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T. J. SIMONS

Abstract

The baroclinic instability problem is formulated as an initial value problem to evaluate the effects of the initial configuration of the wave perturbation. The vertical shape of the initial perturbation is found to be as important as its wavelength in determining the energy conversions during the early stages of its development. The general character of the solution of the initial value problem is compared with normal mode studies of baroclinic instability. It is concluded that the initial value formulation bridges the gap between hydrodynamic stability theory and synoptic studies of cyclone development in the atmosphere.

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T. J. Simons

Abstract

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T. J. Simons

Abstract

A simple two-layer model is constructed to illustrate the combined effect of baroclinicity and topography over an idealized continental slope region. It is shown that an assumed thermohaline circulation leads to topographic vorticity contributions in the southward flowing deep current with the concomitant enhancement of the western boundary transport computed by Holland (1973).

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T. J. Simons

Abstract

Model calculations and current meter observations are analyzed in the spectral domain and in the time domain to investigate effects of topographic waves on the response of nearshore currents to wind. The spectral response is computed for a shelf forced by a progressive atmospheric wave, and effects of friction and alongshore depth variations are considered. Comparisons are made with results for standing atmospheric waves and with the response of closed basins forced by winds uniform in space and periodic in time. It is found that coastline curvature is rather unimportant for the scales under consideration, and that the alongshore wind component represents the crucial forcing. Spectral model results are then compared with current meter spectra to show the resonant topographic wave character of the response of currents to wind.

Time series of observed and computed nearshore currents are compared, and the alongshore momentum balances are considered for models with and without topographic wave effect. It is found that simple models may produce results which seem comparable to those obtained from more complete models, but it is concluded that such simple models are basically erroneous in concept.

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T. J. Simons

Abstract

A critical evaluation is presented of the conventional view that long-term mean circulation of homogeneous basins may be identified with the quasi-steady response of linear models to atmospheric forcing. Based on climatological wind records for the Great Lakes region, it is shown that the mean circulation for unstratified seasons is dominated by rectified effects of nonlinear topographic wave interactions. It is also shown that the dependence of the rectified flow on the frequency of periodic forcing is similar to the linear topographic response to wind with resonance characteristics for certain frequencies determined by topography and friction.

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T. J. Simons

Abstract

Current meter observations in Lake Ontario covering the 140-day period from 4 November 1982 to 23 March 1983 are used to evaluate the performance of circulation models for different time scales. The measurements were taken in a single cross-section of the lake with sufficiently high resolution to verify conservation of total water transport through the section. The results indicate that a typical linear hydrodynamic model can reproduce short- and medium-term circulations induced by wind variations, but that nonlinear effects must be included to simulate seasonal-mean current patterns.

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T. J. Simons

Abstract

Observations of winds and currents along the northshore of Lake Ontario are analyzed to evaluate effects of topographic wave propagation on wind-driven currents. Lagged cross-correlations and spectral transfer functions between winds and currents are found to be consistent with the mechanism of resonant topographic-wave response in the presence of bottom friction. Transfer function models in the time domain are shown to explain 70 to 80 percent of the variance of observed currants.

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T. J. Simons

Abstract

Numerical computations of water circulations, combined with observed temperature distribution, permit estimates of heat transports in Lake Ontario throughout the 1972 International Field Year. Weekly estimates of heat budgets and surface fluxes are available from ship surveys and meteorological observations, thus allowing for validation of model results. Whereas the hydrodynamic model employs a grid mesh of 5 km, the present verification study is based on 21 lake zones, with the mean depth contour separating shore zones from mid-lake areas.

Agreement between simulations and observations ranges from satisfactory to poor, as a function of both time and space. The model is found to perform best along the north shore and in the fall season. Analysis of the results points at low model resolution and gaps in the observational network as the major causes of failure to reproduce south shore heat transports. Persistent overestimates of heat advection by vertical circulations are ascribed to the present structure of the hydrodynamic model.

A brief discussion is included of the present heat transport calculations in the context of estimating vertical mixing properties for different lake zones.

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T. J. Simons

Abstract

No abstract available.

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T. J. Simons

Abstract

The development of atmospheric cyclones is studied from the viewpoint of the instability of large-scale wave perturbations superimposed on a zonal current. The stability properties of the observed mean January flow are investigated and the linear results are extended to include the effects of nonlinear processes on the growth of a cyclone-scale wave. An hemispheric model is employed in this investigation and solutions are obtained by spectral techniques.

It is found that the observed atmospheric zonal current is highly unstable in a hydrodynamic sense. The instability is of a baroclinic character with barotropic stabilizing effects. The nonlinear computations show that the growth of the most unstable waves is brought to a halt when the perturbation kinetic energy reaches a level consistent with atmospheric observations. The barotropic energy exchanges are found to play a major role in this process by feeding a large amount of kinetic energy into the zonal flow when the baroclinic energy conversions reach a maximum. The damping effect of the nonlinear processes on the growth of the unstable wave is found to be slightly reduced when the horizontal resolution of the model is increased in either zonal or latitudinal directions.

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