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Franklin B. Schwing

Abstract

Data collected during the Canadian Atlantic Storms Program (CASP) suggest that two primary current regimes exist on the Scotian Shelf during winter. Moorings inside the 100-m isobath feature currents that are parallel to the general bathymetry and have small eastward mean velocities. The time-varying part of the flow is nearly rectilinear alongshelf at typical maximum velocities of 0.2–0.3 m s−1 and oscillates on 2–5-day periods in response to meteorological forcing. The current at locations beyond the 100-m isobath exhibits a persistent westward flow of about 0.3 m s−1, denoting the dominance of the Nova Scotian Current, although smaller magnitude along- and cross-shelf fluctuations occur at synoptic frequencies. Flow in both regions is largely uniform with depth at this time of year.

The velocity response to local and remote sources of forcing was examined using a frequency (ω)-dependent multiple regression model. Flow is dominated by nonlocal forcing at ω < 0.2 cpd; alongshelf wind stress is the principal local influence at low frequencies. Remote forcing is less important at higher frequencies, while the role of local cross-shelf wind stress is greater. The effect of local wind forcing is generally small and varies substantially on horizontal scales of order 10 km. Current typically lags wind stress by about 30°–45° at ω < 0.5 cpd; the response to alongshelf wind stress also suggests a phase propagation to the west. The typical response to alongshelf wind forcing is consistent with previous quasi-steady observations and model results from the Scotian Shelf. The contribution of cross-shelf wind stress is limited to a near-surface flux in the synoptic band. The remote response decreases offshore and to the west and propagates westward, in the direction that coastally trapped waves propagate. The current response to local and remote forcing is uniform with depth at nearly all locations. Local and remote contributions to alongshelf velocity, integrated across the mooring array, are in geostrophic balance with the cross-shelf subsurface pressure (SSP) gradient. Together, the two wind stress components and a SSP record representing nonlocal forcing explain 40%–80% of alongshelf flow, compared to 95% of the SSP signal.

Some of the large-scale flow features are consistent with variations seen in SSP data. The most obvious similarity is the remotely forced response. However, these results also reveal substantial variations in the current on scales of 10–100 km that are not reflected in the SSP field. Irregular shelf bathymetry is thought to induce the observed variations in circulation. This study also demonstrates the need to include remote forcing effects, in the form of a properly prescribed backward boundary condition, in shelf circulation models.

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Franklin B. Schwing

Abstract

A linear, barotropic numerical model that features realistic bathymetry of the Scotian Shelf provides solutions forced by steady and periodic wind stress that are generally incoherent on spatial scales of the shelf width. Closed circulation cells occur in association with, and on the scales of, major bathymetric features. Bathymetrically steered flow is prevalent, and more evident at lower frequencies. Model transport forced by a coastally trapped wave propagating across the backward boundary is not sensitive to the model structure of the incident wave; bathymetry rapidly modifies the wave form as it propagates in the free direction. Higher modes and reflected waves are probably dissipated over short distances by mattering and strong bottom stress. Patterns seen in the model solutions are consistent with results from more fundamental models, and are readily explained by relatively simple vorticity balances.

Model results generally reproduce the diverse local and nonlocal velocity responses observed on the Scotian Shelf during winter. The model is more effective in accounting for the remote contribution to circulation. Wind-forced results generally reflect the observed directional variability, and are similar in magnitude to observations inside the 100-m isobath but underestimate the actual current response at deeper stations. It is speculated that spatial variability in wind stress, an effect not addressed in this work, may be responsible for some of this difference. The significant of baroclinic effects in deeper water, which cannot be discounted, is also beyond the scope of this model. Nevertheless, the fairly good agreement between the model and observed current responses to local and nonlocal forcing confirms the importance of bathymetric modifications to Scotian Shelf circulation at subtidal frequencies.

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Franklin B. Schwing
and
Jackson O. Blanton

Abstract

The use of land based wind data in nearshore oceanographic work is common, but these winds do not accurately reflect coastal oceanic winds. Ocean winds are often underestimated by a factor of 2 and directional differences are also observed. Wind time series from land and sea regimes in the South Atlantic Bight (SAB) were applied to a reduced form of the momentum equation to estimate the alongshore current. Currents were closely approximated by ocean wind stress, but were consistently underestimated by land data. Further statistical analyses verified this discrepancy in speed and also indicated significant differences between ocean and speed-adjusted land winds. The bottom frictional coefficient required to balance alongshore momentum was unrealistically small when land based wind data were used as input.

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Franklin B. Schwing
,
Lie-Yauw Oey
, and
Jackson O. Blanton

Abstract

Because of relatively strong tidal currants and little notification, the response of continental shelf water to wired off South Carolina is predominantly frictional and barotropic. Currents measured at the 10, 30 and 40 m isobaths were highly coherent with wind and coastal sea level (CSL). Alongshelf current at 10 m responded rapidly to wind oscillations and led CSL by 6–12 hours at periods greater than 2 days. Coastal sea level led, or was in phase with, alongshelf currents at the 30 and 40 m isobaths.

A linear frictional barotropic model that assumes alongshelf homogeneity is used to explain the observed phase relationships. Cross-shelf variation in bottom friction due to the change in water depth and a fluctuating alongshelf pressure gradient at the shelf break which lags slightly behind the wind account for most of the observed features. Nearshore flow is dominated by frictional forces while inertial terms are important on the outer shelf. The boundary separating these two regions is the point at which alongshelf current is in phase with CSL.

Model results are summarized in a response diagram defining five nondimensional quantities: CSL amplitude and phase, width of the nearshore frictional strip, bottom friction coefficient, and alongshelf pressure gradient amplitude at the shelf break. Observations from South Carolina and other broad shelf regions are correlated in the diagram. Observations and model results provide a clear understanding of the frictionally controlled, wind-driven barotropic dynamics in shallow continental shelf regions.

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