Alford Matthew H.

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Matthew H.

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  • Article
    Microstructure mixing observations and finescale parameterizations in the Beaufort Sea
    (American Meteorological Society, 2020-12-22) Fine, Elizabeth C. ; Alford, Matthew H. ; MacKinnon, Jennifer A. ; Mickett, John B.
    In the Beaufort Sea in September of 2015, concurrent mooring and microstructure observations were used to assess dissipation rates in the vicinity of 72°35′N, 145°1′W. Microstructure measurements from a free-falling profiler survey showed very low [O(10−10) W kg−1] turbulent kinetic energy dissipation rates ε. A finescale parameterization based on both shear and strain measurements was applied to estimate the ratio of shear to strain Rω and ε at the mooring location, and a strain-based parameterization was applied to the microstructure survey (which occurred approximately 100 km away from the mooring site) for direct comparison with microstructure results. The finescale parameterization worked well, with discrepancies ranging from a factor of 1–2.5 depending on depth. The largest discrepancies occurred at depths with high shear. Mean Rω was 17, and Rω showed high variability with values ranging from 3 to 50 over 8 days. Observed ε was slightly elevated (factor of 2–3 compared with a later survey of 11 profiles taken over 3 h) from 25 to 125 m following a wind event which occurred at the beginning of the mooring deployment, reaching a maximum of ε= 6 × 10−10 W kg−1 at 30-m depth. Velocity signals associated with near-inertial waves (NIWs) were observed at depths greater than 200 m, where the Atlantic Water mass represents a reservoir of oceanic heat. However, no evidence of elevated ε or heat fluxes was observed in association with NIWs at these depths in either the microstructure survey or the finescale parameterization estimates.
  • Article
    Spectral decomposition of internal gravity wave sea surface height in global models
    (John Wiley & Sons, 2017-10-10) Savage, Anna C. ; Arbic, Brian K. ; Alford, Matthew H. ; Ansong, Joseph ; Farrar, J. Thomas ; Menemenlis, Dimitris ; O’Rourke, Amanda K. ; Richman, James G. ; Shriver, Jay F. ; Voet, Gunnar ; Wallcraft, Alan J. ; Zamudio, Luis
    Two global ocean models ranging in horizontal resolution from 1/12° to 1/48° 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 (>0:87 cpd) 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° HYCOM and 1/48° 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 ∼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.
  • Article
    Hydraulic control of flow in a multi-passage system connecting two basins
    (Cambridge University Press, 2022-04-05) Tan, Shuwen ; Pratt, Lawrence J. ; Voet, Gunnar ; Cusack, Jesse M. ; Helfrich, Karl R. ; Alford, Matthew H. ; Girton, James B. ; Carter, Glenn S.
    When a fluid stream in a conduit splits in order to pass around an obstruction, it is possible that one branch will be critically controlled while the other remains not so. This is apparently the situation in Pacific Ocean abyssal circulation, where most of the northward flow of Antarctic bottom water passes through the Samoan Passage, where it is hydraulically controlled, while the remainder is diverted around the Manihiki Plateau and is not controlled. These observations raise a number of questions concerning the dynamics necessary to support such a regime in the steady state, the nature of upstream influence and the usefulness of rotating hydraulic theory to predict the partitioning of volume transport between the two paths, which assumes the controlled branch is inviscid. Through the use of a theory for constant potential vorticity flow and accompanying numerical model, we show that a steady-state regime similar to what is observed is dynamically possible provided that sufficient bottom friction is present in the uncontrolled branch. In this case, the upstream influence that typically exists for rotating channel flow is transformed into influence into how the flow is partitioned. As a result, the partitioning of volume flux can still be reasonably well predicted with an inviscid theory that exploits the lack of upstream influence.
  • Article
    Internal wave breaking near the foot of a steep East-Pacific continental slope
    (Elsevier, 2022-05-21) van Haren, Hans ; Voet, Gunnar ; Alford, Matthew H. ; Torres, Daniel J.
