Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity
Although atmospheric forcing by wind stress or buoyancy flux is known to change the ocean’s potential vorticity (PV) at the surface, less is understood about PV modification in the bottom boundary layer. The adjustment of a geostrophic current over a sloped bottom in a stratified ocean generates PV...
| Main Authors: | , |
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| Format: | Article |
| Language: | en_US |
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American Meteorological Society
2013
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| Online Access: | http://hdl.handle.net/1721.1/80780 |
| _version_ | 1811076077447020544 |
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| author | Benthuysen, Jessica Thomas, Leif N. |
| author2 | Woods Hole Oceanographic Institution |
| author_facet | Woods Hole Oceanographic Institution Benthuysen, Jessica Thomas, Leif N. |
| author_sort | Benthuysen, Jessica |
| collection | MIT |
| description | Although atmospheric forcing by wind stress or buoyancy flux is known to change the ocean’s potential vorticity (PV) at the surface, less is understood about PV modification in the bottom boundary layer. The adjustment of a geostrophic current over a sloped bottom in a stratified ocean generates PV sources and sinks through friction and diapycnal mixing. The time-dependent problem is solved analytically for a no-slip boundary condition, and scalings are identified for the change in PV that arises during the adjustment to steady state. Numerical experiments are run to test the scalings with different turbulent closure schemes. The key parameters that control whether PV is injected into or extracted from the fluid are the direction of the geostrophic current and the ratio of its initial speed to its steady-state speed. When the current is in the direction of Kelvin wave propagation, downslope Ekman flow advects lighter water under denser water, driving diabatic mixing and extracting PV. For a current in the opposite direction, Ekman advection tends to restratify the bottom boundary layer and increase the PV. Mixing near the bottom counteracts this restratification, however, and an increase in PV will only occur for current speeds exceeding a critical value. Consequently, the change in PV is asymmetric for currents of the opposite sign but the same speed, with a bias toward PV removal. In the limit of a large speed ratio, the change in PV is independent of diapycnal mixing. |
| first_indexed | 2024-09-23T10:15:49Z |
| format | Article |
| id | mit-1721.1/80780 |
| institution | Massachusetts Institute of Technology |
| language | en_US |
| last_indexed | 2024-09-23T10:15:49Z |
| publishDate | 2013 |
| publisher | American Meteorological Society |
| record_format | dspace |
| spelling | mit-1721.1/807802022-09-30T19:58:49Z Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity Benthuysen, Jessica Thomas, Leif N. Woods Hole Oceanographic Institution Benthuysen, Jessica Although atmospheric forcing by wind stress or buoyancy flux is known to change the ocean’s potential vorticity (PV) at the surface, less is understood about PV modification in the bottom boundary layer. The adjustment of a geostrophic current over a sloped bottom in a stratified ocean generates PV sources and sinks through friction and diapycnal mixing. The time-dependent problem is solved analytically for a no-slip boundary condition, and scalings are identified for the change in PV that arises during the adjustment to steady state. Numerical experiments are run to test the scalings with different turbulent closure schemes. The key parameters that control whether PV is injected into or extracted from the fluid are the direction of the geostrophic current and the ratio of its initial speed to its steady-state speed. When the current is in the direction of Kelvin wave propagation, downslope Ekman flow advects lighter water under denser water, driving diabatic mixing and extracting PV. For a current in the opposite direction, Ekman advection tends to restratify the bottom boundary layer and increase the PV. Mixing near the bottom counteracts this restratification, however, and an increase in PV will only occur for current speeds exceeding a critical value. Consequently, the change in PV is asymmetric for currents of the opposite sign but the same speed, with a bias toward PV removal. In the limit of a large speed ratio, the change in PV is independent of diapycnal mixing. Massachusetts Institute of Technology. Woods Hole Oceanographic Institution. Joint Program Stanford University 2013-09-18T11:59:57Z 2013-09-18T11:59:57Z 2012-09 2012-01 Article http://purl.org/eprint/type/JournalArticle 0022-3670 1520-0485 http://hdl.handle.net/1721.1/80780 Benthuysen, Jessica, and Leif N. Thomas. “Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity.” Journal of Physical Oceanography 42, no. 9 (September 2012): 1509-1523. © 2012 American Meteorological Society en_US http://dx.doi.org/10.1175/jpo-d-11-0130.1 Journal of Physical Oceanography Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. application/pdf American Meteorological Society American Meteorological Society |
| spellingShingle | Benthuysen, Jessica Thomas, Leif N. Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity |
| title | Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity |
| title_full | Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity |
| title_fullStr | Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity |
| title_full_unstemmed | Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity |
| title_short | Friction and Diapycnal Mixing at a Slope: Boundary Control of Potential Vorticity |
| title_sort | friction and diapycnal mixing at a slope boundary control of potential vorticity |
| url | http://hdl.handle.net/1721.1/80780 |
| work_keys_str_mv | AT benthuysenjessica frictionanddiapycnalmixingataslopeboundarycontrolofpotentialvorticity AT thomasleifn frictionanddiapycnalmixingataslopeboundarycontrolofpotentialvorticity |