Breaking Topographic Lee Waves in a Tidal Channel in Luzon Strait

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Breaking Topographic Lee Waves in a Tidal Channel in Luzon Strait
S p e C i a L i S S u e O N i N T e r N a L WaV e S aBSTr aCT.Barotropic tides generate energetic internal tides, smaller-scale waves, and turbulence as they flow through Luzon Strait, between Taiwan and the Philippines.Three-dimensional numerical simulations of this process suggest that small-scale lee waves will form and break preferentially in "outflow channels, " trough-like depressions that descend the strait's flanks.In the simulations, these sites are the locations of the most intense dissipation in the eastern strait.To investigate this numerical prediction, an 11-day cruise on R/V Roger Revelle was devoted to exploring an outflow channel on the eastern slope of the strait, north of Batan Island.
Using a rapidly profiling conductivity-temperature-depth sensor and shipboard Doppler sonars, observations of velocity and density fields were made at four sites in the channel.At Site III, approximately 4 km offshore the crest, the generated lee wave was found to occupy much of the water column.It expanded upward from the seafloor as an irregular disturbance with a dominant vertical scale of 250 m.Seasurface horizontal currents exceeded 1.5 m s -1 and were sufficient to cause surface waves to break at 1,300 m above the local topography.Widespread internal wave breaking appeared initially at the seafloor and spread to much of the water column during the outflow phase of the tide.Breaking was also seen to a lesser extent on the inflow phase, as Pacific waters were advected westward toward the crest.The average dissipation rate at Site III, 8 W m -2 , exceeds typical wind energy input rates by four orders of magnitude.
waves of tidal frequency.
Figure 1 presents a numerical illustration of the bore-lee wave phenomenon for the site described below.The offshore flowing currents (red, Figure 1a) separate from the slope in a series of "jumps" that extend nearly 1,000 m above the local seafloor.This pattern is maintained through the peak of the ebb tide.Isopycnal surfaces are tilted to vertical orientation and even inverted in "overturns" that can be several hundred meters high.Modeled dissipation occurs in the overturns, as well as in regions of large isopycnal separation (Figure 1b).
Although lee waves are thought to be widespread (e.g., Nikurashin and Ferrari, 2010), and can be modeled numerically (Klymak et al., 2010), direct observations of the phenomenon in the deep sea are rare.Their presence was inferred in observations from the Floating Instrument Platform (FLIP) in the Hawaii Ocean Mixing Experiment (Pinkel et al., 2000;Klymak et al., 2008).
Recently, Nash et al. (2007) (Aucan, et al., 2006;Klymak, et al., 2011) and can extend for 200-300 m above the seafloor, producing mixing regions of comparable size.Tidal flow over topography such as ridge crests and seamounts can also produce small-scale lee waves (Legg and Klymak, 2008) with horizontal wavelengths much smaller than comparable There, the principal cross-strait pathway bifurcates into two outflow channels.
Each channel appeared sufficiently two dimensional that a series of along-axis measurements might describe the flows.
With the channels cresting at depths less than 1 km, it was hoped that the flows of interest would occur within the depth range of our shipboard instruments.

OBSerVaTiONS
Measurements were collected on a July 11-22 leg of R/V Roger Revelle.
Spring tides peaked on July 14, early in the measurement period.Specific objectives were to observe the evolving vertical structure in the lee waves through the tidal cycle as well as to document the mixing events and the nature of the instabilities that triggered them.Both of these studies can be compared with model predictions.Strain (Figure 5c) is here defined as the instantaneous separation of a pair of isopycnal surfaces divided by their time-mean separation.Density profiles are first Thorpe-sorted (Thorpe, 1977) prior to calculation of isopycnal depths.
When large-scale overturning is prevalent, strain estimates are sensitive to the sorting process.Space-time aliasing also , presented observations from the Oregon slope showing mid-water instabilities of the semidiurnal internal tide at depths of 2,000 m.There remains a need to observe this phenomenon with precision iNTrOduCTiON While the deep sea is generally isolated from the surface layers, localized sites of rapid vertical exchange are now being discovered.These sites are associated with either the generation or dissipation of the internal tide where topography and tides strongly interact.A number of mixing mechanisms have been identified at these sites, including both bores and lee waves.The bores can propagate either upslope or downslope Figure 1.along-channel section of velocity (top) and dissipation bottom from the Massachusetts institute of Technology (MiT) three-dimensional hydrostatic numerical model showing lee wave formation as the flow descends the channel.Vertical excursions of 500 m are anticipated over comparable horizontal distances.in such a flow, variations in ship position scramble spatial and temporal variability.enhanced dissipation (bottom) is found in the model in regions of isopycnal slope and/or dilation.

Figure 3 .
Figure 3. plan view of the experiment site showing depthintegrated time-averaged dissipation from the MiT global circulation model.The concentration of mixing in the outflow channels, as opposed to over topographic crests, is striking.The gray line shows the location of the section in Figure 1.

DecimalFigure 5 .Figure 6 .
Figure 4. a 2.5 day record of along-axis current (positive toward 63°, a, c) and shear (b, d) from the HdSS 140 kHz sonar (a, b) and 50 kHz sonar (c, d) on r/V Revelle.isopycnals, separated in the mean by 10 m (top) and 40 m (bottom), are overplotted.Outflow periods are characterized by rapid vertical motion of the isopycnals, with ~ 200 m vertical coherence scales at depth, diminishing in the upper ocean.Low-frequency motions that are highly coherent in the vertical dominate inflow periods.The dominant shears are not clearly associated with tidal phenomena, although shear layers are advected vertically with tidal motions.Cross-channel shear (not shown) is comparable in magnitude and vertical scale to along-channel shear.