the mechanisms responsible for the circulation of warm Circumpolar Deep Water (CDW) within troughs running perpendicular to the continental shelf. This is examined using process-oriented numerical experiments with an eddy-resolving (1 km) 3D ocean model that includes a static and thermodynamically active ice shelf. Three mechanisms that create a significant onshore flow within the trough are identified: 1) a deep onshore flow driven by the melt of the ice shelf, 2) interaction between the longshore mean flow and the trough, and 3) interaction between a Rossby wave along the shelf break and the trough. In each case the onshore flow is sufficient to maintain the warm temperatures underneath the ice shelf and basal melt rates of O(1 m yr−1). The third mechanism in particular reproduces several features revealed by moorings from Marguerite Trough (Bellingshausen Sea): the temperature maximum at middepth, a stronger intrusion on the downstream edge of the trough, and the appearance of warm anticyclonic anomalies every week. Sensitivity experiments highlight the need to properly resolve the small baroclinic radii of these regions (5 km on the shelf)—simulations at 3-km resolution cannot reproduce mechanism 3 and the associated heat transport.

Intrusion of Upper Circumpolar Deep Water (UCDW), which was derived from the Antarctic Circumpolar Current (ACC), onto the western Antarctic Peninsula (WAP) shelf region in January 1993 provided a reservoir of nutrient-rich, warmer water below 150 m that subsequently upwelled into the upper water column. Four sites, at which topographically-induced upwelling of UCDW occurred, were identi ed in a 50 km by 400 km band along the outer WAP continental shelf. One additional site at which wind-driven upwelling occurred was also identi ed. Diatom-dominated phytoplankton assemblages were always associated with a topographically-induced upwelling site. Such phytoplankton communities were not detected at any other shelf location, although diatoms were present everywhere in the 80,000 km 2 study area and UCDW covered about one-third the area below 150 m. Phytoplankton communities dominated by taxa other than diatoms were restricted to transition waters between the UCDW and shelf waters, the southerly owing waters out of the Gerlache Strait, and/or the summertime glacial ice melt surface waters very near shore. We suggest that in the absence of episodic intrusion and upwelling of UCDW, the growth requirements for elevated silicate/nitrate ratios and/or other upwelled constituents (e.g. trace metals) are not sufficiently met for diatoms to achieve high abundance or community dominance. One consequence of this is that the ice-free regions of the outer WAP continental shelf will not experience predictable spring diatom blooms. Rather, this region will experience episodic diatom blooms that occur at variable intervals and during different seasonal conditions, if the physical structuring events are occurring. Preferential drawdown of silicate relative to nitrate was observed at each of the offshore upwelling sites and resulted in a reduction in the ambient silicate:nitrateratio relative to the correspondingvalue