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  • 1
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    Radiating instability of a meridional boundary current (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 2294-2307, doi:10.1175/2008JPO3853.1.
    Description: A linear stability analysis of a meridional boundary current on the beta plane is presented. The boundary current is idealized as a constant-speed meridional jet adjacent to a semi-infinite motionless far field. The far-field region can be situated either on the eastern or the western side of the jet, representing a western or an eastern boundary current, respectively. It is found that when unstable, the meridional boundary current generates temporally growing propagating waves that transport energy away from the locally unstable region toward the neutral far field. This is the so-called radiating instability and is found in both barotropic and two-layer baroclinic configurations. A second but important conclusion concerns the differences in the stability properties of eastern and western boundary currents. An eastern boundary current supports a greater number of radiating modes over a wider range of meridional wavenumbers. It generates waves with amplitude envelopes that decay slowly with distance from the current. The radiating waves tend to have an asymmetrical horizontal structure—they are much longer in the zonal direction than in the meridional, a consequence of which is that unstable eastern boundary currents, unlike western boundary currents, have the potential to act as a source of zonal jets for the interior of the ocean.
    Description: This work was supported by the National Science Foundation through Grants OCE- 0423975 (MS, HH) and OCE-9901654 (JP). HH would like to thank her thesis committee as well as the MIT– WHOI Joint Program for partial financial support.
    Keywords: Instability ; Boundary currents
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Stability of large-scale oceanic flows and the importance of non-local effects (2009)
    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Details
    Publication Date: 2022-05-25
    Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2009
    Description: My thesis covers two general circulation problems that involve the stability of largescale oceanic flows and the importance of non-local effects. The first problem examines the stability of meridional boundary currents, which are found on both sides of most ocean basins because of the presence of continents. A linear stability analysis of a meridional boundary current on the beta-plane is performed using a quasi-geostrophic model in order to determine the existence of radiating instabilities, a type of instability that propagates energy away from its origin region by exciting Rossby waves and can thus act as a source of eddy energy for the ocean interior. It is found that radiating instabilities are commonly found in both eastern and western boundary currents. However, there are some significant differences that make eastern boundary currents more interesting from a radiation point of view. They possess a larger number of radiating modes, characterized by horizontal wavenumbers which would make them appear like zonal jets as they propagate into the ocean interior. The second problem examines the circulation in a nonlinear thermally-forced two-layer quasi-geostrophic ocean. The only driving force for the circulation in the model is a cross-isopycnal flux parameterized as interface relaxation. This forcing is similar to the radiative damping used commonly in atmospheric models, except that it is applied to the ocean circulation in a closed basin and is meant to represent the large-scale thermal forcing acting on the oceans. It is found that in the strongly nonlinear regime a substantial, not directly thermally-driven barotropic circulation is generated. Its variability in the limit of weak bottom drag is dominated by high-frequency barotropic basin modes. It is demonstrated that the excitation of basin normal modes has significant consequences for the mean state of the system and its variability, conclusions that are likely to apply for any other system whose variability is dominated by basin modes, no matter the forcing. A linear stability analysis performed on a wind- and a thermally-forced double-gyre circulation reveals that under certain conditions the basin modes can arise from local instabilities of the flow.
    Description: I was supported through a graduate research assistantship from the National Science Foundation Grant OCE-0423975 and the Woods Hole Oceanographic Institution Academic Programs Office.
    Keywords: Ocean currents ; Ocean circulation
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
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  • 3
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    Onset of time-dependence in a double-gyre circulation : barotropic basin modes versus classical baroclinic modes (2011)
    Sears Foundation for Marine Research
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © Sears Foundation for Marine Research, 2010. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 68 (2010): 215-236, doi:10.1357/002224010793721424.
    Description: Using a fully-implicit high-resolution two-layer quasi-geostrophic model combined with pseudo-arclength continuation methods, we perform a bifurcation analysis of double-gyre ocean flows to study their initial oscillatory instabilities. In this model, both wind- and thermally-forced flows can be represented. We demonstrate that on the branch of anti-symmetric steady-state solutions the ratio, Ω, of the flow advective speed to the long internal Rossby wave speed determines the type of oscillatory modes to first become unstable. This is the same nondimensional parameter that controls the shape of the geostrophic contours in the linear limit of the circulation. For large values of Ω, the first Hopf bifurcations correspond to the classical baroclinic modes with inter-monthly time periods arising from shear instability of the flow. For small values of Ω, the first Hopf bifurcations correspond instead to barotropic Rossby modes with shorter, monthly periods arising from mixed barotropic-baroclinic instability of the flow. By considering both a wind-forced and a thermally-forced ocean, we show that this is a robust feature that does not depend on the type of forcing driving the circulation.
