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NEMO: results


Based on observations over two summers obtained from a new observing system of two moorings and a glider on the Washington continental shelf, a rich, variable internal wave field is seen, that appears to be modulated in part by the California Current system and its response to the region’s frequent wind reversals. The internal wave continuum spectrum suggests a strong but highly temporally variable semidiurnal internal tide field and an energetic field of  high-frequency nonlinear internal waves (NLIW). Mean semidiurnal energy flux is about 80 Wm-1 to the north-northeast (NNE). The onshore direction of the flux and its lack of a strong spring/neap cycle suggests it is at least partly generated remotely. Nonlinear wave amplitudes reach 38 m in 100 m of water, making them among the strongest observed on continental shelves of similar depth. They often occur each 12.4 hours, clearly linking them to the tide.  Like the internal tide energy flux, the NLIW are also directed toward the NNE. However, their phasing is not constant with respect to either the baroclinic or barotropic currents, and their amplitude is uncorrelated with either internal-tide energy flux or barotropic tidal forcing, suggesting substantial modulation by the low-frequency currents and stratification. More work is required to determine the origin and generation mechanism of the NLIW. However, their large size makes them potentially important in mixing nutrients upward into the euphotic zone and/or transporting nutrients and large laterally on the shelf. Spatial surveys, ideally with microstructure, will be helpful in determining the generation location, propagation pathways, and scalar transport of the waves. We will endeavor to make these measurements in the near future.

Oceanographic Context

The observed low-frequency currents and stratification are typical of the Washington coast (Figure 1) in summer. Winds are predominantly from the northwest with periodic southerly winds typically associated with stormy weather (Figure 2 (a)). Currents are generally toward the southeast. This is the summertime signature of the California current, the eastern boundary current system stretching from Washington all the way down to California (Hickey, 1978). 1-2 days following each of the southerly wind events, currents slacken substantially and even reverse at some depths. The periodic wind reversal events and changing low-frequency currents associated with them are expected to affect the propagation of internal waves profoundly.

Figure 1: Map showing the location of the NEMO moorings(star) and gliderline (blackline), as well as the NDBC Cape Elizabeth meteorological buoy 46021. Observed barotropic tidal flow and time-mean vector of the California Current are shown with  a black ellipse and arrow, respectively, with scaleas indicated. The red and blue arrows are the semidiurnal energy flux computed from observations and model, respectively. The colormap shows conversion of barotropic tidal energy to baroclinic tides. The black and yellow dashed line is the boundary of the Olympic Coast National Marine Sanctuary. Contour levels are each 100m until 500m, then each 200m thereafter.

Figure 2: 2011 time series from the surface mooring, with outer insets showing zoom-ins of 18-hour periods around the four times marked in (d) to demonstrate the variability in the internal tide. Inner insects are zoomed in further on the period indicated in each outer inset, to illustrate the temperature (top) and baroclinic northward velocity (bottom) of the nonlinear internal waves.

Figure 3: Time series of data from the subsurface mooring, corresponding to the last 46days of the period plotted in the previous figure. Panels are windspeed colored by direction as in the previous figure(a), velocity toward 315o true(b), temperature(c), salinity(d), buoyancy frequency(e), dissolved oxygen(f), nitrate(g), and chlorophyll (h). Isopycnals whose mean spacing is10 m are over plotted in each panel in black.

Internal Waves

A rich, variable internal wave field is seen from the record which covers a broad range of frequencies. The spectrum of rotary velocity (Figure 2) which is obtained wherein clockwise and counterclockwise motions appear at negative and positive frequencies, shows strongly clockwise polarized motions at frequencies near f, as expected for internal waves. Superimposed on the "continuum" spectrum are 1) a strong but highly temporally variable semidiurnal internal tide field and 2) an energetic field of high-frequency nonlinear internal waves (NLIW). The internal wave spectral level at intermediate frequencies is consistent with the model spectrum of Levine (2002) developed for continental shelves.

