Instabilities of an eastern boundary current with and without large-scale flow influence
Citable URI
https://hdl.handle.net/1912/4731DOI
10.1575/1912/4731Abstract
Eastern oceanic boundary currents are subject to hydrodynamic instability, generate
small scale features that are visible in satellite images and may radiate westward into
the interior, where they can be modified by the large-scale circulations. This thesis
studies the stability of an eastern boundary current with and without the large-scale
flow influence in an idealized framework represented by barotropic quasi-geostrophic
dynamics.
The linear stability analysis of a meridional current with a continuous velocity
profile shows that meridional eastern and western boundary currents support a limited
number of radiating modes with long meridional and zonal wavelengths and
small growth rates. However, the linearly stable, long radiating modes of an eastern
boundary current can become nonlinearly unstable by resonating with short trapped
unstable modes. This phenomenon is clearly demonstrated in the weakly nonlinear
simulations. Results suggest that linearly stable longwave modes deserve more attention
when the radiating instability of a meridional boundary current is considered.
A large-scale flow affects the short trapped unstable mode and long radiating
mode through different mechanisms. The large-scale flow modifies the structure of
the boundary current to stabilize or destabilize the unstable modes, leading to a
meridionally localized maximum in the perturbation kinetic energy field. The shortwave
mode is accelerated or decelerated by the meridional velocity adjustment of the
large-scale flow to have an elongated or a squeezed meridional structure, which is confirmed both in a linear WKB analysis and in nonlinear simulations. The squeezed or
elongated unstable mode detunes the nonlinear resonance with the longwave modes,
which then become less energetic. These two modes show different meridional structures
in kinetic energy field because of the different mechanisms.
In spite of the model simplicity, these results can potentially explain the formation
of the zonal jets observed in altimeter data, and indicate the influence of the large-scale wind-driven circulation on eastern boundary upwelling systems in the real ocean.
Studies with more realistic configurations remain future challenges.
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 2011
Collections
Suggested Citation
Thesis: Wang, Jinbo, "Instabilities of an eastern boundary current with and without large-scale flow influence", 2011-06, DOI:10.1575/1912/4731, https://hdl.handle.net/1912/4731Related items
Showing items related by title, author, creator and subject.
-
Oceanic fluxes of mass, heat, and freshwater : a global estimate and perspective
Macdonald, Alison M. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1995-08)Data from fifteen globally distributed, modern, high resolution, hydrographic oceanic transects are combined in an inverse calculation using large scale box models. The models provide estimates of the global meridional ... -
Adaptive error estimation in linearized ocean general circulation models
Chechelnitsky, Michael Y. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1999-06)Data assimilation methods, such as the Kalman filter, are routinely used in oceanography. The statistics of the model and measurement errors need to be specified a priori. In this study we address the problem of estimating ... -
On the world ocean circulation. Volume I, some global features/North Atlantic circulation
Schmitz, William J. (Woods Hole Oceanographic Institution, 1996-06)This is the first volume of a "final report" that summarizes, often in a speculative vein, what I have learned over the past 35 years or so about large-scale, low-frequency ocean currents, primarily with support from the ...