    The sloping sides of ocean basins are of particular interest for their potential importance for considerable turbulence generation via internal wave breaking and associated water circulation. The difference with the ocean interior may be manifest in a 10–100 m relatively thin layer above the seafloor. We set up an observational study with high-resolution stand-alone instrumentation attached to a custom-made release-anchor frame sampling to within 0.5 m from the seafloor up to 150 m above it. For two months, the taut wire moored instrumentation was tested in 1100 m water depth of the East-Pacific, off the coast of San Diego (CA, USA). The mooring was oceanward of an underwater bank and near the foot of its steep but gentle two-dimensional slope. Temperature sensor data demonstrate that internal waves peak at semidiurnal frequencies. While short (<1 h) periods show complicated structure, tidally averaged turbulence dissipation rate monotonically increases towards the seafloor over two orders of magnitude. The largest turbulence dissipation rates are observed during the relatively warm phase of an internal wave. Although the local topographic slope is supercritical for semidiurnal internal waves, turbulent bores propagating up the slope and hydraulic jumps are not observed. Most of the turbulence appears to be dominated by shear production, but not related to steady frictional flow near the seafloor.
  • Article
    Global patterns of diapycnal mixing from measurements of the turbulent dissipation rate
    (American Meteorological Society, 2014-07) Waterhouse, Amy F. ; MacKinnon, Jennifer A. ; Nash, Jonathan D. ; Alford, Matthew H. ; Kunze, Eric ; Simmons, Harper L. ; Polzin, Kurt L. ; St. Laurent, Louis C. ; Sun, Oliver M. T. ; Pinkel, Robert ; Talley, Lynne D. ; Whalen, Caitlin B. ; Huussen, Tycho N. ; Carter, Glenn S. ; Fer, Ilker ; Waterman, Stephanie N. ; Naveira Garabato, Alberto C. ; Sanford, Thomas B. ; Lee, Craig M.
    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) m2 s−1 and above 1000-m depth is O(10−5) m2 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.
  • Article
    Estimating dissipation rates associated with double diffusion
    (American Geophysical Union, 2021-06-26) Middleton, Leo ; Fine, Elizabeth C. ; MacKinnon, Jennifer A. ; Alford, Matthew H. ; Taylor, John R.
    Double diffusion refers to a variety of turbulent processes in which potential energy is released into kinetic energy, made possible in the ocean by the difference in molecular diffusivities between salinity and temperature. Here, we present a new method for estimating the kinetic energy dissipation rates forced by double-diffusive convection using temperature and salinity data alone. The method estimates the up-gradient diapycnal buoyancy flux associated with double diffusion, which is hypothesized to balance the dissipation rate. To calculate the temperature and salinity gradients on small scales we apply a canonical scaling for compensated thermohaline variance (or ‘spice’) on sub-measurement scales with a fixed buoyancy gradient. Our predicted dissipation rates compare favorably with microstructure measurements collected in the Chukchi Sea. Fine et al. (2018),, showed that dissipation rates provide good estimates for heat fluxes in this region. Finally, we show the method maintains predictive skill when applied to a sub-sampling of the Conductivity, Temperature, Depth (CTD) data.
  • Article
    On the future of Argo: A global, full-depth, multi-disciplinary array
    (Frontiers Media, 2019-08-02) Roemmich, Dean ; Alford, Matthew H. ; Claustre, Hervé ; Johnson, Kenneth S. ; King, Brian ; Moum, James N. ; Oke, Peter ; Owens, W. Brechner ; Pouliquen, Sylvie ; Purkey, Sarah G. ; Scanderbeg, Megan ; Suga, Koushirou ; Wijffels, Susan E. ; Zilberman, Nathalie ; Bakker, Dorothee ; Baringer, Molly O. ; Belbeoch, Mathieu ; Bittig, Henry C. ; Boss, Emmanuel S. ; Calil, Paulo H. R. ; Carse, Fiona ; Carval, Thierry ; Chai, Fei ; Conchubhair, Diarmuid Ó. ; d’Ortenzio, Fabrizio ; Dall'Olmo, Giorgio ; Desbruyeres, Damien ; Fennel, Katja ; Fer, Ilker ; Ferrari, Raffaele ; Forget, Gael ; Freeland, Howard ; Fujiki, Tetsuichi ; Gehlen, Marion ; Geenan, Blair ; Hallberg, Robert ; Hibiya, Toshiyuki ; Hosoda, Shigeki ; Jayne, Steven R. ; Jochum, Markus ; Johnson, Gregory C. ; Kang, KiRyong ; Kolodziejczyk, Nicolas ; Körtzinger, Arne ; Le Traon, Pierre-Yves ; Lenn, Yueng-Djern ; Maze, Guillaume ; Mork, Kjell Arne ; Morris, Tamaryn ; Nagai, Takeyoshi ; Nash, Jonathan D. ; Naveira Garabato, Alberto C. ; Olsen, Are ; Pattabhi Rama Rao, Eluri ; Prakash, Satya ; Riser, Stephen C. ; Schmechtig, Catherine ; Schmid, Claudia ; Shroyer, Emily L. ; Sterl, Andreas ; Sutton, Philip J. H. ; Talley, Lynne D. ; Tanhua, Toste ; Thierry, Virginie ; Thomalla, Sandy J. ; Toole, John M. ; Troisi, Ariel ; Trull, Thomas W. ; Turton, Jon ; Velez-Belchi, Pedro ; Walczowski, Waldemar ; Wang, Haili ; Wanninkhof, Rik ; Waterhouse, Amy F. ; Waterman, Stephanie N. ; Watson, Andrew J. ; Wilson, Cara ; Wong, Annie P. S. ; Xu, Jianping ; Yasuda, Ichiro
    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.
