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2019
Schlosser, TL, Jones NL, Bluteau CE, Alford MH, Ivey GN, Lucas AJ.  2019.  Generation and propagation of near-inertial waves in a baroclinic current on the Tasmanian Shelf. Journal of Physical Oceanography.   10.1175/jpo-d-18-0208.1   Abstract

Near-inertial waves (NIWs) are often an energetic component of the internal wave field on windy continental shelves. The effect of baroclinic geostrophic currents, which introduce both relative vorticity and baroclinicity, on NIWs is not well understood. Relative vorticity affects the resonant frequency, feff, while both relative vorticity and baroclinicity modify the minimum wave frequency of freely-propagating waves, ωmin. On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where feff was less than the Coriolis frequency f. If everywhere feff > f then NIWs were generated where ωmin < f and feff was smallest. The background current not only affected the location of generation, but also the NIWs’ propagation direction. The estimated NIW energy fluxes show that NIWs propagated predominantly towards the equator because ωmin > f on the continental slope for the entire sample period. In addition to being laterally trapped on the shelf, we observed vertically trapped and intensified NIWs that had a frequency, ω, within the anomalously low-frequency band (i.e. ωmin <ω < feff), which only exists if the baroclinicity is non-zero. We observed two periods when ωmin < f on the shelf, but the relative vorticity was positive (i.e. feff > f) for one of these periods. The process of NIW propagation remained consistent with the local ωmin, and not feff, emphasizing the importance of baroclinicity on the NIW dynamics. We conclude that windy shelves with baroclinic background currents are likely to have energetic NIWs, but the current and seabed will adjust the spatial distribution and energetics of these NIWs.

Roemmich, D, Alford MH, Claustre H, Johnson K, King B, Moum J, Oke P, Owens WB, Pouliquen S, Purkey S, Scanderbeg M, Suga T, Wijffels S, Zilberman N, Bakker D, Baringer M, Belbeoch M, Bittig HC, Boss E, Calil P, Carse F, Carval T, Chai F, Conchubhair DO, D'Ortenzio F, Dall'Olmo G, Desbruyeres D, Fennel K, Fer I, Ferrari R, Forget G, Freeland H, Fujiki T, Gehlen M, Greenan B, Hallberg R, Hibiya T, Hosoda S, Jayne S, Jochum M, Johnson GC, Kang K, Kolodziejczyk N, Kortzinger A, Le Traon PY, Lenn YD, Maze G, Mork KA, Morris T, Nagai T, Nash J, Garabato AN, Olsen A, Pattabhi RR, Prakash S, Riser S, Schmechtig C, Schmid C, Shroyer E, Sterl A, Sutton P, Talley L, Tanhua T, Thierry V, Thomalla S, Toole J, Troisi A, Trull TW, Turton J, Velez-Belchi PJ, Walczowski W, Wang HL, Wanninkhof R, Waterhouse AF, Waterman S, Watson A, Wilson C, Wong APS, Xu JP, Yasuda I.  2019.  On the future of Argo: A global, full-depth, multi-disciplinary array. Frontiers in Marine Science. 6   10.3389/fmars.2019.00439   AbstractWebsite

The Argo Program has been implemented and sustained for almost two decades, as a global array of about 4000 profiling floats. Argo provides continuous observations of ocean temperature and salinity versus pressure, from the sea surface to 2000 dbar. The successful installation of the Argo array and its innovative data management system arose opportunistically from the combination of great scientific need and technological innovation. Through the data system, Argo provides fundamental physical observations with broad societally-valuable applications, built on the cost-efficient and robust technologies of autonomous profiling floats. Following recent advances in platform and sensor technologies, even greater opportunity exists now than 20 years ago to (i) improve Argo's global coverage and value beyond the original design, (ii) extend Argo to span the full ocean depth, (iii) add biogeochemical sensors for improved understanding of oceanic cycles of carbon, nutrients, and ecosystems, and (iv) consider experimental sensors that might be included in the future, for example to document the spatial and temporal patterns of ocean mixing. For Core Argo and each of these enhancements, the past, present, and future progression along a path from experimental deployments to regional pilot arrays to global implementation is described. The objective is to create a fully global, top-to-bottom, dynamically complete, and multidisciplinary Argo Program that will integrate seamlessly with satellite and with other in situ elements of the Global Ocean Observing System (Legler et al., 2015). The integrated system will deliver operational reanalysis and forecasting capability, and assessment of the state and variability of the climate system with respect to physical, biogeochemical, and ecosystems parameters. It will enable basic research of unprecedented breadth and magnitude, and a wealth of ocean-education and outreach opportunities.

