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Diane Masson

Abstract

A data-adaptive technique, the iterative maximum likelihood method, is used to estimate directional wave spectra from data collected by three pitch-and-roll buoys, during two turning wind events of the CASP 86 experiment. A relaxation coefficient is estimated for the response of both the mean wave direction for each frequency band and the main wave direction of the wind sea spectrum to a change in wind direction. The values of the coefficients are found to be consistent with each other, and within the scatter of previous estimates. Also, a definite correlation is established between the wave rms angular spread and the rate of change of wind direction. Finally, the shapes of the estimated directional distributions are tested for symmetry and unimodality. A significant proportion of the estimated distributions are found to be neither unimodal nor symmetric.

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Diane Masson

Abstract

During August 1991, a field program was carried out in the vicinity of Cape St. James, off the British Columbia coast, where a strong tidally driven flow interacts with an active wave climate. Surface current maps were obtained from a CODAR-type HF radar (Seasonde) over an area of about 350 km2 around the cape. A series of Loran-C drifters were also deployed during the experiment and used as ground truth for the radar. A comparison between the drifter and the radar surface currents indicates reasonable agreement.

Wave information was acquired with three Waverider buoys deployed around the cape. A significant modulation of the wave properties at the tidal period was observed for the buoy located in the area where the currents are maximum. The tidally induced changes in the wave field are modeled with a local wave–current interaction model based on wave action conservation and on a high-frequency limiting spectral shape. The model is applied on a period of 11 days for which the wind was relatively steady. The magnitude of the modeled tidal modulation of the wave field is of the same order of magnitude as the measurements but, in general, underestimates the measured tidally induced changes. However, during the first half of the period, the modulation of the total wave energy is significantly out of phase with the buoy data. The effect of refraction by the current on the waves is assessed using a backward ray tracing method and two-dimensional surface current maps. It is proposed that refraction effects are important during the first part of the study period and are a plausible cause for the phase discrepancy between the measurements and the results of the local model.

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Diane Masson

Abstract

The effect of nonlinear coupling due to resonant interactions on a bimodal spectrum is examined in the case of deep-water waves. Following Hasselmann, a swell decay time scale is first estimated for a monochromatic swell coupled with a typical wind wave spectrum. This time scale indicates that the nonlinear coupling generally causes the swell to decay at a rate that decreases rapidly as the swell frequency moves away from the peak frequency of the short waves. It is also shown, however, that the coupling makes the swell grow at the expense of the local sea in the frequency range just below the peak frequency of the short waves. Estimation of the nonlinear transfer for a swell of finite bandwidth confirms these results and also indicates a maximum coupling when the swell direction is about 40° to the mean direction of the short waves.

When a double peaked spectrum is time integrated under the influence of the nonlinear interactions, the spectral distribution gradually changes into a unimodal shape where the local sea peak has disappeared and the swell has significantly broadened. These results are compared with laboratory observations where the wind wave growth has been shown to be greatly altered by a train of long waves. It is shown that the nonlinear coupling produces an energy flux that smooths out the high-frequency peak in agreement with the observations. Finally, the nonlinear coupling is discussed in the case of sea and swell in the ocean for which it is found that the coupling is generally negligible unless the two peaks are so close that the spectrum can be hardly qualified as bimodal.

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Diane Masson

Abstract

Wind waves are commonly ignored when modeling the ice motion in the marginal ice zone. In order to estimate the importance of the wave forcing, an expression for the second-order wave-induced drift force on a floe exposed to a full directional wave spectrum is obtained in terms of a quadratic transfer function. For a given floe shape, the transfer function generally augments with the incident wave frequency, with a sharp increase near the resonant frequency of the pitch motion. The short wave limit of this function is determined by the shape of the horizontal contour of the floe. The value corresponding to the truncated cylindrical floe used here is two-thirds of the value obtained by the two-dimensional approximation. The total drift force is computed for two situations; an off-ice wind over a large polynya, and an on-ice wind at the extreme ice edge. In the first case, the drift force induced by the short fetch waves represents a significant fraction of the direct wind forcing and may be partly responsible for the formation of ice edge bands. In the second case, the very large drift force on a floe exposed to the high frequency components of the open water spectrum rapidly decreases (in the first few hundred meters) as these short waves are efficiently attenuated by the ice. This rapid decrease of the force generates a large compressive stress that is important in compacting the ice at the extreme ice edge.

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Diane Masson and Patrick F. Cummins

Abstract

A three-dimensional prognostic numerical model has been developed to study the ocean circulation around Vancouver Island, British Columbia. In a series of simulations, the model is applied to examine the role of buoyancy forcing in the dynamics of the summer coastal countercurrent found off the west coast of Vancouver Island. The forcing is provided by the Fraser River discharge into the Strait of Georgia. An estuarine circulation establishes itself in Juan de Fuca Strait, from which a distinctive right-bounded current is formed and advances along the coast. Sensitivity studies are conducted to determine the robustness of this current to initial conditions, opposing wind, enhanced vertical mixing, and grid resolution. Finally, various characteristics of the numerically modeled coastal flow are compared with observations.

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Patrick F. Cummins, Diane Masson, and Michael G. G. Foreman

Abstract

A series of numerical experiments with a three-dimensional, baroclinic model were conducted to study the influences of density stratification and wind-driven currents on the K 1 tide over the continental shelf off Vancouver Island. The region is one of anomalously large diurnal tidal currents due to the generation of coastally trapped waves at the entrance to Juan de Fuca Strait. Model results are compared with data obtained from a number of moorings deployed over the shelf, including a line extending for over 300 km in the alongshore direction. The results show that inclusion of stratification significantly improves the representation of K 1 currents in the model, particularly with respect to the alongshore phase propagation of the clockwise and counterclockwise rotary components. With homogeneous water, the coastally trapped waves propagate relatively slowly and are dissipated before reaching Brooks Peninsula. In contrast, the ambient stratification permits coastally trapped motions to propagate beyond Brooks Peninsula, in agreement with observations.

The seasonal variability of the K 1 currents is also examined from long records obtained at a number of sites over the shelf. The observations indicate a seasonal modulation in the phase of the rotary components that increases in amplitude with distance from the generation region of the shelf waves. Attempts to model the observed summer/winter phase difference by including a seasonal-mean wind stress are discussed.

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