  • Preprint
    The formation and fate of internal waves in the South China Sea
    ( 2015-03) Alford, Matthew H. ; Peacock, Thomas ; MacKinnon, Jennifer A. ; Nash, Jonathan D. ; Buijsman, Maarten C. ; Centurioni, Luca R. ; Chao, Shenn-Yu ; Chang, Ming-Huei ; Farmer, David M. ; Fringer, Oliver B. ; Fu, Ke-Hsien ; Gallacher, Patrick C. ; Graber, Hans C. ; Helfrich, Karl R. ; Jachec, Steven M. ; Jackson, Christopher R. ; Klymak, Jody M. ; Ko, Dong S. ; Jan, Sen ; Johnston, T. M. Shaun ; Legg, Sonya ; Lee, I-Huan ; Lien, Ren-Chieh ; Mercier, Matthieu J. ; Moum, James N. ; Musgrave, Ruth C. ; Park, Jae-Hun ; Pickering, Andrew I. ; Pinkel, Robert ; Rainville, Luc ; Ramp, Steven R. ; Rudnick, Daniel L. ; Sarkar, Sutanu ; Scotti, Alberto ; Simmons, Harper L. ; St Laurent, Louis C. ; Venayagamoorthy, Subhas K. ; Wang, Yu-Huai ; Wang, Joe ; Yang, Yiing-Jang ; Paluszkiewicz, Theresa ; Tang, Tswen Yung
    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 impact a panoply of ocean processes, such as the supply of nutrients for photosynthesis1, sediment and pollutant transport2 and acoustic transmission3; they also pose hazards for manmade structures in the ocean4. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking5, posing severe challenges for their observation and their inclusion in numerical climate models, which are sensitive to their effects6-7. Over a decade of studies8-11 have targeted the South China Sea, where the oceans’ most powerful internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their generation mechanism, variability and energy budget, however, due to the lack of in-situ data from the Luzon Strait, where extreme flow conditions make measurements challenging. Here we employ new observations and numerical models to (i) show that the waves begin as sinusoidal disturbances rather than from sharp hydraulic phenomena, (ii) reveal the existence of >200-m-high breaking internal waves in the generation region that give rise to turbulence levels >10,000 times that in the open ocean, (iii) determine that the Kuroshio western boundary current significantly refracts the internal wave field emanating from the Luzon Strait, and (iv) demonstrate a factor-of-two agreement between modelled and observed energy fluxes that enables the first 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.
  • Article
    A tale of two spicy seas
    (The Oceanography Society, 2016-06) MacKinnon, Jennifer A. ; Nash, Jonathan D. ; Alford, Matthew H. ; Lucas, Andrew J. ; Mickett, John B. ; Shroyer, Emily L. ; Waterhouse, Amy F. ; Tandon, Amit ; Sengupta, Debasis ; Mahadevan, Amala ; Ravichandran, M. ; Pinkel, Robert ; Rudnick, Daniel L. ; Whalen, Caitlin B. ; Alberty, Marion S. ; Lekha, J. Sree ; Fine, Elizabeth C. ; Chaudhuri, Dipayan ; Wagner, Gregory L.
    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.
  • Article
    Observations of double diffusive staircase edges in the Arctic Ocean
    (American Geophysical Union, 2022-10-12) Boury, Samuel ; Supekar, Rohit ; Fine, Elizabeth C. ; Musgrave, Ruth C. ; Mickett, John B. ; Voet, Gunnar ; Odier, Philippe ; Peacock, Thomas ; MacKinnon, Jennifer A. ; Alford, Matthew H.