Alford, MH, Simmons HL, Marques OB, Girton JB.  2019.  Internal tide attenuation in the North Pacific. Geophysical Research Letters. 46:8205-8213.   10.1029/2019gl082648   Abstract

Multisatellite altimetry and an eddy-resolving model with tides are used to quantify the attenuation of the mode-1 M2 internal tide as it propagates from three major sources in the North Pacific. The model is used to correct the altimetric fluxes for the nonstationary signal that altimeters cannot detect. Because internal tides in the North Pacific are highly stationary, these corrections do not materially impact the decay rate estimates. Fluxes are integrated in wedges extending from the sources to account for interference and radial spreading. Observed attenuation rates are consistent with e-folding scales between 750 and 3,000 km, suggesting weak dissipation rates (≤10−10 W/kg or 0.75×10−3 W/m2) compared to typical open-ocean turbulence levels, implicating near-inertial waves and higher-mode internal tides in providing the balance of the dissipation in the ocean interior.

Brizuela, N, Filonov A, Alford MH.  2019.  Internal tsunami waves transport sediment released by underwater landslides. Scientific Reports. 9   10.1038/s41598-019-47080-0   AbstractWebsite

Accelerated by gravity, submarine landslides transfer energy to the marine environment, most notably leading to catastrophic tsunamis. While tsunamis are thought to use less than 15% of the total energy released by landslides, little is known about subsurface processes comprising the rest of their energy budgets. Here, we analyze the first set of observations depicting a lake's interior response to underwater landslides and find that sediment transport is modulated by baroclinic waves that propagate along vertical gradients in temperature and sediment concentration. When traveling along a shallow thermocline, these waves can reach past topographic features that bound turbidity currents and thus expand the influence area of underwater landslides. With order of magnitude calculations, we estimate that observed thermocline internal waves received roughly 0.7% of available landslide energy and infer their contribution to homogenize the lake's thermodynamical properties by means of turbulent mixing. Lastly, we show that landslides in our data set modified the lake's intrinsic dynamical modes and thus had a permanent impact on its circulation. This suggests that measurements of subsurface wave propagation are sufficient to diagnose bathymetric transformations. Our experiment constitutes the first direct observation of both internal tsunami waves and turbidity current reflection. Moreover, it demonstrates that background density stratification has a significant effect on the transport of sediment after submarine landslides and provides a valuable reference for numerical models that simulate submarine mass failures.

MacKinnon, JA, Alford MH, Voet G, Zeiden K, Johnston STM, Siegelman M, Merrifield S, Merrifield M.  2019.  Eddy wake generation from broadband currents near Palau. Journal of Geophysical Research: Oceans.   10.1029/2019jc014945   Abstract

Wake eddies are frequently created by flow separation where ocean currents encounter abrupt topography in the form of islands or headlands. Most previous work has concentrated on wake eddy generation by either purely oscillatory (usually tidal) currents, or quasi-steady mean flows. Here we report measurements near the point of flow separation at the northern end of the Palau island chain, where energetic tides and vertically sheared low-frequency flows are both present. Energetic turbulence measured near the very steeply sloping ocean floor varied cubically with the total flow speed (primarily tidal). The estimated turbulent viscosity suggests a regime of flow separation and eddying wake generation for flows that directly feel this drag. Small-scale (∼ 1 km), vertically sheared wake eddies of different vorticity signs were observed with a ship-board survey on both sides of the separation point, and significantly evolved over several tidal periods. The net production and export of vorticity into the wake, expected to sensitively depend on the interplay of tidal and low frequency currents, is explored here with a simple conceptual model. Application of the model to a 10-month mooring record suggests that inclusion of high frequency oscillatory currents may boost the net flux of vorticity into the ocean interior by a depth dependent factor of 2 to 25. Models that do not represent the effect of these high frequency currents may not accurately infer the net momentum or energy losses felt where strong flows encounter steep island or headland topography.

