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Alford, MH, Klymak JM, Carter GS.  2014.  Breaking internal lee waves at Kaena Ridge, Hawaii. Geophysical Research Letters. 41:906-912.   10.1002/2013GL059070   AbstractWebsite

Shallow water oscillatory flows and deep ocean steady flows have both been observed to give rise to breaking internal lee waves downstream of steep seafloor obstacles. A recent theory also predicts the existence of high-mode oscillatory internal lee waves in deep water, but they have not previously been directly observed. Here we present repeated spatial transects of velocity, isopycnal displacement, and dissipation rate measured with towed instruments on the south flank of a supercritical ridge in Hawaii known as Kaena Ridge and compare them with predictions from a 3-D numerical model with realistic tidal forcing, bathymetry, and stratification. The measured and modeled flow and turbulence agree well in their spatial structure, time dependence, and magnitude, confirming the existence and predicted nature of high-mode internal lee waves. Turbulence estimated from Thorpe scales increases 2 orders of magnitude following downslope tidal flow, when the internal lee wave begins to propagate upslope and breaks.

Alford, MH, Gregg MC, Zervakis V, Kontoyiannis H.  2012.  Internal wave measurements on the Cycladic Plateau of the Aegean Sea. Journal of Geophysical Research: Oceans. 117:C01015.   10.1029/2011JC007488   AbstractWebsite

The internal wave climate in the southern Aegean Sea is examined with an array of two bottom-mounted acoustic Doppler current profilers and three profiling moorings deployed on the northern continental slope of the Cretan Sea for 3 months. Frequency spectra indicate an extremely weak internal wave continuum, about 4–10 times weaker than the Garrett-Munk and Levine reference levels. Spectra are instead dominated by semidiurnal internal tides and near-inertial waves, which are examined in detail by bandpass filtering. In the semidiurnal band, a barotropic tidal flow of ≈2 cm s−1 is observed, with a pronounced spring/neap modulation in phase with the lunar fortnightly cycle. One to two days following several of these spring tide periods, a distinct internal tide featuring 10–20 m vertical displacements and 15–20 cm s−1baroclinic velocities is detectable propagating upward and to the southeast. Time-mean energy increases a factor of 2–5 within about 100 m from the bottom, implying generation and/or scattering from the bottom, whose slope is nearly critical to semidiurnal internal waves over much of the array. Several strong, downward propagating near-inertial events are also seen, each of which occurs following a period of work done by the wind on the mixed layer as estimated from a nearby surface mooring. The high-frequency internal wave continuum is more temporally constant but increases substantially toward the end of the deployment. Significant but unexplained differences in kinetic energy occur between successive spring tide periods in the case of the internal tides and between successive wind events in the case of the near-inertial signals. Substantial variability is observed in the low-frequency flows, which likely contributes to the time variability of the internal wave signals.

Chinn, BS, Girton JB, Alford MH.  2012.  Observations of internal waves and parametric subharmonic instability in the Philippines archipelago. Journal of Geophysical Research: Oceans. 117:C05019.   10.1029/2011JC007392   AbstractWebsite

Internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing (by wind and tide) and interior diapycnal mixing. We report here on measurements from Mindoro Strait in the Philippines (connecting the South China Sea to the Sulu Sea) of an internal wave field with a number of surprising properties that point to previously-unrecognized processes at work in the region. Continuum spectral levels are very close to typical “background” values found in the open ocean, but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated—and higher at the northern mooring (MP1) than the southern (MP2). Two particularly energetic depth ranges stand out at MP1: an upper layer centered near 300 m, and one at the bottom of the water column, near 1800 m. The upper layer contains both internal tides and a near-inertial band with upward and downward propagating waves and an apparent spring-neap cycle. The combination is suggestive of Parametric Subharmonic Instability as the forcing for the near-inertial band—a conclusion supported by bicoherence estimates. Mixing, estimated from density overturns, is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide—both diurnal and semidiurnal. Near-inertial currents in this deep layer are dominantly rectilinear and not well-correlated with the mixing. Bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography, suggesting additional enhancement elsewhere in the archipelago.

