Climatological Mean Circulation at the New England Shelf Break

Zhang, Weifeng G.; Gawarkiewicz, Glen; McGillicuddy, Dennis J.; Wilkin, John

doi:10.1175/2011jpo4604.1

Cited by 51 publications

(69 citation statements)

References 50 publications

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“…This sea level gradient is consistent with the value estimated by Xu and Oey (2011), which reported a slope of 4.8 × 10 −8 with a range of 10 −7 , and is also consistent with the estimate by Zhang et al (2011), which is 0.2-2.5 × 10 −7 . It is equivalent to a 0.36 m mean sea level difference over the 1600 km distance from the Nova Scotia shelf to the North Carolina shelf, arguably contributing to the equatorward mean shelf circulation.…”

Section: Mean Dynamical Balances Along the 200 M Isobathsupporting

confidence: 89%

Mean circulation in the coastal ocean off northeastern North America from a regional-scale ocean model

Chen

2015

Ocean Sci.

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Abstract. A regional-scale ocean model was used to hindcast the coastal circulation over the Middle Atlantic Bight (MAB) and Gulf of Maine (GOM) from 2004 to 2013. The model was nested inside a data assimilative global ocean model that provided initial and open boundary conditions. Realistic atmospheric forcing, tides and observed river runoff were also used to drive the model. Hindcast solutions were compared against observations, which included coastal sea levels, satellite altimetry sea surface height, in situ temperature and salinity measurements in the GOM, and observed mean depth-averaged velocities. Good agreements with observations suggest that the hindcast model is capable of capturing the major circulation variability in the MAB and GOM. Time-and space-continuous hindcast fields were used to depict the mean circulation, along-and cross-shelf transport and the associated momentum balances. The hindcast confirms the presence of the equatorward mean shelf circulation, which varies from 2.33 Sv over the Scotian Shelf to 0.22 Sv near Cape Hatteras. Using the 200 m isobath as the shelf/slope boundary, the mean cross-shelf transport calculations indicate that the shelfbreak segments off the Gulf of Maine (including the southern flank of Georges Bank and the Northeast Channel) and Cape Hatteras are the major sites for shelf water export. The momentum analysis reveals that the along-shelf sea level difference from Nova Scotia to Cape Hatteras is about 0.36 m. The nonlinear advection, stress, and horizontal viscosity terms all contribute to the ageostrophic circulation in the along-isobath direction, whereas the nonlinear advection plays a dominant role in determining the ageostrophic current in the cross-isobath direction.

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Section: Mean Dynamical Balances Along the 200 M Isobathsupporting

confidence: 89%

Mean circulation in the coastal ocean off northeastern North America from a regional-scale ocean model

Chen

2015

Ocean Sci.

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“…In the model results, the source of the upwelled water is solely from the continental shelf, whereas some observations infer a double-sided convergence (Pickart 2000). A model of the Middle Atlantic Bight shelf break forced by climatology (temperature, salinity, and wind stress) indicates upwelling with the dominant source from offshore of the shelf break (Zhang et al 2011). An alongshelf pressure gradient supports an onshore geostrophic flow over the shelf (Lentz 2008).…”

Section: A Comparison With Observations and A Climatology-forced Modelmentioning

confidence: 84%

“…Upwelling rates range from vertical velocities of 9 6 2 (Barth et al 1998) and 23 m day 21 (Pickart 2000), inferred from ADCP measurements, 4 to 7 m day 21 (Houghton and Visbeck 1998) and 6 to 10 m day 21 (Houghton et al 2006) from dye tracer experiments, and an along-isopycnal vertical velocity of 17.5 m day 21 from a subsurface isopycnal float (Barth et al 2004). With a shelfbreak model forced by climatology, vertical velocities reach 2 m day 21 in winter, and offshore sources rather than the shelf contribute to most of the upwelling (Zhang et al 2011). These results motivate further investigation of the mechanisms driving upwelling at the shelf break and source region for this vertical flux.…”