    Recent observational studies have provided detailed descriptions of double‐diffusive staircases in the Beaufort Sea, characterized by well‐mixed intrusions between high‐gradient interfaces. These structures result from double‐diffusive convection, occurring when cooler fresh water lies atop the warmer saltier Atlantic water layer. In the present study, we investigate the spatial structure of such layers, by analyzing combined high resolution data from a subsurface mooring, a ship‐towed profiling conductivity‐temperature‐depth/ADCP package, and a free‐falling microstructure profiler. At large scale, the modular microstructure profiler data suggest a horizontal “ragged edge” of the layered water masses near the basin boundary. At smaller scales, the mooring data indicate that, in the 300–400 m depth interval, regions of layers abruptly appear. This laterally sharp (of the order of 100 m) interface is advected southwards, as shown by the shallow water integrated mapping system survey conducted nearby. Neither disruption nor formation of layers is directly observed in our data, and we thus interpret our observations as the stable and possibly recent abutment of a layered and an unlayered water masses, now globally advected southwards by a large scale flow.
  • Article
    Parametric subharmonic instability of the internal tide at 29°N
    (American Meteorological Society, 2013-01) MacKinnon, Jennifer A. ; Alford, Matthew H. ; Sun, Oliver M. T. ; Pinkel, Robert ; Zhao, Zhongxiang ; Klymak, Jody M.
    Observational evidence is presented for transfer of energy from the internal tide to near-inertial motions near 29°N in the Pacific Ocean. The transfer is accomplished via parametric subharmonic instability (PSI), which involves interaction between a primary wave (the internal tide in this case) and two smaller-scale waves of nearly half the frequency. The internal tide at this location is a complex superposition of a low-mode waves propagating north from Hawaii and higher-mode waves generated at local seamounts, making application of PSI theory challenging. Nevertheless, a statistically significant phase locking is documented between the internal tide and upward- and downward-propagating near-inertial waves. The phase between those three waves is consistent with that expected from PSI theory. Calculated energy transfer rates from the tide to near-inertial motions are modest, consistent with local dissipation rate estimates. The conclusion is that while PSI does befall the tide near a critical latitude of 29°N, it does not do so catastrophically.
  • Article
    Flow-topography interactions in the Samoan Passage
    (Oceanography Society, 2019-12-11) Girton, James B. ; Mickett, John B. ; Zhao, Zhongxiang ; Alford, Matthew H. ; Voet, Gunnar ; Cusack, Jesse M. ; Carter, Glenn S. ; Pearson-Potts, Kelly A. ; Pratt, Lawrence J. ; Tan, Shuwen ; Klymak, Jody M.
    Mixing in the Samoan Passage has implications for the abyssal water properties of the entire North Pacific—nearly 20% of the global ocean’s volume. Dense bottom water formed near Antarctica encounters the passage—a gap in a ridge extending from north of Samoa eastward across the Pacific at around 10°S—and forms an energetic cascade much like a river flowing through a canyon. The 2011–2014 Samoan Passage Abyssal Mixing Experiment explored the importance of topography to the dense water flow on a wide range of scales, including (1) constraints on transport due to the overall passage shape and the heights of its multiple sills, (2) rapid changes in water properties along particular pathways at localized mixing hotspots where there is extreme topographic roughness and/or downslope flow acceleration, and (3) diversion and disturbance of flow pathways and density surfaces by small-scale seamounts and ridges. The net result is a complex but fairly steady picture of interconnected pathways with a limited number of intense mixing locations that determine the net water mass transformation. The implication of this set of circumstances is that the dominant features of Samoan Passage flow and mixing (and their responses to variations in incoming or background properties) can be described by the dynamics of a single layer of dense water flowing beneath a less-dense one, combined with mixing and transformation that is determined by the small-scale topography encountered along flow pathways.
  • Article
    Climate Process Team on internal wave–driven ocean mixing
    (American Meteorological Society, 2017-12-01) MacKinnon, Jennifer A. ; Zhao, Zhongxiang ; Whalen, Caitlin B. ; Waterhouse, Amy F. ; Trossman, David S. ; Sun, Oliver M. ; St. Laurent, Louis C. ; Simmons, Harper L. ; Polzin, Kurt L. ; Pinkel, Robert ; Pickering, Andrew I. ; Norton, Nancy J. ; Nash, Jonathan D. ; Musgrave, Ruth C. ; Merchant, Lynne M. ; Melet, Angelique ; Mater, Benjamin D. ; Legg, Sonya ; Large, William G. ; Kunze, Eric ; Klymak, Jody M. ; Jochum, Markus ; Jayne, Steven R. ; Hallberg, Robert ; Griffies, Stephen M. ; Diggs, Stephen ; Danabasoglu, Gokhan ; Chassignet, Eric P. ; Buijsman, Maarten C. ; Bryan, Frank O. ; Briegleb, Bruce P. ; Barna, Andrew ; Arbic, Brian K. ; Ansong, Joseph ; Alford, Matthew H.
