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2018
Thorpe, SA, Malarkey J, Voet G, Alford MH, Girton JB, Carter GS.  2018.  Application of a model of internal hydraulic jumps. Journal of Fluid Mechanics. 834:125-148.   10.1017/jfm.2017.646   AbstractWebsite

A model devised by Thorpe & Li (J. Fluid Mech., vol. 758, 2014, pp. 94-120) that predicts the conditions in which stationary turbulent hydraulic jumps can occur in the flow of a continuously stratified layer over a horizontal rigid bottom is applied to, and its results compared with, observations made at several locations in the ocean. The model identifies two positions in the Samoan Passage at which hydraulic jumps should occur and where changes in the structure of the flow are indeed observed. The model predicts the amplitude of changes and the observed mode 2 form of the transitions. The predicted dissipation of turbulent kinetic energy is also consistent with observations. One location provides a particularly well-defined example of a persistent hydraulic jump. It takes the form of a 390 m thick and 3.7 km long mixing layer with frequent density inversions separated from the seabed by some 200 m of relatively rapidly moving dense water, thus revealing the previously unknown structure of an internal hydraulic jump in the deep ocean. Predictions in the Red Sea Outflow in the Gulf of Aden are relatively uncertain. Available data, and the model predictions, do not provide strong support for the existence of hydraulic jumps. In the Mediterranean Outflow, however, both model and data indicate the presence of a hydraulic jump.

2017
Alford, MH, MacKinnon JA, Pinkel R, Klymak JM.  2017.  Space-time scales of shear in the North Pacific. Journal of Physical Oceanography. 47:2455-2478.   10.1175/jpo-d-17-0087.1   AbstractWebsite

The spatial, temporal, and directional characteristics of shear are examined in the upper 1400m of the North Pacific during late spring with an array of five profiling moorings deployed from 25 degrees to 37 degrees N (1330 km) and simultaneous shipboard transects past them. The array extended from a regime of moderate wind generation at the north to south of the critical latitude 28.8 degrees N, where parametric subharmonic instability (PSI) can transfer energy from semidiurnal tides to near-inertial motions. Analyses are done in an isopycnal-following frame to minimize contamination by Doppler shifting. Approximately 60% of RMS shear at vertical scales >20m (and 80% for vertical scales >80 m) is contained in near-inertial motions. An inertial back-rotation technique is used to index shipboard observations to a common time and to compute integral time scales of the shear layers. Persistence times are O(7) days at most moorings but O(25) days at the critical latitude. Simultaneous shipboard transects show that these shear layers can have lateral scales >= 100 km. Layers tend to slope downward toward the equator north of the critical latitude and are more flat to its south. Phase between shear and strain is used to infer lateral propagation direction. Upgoing waves are everywhere laterally isotropic. Downgoing waves propagate predominantly equatorward north and south of the critical latitude but are isotropic near it. Broadly, results are consistent with wind generation north of the critical latitude and PSI near it-and suggest a more persistent and laterally coherent near-inertial wave field than previously thought.

Savage, AC, Arbic BK, Alford MH, Ansong JK, Farrar JT, Menemenlis D, O'Rourke AK, Richman JG, Shriver JF, Voet G, Wallcraft AJ, Zamudio L.  2017.  Spectral decomposition of internal gravity wave sea surface height in global models. Journal of Geophysical Research-Oceans. 122:7803-7821.   10.1002/2017jc013009   AbstractWebsite

Two global ocean models ranging in horizontal resolution from 1/12 degrees to 1/48 degrees are used to study the space and time scales of sea surface height (SSH) signals associated with internal gravity waves (IGWs). Frequency-horizontal wavenumber SSH spectral densities are computed over seven regions of the world ocean from two simulations of the HYbrid Coordinate Ocean Model (HYCOM) and three simulations of the Massachusetts Institute of Technology general circulation model (MITgcm). High wavenumber, high-frequency SSH variance follows the predicted IGW linear dispersion curves. The realism of high-frequency motions (> 0.87cpd) in the models is tested through comparison of the frequency spectral density of dynamic height variance computed from the highest-resolution runs of each model (1/25 degrees HYCOM and 1/48 degrees MITgcm) with dynamic height variance frequency spectral density computed from nine in situ profiling instruments. These high-frequency motions are of particular interest because of their contributions to the small-scale SSH variability that will be observed on a global scale in the upcoming Surface Water and Ocean Topography (SWOT) satellite altimetry mission. The variance at supertidal frequencies can be comparable to the tidal and low-frequency variance for high wavenumbers (length scales smaller than approximate to 50 km), especially in the higher-resolution simulations. In the highest-resolution simulations, the high-frequency variance can be greater than the low-frequency variance at these scales.