Section: Introductionmentioning

confidence: 99%

Rapid Generation of Upwelling at a Shelf Break Caused by Buoyancy Shutdown

Benthuysen

Thomas

Lentz

2015

Journal of Physical Oceanography

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Model analyses of an alongshelf flow over a continental shelf and slope reveal upwelling near the shelf break. A stratified, initially uniform, alongshelf flow undergoes a rapid adjustment with notable differences onshore and offshore of the shelf break. Over the shelf, a bottom boundary layer and an offshore bottom Ekman transport develop within an inertial period. Over the slope, the bottom offshore transport is reduced from the shelf's bottom transport by two processes. First, advection of buoyancy downslope induces vertical mixing, destratifying, and thickening the bottom boundary layer. The downward-tilting isopycnals reduce the geostrophic speed near the bottom. The reduced bottom stress weakens the offshore Ekman transport, a process known as buoyancy shutdown of the Ekman transport. Second, the thickening bottom boundary layer and weakening near-bottom speeds are balanced by an upslope ageostrophic transport. The convergence in the bottom transport induces adiabatic upwelling offshore of the shelf break. For a time period after the initial adjustment, scalings are identified for the upwelling speed and the length scale over which it occurs. Numerical experiments are used to test the scalings for a range of initial speeds and stratifications. Upwelling occurs within an inertial period, reaching values of up to 10 m day 21 within 2 to 7 km offshore of the shelf break. Upwelling drives an interior secondary circulation that accelerates the alongshelf flow over the slope, forming a shelfbreak jet. The model results are compared with upwelling estimates from other models and observations near the Middle Atlantic Bight shelf break.

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“…Parameters for the base case and model runs with wind stress magnitude, initial stratification, and bottom slope variation were chosen to match those used by Tilburg (2003 (Lentz 2008) and density (Zhang et al 2011) indicate that uniformly sloped isopycnals are representative of the seasonal mean near Martha's Vineyard. A thermal wind shear is spun up in the first inertial period of the model run.…”

Section: Model Runsmentioning

confidence: 99%

Inner-Shelf Response to Cross-Shelf Wind Stress: The Importance of the Cross-Shelf Density Gradient in an Idealized Numerical Model and Field Observations

Horwitz

Lentz

2014

Journal of Physical Oceanography

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This study investigates the effects of horizontal and vertical density gradients on the inner-shelf response to cross-shelf wind stress by using an idealized numerical model and observations from a moored array deployed south of Martha's Vineyard, Massachusetts. In two-dimensional (no along-shelf variation) numerical model runs of an initially stratified shelf, a cross-shelf wind stress drives vertical mixing that results in a nearly wellmixed inner shelf with a cross-shelf density gradient because of the sloping bottom. The cross-shelf density gradient causes an asymmetric response to on-and offshore wind stresses. For density increasing offshore, an offshore wind stress drives a near-surface offshore flow and near-bottom onshore flow that slightly enhances the vertical stratification and the cross-shelf circulation. An onshore wind stress drives the reverse cross-shelf circulation reducing the vertical stratification and the cross-shelf circulation. A horizontal Richardson number is shown to be the nondimensional parameter that controls the dependence of the wind-driven nondimensional cross-shelf transport on the cross-shelf density gradient. Field observations show the same empirical relationship between the horizontal Richardson number and transport fraction as the model predicts. These results show that it is the cross-shelf rather than vertical density gradient that is critical to predicting the inner-shelf cross-shelf transport driven by a cross-shelf wind stress.

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Climatological Mean Circulation at the New England Shelf Break

Cited by 51 publications

References 50 publications

Mean circulation in the coastal ocean off northeastern North America from a regional-scale ocean model

Mean circulation in the coastal ocean off northeastern North America from a regional-scale ocean model

Rapid Generation of Upwelling at a Shelf Break Caused by Buoyancy Shutdown

Inner-Shelf Response to Cross-Shelf Wind Stress: The Importance of the Cross-Shelf Density Gradient in an Idealized Numerical Model and Field Observations

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