    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 spatiotemporal 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 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (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.
  • Article
    Flow Encountering Abrupt Topography (FLEAT): a multiscale observational and modeling program to understand how topography affects flows in the western North Pacific
    (Oceanography Society, 2019-12-11) Johnston, T. M. Shaun ; Schönau, Martha ; Paluszkiewicz, Theresa ; MacKinnon, Jennifer A. ; Arbic, Brian K. ; Colin, Patrick L. ; Alford, Matthew H. ; Andres, Magdalena ; Centurioni, Luca R. ; Graber, Hans C. ; Helfrich, Karl R. ; Hormann, Verena ; Lermusiaux, Pierre F. J. ; Musgrave, Ruth C. ; Powell, Brian S. ; Qiu, Bo ; Rudnick, Daniel L. ; Simmons, Harper L. ; St. Laurent, Louis C. ; Terrill, Eric ; Trossman, David S. ; Voet, Gunnar ; Wijesekera, Hemantha W. ; Zeide, Kristin L.
    Using a combination of models and observations, the US Office of Naval Research Flow Encountering Abrupt Topography (FLEAT) initiative examines how island chains and submerged ridges affect open ocean current systems, from the hundreds of kilometer scale of large current features to the millimeter scale of turbulence. FLEAT focuses on the western Pacific, mainly on equatorial currents that encounter steep topography near the island nation of Palau. Wake eddies and lee waves as small as 1 km were observed to form as these currents flowed around or over the steep topography. The direction and vertical structure of the incident flow varied over tidal, inertial, seasonal, and interannual timescales, with implications for downstream flow. Models incorporated tides and had grids with resolutions of hundreds of meters to enable predictions of flow transformations as waters encountered and passed around Palau’s islands. In addition to making scientific advances, FLEAT had a positive impact on the local Palauan community by bringing new technology to explore local waters, expanding the country’s scientific infrastructure, maintaining collaborations with Palauan partners, and conducting outreach activities aimed at elementary and high school students, US embassy personnel, and Palauan government officials.
  • Article
    Persistent turbulence in the Samoan Passage
    (American Meteorological Society, 2019-12-09) Cusack, Jesse M. ; Voet, Gunnar ; Alford, Matthew H. ; Girton, James B. ; Carter, Glenn S. ; Pratt, Lawrence J. ; Pearson-Potts, Kelly A. ; Tan, Shuwen
    Abyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe-scale-based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the passage. Overturns were observed in a density band in which the Richardson number was often smaller than ¼, consistent with shear instability occurring at the upper interface of the fast-flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales from days to hours. At some mooring locations, near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealized 2D numerical simulation that includes a barotropic tidal forcing. Depth-integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously.
  • Article
    Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies
    (John Wiley & Sons, 2017-03-28) Savage, Anna C. ; Arbic, Brian K. ; Richman, James G. ; Shriver, Jay F. ; Alford, Matthew H. ; Buijsman, Maarten C. ; Farrar, J. Thomas ; Sharma, Hari ; Voet, Gunnar ; Wallcraft, Alan J. ; Zamudio, Luis
    High horizontal-resolution (1=12:5° and 1=25°) 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 cm2, 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.
  • Article
    A spatial geography of abyssal turbulent mixing in the Samoan passage
    (Oceanography Society, 2019-12-11) Carter, Glenn S. ; Voet, Gunnar ; Alford, Matthew H. ; Girton, James B. ; Mickett, John B. ; Klymak, Jody M. ; Pratt, Lawrence J. ; Pearson-Potts, Kelly A. ; Cusack, Jesse M. ; Tan, Shuwen
    High levels of turbulent mixing have long been suspected in the Samoan Passage, an important topographic constriction in the deep limb of the Pacific Meridional Overturning Circulation. Along the length of the passage, observations undertaken in 2012 and 2014 showed the bottom water warmed by ~55 millidegrees Celsius and decreased in density by 0.01 kg m–3. Spatial analysis of this first-ever microstructure survey conducted in the Samoan Passage confirmed there are multiple hotspots of elevated abyssal mixing. This mixing was not just confined to the four main sills—even between sills, the nature of the mixing processes appeared to differ: for example, one sill is clearly a classical hydraulically controlled overflow, whereas another is consistent with mode-2 hydraulic control. When microstructure casts were averaged into 0.1°C conservative temperature classes, the largest dissipation rates and diapycnal diffusivity values (>10–7 W kg–1 and 10–2 m2 s–1, respectively) occurred immediately downstream of the northern sill in the eastern and deepest channel. Although topographic blocking is the primary reason that no water colder than Θ = 0.7°C is found in the western channel, intensive mixing at the entrance sills appeared to be responsible for eroding an approximately 100 m thick layer of Θ < 0.7°C water. Three examples highlighting weak temporal variability, and hence suggesting that the observed spatial patterns are robust, are presented. The spatial variability in mixing over short lateral scales suggests that any simple parameterization of mixing within the Samoan Passage may not be applicable.