Alberty, MS, Billheimer S, Hamann MM, Ou CY, Tamsitt V, Lucas AJ, Alford MH.  2017.  A reflecting, steepening, and breaking internal tide in a submarine canyon. Journal of Geophysical Research-Oceans. 122:6872-6882.   10.1002/2016jc012583   AbstractWebsite

Submarine canyons are common features of the coastal ocean. Although they are known to be hotspots of turbulence that enhance diapycnal transport in their stratified waters, the dynamics of canyon mixing processes are poorly understood. Most studies of internal wave dynamics within canyons have focused on a handful of canyons with along-axis slopes less steep than semidiurnal (D-2) internal wave characteristics (subcritical). Here, we present the first tidally resolving observations within a canyon with a steeply sloping axis (supercritical). A process study consisting of two 24 h shipboard stations and a profiling mooring was conducted in the La Jolla Canyon off the coast of La Jolla, CA. Baroclinic energy flux is oriented up-canyon and decreases from 182 +/- 18 W m(-1) at the canyon mouth to 46 +/- 5 W m(-1) near the head. The ratio of horizontal kinetic energy to available potential energy and the observed group speed of each mode are lower than expected for freely propagating D-2 internal waves at each station, indicating partial reflection. Harmonic analysis reveals that variance is dominated by the D-2 tide. Moving up-canyon, the relative importance of D-2 decreases and its higher harmonics are needed to account for a majority of the observed variance, indicating steepening. Steep internal tides cause large isopycnal displacements (approximate to 50 m in 100 m water depth) and high strain events. These events coincide with enhanced O( 10-7-10-5 m(2) s(-3)) dissipation of turbulent kinetic energy at mid-depths.

MacKinnon, JA, Alford MH, Ansong JK, Arbic BK, Barna A, Briegleb BP, Bryan FO, Buijsman MC, Chassignet EP, Danabasoglu G, Diggs S, Griffies SM, Hallberg RW, Jayne SR, Jochum M, Klymak JM, Kunze E, Large WG, Legg S, Mater B, Melet AV, Merchant LM, Musgrave R, Nash JD, Norton NJ, Pickering A, Pinkel R, Polzin K, Simmons HL, Laurent LSC, Sun OM, Trossman DS, Waterhouse AF, Whalen CB, Zhao Z.  2017.  Climate process team on internal-wave driven ocean mixing. Bulletin of the American Meteorological Society.   10.1175/bams-d-16-0030.1   Abstract

Recent advances in our understanding of internal-wave driven turbulent mixing in the ocean interior are summarized. New parameterizations for global climate ocean models, and their climate impacts, are introduced.Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF- and NOAA-supported Climate Process Team has been engaged in developing, implementing and testing dynamics-based parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions.

Savage, AC, Arbic BK, Richman JG, Shriver JF, Alford MH, Buijsman MC, Farrar JT, Sharma H, Voet G, Wallcraft AJ, Zamudio L.  2017.  Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies. Journal of Geophysical Research-Oceans. 122:2519-2538.   10.1002/2016jc012331   AbstractWebsite

High horizontal-resolution (1/12: 5 degrees and 1/25 degrees) 41-layer global simulations of the HYbrid Coordinate Ocean Model (HYCOM), forced by both atmospheric fields and the astronomical tidal potential, are used to construct global maps of sea surface height (SSH) variability. The HYCOM output is separated into steric and nonsteric and into subtidal, diurnal, semidiurnal, and supertidal frequency bands. The model SSH output is compared to two data sets that offer some geographical coverage and that also cover a wide range of frequencies-a set of 351 tide gauges that measure full SSH and a set of 14 in situ vertical profilers from which steric SSH can be calculated. Three of the global maps are of interest in planning for the upcoming Surface Water and Ocean Topography (SWOT) two-dimensional swath altimeter mission: (1) maps of the total and (2) nonstationary internal tidal signal (the latter calculated after removing the stationary internal tidal signal via harmonic analysis), with an average variance of 1: 05 and 0: 43 cm(2), respectively, for the semidiurnal band, and (3) a map of the steric supertidal contributions, which are dominated by the internal gravity wave continuum, with an average variance of 0: 15 cm2. Stationary internal tides (which are predictable), nonstationary internal tides (which will be harder to predict), and nontidal internal gravity waves (which will be very difficult to predict) may all be important sources of high-frequency "noise" that could mask lower frequency phenomena in SSH measurements made by the SWOT mission.