Rotary frequency spectrum of baroclinic velocity at 30 m, measured at the surface mooring during 2010 (top) and 2011 (bottom). The blue and green curves correspond to clockwise and counterclockwise, respectively.

Figure 4. Rotary frequency spectrum of baroclinic velocity at 30m, measured at the surface mooring during 2010 (top) and 2011(bottom). The blue and green curves correspond to clockwise and counter clockwise motions, respectively. Vertical dashed lines indicate the near-inertial and tidal frequencies. The Levine (2002) and GM76 model spectra are overlain with red and gray dashed lines, respectively.

Internal Tides
The internal tide contains much of the variance in the velocity spectrum. Four periods of the baroclinic velocity record plotted in closeup in Figure 2, indicates the dominance of mode-1 motions. The fact that isotherms tend to be downward during velocity to the north is a signature of a wave traveling to the north. There is, however, considerable variability in the strength of the semidiurnal motions (with the first period showing significantly stronger velocities than the rest), as well as the depth/time structure, with some periods showing more vertically-standing signals and others (such as the fourth) showing downward phase propagation, implying upward energy propagation.

Quantitative estimates of the energy and energy flux in the internal tide are made following now-standard methods described in Alford (2003), Nash et al. (2005) and Alford and Zhao (2007a). The baroclinic energy flux bears little relation to the barotropic forcing, suggesting the presence of remotely incident and/or modulation of the generation and propagation of the local signals by the low-frequency flows as opposed to the open ocean. More definitive statements may be possible as the length of the time series grows.

The time-mean semidiurnal energy flux is toward the NNE in both 2010 and 2011, with magnitude of 80 and 70 Wm-1, respectively. These values are similar to those seen on other continental shelves, but much smaller than typical values seen in the open ocean or near strong sources. The observed fluxes in both years are substantially greater than the fluxes from the MOSSea regional numerical model (Sutherland et al., 2011) and directed more to the north (Figure 3), suggesting at least partly of remote origin, possibly from Cape Mendocino (Althaus et al., 2003; Alford, 2010).

Nonlinear Internal Waves (NLIWs)
The 3.5-hour period closeups of nonlinear internal waves in Figure 2 indicate sharp waves of depression as bursts of warm water observed at much greater depth than previously, implying downward displacements of 23-38 m in only 100 m of water. Individual waves last 5-10 minutes, and are therefore well resolved in temperature which is sampled every minute. Assuming the waves are propagating at a bout mode-1 phase speed for high-frequency waves of 0.5 m/s, they have horizontal wavelengths of about 150-300m.

Waves occur at times singly, or in dramatic sets of many waves shown a "rank ordering" pattern with a clear signature in velocity. Direction of these and nearly all waves clusters toward the NNE, similar to the direction of semidiurnal flux (Figure 1). The generation mechanism of the NLIW is yet unknown, but transformation of the shoaling internal tide into internal bores as it transits the shelf break and/or lee-wave generation by the barotropic tide flowing past the shelf break are the most likely candidates. The along-shelf orientation of the barotropic tidal ellipses and the propagation of the NLIW in the same direction as the internal tide are suggestive of the former mechanism, but more work is required to be sure.

The amplitude of the nonlinear waves is computed by tracking the depth of each isotherm relative 9 to a 2-day mean, which is intended to represent the slowly-varying background state without the
displacements of internal waves. The four waves shown range from 23-38 m, in the range of sizes seen on other shelves and continental slopes (Table 1). Expressed as a fraction of the water depth,
the largest waves observed here appear to be among the largest anywhere, exceeding even those on the South China Sea continental slope. The magnitude and frequency of the NLIW shows modulation on a variety of timescales, but appears to bear little relationship with either the barotropic forcing (spring tide times given with dashed lines) or the energy or energy flux of the semidiurnal internal tide (Figure 2d).

Table 1.Selected studies of nonlinear waves observed on continental shelves and their amplitudes in relation to water depth.