Gregg, MC, Hall RA, Carter GS, Alford MH, Lien RC, Winkel DP, Wain DJ.  2011.  Flow and mixing in Ascension, a steep, narrow canyon. Journal of Geophysical Research: Oceans. 116:C07016.   10.1029/2010JC006610   AbstractWebsite

A thin gash in the continental slope northwest of Monterey Bay, Ascension Canyon, is steep, with sides and axis both strongly supercritical to M2 internal tides. A hydrostatic model forced with eight tidal constituents shows no major sources feeding energy into the canyon, but significant energy is exchanged between barotropic and baroclinic flows along the tops of the sides, where slopes are critical. Average turbulent dissipation rates observed near spring tide during April are half as large as a two week average measured during August in Monterey Canyon. Owing to Ascension's weaker stratification, however, its average diapycnal diffusivity, 3.9 × 10−3 m2 s−1, exceeded the 2.5 × 10−3 m2 s−1 found in Monterey. Most of the dissipation occurred near the bottom, apparently associated with an internal bore, and just below the rim, where sustained cross-canyon flow may have been generating lee waves or rotors. The near-bottom mixing decreased sharply around Ascension's one bend, as did vertically integrated baroclinic energy fluxes. Dissipation had a minor effect on energetics, which were controlled by flux divergences and convergences and temporal changes in energy density. In Ascension, the observed dissipation rate near spring tide was 2.1 times that predicted from a simulation using eight tidal constituents averaged over a fortnightly period. The same observation was 1.5 times the average of an M2-only prediction. In Monterey, the previous observed average was 4.9 times the average of an M2-only prediction.

Martini, KI, Alford MH, Nash JD, Kunze E, Merrifield MA.  2007.  Diagnosing a partly standing internal wave in Mamala Bay, Oahu. Geophysical Research Letters. 34:L17604.   10.1029/2007GL029749   AbstractWebsite

An internal partly standing wave in Mamala Bay, Hawaii is studied using new observations and the Princeton Ocean Model (POM). Previous work suggested a convergence in the bay of east- and westbound waves emanating from Kaena Ridge and Makapuu Point, respectively. New energy flux measurements with shipboard ADCP/CTD confirm that Makapuu Point is the eastern source. After validating the POM results against observations, the model output is modally decomposed and compared with the expected theoretical patterns of kinetic and available potential energy, energy flux, and group velocity for a partly standing wave. Agreement is seen for the first baroclinic mode, which also contains most of the energy. The results confirm previous suggestions of standing wave dynamics in Mamala Bay.

Nash, JD, Alford MH, Kunze E, Martini K, Kelly S.  2007.  Hotspots of deep ocean mixing on the Oregon continental slope. Geophysical Research Letters. 34:L01605.   10.1029/2006GL028170   AbstractWebsite

Two deep ocean hotspots of turbulent mixing were found over the Oregon continental slope. Thorpe-scale analyses indicate time-averaged turbulent energy dissipation rates of ε > 10−7 W/kg and eddy diffusivities of Kρ ∼ 10−2 m2/s at both hotspots. However, the structure of turbulence and its generation mechanism at each site appear to be different. At the 2200-m isobath, sustained >100-m high turbulent overturns occur in stratified fluid several hundred meters above the bottom. Turbulence shows a clear 12.4-h periodicity proposed to be driven by flow over a nearby 100-m tall ridge. At the 1300-m isobath, tidally-modulated turbulence of similar intensity is confined within a stratified bottom boundary layer. Along-slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed. Such topography is common to most continental slopes, providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along-isobath.

Zhao, Z, Alford MH.  2006.  Source and propagation of internal solitary waves in the northeastern South China Sea. Journal of Geophysical Research: Oceans. 111:C11012.   10.1029/2006JC003644   AbstractWebsite

Large-amplitude internal solitary waves (ISWs) observed near Dongsha Island in the South China Sea originate in tide-topography interactions at Luzon Strait. Their arrival times at two moorings (S7 at 117°17′E, 21°37′N, and Y at 117°13.2′E, 21°2.8′N) are investigated, with respect to model-predicted barotropic tidal currents over Lan-Yu ridge at Luzon Strait. Each ISW packet can be associated with a westward tidal current peak. The time lags between the ISWs and the barotropic tidal currents are 57.6 ± 0.9 hours at S7 and 55.1 ± 1.0 hours at Y, consistent with the mode-one internal waves propagating nondispersively through the region's bathymetry and climatological stratification. Larger ISWs usually arrive earlier than smaller ones, consistent with the theoretical relation between nonlinear wave speed and wave amplitude. The observation that the ISWs are associated with westward tidal currents, with/without the presence of earlier eastward tidal currents, suggests that they are generated by nonlinear steepening of internal tides, rather than by the lee-wave mechanism. An idealized nonlinearization distance, over which the ISWs are generated in internal tide troughs, is estimated to be 260 ± 40 km from Luzon Strait.