Ansong, JK, Arbic BK, Alford MH, Buijsman MC, Shriver JF, Zhao ZX, Richman JG, Simmons HL, Timko PG, Wallcraft AJ, Zamudio L.  2017.  Semidiurnal internal tide energy fluxes and their variability in a Global Ocean Model and moored observations. Journal of Geophysical Research-Oceans. 122:1882-1900.   10.1002/2016jc012184   AbstractWebsite

We examine the temporal means and variability of the semidiurnal internal tide energy fluxes in 1/25 degrees global simulations of the Hybrid Coordinate Ocean Model (HYCOM) and in a global archive of 79 historical moorings. Low-frequency flows, a major cause of internal tide variability, have comparable kinetic energies at the mooring sites in model and observations. The computed root-mean-square (RMS) variability of the energy flux is large in both model and observations and correlates positively with the time-averaged flux magnitude. Outside of strong generation regions, the normalized RMS variability (the RMS variability divided by the mean) is nearly independent of the flux magnitudes in the model, and of order 23% or more in both the model and observations. The spatially averaged flux magnitudes in observations and the simulation agree to within a factor of about 1.4 and 2.4 for vertical mode-1 and mode-2, respectively. The difference in energy flux computed from the full-depth model output versus model output subsampled at mooring instrument depths is small. The global historical archive is supplemented with six high-vertical resolution moorings from the Internal Waves Across the Pacific (IWAP) experiment. The model fluxes agree more closely with the high-resolution IWAP fluxes than with the historical mooring fluxes. The high variability in internal tide energy fluxes implies that internal tide fluxes computed from short observational records should be regarded as realizations of a highly variable field, not as "means" that are indicative of conditions at the measurement sites over all time.

2016
Chinn, BS, Girton JB, Alford MH.  2016.  The impact of observed variations in the shear-to-strain ratio of internal waves on inferred turbulent diffusivities. Journal of Physical Oceanography. 46:3299-3320.   10.1175/jpo-d-15-0161.1   AbstractWebsite

The most comprehensive studies of the spatial and temporal scales of diffusivity rely on internal wave parameterizations that require knowledge of finescale shear and strain. Studies lacking either shear or strain measurements have to assume a constant ratio between shear and strain (R-omega). Data from 14 moorings collected during five field programs are examined to determine the spatial and temporal patterns in R-omega and the influence of these patterns on parameterized diffusivity. Time-mean R-omega ranges from 1 to 10, with changes of order 10 observed over a broad range of scales. Temporal variability in R-omega is observed at daily, weekly, and monthly scales. Observed changes in R-omega could produce a 2-3 times change in parameterized diffusivity. Vertical profiles of R-omega, E-shear, and E-strain (shear or strain variance relative to Garret-Munk) reveal that both local topographic properties and wind variability impact the internal wave field. Time series of R-omega from each mooring have strong correlations to either shear or strain, often only at a specific range of vertical wave-numbers. Sites fall into two categories, in which R-omega variability is dominated by either shear or strain. Linear fits to the dominant property (i.e., shear or strain) can be used to estimate a time series of R-omega that has an RMS error that is 30% less than the RMS error from assuming R-omega = 3. Shear and strain level vary in concert, as predicted by the Garret-Munk model, at high E-shear values. However, at E-shear, 5, strain variations are 3 times weaker than shear.

