Generation of midocean eddies : the local baroclinic instability hypothesis
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http://hdl.handle.net/1912/4085DOI
10.1575/1912/4085Abstract
The plausibility of local baroclinic instability as a generation mechanism for midocean
mesoscale eddies is examined with a twolayer, quasigeostrophic (QG) model
forced by an imposed, horizontally homogeneous, vertically sheared mean flow and
dissipated through bottom Ekman friction, Explanations are sought for two observed
features of midocean eddies: 1) substantial energy is retained in the baroclinic
mode and in the associated deformation radius (Rd) scale, and 2) the ratio of
eddy to mean kinetic energy is much larger than one,
The tendency of QG to cascade energy into the barotropic mode and into scales
larger than Rd can be counteracted when stratification is surfacetrapped, for then
the baroclinic mode is weakly damped, and hence enhanced, Numerical experiments
are performed with both surfacetrapped and uniform stratification to quantify this,
Experiments with equal Ekman frictions in the two layers are also performed for
purposes of contrast, Interpretation is aided with an inequality derived from the
energy and enstrophy equations, The inequality forbids the simultaneous retention
of substantial energy in the baroclinic mode and in scales near Rd when Ekman
friction is symmetric, but points towards surfacetrapped stratification and bottomtrapped
friction as an environment in which both of these can be achieved,
The dissertation also contains a systematic study of geostrophic turbulence forced
by nonzonal flows, Narrow zonal jets emerge when shearinduced mean potential
vorticity (PV) gradients are small compared to the planetary gradient (β), and
energy is a strong function of the angle shear presents to the eastwest direction,
When shearinduced PV gradients are comparable to β, and the mean shear has a
westward component, fields of monopolar vortices form and persist, Energy is asymmetric
between fields of cyclones and anticyclones, Such asymmetry was commonly
thought not to occur in QG, but is shown here to be introduced by the nonzonal
basic state, In both jet and vortex regimes, eddy energy can be much larger than
mean kinetic energy, contrary to the expectation that β stabilizes weak shear flows,
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 August 2000
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