  • Article
    Pacific abyssal transport and mixing: Through the Samoan Passage versus around the Manihiki Plateau
    (American Meteorological Society, 2019-01-11) Pratt, Lawrence J. ; Voet, Gunnar ; Pacini, Astrid ; Tan, Shuwen ; Alford, Matthew H. ; Carter, Glenn S. ; Girton, James B. ; Menemenlis, Dimitris
    The 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.
  • Article
    The impact of oceanic near-inertial waves on climate
    (American Meteorological Society, 2013-05-01) Jochum, Markus ; Briegleb, Bruce P. ; Danabasoglu, Gokhan ; Large, William G. ; Norton, Nancy J. ; Jayne, Steven R. ; Alford, Matthew H. ; Bryan, Frank O.
    The Community Climate System Model, version 4 (CCSM4) is used to assess the climate impact of wind-generated near-inertial waves (NIWs). Even with high-frequency coupling, CCSM4 underestimates the strength of NIWs, so that a parameterization for NIWs is developed and included into CCSM4. Numerous assumptions enter this parameterization, the core of which is that the NIW velocity signal is detected during the model integration, and amplified in the shear computation of the ocean surface boundary layer module. It is found that NIWs deepen the ocean mixed layer by up to 30%, but they contribute little to the ventilation and mixing of the ocean below the thermocline. However, the deepening of the tropical mixed layer by NIWs leads to a change in tropical sea surface temperature and precipitation. Atmospheric teleconnections then change the global sea level pressure fields so that the midlatitude westerlies become weaker. Unfortunately, the magnitude of the real air-sea flux of NIW energy is poorly constrained by observations; this makes the quantitative assessment of their climate impact rather uncertain. Thus, a major result of the present study is that because of its importance for global climate the uncertainty in the observed tropical NIW energy has to be reduced.
  • Article
    Eddies, topography, and the abyssal flow by the Kyushu-Palau Ridge near Velasco Reef
    (The Oceanography Society, 2019-12-11) Andres, Magdalena ; Siegelman, Mika ; Hormann, Verena ; Musgrave, Ruth C. ; Merrifield, Sophia T. ; Rudnick, Daniel L. ; Merrifield, Mark ; Alford, Matthew H. ; Voet, Gunnar ; Wijesekera, Hemantha W. ; MacKinnon, Jennifer A. ; Centurioni, Luca R. ; Nash, Jonathan D. ; Terrill, Eric
    Palau, an island group in the tropical western North Pacific at the southern end of Kyushu-Palau Ridge, sits near the boundary between the westward-​flowing North Equatorial Current (NEC) and the eastward-flowing North Equatorial Countercurrent. Combining remote-sensing observations of the sea surface with an unprecedented in situ set of subsurface measurements, we examine the flow near Palau with a particular focus on the abyssal circulation and on the deep expression of mesoscale eddies in the region. We find that the deep currents time-averaged over 10 months are generally very weak north of Palau and not aligned with the NEC in the upper ocean. This weak abyssal flow is punctuated by the passing of mesoscale eddies, evident as sea surface height anomalies, that disrupt the mean flow from the surface to the seafloor. Eddy influence is observed to depths exceeding 4,200 m. These deep-​reaching mesoscale eddies typically propagate westward past Palau, and as they do, any associated deep flows must contend with the topography of the Kyushu-Palau Ridge. This interaction leads to vertical structure far below the main thermocline. Observations examined here for one particularly strong and well-sampled eddy suggest that the flow was equivalent barotropic in the far field east and west of the ridge, with a more complicated vertical structure in the immediate vicinity of the ridge by the tip of Velasco Reef.