Klymak, JM, Simmons HL, Braznikov D, Kelly S, MacKinnon JA, Alford MH, Pinkel R, Nash JD.  2016.  Reflection of linear internal tides from realistic topography: The Tasman continental slope. Journal of Physical Oceanography. 46:3321-3337.   10.1175/jpo-d-16-0061.1   AbstractWebsite

The reflection of a low-mode internal tide on the Tasman continental slope is investigated using simulations of realistic and simplified topographies. The slope is supercritical to the internal tide, which should predict a large fraction of the energy reflected. However, the response to the slope is complicated by a number of factors: the incoming beam is confined laterally, it impacts the slope at an angle, there is a roughly cylindrical rise directly offshore of the slope, and a leaky slope-mode wave is excited. These effects are isolated in simulations that simplify the topography. To separate the incident from the reflected signal, a response without the reflector is subtracted from the total response to arrive at a reflected signal. The real slope reflects approximately 65% of themode-1 internal tide asmode 1, less than two-dimensional linear calculations predict, because of the three-dimensional concavity of the topography. It is also less than recent glider estimates, likely as a result of along-slope inhomogeneity. The inhomogeneity of the response comes from the Tasman Rise that diffracts the incoming tidal beam into two beams: one focused along beam and one diffracted to the north. Along-slope inhomogeneity is enhanced by a partially trapped, superinertial slope wave that propagates along the continental slope, locally removing energy from the deep-water internal tide and reradiating it into the deep water farther north. This wave is present even in a simplified, straight slope topography; its character can be predicted from linear resonance theory, and it represents up to 30% of the local energy budget.

Voet, G, Alford MH, Girton JB, Carter GS, Mickett JB, Klymak JM.  2016.  Warming and weakening of the abyssal flow through Samoan Passage. Journal of Physical Oceanography. 46:2389-2401.   10.1175/jpo-d-16-0063.1   AbstractWebsite

The abyssal flow of water through the Samoan Passage accounts for the majority of the bottom water renewal in the North Pacific, thereby making it an important element of the meridional overturning circulation. Here the authors report recent measurements of the flow of dense waters of Antarctic and North Atlantic origin through the Samoan Passage. A 15-month long moored time series of velocity and temperature of the abyssal flow was recorded between 2012 and 2013. This allows for an update of the only prior volume transport time series from the Samoan Passage from WOCE moored measurements between 1992 and 1994. While highly variable on multiple time scales, the overall pattern of the abyssal flow through the Samoan Passage was remarkably steady. The time-mean northward volume transport of about 5.4 Sv (1 Sv = 10(6) m(3) s(-1)) in 2012/13 was reduced compared to 6.0 Sv measured between 1992 and 1994. This volume transport reduction is significant within 68% confidence limits (60.4 Sv) but not at 95% confidence limits (+/-0.6 Sv). In agreement with recent studies of the abyssal Pacific, the bottom flow through the Samoan Passage warmed significantly on average by 1 x 10(-38)Cyr(-1) over the past two decades, as observed both in moored and shipboard hydrographic observations. While the warming reflects the recently observed increasing role of the deep oceans for heat uptake, decreasing flow through Samoan Passage may indicate a future weakening of this trend for the abyssal North Pacific.

MacKinnon, JA, Nash JD, Alford MH, Lucas AJ, Mickett JB, Shroyer EL, Waterhouse AF, Tandon A, Sengupta D, Mahadevan A, Ravichandran M, Pinkel R, Rudnick DL, Whalen CB, Alberty MS, Lekha JS, Fine EC, Chaudhuri D, Wagner GL.  2016.  A tale of two spicy seas. Oceanography. 29:50-61.   10.5670/oceanog.2016.38   AbstractWebsite

Upper-ocean turbulent heat fluxes in the Bay of Bengal and the Arctic Ocean drive regional monsoons and sea ice melt, respectively, important issues of societal interest. In both cases, accurate prediction of these heat transports depends on proper representation of the small-scale structure of vertical stratification, which in turn is created by a host of complex submesoscale processes. Though half a world apart and having dramatically different temperatures, there are surprising similarities between the two: both have (1) very fresh surface layers that are largely decoupled from the ocean below by a sharp halocline barrier, (2) evidence of interleaving lateral and vertical gradients that set upper-ocean stratification, and (3) vertical turbulent heat fluxes within the upper ocean that respond sensitively to these structures. However, there are clear differences in each ocean's horizontal scales of variability, suggesting that despite similar background states, the sharpening and evolution of mesoscale gradients at convergence zones plays out quite differently. Here, we conduct a qualitative and statistical comparison of these two seas, with the goal of bringing to light fundamental underlying dynamics that will hopefully improve the accuracy of forecast models in both parts of the world.