Pratt, LJ, Voet G, Pacini A, Tan S, Alford MH, Carter GS, Girton JB, Menemenlis D.  2019.  Pacific Abyssal Transport and Mixing: Through the Samoan Passage versus around the Manihiki Plateau. Journal of Physical Oceanography. 49:1577-1592.   10.1175/jpo-d-18-0124.1   AbstractWebsite

AbstractThe main source feeding the abyssal circulation of the North Pacific is the deep, northward flow of 5–6 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) through the Samoan Passage. A recent field campaign has shown that this flow is hydraulically controlled and that it experiences hydraulic jumps accompanied by strong mixing and dissipation concentrated near several deep sills. By our estimates, the diapycnal density flux associated with this mixing is considerably larger than the diapycnal flux across a typical isopycnal surface extending over the abyssal North Pacific. According to historical hydrographic observations, a second source of abyssal water for the North Pacific is 2.3–2.8 Sv of the dense flow that is diverted around the Manihiki Plateau to the east, bypassing the Samoan Passage. This bypass flow is not confined to a channel and is therefore less likely to experience the strong mixing that is associated with hydraulic transitions. The partitioning of flux between the two branches of the deep flow could therefore be relevant to the distribution of Pacific abyssal mixing. To gain insight into the factors that control the partitioning between these two branches, we develop an abyssal and equator-proximal extension of the “island rule.” Novel features include provisions for the presence of hydraulic jumps as well as identification of an appropriate integration circuit for an abyssal layer to the east of the island. Evaluation of the corresponding circulation integral leads to a prediction of 0.4–2.4 Sv of bypass flow. The circulation integral clearly identifies dissipation and frictional drag effects within the Samoan Passage as crucial elements in partitioning the flow.

Wagner, GL, Flierl G, Ferrari R, Voet G, Carter GS, Alford MH, Girton JB.  2019.  Squeeze dispersion and the effective diapycnal diffusivity of oceanic tracers. Geophysical Research Letters. 46:5378-5386.   10.1029/2019gl082458   Abstract

Abstract We describe a process called “squeeze dispersion” in which the squeezing of oceanic tracer gradients by waves, eddies, and bathymetric flow modulates diapycnal diffusion by centimeter to meter-scale turbulence. Due to squeeze dispersion, the effective diapycnal diffusivity of oceanic tracers is different and typically greater than the average “local” diffusivity, especially when local diffusivity correlates with squeezing. We develop a theory to quantify the effects of squeeze dispersion on diapycnal oceanic transport, finding formulas that connect density-averaged tracer flux, locally measured diffusivity, large-scale oceanic strain, the thickness-weighted average buoyancy gradient, and the effective diffusivity of oceanic tracers. We use this effective diffusivity to interpret observations of abyssal flow through the Samoan Passage reported by Alford et al. (2013, https://doi.org/10.1002/grl.50684) and find that squeezing modulates diapycnal tracer dispersion by factors between 0.5 and 3.

2018
Fine, EC, MacKinnon JA, Alford MH, Mickett JB.  2018.  Microstructure observations of turbulent heat fluxes in a warm-core Canada Basin eddy. Journal of Physical Oceanography. 48:2397-2418.   10.1175/jpo-d-18-0028.1   AbstractWebsite

An intrahalocline eddy was observed on the Chukchi slope in September of 2015 using both towed CTD and microstructure temperature and shear sections. The core of the eddy was 6 degrees C, significantly warmer than the surrounding -1 degrees C water and far exceeding typical temperatures of warm-core Arctic eddies. Microstructure sections indicated that outside of the eddy the rate of dissipation of turbulent kinetic energy epsilon was quite low . Three different processes were associated with elevated epsilon. Double-diffusive steps were found at the eddy's top edge and were associated with an upward heat flux of 5 W m(-2). At the bottom edge of the eddy, shear-driven mixing played a modest role, generating a heat flux of approximately 0.5 W m(-2) downward. Along the sides of the eddy, density-compensated thermohaline intrusions transported heat laterally out of the eddy, with a horizontal heat flux of 2000 W m(-2). Integrating these fluxes over an idealized approximation of the eddy's shape, we estimate that the net heat transport due to thermohaline intrusions along the eddy flanks was 2 GW, while the double-diffusive flux above the eddy was 0.4 GW. Shear-driven mixing at the bottom of the eddy accounted for only 0.04 GW. If these processes continued indefinitely at the same rate, the estimated life-span would be 1-2 years. Such eddies may be an important mechanism for the transport of Pacific-origin heat, freshwater, and nutrients into the Canada Basin.

