Spatial and Temporal Variability of the M2 Internal Tide Generation and Propagation on the Oregon Shelf

Abstract

A 1-km-horizontal-resolution model based on the Regional Ocean Modeling System is implemented along the Oregon coast to study average characteristics and intermittency of the M 2 internal tide during summer upwelling. Wind-driven and tidally driven flows are simulated in combination, using realistic bathymetry, atmospheric forcing, and boundary conditions. The study period is April through August 2002, when mooring velocities are available for comparison. Modeled subtidal and tidal variability on the shelf are in good quantitative agreement with moored velocity time series observations. Depth-integrated baroclinic tidal energy flux (EF), its divergence, and topographic energy conversion (TEC) from the barotropic to baroclinic tide are computed from high-pass-filtered, harmonically analyzed model results in a series of 16-day time windows. Model results reveal several ''hot spots'' of intensive TEC on the slope. At these locations, TEC is well balanced by EF divergence. Changes in background stratification and currents associated with winddriven upwelling and downwelling do not appreciably affect TEC hot spot locations but may affect intensity of internal tide generation at those locations. Relatively little internal tide is generated on the shelf. Areas of supercritical slope near the shelf break partially reflect baroclinic tidal energy to deeper water, contributing to spatial variability in seasonally averaged on-shelf EF. Despite significant temporal and spatial variability in the internal tide, the alongshore-integrated flux of internal tide energy onto the Oregon shelf, where it is dissipated, does not vary much with time. Approximately 65% of the M 2 baroclinic tidal energy generated on the slope is dissipated there, and the rest is radiated toward the shelf and interior ocean in roughly equal proportions. An experiment with smoother bathymetry reveals that slope-integrated TEC is more sensitive to bathymetric roughness than on-shelf EF.

The Cascade of Tidal Energy from Low to High Modes on a Continental Slope

“…The resulting internal-tide energy flux radiates out of the domain approximately equally in the along-isobath and offshore directions, with very little energy flux onto the continental shelf. The along-isobath divergence of energy flux indicates that the study site generates larger internal tides than regions 20-km north or south (see also Osborne et al 2011).…”

“…3a), is forced at the boundaries by realistic spring-tide surface-tide velocities (TPXO7.2, Egbert 1997), which produce a locally generated internal tide that radiates primarily offshore (see also Osborne et al 2011). Because the amplitude and phase of the simulated internal tide are determined by the prescribed surface tide, which is well known and highly predictable from observations alone, this simulation represents tidal dynamics on the Oregon slope in the absence of remotely generated incident internal tides.…”

The Unpredictable Nature of Internal Tides on Continental Shelves

“…Similarly, internal tides on continental shelves and near coastal margins typically have significant low-frequency variability (MacKinnon and Gregg 2003;Lerczak et al 2003;Sherwin et al 2002;Savidge et al 2007;Osborne et al 2011). Much of this variability has historically been associated with changes in wind-and freshwater-driven mesoscale circulation, which alter the stratification, background currents and shear.…”

Are Any Coastal Internal Tides Predictable?

“…Indeed, observations that demonstrate a highly stationary internal tide tend to be located in semi-enclosed basins and marginal seas, such as Massachusetts Bay (Scotti et al, 2008), the Bay of Biscay (Pingree and New, 1991;Gerkema et al, 2004), and the South China Sea (Ramp et al, 2010;Li and Farmer, 2011). Conversely, several studies over open continental shelves have noted that the internal tide may exhibit significant low-frequency variability that does not follow the local barotropic spring/neap cycle (Inall et al, 2000;Sherwin et al, 2002;MacKinnon and Gregg, 2003;Lerczak et al, 2003;Savidge et al, 2007;Osborne et al, 2011), suggesting that shoaling internal tides may complicate local dynamics.…”