Zhao, Z, Alford MH, Girton JB, Rainville L, Simmons HL.  2016.  Global observations of open-ocean mode-1 M2 internal tides. Journal of Physical Oceanography. 46:1657-1684.   10.1175/JPO-D-15-0105.1   AbstractWebsite

AbstractA global map of open-ocean mode-1 M2 internal tides is constructed using sea surface height (SSH) measurements from multiple satellite altimeters during 1992–2012, representing a 20-yr coherent internal tide field. A two-dimensional plane wave fit method is employed to 1) suppress mesoscale contamination by extracting internal tides with both spatial and temporal coherence and 2) separately resolve multiple internal tidal waves. Global maps of amplitude, phase, energy, and flux of mode-1 M2 internal tides are presented. The M2 internal tides are mainly generated over topographic features, including continental slopes, midocean ridges, and seamounts. Internal tidal beams of 100–300 km width are observed to propagate hundreds to thousands of kilometers. Multiwave interference of some degree is widespread because of the M2 internal tide’s numerous generation sites and long-range propagation. The M2 internal tide propagates across the critical latitudes for parametric subharmonic instability (28.8°S/N) with little energy loss, consistent with the 2006 Internal Waves across the Pacific (IWAP) field measurements. In the eastern Pacific Ocean, the M2 internal tide loses significant energy in propagating across the equator; in contrast, little energy loss is observed in the equatorial zones of the Atlantic, Indian, and western Pacific Oceans. Global integration of the satellite observations yields a total energy of 36 PJ (1 PJ = 1015 J) for all the coherent mode-1 M2 internal tides. Finally, satellite observed M2 internal tides compare favorably with field mooring measurements and a global eddy-resolving numerical model.

Alford, MH, MacKinnon JA, Simmons HL, Nash JD.  2016.  Near-inertial internal gravity waves in the ocean. Annual Review of Marine Science, Vol 8. 8( Carlson CA, Giovannoni SJ, Eds.).:95-123., Palo Alto: Annual Reviews   10.1146/annurev-marine-010814-015746   Abstract

We review the physics of near-inertial waves (NIWs) in the ocean and the observations, theory, and models that have provided our present knowledge. NIWs appear nearly everywhere in the ocean as a spectral peak at and just above the local inertial period f, and the longest vertical wavelengths can propagate at least hundreds of kilometers toward the equator from their source regions; shorter vertical wavelengths do not travel as far and do not contain as much energy, but lead to turbulent mixing owing to their high shear. NIWs are generated by a variety of mechanisms, including the wind, nonlinear interactions with waves of other frequencies, lee waves over bottom topography, and geostrophic adjustment; the partition among these is not known, although the wind is likely the most important. NIWs likely interact strongly with mesoscale and submesoscale motions, in ways that are just beginning to be understood.

2015
Alford, MH, McGinnis T, Howe BM.  2015.  An inductive charging and real-time communications system for profiling moorings. Journal of Atmospheric and Oceanic Technology. 32:2243-2252.   10.1175/jtech-d-15-0103.1   AbstractWebsite

This paper describes a system for providing power and communications to moored profiling vehicles. A McLane Moored Profiler (MP) was equipped with a rechargeable battery pack and an inductive charging system to allow it to move periodically to a charging dock at the top of a subsurface mooring. Power was provided from a large bank of alkaline batteries housed in two 0.94-m steel spheres. Data were transferred inductively from the profiler to a mooring controller, and from there back to shore via radio and Iridium satellite modems housed in a small surface communications float on an L tether. An acoustic modem provided backup communications to a nearby ship in the event of loss or damage to the surface float. The system was tested in a 180-m-deep fjord (Puget Sound, Washington) and at Station ALOHA (A Long-Term Oligotrophic Habitat Assessment), a 4748-m-deep open-ocean location north of Hawaii. Basic functionality of the system was demonstrated, with the profiler repeatedly recharging at about 225 W (with an overall efficiency of about 70%). Data were relayed back to shore via Iridium and to a nearby ship via the radio and acoustic modems. The system profiled flawlessly for the entire 6-week test in Puget Sound, but charging at the deep site stopped after only 9 days in the deep-ocean deployment owing to damage to the charging station, possibly by surface wave action.

