Subtidal inner-shelf circulation near Point Conception, California

Abstract 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 well-mixed 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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Alongshelf Variability of Inner-Shelf Circulation along the Central Oregon Coast during Summer

Journal of Physical Oceanography ◽

10.1175/2008jpo3760.1 ◽

2009 ◽

Vol 39 (6) ◽

pp. 1380-1398 ◽

Cited By ~ 25

Author(s):

Anthony R. Kirincich ◽

John A. Barth

Keyword(s):

Momentum Equation ◽

Wind Forcing ◽

Spatial And Temporal Variability ◽

Local Wind ◽

Inner Shelf ◽

Oregon Coast ◽

Bottom Stress ◽

Shelf Circulation ◽

Invertebrate Species ◽

Local Wind Forcing

Abstract The spatial and temporal variability of inner-shelf circulation along the central Oregon coast during the 2004 upwelling season is described using a 70-km-long array of moorings along the 15-m isobath. Circulation at three stations located onshore of a submarine bank differed from that of a station north of the bank, despite the relatively uniform wind forcing and inner-shelf bathymetry present. During upwelling-favorable winds, strong southward alongshelf flow occurred north of the bank, no alongshelf flow occurred onshore of the northern part of the bank, and increasing southward flow occurred onshore of the southern part of the bank. During downwelling-favorable winds, strong northward flow occurred in the inner shelf onshore of the bank while weak flow occurred north of the bank. These alongshelf differences in inner-shelf circulation were due to the effects of the bank, which isolated the inner shelf onshore of the bank from the regional upwelling circulation that was evident at the northernmost station. As a result, circulation onshore of the bank was driven primarily by local wind forcing, while flow north of the bank was only partially driven by local winds. A secondary mode of variability, attributed to the movement of the regional upwelling jet due to remote forcings, contributed the bulk of the variability observed north of the bank. With the time-dependent wind forcing present, acceleration was an important term in the depth-averaged alongshelf momentum equation at all stations. During upwelling, bottom stress and acceleration opposed the wind stress north of the bank, while bottom stress was weaker onshore of the bank where the across-shelf momentum flux and the alongshelf pressure gradient balanced the residual of the acceleration and stresses. During downwelling, waters onshore of the bank surged northward at magnitudes much larger than that found north of the bank. These spatial variations developed as the season progressed and the regional upwelling circulation intensified, explaining known variations in growth and recruitment of nearshore invertebrate species.

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The Effects of Tides and Oscillatory Winds on the Subtidal Inner-Shelf Cross-Shelf Circulation

Journal of Physical Oceanography ◽

10.1175/2009jpo4273.1 ◽

2010 ◽

Vol 40 (4) ◽

pp. 775-788 ◽

Cited By ~ 7

Author(s):

Renato Castelao ◽

Robert Chant ◽

Scott Glenn ◽

Oscar Schofield

Keyword(s):

Bottom Topography ◽

Surface Wind ◽

Low Frequency ◽

Inner Shelf ◽

Tidal Forcing ◽

Shelf Circulation ◽

Viscosity Coefficients ◽

Middle Atlantic ◽

The Cross ◽

Shelf Flow

Abstract A two-dimensional numerical model is used to investigate the effects of tidal forcing and oscillatory winds on the subtidal cross-shelf circulation on the inner shelf. Bottom topography and initial stratification are representative of the South and Middle Atlantic Bights along the U.S. east coast. Results from simulations forced by upwelling winds and no tides are consistent with previous studies of inner-shelf circulation. The inclusion of tidal forcing leads to increased mixing, larger eddy viscosity coefficients, and reduced stratification over the shallow regions, effectively reducing the wind efficiency to drive cross-shelf currents on the inner shelf. Tidally averaged cross-shelf currents are weaker compared to when no tides are considered. There is an increase in the width of the region of surface wind-driven transport divergence, which changes the cross-shelf location where upwelling occurs. Lagrangian analyses indicate that tidal forcing substantially reduces the transport of offshore waters toward the coast and increases the residence time over the inner shelf by up to 70%. Fluctuating winds with zero mean lead to a rectification of the cross-shelf flow on the inner shelf, resulting in net upwelling. The rectification occurs because the cross-shelf transport is nonzero during upwelling wind forcing (since dense water is brought to the inner shelf maintaining the stratification), but is approximately zero during downwelling winds (since surface water is forced under near-bottom water, destroying the stratification). The rectification is more clearly observed when stratification is strong, when tidal forcing is weak or absent, and when the wind fluctuates at low frequency.

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