Search Results

You are looking at 1 - 5 of 5 items for

  • Author or Editor: Paul H. LeBlond x
  • Refine by Access: All Content x
Clear All Modify Search
Toshiyuki Hibiya and Paul H. LeBlond

Abstract

Tidal processes that control basin–ocean water exchange across a sill, namely, the fortnightly modulation of mixing intensity associated with the spring-neap tidal cycle, are examined. The study proceeds analytically by modeling flow in a vertical two-dimensional plane where a zone of enhanced mixing of finite horizontal extent is embedded within a vertically sheared steady estuarine flow. The intensity of mixing is allowed to vary with the fortnightly cycle.

As the mixing intensity increases and hence approaches the maximum at spring tides, landward propagating and seaward propagating internal waves are generated in response to the horizontal density gradients induced by the localized mixing. Furthermore, these internal waves interact with the ambient steady sheer flow to create strong upwelling over the mixing zone, which is accompanied by bottom-intensified seaward currents on the landward side of the mixing zone and surface-intensified seaward currents on the seaward side of the mixing zone, and which are superimposed on the background estuarine flow. Accordingly, a considerable fraction of the bottom water flowing landward into the mixing zone is refluxed back seaward as part of the surface water at spring tides. As the mixing intensity decreases, the internal disturbances propagate away in both directions from the mixing zone, so that the bottom inflow from outside continues landward at neap tides.

This model reproduce observations of deep-water replacement in coastal basins and shows that bottom water intrusions from outside occur during periods of weak tidal currents.

Full access
Lin Jiang and Paul H. Leblond

Abstract

Submarine landslides are a common cause of tsunamis in coastal and estuarine areas. To study this phenomenon, a numerical model is developed to simulate tsunami generation due to a viscous mudslide on a gentle uniform slope. A formulation of the dynamics of the problem is presented, where the mudslide is treated as an incompressible three-dimensional viscous flow. Seawater is treated as an inviscid fluid, and the water motion is assumed irrotational. The long-wave approximation is adopted for both water waves and the mudslide. The resulting differential equations are solved by a finite-difference method. The focus of this paper is to examine the effects of the longitudinal spreading as well as the transversal spreading of the slide upon surface wave generation, and the spreading of water waves sideways. Three-dimensional pictures are presented for successive profiles of the mud surface, the horizontal velocities of the mudslide, the evolution of the surface elevations, and the velocities of the water motion. Comparisons of the present three-dimensional calculations with previously published two-dimensional results indicate small differences for large length/width ratios for a small time after the initiation of the slide. Generally, however, the water surface profiles deviate significantly from the two-dimensional results. Adequate simulations thus require accurate representation of the aspect ratio of the sliding mass.

Full access
Yves Gratton and Paul H. LeBlond

Abstract

Analytical solutions are found for topographic waves propagating over step bottom slopes in a two-layer infinite channel. From the inviscid unforced long-wave equation for a two-layer fluid on an f-plane, it is shown, under the assumption of a relatively thin upper layer, that barotropic waves force a baroclinic response through topographic coupling, resulting in surface intensified motion. Solutions are found with and without the small slope approximation. It is shown that the small slope approximation underestimates the frequency of low-frequency topographic waves, even when the slope is small. The theory is compared with observations from the Strait of Georgia and with a numerical model of the St. Lawrence estuary.

Full access
Lawrence A. Mysak, Paul H. Leblond, and William J. Emery

Abstract

The cross sections of four trenches peripheral to the Pacific Ocean are fitted by a double exponential depth profile. Nondivergent trapped wave propagation is shown to be possible in two directions along such a profile. In addition to the familiar shelf waves, only slightly modified by the presence of a trench, “trench waves” propagating in the direction opposite to that of shelf waves and at speeds lower by an order of magnitude are also possible.

Dispersion curves and eigenfunctions are presented for the Peru-Chile and Japan-Kuril trenches. Coastal sea level records are used to demonstrate phase propagation at “trench wave” phase speeds off both Japan and Peru. The fundamental mode speeds predominate in the phase spectra off both Japan and Peru.

Full access
Michael W. Stacey, Stephen Pond, Paul H. LeBlond, Howard J. Freeland, and David M. Farmer

Abstract

A description of the low-frequency (ap;10 to 30 days period) current fluctuations in the Strait of Georgia is presented. Velocity time series from four cyclesonde moorings and five current meter mooring, spanning the time interval from June 1984 until January 1985, are analyzed. Emphasis is placed on identifying the forcing mechanisms and determining the spatial structure of low-frequency flow.

The nonlinear interaction of semidiurnal tidal constituents with bottom topography caused a near-bottom, low-frequency oscillation that was coherent over the span of the experimental array (ap;11 km). The tides were important elsewhere in the water column too, and altogether directly accounted for 37% of the low-frequency energy in the Strait.

There is evidence of significant wind forcing. An empirical orthogonal function analysis of the vertical structure of the current fluctuations yields strong evidence for the existence of wind-forced Ekman spirals. Typically, the orthogonal modes that dominate the variance near the surface rotate clockwise with depth and are coherent with the wind.

Longitudinal and transverse velocity correlations imply that at some depths the low-frequency current fluctuations are consistent with horizontally nondivergent, isotropic flow. They also suggest horizontal scales of less than 8 km.

Full access