Near-inertial waves in oceanic fronts : from generation to dissipation
Abstract/Contents
- Abstract
- The question of how wind-forced near-inertial motions interact with strongly baroclinic upper-ocean fronts, like those occurring in wintertime western boundary current extension zones, is investigated theoretically, computationally and observationally. The investigation considers the full life cycle of near-inertial waves in this context, from generation to dissipation. Winds with frequency content near the local Coriolis frequency accelerate near-inertial motions in the ocean boundary layer. Due to the strong and variable vertical vorticity of the frontal jet, the dominant frequency of the near-inertial motion varies across the front and the horizontal length scales of the oscillations shrink with time. As a result, wave energy radiates into the upper thermocline with an e-folding time scale similar to an inertial period. In the thermocline, wave propagation is substantially modified by the presence of the background flow. Wave energy tends to pile up at turning points and slantwise critical layers where the wave group velocity goes to zero and, in the latter case, the ray tube area also goes to zero. In these regions, numerical models and observations exhibit strong, banded, and coherent wave shear nearly parallel to isopycnals oscillating at a frequency near the local minimum permitted frequency of inertia-gravity waves, which may be less than the local Coriolis frequency. Furthermore, these amplified waves are associated with banded regions of low and in some cases sub-critical Richardson numbers leading to enhanced dissipation and mixing in the thermocline. Three key results emerge from this narrative: 1) Although the dynamics of inertia-gravity wave propagation in strong fronts is qualitatively different from propagation in the absence of a background flow, the essential physics of near-inertial wave propagation can still be characterized by two conservation laws: i) conservation of absolute momentum, and ii) conservation of buoyancy. 2) The strong and variable vertical vorticity of the front facilitates energy exchange between the wave motions and the geostrophic frontal jet. In fact, a large ensemble of wind-generated near-inertial waves always extracts energy from the geostrophic flow on average. With the addition of wave dissipation, the energy loss from the geostrophic flow becomes permanent. 3) The ubiquitous banded near-inertial shear observed in the upper thermocline of the north wall front in the winter Gulf Stream over the course of a month-long observational campaign in the winter of 2012, leads to the conclusion that western boundary current fronts are undersea "surf-zones" for near-inertial waves. That is, in the presence of strong and persistent atmospheric forcing, they are regions of persistently amplified wave shear, strong vertical velocities, enhanced turbulence and mixing.
Description
| Type of resource | text |
|---|---|
| Form | electronic; electronic resource; remote |
| Extent | 1 online resource. |
| Publication date | 2014 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Associated with | Whitt, Daniel Bridger |
|---|---|
| Associated with | Stanford University, Department of Environmental Earth System Science. |
| Primary advisor | Thomas, Leif N |
| Thesis advisor | Thomas, Leif N |
| Thesis advisor | Arrigo, Kevin R |
| Thesis advisor | Fringer, Oliver B. (Oliver Bartlett) |
| Advisor | Arrigo, Kevin R |
| Advisor | Fringer, Oliver B. (Oliver Bartlett) |
Subjects
| Genre | Theses |
|---|
Bibliographic information
| Statement of responsibility | Daniel Bridger Whitt. |
|---|---|
| Note | Submitted to the Department of Environmental Earth System Science. |
| Thesis | Thesis (Ph.D.)--Stanford University, 2014. |
| Location | electronic resource |
Access conditions
- Copyright
- © 2014 by Daniel Bridger Whitt
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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