Eich, ML, Merrifield MA, Alford MH.  2004.  Structure and variability of semidiurnal internal tides in Mamala Bay, Hawaii. Journal of Geophysical Research: Oceans. 109:C05010.   10.1029/2003JC002049   AbstractWebsite

Moored current meter and temperature observations and results from a three-dimensional primitive equation model are used to examine the energetic semidiurnal internal tides present in Mamala Bay on the south coast of Oahu, Hawaii. The steady, harmonic component of the internal tide is characterized by large vertical displacements in the central region of the bay (35 m amplitude for the M2 constituent), and enhanced alongshelf baroclinic currents at the headlands on either end of the bay (0.27 m s−1). Seasonal changes in amplitude and phase are observed. The model captures the qualitative spatial structure of the observations. Baroclinic energy flux estimates, from the mooring observations and the numerical simulations, suggest that internal tide energy propagates into the bay and does not originate within the bay. The model indicates that internal wave generation occurs over the flanks (500–1000 m depth) of the ridge, predominantly on the east side, with perhaps some additional contribution on the west from an energetic internal tide generated north of Oahu. Wave superposition is believed to account for the alongshelf spatial structure of currents and displacements. Incoherent modulations of the internal tide occur that are not related to local changes in stratification, at least on superannual timescales. Factors contributing to this signal may include stratification variations at the deep generation sites, mesoscale activity, and/or the shoaling of a random internal wave field into the bay from the open ocean.

Alford, MH.  2003.  Improved global maps and 54-year history of wind-work on ocean inertial motions. Geophysical Research Letters. 30:1424.   10.1029/2002GL016614   AbstractWebsite

The global distribution and 54-year time dependence of the energy-flux from the wind to near-inertial motions is computed by driving a slab mixed-layer model with NCEP/NCAR Reanalysis winds, improving upon previous estimates [Alford, 2001; Watanabe and Hibiya, 2002]. The slab model is solved spectrally with frequency-dependent damping. The resulting solutions are more physically sensible than the previous, and more skillful at high latitudes, where the inertial frequency approaches the 4×-daily sampling of the Reanalysis winds. This enables Alford's calculation, whose domain was limited to ±50°, to be extended to the poles. The high-latitude reliability is demonstrated by direct comparison with a high-resolution regional model (REMO) in the NE Atlantic. The total power input, 0.47 TW, has increased by 25% since 1948, paralleling observed increases in extratropical cyclone frequency and intensity. If believable, the trend may have important consequences for modulation of the meridional overturning circulation.

Alford, MH, Gregg MC.  2001.  Near-inertial mixing: Modulation of shear, strain and microstructure at low latitude. Journal of Geophysical Research: Oceans. 106:16947-16968.   10.1029/2000JC000370   AbstractWebsite

We report direct, quantitative measurements of mixing associated with three cycles of a single, energetic, downward-propagating near-inertial wave in the Banda Sea at 6.5°S, 128°E during October 1998. The wave dominates the shear, containing 70% of the total variance. Simultaneous depth/time series of shear, strain, Froude number (Fr), and microstructure allow direct computation of their coherence and phase from 50–120 m, for 14 days. In this depth range, 72% of diapycnal diffusivity (68% of dissipation) occurs in three distinct pulses, spaced at the inertial period of 4.4 days. These are collocated with maxima of transverse shear, strain and Fr. Inertial-band log diapycnal diffusivity, log10 Kρ, is coherent at the 95% confidence level with both components of shear and Froude number. In this data set, strain is more important than shear in modulating Fr. Owing to the low latitude, the inertial frequency (fo = 1/4.4 cycles per day) is much smaller than the diurnal and tidal frequencies. Consequently, near-inertial motions may be studied separately from tides and other motions via time-domain filtering. Semiempirical WKB plane-wave solutions with observed frequency ωo = 1.02fo and vertical scale 100 m explain 66% and 42% of inertial-band shear and strain variance, respectively. On the basis of the observed phase relationship between shear and strain, the wave is propagating equatorward, toward 295° true. Ratios of shear to strain and of parallel to transverse shear suggest that the wave's intrinsic frequency ωI ≈ 1.18feff. This indicates that background vorticity ζ has lowered the effective Coriolis frequency, feff = fo + ζ/2, relative to its planetary value, fo [Kunze, 1985]. Ray tracing suggests that the wave was generated near 6.9°S, 130.6°E, ∼20 days prior to the cruise, coincident with the end of high winds associated with the SE monsoon. A slab mixed layer model [Pollard and Millard, 1970], forced with National Center for Environmental Prediction (NCEP) model surface winds, confirms that fluxes from the wind to the ocean at this time were sufficient to generate the wave. A very simple model shows that mixing by monsoon-generated inertial waves may add an important and strongly time-dependent aspect to some regions' energy budgets.