Zhang, S, Alford MH.  2015.  Instabilities in nonlinear internal waves on the Washington continental shelf. Journal of Geophysical Research-Oceans. 120:5272-5283.   10.1002/2014jc010638   AbstractWebsite

Previous studies have identified two primary mechanisms (shear instability and convective instability) by which nonlinear internal waves (NLIWs) induce mixing on continental shelves. To determine the relative importance of these and their dependence on background flow conditions, we examine a much longer (6 month) data set from a moored ADCP/thermistor chain with 2 m vertical spacing in which over 600 NLIWs are detected. Turbulent properties of the 318 waves with detectable overturning instabilities are documented using Thorpe scales. The 130 waves detected while an ADCP was functioning are classified based on a Froude number criterion (Fr = u/c, where u is velocity in the wave propagation direction, c is the wave phase speed). Of these, 108 waves are identified as shear-instability (Type I; Fr < 1) waves and 22 as convective instability (Type II; Fr > 1). Composites are constructed by averaging in a wave coordinate frame over all waves in each category, showing the mean spatial structure of dissipation and other wave quantities. Turbulence is highest at the sheared interface for Type I waves and throughout the wave core for Type II waves. No relationship between wave instability mechanisms and wave/background parameters such as wave steepness, stratification, or mean flow is found, except that unstable waves tend to be more energetic, demonstrating a need to better understand wave propagation and breaking in complex and variable coastal oceanographic background flows.

Pickering, A, Alford M, Nash J, Rainville L, Buijsman M, Ko DS, Lim B.  2015.  Structure and variability of internal tides in Luzon Strait. Journal of Physical Oceanography. 45:1574-1594.   10.1175/jpo-d-14-0250.1   AbstractWebsite

The Luzon Strait is the generation region for strong internal tides that radiate westward into the South China Sea and eastward into the western Pacific. Intrusions of the Kuroshio and strong mesoscale variability in the Luzon Strait can influence their generation and propagation. Here, the authors use eight moorings and two numerical models to investigate these relationships by quantifying the coherence of the diurnal and semidiurnal internal tides in the Luzon Strait. This study finds that the level of coherence of internal tide generation, energy, and energy flux is quite variable, depending on the specific location within the Luzon Strait. Large-scale spatial patterns in internal tide pressure and velocity exist across the region, shaped by the bathymetry, mean flow, and stratification. Internal tide coherence is lower (<30%) near large gradients in this pattern (predominantly along the eastern ridge), which are shifted by the variable Kuroshio and mesoscale fields. At other locations within the Luzon Strait, the internal tide is largely coherent (>80%), and simple calculations suggest that remote sources of internal tides could account for these small decreases in coherence. To the west of the Luzon Strait (away from the primary generation regions), the model suggests that diurnal internal tide energy is more coherent than semidiurnal.

Alford, MH, Peacock T, MacKinnon JA, Nash JD, Buijsman MC, Centuroni LR, Chao SY, Chang MH, Farmer DM, Fringer OB, Fu KH, Gallacher PC, Graber HC, Helfrich KR, Jachec SM, Jackson CR, Klymak JM, Ko DS, Jan S, Johnston TMS, Legg S, Lee IH, Lien RC, Mercier MJ, Moum JN, Musgrave R, Park JH, Pickering AI, Pinkel R, Rainville L, Ramp SR, Rudnick DL, Sarkar S, Scotti A, Simmons HL, St Laurent LC, Venayagamoorthy SK, Hwang Y, Wang J, Yang YJ, Paluszkiewicz T, Tang TY.  2015.  The formation and fate of internal waves in the South China Sea. Nature. 521:65-U381.   10.1038/nature14399   AbstractWebsite

Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis(1), sediment and pollutant transport(2) and acoustic transmission(3); they also pose hazards for man-made structures in the ocean(4). Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking(5), making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects(6,7). For over a decade, studies(8-11) have targeted the South China Sea, where the oceans' most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

Klymak, JM, Crawford W, Alford MH, MacKinnon JA, Pinkel R.  2015.  Along-isopycnal variability of spice in the North Pacific. Journal of Geophysical Research-Oceans. 120:2287-2307.   10.1002/2013jc009421   AbstractWebsite