    Description: NSF Grant OCE-0423975, NSF Grants OCE-042975 and OCE-0850416
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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    Topographic and Mixed Layer Submesoscale Currents in the Near-Surface Southwestern Tropical Pacific (2017)
    American Meteorological Society
    Details
    Publication Date: 2017-05-19
    Description: The distribution and strength of submesoscale (SM) surface layer fronts and filaments generated through mixed layer baroclinic energy conversion and submesoscale coherent vortices (SCVs) generated by topographic drag are analyzed in numerical simulations of the near-surface southwestern Pacific, north of 16°S. In the Coral Sea a strong seasonal cycle in the surface heat flux leads to a winter SM “soup” consisting of baroclinic mixed layer eddies (MLEs), fronts, and filaments similar to those seen in other regions farther away from the equator. However, a strong wind stress seasonal cycle, largely in sync with the surface heat flux cycle, is also a source of SM processes. SM restratification fluxes show distinctive signatures corresponding to both surface cooling and wind stress. The winter peak in SM activity in the Coral Sea is not in phase with the summer dominance of the mesoscale eddy kinetic energy in the region, implying that local surface layer forcing effects are more important for SM generation than the nonlocal eddy deformation field. In the topographically complex Solomon and Bismarck Seas, a combination of equatorial proximity and boundary drag generates SCVs with large-vorticity Rossby numbers (Ro ~ 10). River outflows in the Bismarck and Solomon Seas make a contribution to SM generation, although they are considerably weaker than the topographic effects. Mean to eddy kinetic energy conversions implicate barotropic instability in SM topographic wakes, with the strongest values seen north of the Vitiaz Strait along the coast of Papua New Guinea.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 5
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    Surface Circulation in the Solomon Sea Derived from Lagrangian Drifter Observations* (2012)
    American Meteorological Society
    Details
    Publication Date: 2012-03-01
    Description: Velocity measurements from satellite-tracked surface drifters collected between 1994 and 2010 are used to map the surface circulation in the Solomon Sea, the last passageway for waters of subtropical origin flowing northward toward the equator, where they replenish the Pacific warm pool. Pseudo-Eulerian statistics of the drifter observations show a strong seasonal cycle in both the mean circulation and the eddy kinetic energy in the region. The circulation is characterized by a strong northward flow from June to November (the season of strong southeasterly trade winds over the Solomon Sea) and a mostly southward flow with increased variability from December to May (when the winds over the sea are weak). The seasonal velocity signal has the largest magnitude narrowly along the double western boundary formed by the eastern coastlines of New Guinea and the Solomon Islands, suggesting that direct wind driving with its much larger spatial scales is not the main influence. In addition, the surface circulation exhibits substantial interannual variability of magnitude comparable to that of the seasonal cycle with velocity and temperature anomalies consistent with changes in the western boundary current acting to compensate for the discharge and recharge of the Pacific warm pool during ENSO.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 6
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    Radiating Instability of a Meridional Boundary Current (2008)
    American Meteorological Society
    Details
    Publication Date: 2008-10-01
    Description: A linear stability analysis of a meridional boundary current on the beta plane is presented. The boundary current is idealized as a constant-speed meridional jet adjacent to a semi-infinite motionless far field. The far-field region can be situated either on the eastern or the western side of the jet, representing a western or an eastern boundary current, respectively. It is found that when unstable, the meridional boundary current generates temporally growing propagating waves that transport energy away from the locally unstable region toward the neutral far field. This is the so-called radiating instability and is found in both barotropic and two-layer baroclinic configurations. A second but important conclusion concerns the differences in the stability properties of eastern and western boundary currents. An eastern boundary current supports a greater number of radiating modes over a wider range of meridional wavenumbers. It generates waves with amplitude envelopes that decay slowly with distance from the current. The radiating waves tend to have an asymmetrical horizontal structure—they are much longer in the zonal direction than in the meridional, a consequence of which is that unstable eastern boundary currents, unlike western boundary currents, have the potential to act as a source of zonal jets for the interior of the ocean.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 7
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    Variability and Trends of the Alaska Gyre From Argo and Satellite Altimetry (2019)
    American Geophysical Union
    Details
    Publication Date: 2019-08-01
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 8
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    Mesoscale variability and its seasonality in the Solomon and Coral Seas (2014)
    American Geophysical Union
    Details
    Publication Date: 2014-07-01
    Print ISSN: 2169-9275
    Electronic ISSN: 2169-9291
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 9
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    Equatorward western boundary transport from the South Pacific: Glider observations, dynamics and consequences (2019)
    Elsevier
    Details
    Publication Date: 2019-07-01
    Print ISSN: 0079-6611
    Electronic ISSN: 1873-4472
    Topics: Geosciences , Physics
    Published by Elsevier
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  • 10
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    Observations of the Alaskan Stream near Samalga Pass and its connection to the Bering Sea: 2001–2004 (2014)
    Elsevier
    Details
    Publication Date: 2014-06-01
    Print ISSN: 0967-0637
    Electronic ISSN: 1879-0119
    Topics: Biology , Geosciences , Physics
    Published by Elsevier
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