Ansong, JK, Arbic BK, Simmons HL, Alford MH, Buijsman MC, Timko PG, Richman JG, Shriver JF, Wallcraft AJ.  2018.  Geographical distribution of diurnal and semidiurnal parametric subharmonic instability in a global ocean circulation model. Journal of Physical Oceanography. 48:1409-1431.   10.1175/jpo-d-17-0164.1   Abstract

The evidence for, baroclinic energetics of, and geographic distribution of parametric subharmonic instability (PSI) arising from both diurnal and semidiurnal tides in a global ocean general circulation model is investigated using 1/12.5° and 1/25° simulations that are forced by both atmospheric analysis fields and the astronomical tidal potential. The paper examines whether PSI occurs in the model, and whether it accounts for a significant fraction of the tidal baroclinic energy loss. Using energy transfer calculations and bispectral analyses, evidence is found for PSI around the critical latitudes of the tides. The intensity of both diurnal and semidiurnal PSI in the simulations is greatest in the upper ocean, consistent with previous results from idealized simulations, and quickly drops off about 5° from the critical latitudes. The sign of energy transfer depends on location; the transfer is positive (from the tides to subharmonic waves) in some locations and negative in others. The net globally integrated energy transfer is positive in all simulations and is 0.5%–10% of the amount of energy required to close the baroclinic energy budget in the model. The net amount of energy transfer is about an order of magnitude larger in the 1/25° semidiurnal simulation than the 1/12.5° one, implying the dependence of the rate of energy transfer on model resolution.

Waterhouse, AF, Kelly SM, Zhao Z, MacKinnon JA, Nash JD, Simmons H, Brahznikov D, Rainville L, Alford M, Pinkel R.  2018.  Observations of the Tasman Sea internal tide beam. Journal of Physical Oceanography. 48:1283-1297.   10.1175/jpo-d-17-0116.1   Abstract

AbstractLow-mode internal tides, a dominant part of the internal wave spectrum, carry energy over large distances, yet the ultimate fate of this energy is unknown. Internal tides in the Tasman Sea are generated at Macquarie Ridge, south of New Zealand, and propagate northwest as a focused beam before impinging on the Tasmanian continental slope. In situ observations from the Tasman Sea capture synoptic measurements of the incident semidiurnal mode-1 internal-tide, which has an observed wavelength of 183 km and surface displacement of approximately 1 cm. Plane-wave fits to in situ and altimetric estimates of surface displacement agree to within a measurement uncertainty of 0.3 cm, which is the same order of magnitude as the nonstationary (not phase locked) mode-1 tide observed over a 40-day mooring deployment. Stationary energy flux, estimated from a plane-wave fit to the in situ observations, is directed toward Tasmania with a magnitude of 3.4 ± 1.4 kW m−1, consistent with a satellite estimate of 3.9 ± 2.2 kW m−1. Approximately 90% of the time-mean energy flux is due to the stationary tide. However, nonstationary velocity and pressure, which are typically 1/4 the amplitude of the stationary components, sometimes lead to instantaneous energy fluxes that are double or half of the stationary energy flux, overwhelming any spring–neap variability. Despite strong winds and intermittent near-inertial currents, the parameterized turbulent-kinetic-energy dissipation rate is small (i.e., 10−10 W kg−1) below the near surface and observations of mode-1 internal tide energy-flux convergence are indistinguishable from zero (i.e., the confidence intervals include zero), indicating little decay of the mode-1 internal tide within the Tasman Sea.

Hamann, MM, Alford MH, Mickett JB.  2018.  Generation and propagation of nonlinear internal waves in sheared currents over the Washington Continental Shelf. Journal of Geophysical Research-Oceans. 123:2381-2400.   10.1002/2017jc013388   AbstractWebsite

The generation, propagation, and dissipation of nonlinear internal waves (NLIW) in sheared background currents is examined using 7 days of shipboard microstructure surveys and two moorings on the continental shelf offshore of Washington state. Surveys near the hypothesized generation region show semi-diurnal (D2) energy flux is onshore and that the ratio of energy flux to group speed times energy (F/cgE) increases sharply at the shelf break, suggesting that the incident D2 internal tide is partially reflected and partially transmitted. NLIW appear at an inshore mooring at the leading edge of the onshore phase of the baroclinic tide, consistent with nonlinear transformation of the shoaling internal tide as their generation mechanism. Of the D2 energy flux observed at the eastern extent of the generation region (13318 Wm(-1)), approximately 30% goes into the NLIW observed inshore (3611 Wm(-1)). Inshore of the moorings, 7 waves are tracked into shallow (30-40 m) water, where a vertically sheared, southward current becomes strong. As train-like waves propagate onshore, wave amplitudes of 25-30 m and energies of 5 MJ decrease to 12 m and 10 kJ, respectively. The observed direction of propagation rotates from 30 degrees N of E to approximate to 30 degrees S of E in the strongly sheared region. Linear ray tracing using the Taylor-Goldstein equation to incorporate parallel shear effects accounts for only a small portion of the observed rotation, suggesting that three-dimensionality of the wave crests and the background currents is important here.