Two hydrographic surveys in the Gulf of Alaska and the North Pacific subtropical gyre are presented. Both surveys are roughly perpendicular to lateral temperature gradients, and were collected in the summer when there was a shallow mixed layer and a seasonal thermocline. Isopycnal displacements and horizontal velocities are dominated by internal waves. Spice anomalies along isopycnals are examined to diagnose lateral stirring mechanisms. The spectra of spice anomaly gradients along near-surface isopycnals roughly follow power laws of similar to k(X)(0.6+/-0.2) (variance spectra power laws similar to k(X)(1.4+/-0.2)), and in most cases, the spectra become redder at depth. The near-surface spectra are possibly consistent with the predictions of quasi-geostrophic turbulence theory (when surface buoyancy effects are accounted for), but the spectra at depth are inconsistent with any quasi-geostrophic theory. Probability distributions of spice gradients exhibit a large peak at low gradients and long tails for large gradients, symptomatic of fronts. Vertical coherence of the spice signal falls off with a decorrelation depth scale that has a maximum of about 80 m at 100 km wavelengths and depends on horizontal wavelength with a power law of approximately k(x)(-1/2). Lateral decorrelation length scales are 20-40 km, close to the baroclinic Rossby radius. Lateral stirring occurs over large scales, with average lateral displacements of about 200 km in the upper 75 m, decreasing to 100 km at greater depths. The depth variation of the statistics indicates that time history of tracer stirring on each isopycnal is important, or that there are unconsidered depth-dependent stirring mechanisms.

Voet, G, Girton JB, Alford MH, Carter GS, Klymak JM, Mickett JB.  2015.  Pathways, volume transport, and mixing of abyssal water in the samoan Passage. Journal of Physical Oceanography. 45:562-588.   10.1175/jpo-d-14-0096.1   AbstractWebsite

The flow of dense water through the Samoan Passage accounts for the major part of the bottom water renewal in the North Pacific and is thus an important element of the Pacific meridional overturning circulation. A recent set of highly resolved measurements used CTD/LADCP, a microstructure profiler, and moorings to constrain the complex pathways and variability of the abyssal flow. Volume transport estimates for the dense northward current at several sections across the passage, calculated using direct velocity measurements from LADCPs, range from 3.9x10(6) to 6.0 x 10(6) +/- 1 x 10(6) m(3) s(-1). The deep channel to the east and shallower pathways to the west carried about equal amounts of this volume transport, with the densest water flowing along the main eastern channel. Turbulent dissipation rates estimated from Thorpe scales and direct microstructure agree to within a factor of 2 and provide a region-averaged value of O(10(-8)) Wkg(-1) for layers colder than 0.8 degrees C. Associated diapycnal diffusivities and downward turbulent heat fluxes are about 5 x 10(-3) m(2) s(-1) and O(10) Wm(-2), respectively. However, heat budgets suggest heat fluxes 2-6 times greater. In the vicinity of one of the major sills of the passage, highly resolved Thorpe-inferred diffusivity and heat flux were over 10 times larger than the region-averaged values, suggesting the mismatch is likely due to undersampled mixing hotspots.

2014
Waterhouse, AF, MacKinnon JA, Nash JD, Alford MH, Kunze E, Simmons HL, Polzin KL, St Laurent LC, Sun OM, Pinkel R, Talley LD, Whalen CB, Huussen TN, Carter GS, Fer I, Waterman S, Garabato ACN, Sanford TB, Lee CM.  2014.  Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate. Journal of Physical Oceanography. 44:1854-1872.   10.1175/jpo-d-13-0104.1   AbstractWebsite

The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from(i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10(-4))m(2) s(-1) and above 1000-m depth is O(10(-5))m(2) s(-1). The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

Buijsman, MC, Klymak JM, Legg S, Alford MH, Farmer D, MacKinnon JA, Nash JD, Park JH, Pickering A, Simmons H.  2014.  Three-dimensional double-ridge internal tide resonance in Luzon Strait. Journal of Physical Oceanography. 44:850-869.   10.1175/jpo-d-13-024.1   AbstractWebsite

