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growth rates and spectral fluxes of baroclinic instability in the ocean
- J. Phys. Oceanogr
, 2011
"... An observational, modeling, and theoretical study of the scales, growth rates, and spectral fluxes of baro-clinic instability in the ocean is presented, permitting a discussion of the relation between the local instability scale; the first baroclinic deformation scale Rdef; and the equilibrated, obs ..."
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Cited by 13 (5 self)
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An observational, modeling, and theoretical study of the scales, growth rates, and spectral fluxes of baro-clinic instability in the ocean is presented, permitting a discussion of the relation between the local instability scale; the first baroclinic deformation scale Rdef; and the equilibrated, observed eddy scale. The geography of the large-scale, meridional quasigeostrophic potential vorticity (QGPV) gradient is mapped out using a cli-matological atlas, and attention is drawn to asymmetries between midlatitude eastward currents and sub-tropical return flows, the latter of which has westward and eastward zonal velocity shears. A linear stability analysis of the climatology, under the ‘‘local approximation,’ ’ yields the growth rates and scales of the fastest-growing modes. Fastest-growing modes on eastward-flowing currents, such as the Kuroshio and the Antarctic Circumpolar Current, have a scale somewhat larger (by a factor of about 2) than Rdef. They are rapidly growing (e folding in 1–3 weeks) and deep reaching, and they can be characterized by an interaction between interior QGPV gradients, with a zero crossing in the QGPV gradient at depth. In contrast, fastest-growing modes in the subtropical return flows (as well as much of the gyre interiors) have a scale smaller thanRdef (by a factor of between 0.5 and 1), growmore slowly (e-folding scale of several weeks), and owe their existence to the interaction of a positive surface QGPV gradient and a negative gradient beneath.
A geostrophic vortex over a slope
- J. Phys. Oceanogr
, 1998
"... ABSTRACT Nonlinear, quasigeostrophic, f -plane vortices in two layers over a topographic slope are considered. Scaling arguments suggest two parameters that dictate the effective strength of the slope: the first indicates the likelihood of dispersion at depth, and the second relates to baroclinic s ..."
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Cited by 11 (2 self)
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ABSTRACT Nonlinear, quasigeostrophic, f -plane vortices in two layers over a topographic slope are considered. Scaling arguments suggest two parameters that dictate the effective strength of the slope: the first indicates the likelihood of dispersion at depth, and the second relates to baroclinic stability. If the deep flow is only weakly dispersive (weak slopes), an initially barotropic vortex can translate barotropically across the isobaths, provided the vortex scale exceeds the deformation scale. Over stronger slopes, the vortex separates into topographic waves and a stationary, surface-trapped vortex. An initially surface-trapped vortex larger than deformation scale becomes unstable over a weak slope, as it does over a flat bottom. However, a strong slope can stabilize the vortex to small perturbations, despite the large vortex scale. The effective slope parameters depend not only on topographic grade, but on vortex strength and size, and on the ambient stratification. Parameters obtained with representative oceanic values suggest that topographically induced vertical decoupling may be quite common.
2013: Nonlinear radiating instability of a barotropic eastern boundary current
- J. Phys. Oceanogr
"... Linear and nonlinear radiating instabilities of an eastern boundary current are studied using a barotropic quasigeostrophic model in an idealized meridional channel. The eastern boundary current is meridionally uniform and produces unstable modes in which long waves are most able to radiate. These l ..."
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Cited by 5 (1 self)
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Linear and nonlinear radiating instabilities of an eastern boundary current are studied using a barotropic quasigeostrophic model in an idealized meridional channel. The eastern boundary current is meridionally uniform and produces unstable modes in which long waves are most able to radiate. These long radiating modes are easily suppressed by friction because of their small growth rates. However, the long radiating modes can overcome friction by nonlinear energy input transferred from themore unstable trappedmode and play an important role in the energy budget of the boundary current system. The nonlinearly powered long radiating modes take away part of the perturbation energy from the instability origin to the ocean interior. The radiated instabilities can generate zonal striations in the ocean interior that are comparable to features observed in the ocean. Subharmonic instability is identified to be responsible for the nonlinear resonance between the radiating and trappedmodes, butmore general nonlinear triad interactions are expected to apply in a highly nonlinear environment. 1.
2013: Effects of topography on baroclinic instability
- J. Phys. Oceanogr
"... The importance of bottom topography in the linear baroclinic instability of zonal flows on the b plane is examined by using analytical calculations and a quasigeostrophic eddy-resolving numerical model. The particular focus is on the effects of a zonal topographic slope, comparedwith the effects of ..."
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The importance of bottom topography in the linear baroclinic instability of zonal flows on the b plane is examined by using analytical calculations and a quasigeostrophic eddy-resolving numerical model. The particular focus is on the effects of a zonal topographic slope, comparedwith the effects of ameridional slope.A zonal slope always destabilizes background zonal flows that are otherwise stable in the absence of topography regardless of the slope magnitude, whereas the meridional slopes stabilize/destabilize zonal flows only through changing the lower-level background potential vorticity gradient beyond a known critical value. Growth rates, phase speeds, and vertical structure of the growing solutions strongly depend on the slope magnitude. In the numerical simulations configuredwith an isolatedmeridional ridge, unstablemodes develop onboth sides of the ridge and propagate eastward of the ridge, in agreement with analytical results. 1.
Radiating Instability of a Meridional Boundary Current
, 2007
"... 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 je ..."
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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 dif-ferences 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. 1.
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, 2009
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, 2009
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"... Ocean is studied in an eddy-resolving ocean model simulation. The meandering model Gulf Stream radiates barotropic Rossby waves southward through preferred corridors defined by topographic features. The smoother region between the Bermuda Rise and the mid-Atlantic Ridge is a particularly striking co ..."
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Ocean is studied in an eddy-resolving ocean model simulation. The meandering model Gulf Stream radiates barotropic Rossby waves southward through preferred corridors defined by topographic features. The smoother region between the Bermuda Rise and the mid-Atlantic Ridge is a particularly striking corridor of barotropic wave radiation in the 20–50 day period band. Barotropic Rossby waves are also preferentially excited at higher frequencies over the Bermuda Rise, suggesting resonant excitation of topographic Rossby normal modes. The prevalence of these radiated waves suggests that they may be an important energy sink for the equilibrium state of the Gulf Stream. Citation: Miller, A. J., et al. (2007), Barotropic Rossby wave radiation from a model Gulf Stream, Geophys. Res. Lett., 34,
