Stability of large-scale oceanic flows and the importance of non-local effects
Stability of large-scale oceanic flows and the importance of non-local effects
Date
2009-06
Authors
Hristova, Hristina G.
Linked Authors
Person
Alternative Title
Citable URI
As Published
Date Created
Location
DOI
10.1575/1912/3001
Related Materials
Replaces
Replaced By
Keywords
Ocean currents
Ocean circulation
Ocean circulation
Abstract
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
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
Embargo Date
Citation
Hristova, H. G. (2009). Stability of large-scale oceanic flows and the importance of non-local effects [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/3001