Observations of Internal Tides on the Oregon Continental Slope

Abstract

A complex superposition of locally forced and shoaling remotely generated semidiurnal internal tides occurs on the Oregon continental slope. Presented here are observations from a zonal line of five profiling moorings deployed across the continental slope from 500 to 3000 m, a 24-h expendable current profiler (XCP) survey, and five 15-48-h lowered ADCP (LADCP)/CTD stations. The 40-day moored deployment spans three spring and two neap tides, during which the proportions of the locally and remotely forced internal tides vary. Baroclinic signals are strong throughout spring and neap tides, with 4-5-day-long bursts of strong crossslope baroclinic semidiurnal velocity (u M 2. 0:05 m s 21 ) and vertical displacement (z M 2 . 100 m). Energy fluxes exhibit complex spatial and temporal patterns throughout both tidal periods. During spring tides, local barotropic forcing is strongest and energy flux over the slope is predominantly offshore (westward). During neap tides, shoaling remotely generated internal tides dominate and energy flux is predominantly onshore (eastward). Shoaling internal tides do not exhibit a strong spring-neap cycle and are also observed during the first spring tide, indicating that they originate from multiple sources. The bulk of the remotely generated internal tide is hypothesized to be generated from south of the array (e.g., Mendocino Escarpment), because energy fluxes at the deep mooring 100 km offshore are always directed northward. However, fluxes on the slope suggest that the northbound internal tide is turned onshore, most likely by reflection from large-scale bathymetry. This is verified with a simple three-dimensional model of mode-1 internal tides propagating obliquely onto a near-critical slope, whose output conforms fairly well to observations, in spite of its simplicity.

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

“…Here, observations of TKE dissipation (Moum et al 2002) and internal tides (Nash et al 2007;Martini et al 2011Martini et al , 2012, manuscript submitted to J. Phys. Oceanogr., hereafter MAK) provide an opportunity to describe many of the processes relevant to the global cascade of tidal energy.…”

“…Previous studies have examined the time variability of internal-tide generation (Kelly and Nash 2010;Zilberman et al 2011), propagation (Rainville and Pinkel 2006) and incidence on continental slopes (Martini et al 2011), but little is understood about the time variability of tidally driven TKE dissipation. Here, terms are derived that represent modal-energy conversion (section 2a) and the energy conversion to high modes is characterized in terms of internal-tide intermittency (section 3c).…”

“…In a 40-day mooring record, Martini et al (2011) identified a superposition of locally generated and incident internal tides that produced time-variable onshore energy flux (Fig. 1b).…”

“…The data analyzed here are described by Nash et al (2007), Martini et al (2011), and MAK, who examined turbulent density overturns, 40-day mooring records, and upslope-propagating bores, respectively. This study complements the previous studies by examining the cascade of tidal energy during a high-resolution crossslope snapshot.…”

“…Sampling stations were spaced approximately 3 km apart, between the shelf break and abyssal plain, to produce a cross-slope picture of tidal dynamics. Three sampling schemes were employed to obtain almost full-water-column measurements (see also Nash et al 2007;Martini et al 2011;MAK): (i) At five locations, McLane moored profilers (MPs) continuously traversed the water column, obtaining vertical profiles of velocity, temperature, and salinity every 3 h or less over a 40-day deployment. (Two of these moorings, MP1 and MP6, are omitted from this analysis because their water-column coverage is inadequate for a vertical-mode decomposition.)…”