This project was part of my PhD thesis work with Dr. Steven Lentz at the Woods Hole Oceanographic Institute.
inner shelf circulation
The inner shelf is a critical link in the cross-shelf transport pathway, connecting the surfzone and the continental shelf, but the mechanisms that drive cross-shelf circulation on the inner shelf are not well understood. On mid and outer shelves, along-shelf winds typically drive cross-shelf transport following classic Ekman theory.
Where the water is shallower, momentum from the wind mixes to the bottom faster than the Coriolis acceleration can turn it, so the along-shelf wind stress is ineffective at driving cross-shelf transport. Recent observational (Fewings et al., 2008) and modeling (Tilburg, 2003) studies focused specifically on the cross-shelf wind stress have shown it to be a significant mechanism for cross-shelf transport on the inner shelf.
effect of Density gradients on transport by cross-shelf wind stress
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, MA. In two-dimensional (no along-shelf variation) numerical model (ROMS) 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 due to the sloping bottom. The cross-shelf density gradient causes an asymmetric response to onshore 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.
Combined along- and cross-shelf wind stresses
The wind on the Martha's Vineyard inner shelf is rarely purely along- or cross-shore. The second part of this project describes the effect of an upwelling or downwelling wind stress on the inner shelf response to cross-shelf wind stress. When there is an upwelling circulation, the cross-shelf wind either enhances or opposes the upwelling circulation, which can either extend the reach of the upwelling cell all the way across the inner shelf or act as a kinematic barrier to cross-shelf transport. As a result of this interaction, both components of the wind stress must be considered to predict the cross-shelf transport.
For more details, see:
Horwitz, Rachel M., Steven J. Lentz, 2016: The effect of wind direction on cross-shelf transport on an initially stratified inner shelf, Journal of Marine Research, Volume 74, Number 4, July and September 2016, pp. 201-227(27). https://doi.org/10.1357/002224016820870648
Horwitz, Rachel, Steven J. Lentz, 2014: Inner-Shelf Response to Cross-Shelf Wind Stress: The Importance of the Cross-Shelf Density Gradient in an Idealized Numerical Model and Field Observations. J. Phys. Oceanogr., 44, 86–103. http://dx.doi.org/10.1175/JPO-D-13-075.1
PhD thesis available here: http://hdl.handle.net/1912/5283
Horwitz, Rachel M., Steven J. Lentz, 2016: The effect of wind direction on cross-shelf transport on an initially stratified inner shelf, Journal of Marine Research, Volume 74, Number 4, July and September 2016, pp. 201-227(27). https://doi.org/10.1357/002224016820870648
Horwitz, Rachel, Steven J. Lentz, 2014: Inner-Shelf Response to Cross-Shelf Wind Stress: The Importance of the Cross-Shelf Density Gradient in an Idealized Numerical Model and Field Observations. J. Phys. Oceanogr., 44, 86–103. http://dx.doi.org/10.1175/JPO-D-13-075.1
PhD thesis available here: http://hdl.handle.net/1912/5283