Zhao, ZX, Alford MH, Simmons HL, Brazhnikov D, Pinkel R.  2018.  Satellite investigation of the M-2 Internal Tide in the Tasman Sea. Journal of Physical Oceanography. 48:687-703.   10.1175/jpo-d-17-0047.1   AbstractWebsite

The M-2 internal tide in the Tasman Sea is investigated using sea surface height measurements made by multiple altimeter missions from 1992 to 2012. Internal tidal waves are extracted by two-dimensional plane wave fits in 180 km by 180 km windows. The results show that the Macquarie Ridge radiates three internal tidal beams into the Tasman Sea. The northern and southern beams propagate respectively into the East Australian Current and the Antarctic Circumpolar Current and become undetectable to satellite altimetry. The central beam propagates across the Tasman Sea, impinges on the Tasmanian continental slope, and partially reflects. The observed propagation speeds agree well with theoretical values determined from climatological ocean stratification. Both the northern and central beams refract about 158 toward the equator because of the beta effect. Following a concave submarine ridge in the source region, the central beam first converges around 45.5 degrees S, 155.5 degrees E and then diverges beyond the focal region. The satellite results reveal two reflected internal tidal beams off the Tasmanian slope, consistent with previous numerical simulations and glider measurements. The total energy flux from the Macquarie Ridge into the Tasman Sea is about 2.2 GW, of which about half is contributed by the central beam. The central beam loses little energy in its first 1000-km propagation, for which the likely reasons include flat bottom topography and weak mesoscale eddies.

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
Lucas, AJ, Pinkel R, Alford M.  2017.  Ocean wave energy for long endurance, broad bandwidth ocean monitoring. Oceanography. 30:126-127.   10.5670/oceanog.2017.232   AbstractWebsite
n/a
Alford, MH, Sloyan BM, Simmons HL.  2017.  Internal waves in the East Australian Current. Geophysical Research Letters. 44:12280-12288.   10.1002/2017gl075246   AbstractWebsite

Internal waves, which drive most ocean turbulence and add noise to lower-frequency records, interact with low-frequency current systems and topography in yet poorly known ways. Taking advantage of a heavily instrumented, 14 month mooring array, internal waves in the East Australian Current (EAC) are examined for the first time. Internal wave horizontal kinetic energy (HKE) is within a factor of 2 of the Garrett-Munk (1976) spectrum. Continuum internal waves, near-inertial waves, and internal tides together constitute a significant percentage of the total velocity variance. Mode-1 internal tide energy fluxes are southward and much smaller than energy times group velocity, consistent with reflection at the continental slope of incident waves generated from near New Caledonia and the Solomon Islands. Internal tide HKE is highly phase variable, consistent with refraction by the variable EAC. Mode-1 near-inertial wave energy fluxes are of comparable magnitude and are equatorward and episodic, consistent with generation by storms farther poleward. These processes are considered together in the complex environment of the EAC.

Luecke, CA, Arbic BK, Bassette SL, Richman JG, Shriver JF, Alford MH, Smedstad OM, Timko PG, Trossman DS, Wallcraft AJ.  2017.  The global mesoscale eddy available potential energy field in models and observations. Journal of Geophysical Research-Oceans. 122:9126-9143.   10.1002/2017jc013136   AbstractWebsite

Global maps of the mesoscale eddy available potential energy (EAPE) field at a depth of 500 m are created using potential density anomalies in a high-resolution 1/12.5 degrees global ocean model. Maps made from both a free-running simulation and a data-assimilative reanalysis of the HYbrid Coordinate Ocean Model (HYCOM) are compared with maps made by other researchers from density anomalies in Argo profiles. The HYCOM and Argo maps display similar features, especially in the dominance of western boundary currents. The reanalysis maps match the Argo maps more closely, demonstrating the added value of data assimilation. Global averages of the simulation, reanalysis, and Argo EAPE all agree to within about 10%. The model and Argo EAPE fields are compared to EAPE computed from temperature anomalies in a data set of moored historical observations (MHO) in conjunction with buoyancy frequencies computed from a global climatology. The MHO data set allows for an estimate of the EAPE in high-frequency motions that is aliased into the Argo EAPE values. At MHO locations, 15-32% of the EAPE in the Argo estimates is due to aliased motions having periods of 10 days or less. Spatial averages of EAPE in HYCOM, Argo, and MHO data agree to within 50% at MHO locations, with both model estimates lying within error bars observations. Analysis of the EAPE field in an idealized model, in conjunction with published theory, suggests that much of the scatter seen in comparisons of different EAPE estimates is to be expected given the chaotic, unpredictable nature of mesoscale eddies.

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.