The three-dimensional (3D) double-ridge internal tide interference in the Luzon Strait in the South China Sea is examined by comparing 3D and two-dimensional (2D) realistic simulations. Both the 3D simulations and observations indicate the presence of 3D first-mode (semi)diurnal standing waves in the 3.6-km-deep trench in the strait. As in an earlier 2D study, barotropic-to-baroclinic energy conversion, flux divergence, and dissipation are greatly enhanced when semidiurnal tides dominate relative to periods dominated by diurnal tides. The resonance in the 3D simulation is several times stronger than in the 2D simulations for the central strait. Idealized experiments indicate that, in addition to ridge height, the resonance is only a function of separation distance and not of the along-ridge length; that is, the enhanced resonance in 3D is not caused by 3D standing waves or basin modes. Instead, the difference in resonance between the 2D and 3D simulations is attributed to the topographic blocking of the barotropic flow by the 3D ridges, affecting wave generation, and a more constructive phasing between the remotely generated internal waves, arriving under oblique angles, and the barotropic tide. Most of the resonance occurs for the first mode. The contribution of the higher modes is reduced because of 3D radiation, multiple generation sites, scattering, and a rapid decay in amplitude away from the ridge.

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, MacCready P.  2014.  Flow and mixing in Juan de Fuca Canyon, Washington. Geophysical Research Letters. 41:1608-1615.   10.1002/2013GL058967   AbstractWebsite

We report breaking internal lee waves, strong mixing, and hydraulic control associated with wind-driven up-canyon flow in Juan de Fuca Canyon, Washington. Unlike the flow above the canyon rim, which shows a tidal modulation typical on continental shelves, the flow within the canyon is persistently up-canyon during our observations, with isopycnals tilted consistent with a geostrophic cross-canyon momentum balance. As the flow encounters a sill near the canyon entrance at the shelf break, it accelerates significantly and undergoes elevated mixing on the upstream and downstream sides of the sill. On the downstream side, a strong lee wave response is seen, with displacements of O(100 m) and overturns tens of meters high. The resulting diffusivity is O(10−2 m2 s−1), sufficient to substantially modify coastal water masses as they transit the canyon and enter the Salish Sea estuarine system.

2013
Hosegood, PJ, Gregg MC, Alford MH.  2013.  Wind-driven submesoscale subduction at the north Pacific subtropical front. Journal of Geophysical Research: Oceans. 118:5333-5352.   10.1002/jgrc.20385   AbstractWebsite

Upper ocean observations from the north Pacific subtropical front during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss. Prior to generation, a sharp thermohaline front was intensified by confluent flow of 1–2 × 10−5 s−1. Relative vertical vorticity, ζ, across a surface-intensified, along-front jet on the warm side of a frontal trough was 0.5 f. During the storm, buoyancy loss arose due to cooling of ∼650 W m−2 and down-front wind stress <0.5 N m−2 that generated a southward, cross-front Ekman transport of dense water over light. The resulting wind-driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude. The intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline. They were approximately 20 m thick and horizontally elongated in the cross-frontal direction. The near-surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline, whereas the subsurface intrusions were composed of warm and saline water from the surface. The apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterized by O(1) Rossby and Richardson numbers, pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer, and sloping isopycnals. The intrusion properties, background environmental context, and forcing history are consistent with prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss, possibly resulting in symmetric instability.

Johnston, TMS, Rudnick DL, Alford MH, Pickering A, Simmons HL.  2013.  Internal tidal energy fluxes in the South China Sea from density and velocity measurements by gliders. Journal of Geophysical Research-Oceans. 118:3939-3949. AbstractWebsite

Internal tidal energy fluxes were obtained from June 2011 to August 2011 using underwater gliders in the South China Sea. Spray gliders profiled every approximate to 2 h to 500 m, which is deep enough given the shallow thermocline to compute mode-1 fluxes from vertical mode fits to tidal displacements and currents. Westward, mode-1 diurnal and semidiurnal fluxes exceeded 40 and 30 kW m(-1). To our knowledge, these flux observations are the first from both velocity and density measurements by gliders. Fluxes compare well with a mooring near a generation site in southern Luzon Strait and a regional model. Furthermore, the zonal-depth structure of the internal tide is obtained by binning measurements, which cover four spring-neap cycles and over 100 km along 20 degrees 39N. Westward phase propagation is found for currents and displacements, while roughly constant phase is found along beams. Both these features of the phase suggest a narrow-banded internal tide. Semidiurnal energy density is largest along a raypath which coincides with generation sites on both the eastern and western ridges in Luzon Strait. Diurnal energy density is surface-intensified consistent with relatively shallower diurnal raypaths emanating from the eastern ridge.