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  • Author or Editor: Glen G. Gawarkiewicz x
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Weifeng G. Zhang and Glen G. Gawarkiewicz

Abstract

Through combining analytical arguments and numerical models, this study investigates the finite-amplitude meanders of shelfbreak fronts characterized by sloping isopycnals outcropping at both the surface and the shelfbreak bottom. The objective is to provide a formula for the meander length scale that can explain observed frontal length scale variability and also be verified with observations. Considering the frontal instability to be a mixture of barotropic and baroclinic instability, the derived along-shelf meander length scale formula is [b 1/(1 + a 1 S 1/2)]NH/f, where N is the buoyancy frequency; H is the depth of the front; f is the Coriolis parameter; S is the Burger number measuring the ratio of energy conversion associated with barotropic and baroclinic instability; and a 1 and b 1 are empirical constants. Initial growth rate of the frontal instability is formulated as [b 2(1 + a 1 S 1/2)/(1 + a 2 α S 1/2)]NH/L, where α is the bottom slope at the foot of the front, and a 2 and b 2 are empirical constants. The formulas are verified using numerical sensitivity simulations, and fitting of the simulated and formulated results gives a 1 = 2.69, b 1 = 14.65, a 2 = 5.1 × 103, and b 2 = 6.2 × 10−2. The numerical simulations also show development of fast-growing frontal symmetric instability when the minimum initial potential vorticity is negative. Although frontal symmetric instability leads to faster development of barotropic and baroclinic instability at later times, it does not significantly influence the meander length scale. The derived meander length scale provides a framework for future studies of the influences of external forces on shelfbreak frontal circulation and cross-frontal exchange.

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M. Susan Lozier, Mark S. C. Reed, and Glen G. Gawarkiewicz

Abstract

In an attempt to understand whether local instabilities can account for the observed frontal variability in the Middle Atlantic Bight, a linear stability analysis was conducted for a wide range of background density and velocity fields. Three-dimensional perturbations superposed on a continuously stratified shelfbreak front were investigated using the hydrostatic primitive equations. Model results indicate that the shelfbreak frontal jet is unstable over the wide parameter range dictated by the observed velocity and density structure. Model growth rates, on the order of one day, and wavelengths of ∼10–50 km compare favorably to observations, suggesting that local baroclinic/barotropic instabilities are a likely source for the strong temporal and spatial variability of the shelfbreak front in the Middle Atlantic Bight.

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Robert E. Todd, Glen G. Gawarkiewicz, and W. Brechner Owens

Abstract

Observations with fine horizontal resolution are used to identify the horizontal scales of variability over the Middle Atlantic Bight (MAB) shelf break and continental rise. Spray gliders collected observations along two alongshelf transects over the continental rise in March–April 2006 and along 16 cross-shelf transects over the shelf break and continental rise during July–October 2007. Horizontal resolution varied from 1 km or finer over the shelf to 6 km in deep water. These observations allow horizontal thermohaline variability offshore of the MAB shelf break to be examined for the first time. Structure functions of temperature and salinity, the mean square difference between observations separated by specified distances, reveal the horizontal spatial scales in the region. Exponential (e-folding) scales of temperature and salinity increase from 8–13 km near the shelf break to about 30 km over the continental rise. Just offshore of the shelf break, alongshelf structure functions exhibit periodicity with a 40–50-km wavelength that matches the wavelength of shelfbreak frontal meanders. Farther offshore, alongshelf structure functions suggest a dominant wavelength of 175–250 km, but these scales are only marginally resolved by the available observations. Examination of structure functions of along-isopycnal salinity (i.e., spice) suggests that interleaving of shelf and slope water masses contributes most of the horizontal variability near the MAB shelf break, but heaving of isopycnals is the primary source of horizontal variability over the continental rise.

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Weifeng G. Zhang, Glen G. Gawarkiewicz, Dennis J. McGillicuddy Jr., and John L. Wilkin

Abstract

A two-dimensional cross-shelf model of the New England continental shelf and slope is used to investigate the mean cross-shelf and vertical circulation at the shelf break and their seasonal variation. The model temperature and salinity fields are nudged toward climatology. Annual and seasonal mean wind stresses are applied on the surface in separate equilibrium simulations. The along-shelf pressure gradient force associated with the along-shelf sea level tilt is tuned to match the modeled and observed depth-averaged along-shelf velocity. Steady-state model solutions show strong seasonal variation in along-shelf and cross-shelf velocity, with the strongest along-shelf jet and interior onshore flow in winter, consistent with observations. Along-shelf sea level tilt associated with the tuned along-shelf pressure gradient increases shoreward because of decreasing water depth. The along-shelf sea level tilt varies seasonally with the wind and is the strongest in winter and weakest in summer. A persistent upwelling is generated at the shelf break with a maximum strength of 2 m day−1 at 50-m depth in winter. The modeled shelfbreak upwelling differs from the traditional view in that most of the upwelled water is from the upper continental slope instead of from the shelf in the form of a detached bottom boundary layer.

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Claudia Cenedese, Robert E. Todd, Glen G. Gawarkiewicz, W. Brechner Owens, and Andrey Y. Shcherbina

Abstract

Interactions between vortices and a shelfbreak current are investigated, with particular attention to the exchange of waters between the continental shelf and slope. The nonlinear, three-dimensional interaction between an anticyclonic vortex and the shelfbreak current is studied in the laboratory while varying the ratio ε of the maximum azimuthal velocity in the vortex to the maximum alongshelf velocity in the shelfbreak current. Strong interactions between the shelfbreak current and the vortex are observed when ε > 1; weak interactions are found when ε < 1. When the anticyclonic vortex comes in contact with the shelfbreak front during a strong interaction, a streamer of shelf water is drawn offshore and wraps anticyclonically around the vortex. Measurements of the offshore transport and identification of the particle trajectories in the shelfbreak current drawn offshore from the vortex allow quantification of the fraction of the shelfbreak current that is deflected onto the slope; this fraction increases for increasing values of ε. Experimental results in the laboratory are strikingly similar to results obtained from observations in the Middle Atlantic Bight (MAB); after proper scaling, measurements of offshore transport and offshore displacement of shelf water for vortices in the MAB that span a range of values of ε agree well with laboratory predictions.

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