Horwitz Rachel M.

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Horwitz
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Rachel M.
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  • Article
    Inner-shelf response to cross-chelf wind stress : the importance of the cross-shelf density gradient in an idealized numerical model and field observations
    (American Meteorological Society, 2014-01) Horwitz, Rachel M. ; Lentz, Steven J.
    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.
  • Thesis
    The effect of stratification on wind-driven, cross-shelf circulation and transport on the inner continental shelf
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2012-06) Horwitz, Rachel M.
    Observations from a three-year field program on the inner shelf south of Martha's Vineyard, MA and a numerical model are used to describe the effect of stratification on inner shelf circulation, transport, and sediment resuspension height. Thermal stratification above the bottom mixed layer is shown to cap the height to which sediment is resuspended. Stratification increases the transport driven by cross-shelf wind stresses, and this effect is larger in the response to offshore winds than onshore winds. However, a one-dimensional view of the dynamics is not sufficient to explain the relationship between circulation and stratification. An idealized, cross-shelf transect in a numerical model (ROMS) is used to isolate the effects of stratification, wind stress magnitude, surface heat flux, cross-shelf density gradient, and wind direction on the inner shelf response to the cross-shelf component of the wind stress. In well mixed and weakly stratified conditions, the cross-shelf density gradient can be used to predict the transport efficiency of the cross-shelf wind stress. In stratified conditions, the presence of an along-shelf wind stress component makes the inner shelf response to cross-shelf wind stress strongly asymmetric.