Arbic Brian K.

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Brian K.

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  • Article
    On eddy viscosity, energy cascades, and the horizontal resolution of gridded satellite altimeter products
    (American Meteorological Society, 2013-02) Arbic, Brian K. ; Polzin, Kurt L. ; Scott, Robert B. ; Richman, James G. ; Shriver, Jay F.
    Motivated by the recent interest in ocean energetics, the widespread use of horizontal eddy viscosity in models, and the promise of high horizontal resolution data from the planned wide-swath satellite altimeter, this paper explores the impacts of horizontal eddy viscosity and horizontal grid resolution on geostrophic turbulence, with a particular focus on spectral kinetic energy fluxes Π(K) computed in the isotropic wavenumber (K) domain. The paper utilizes idealized two-layer quasigeostrophic (QG) models, realistic high-resolution ocean general circulation models, and present-generation gridded satellite altimeter data. Adding horizontal eddy viscosity to the QG model results in a forward cascade at smaller scales, in apparent agreement with results from present-generation altimetry. Eddy viscosity is taken to roughly represent coupling of mesoscale eddies to internal waves or to submesoscale eddies. Filtering the output of either the QG or realistic models before computing Π(K) also greatly increases the forward cascade. Such filtering mimics the smoothing inherent in the construction of present-generation gridded altimeter data. It is therefore difficult to say whether the forward cascades seen in present-generation altimeter data are due to real physics (represented here by eddy viscosity) or to insufficient horizontal resolution. The inverse cascade at larger scales remains in the models even after filtering, suggesting that its existence in the models and in altimeter data is robust. However, the magnitude of the inverse cascade is affected by filtering, suggesting that the wide-swath altimeter will allow a more accurate determination of the inverse cascade at larger scales as well as providing important constraints on smaller-scale dynamics.
  • 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.
  • Thesis
    Generation of mid-ocean eddies : the local baroclinic instability hypothesis
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2000-08) Arbic, Brian K.
    The plausibility of local baroclinic instability as a generation mechanism for midocean mesoscale eddies is examined with a two-layer, quasi-geostrophic (QG) model forced by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated through bottom Ekman friction, Explanations are sought for two observed features of mid-ocean eddies: 1) substantial energy is retained in the baroclinic mode and in the associated deformation radius (Rd) scale, and 2) the ratio of eddy to mean kinetic energy is much larger than one, The tendency of QG to cascade energy into the barotropic mode and into scales larger than Rd can be counteracted when stratification is surface-trapped, for then the baroclinic mode is weakly damped, and hence enhanced, Numerical experiments are performed with both surface-trapped and uniform stratification to quantify this, Experiments with equal Ekman frictions in the two layers are also performed for purposes of contrast, Interpretation is aided with an inequality derived from the energy and enstrophy equations, The inequality forbids the simultaneous retention of substantial energy in the baroclinic mode and in scales near Rd when Ekman friction is symmetric, but points towards surface-trapped stratification and bottomtrapped friction as an environment in which both of these can be achieved, The dissertation also contains a systematic study of geostrophic turbulence forced by nonzonal flows, Narrow zonal jets emerge when shear-induced mean potential vorticity (PV) gradients are small compared to the planetary gradient (β), and energy is a strong function of the angle shear presents to the east-west direction, When shear-induced PV gradients are comparable to β, and the mean shear has a westward component, fields of monopolar vortices form and persist, Energy is asymmetric between fields of cyclones and anticyclones, Such asymmetry was commonly thought not to occur in QG, but is shown here to be introduced by the nonzonal basic state, In both jet and vortex regimes, eddy energy can be much larger than mean kinetic energy, contrary to the expectation that β stabilizes weak shear flows,
  • Article
    Effects of grid spacing on high-frequency precipitation variance in coupled high-resolution global ocean–atmosphere models
    (Springer, 2022-03-29) Light, Charles X. ; Arbic, Brian K. ; Martin, Paige E. ; Brodeau, Laurent ; Farrar, J. Thomas ; Griffies, Stephen M. ; Kirtman, Benjamin ; Laurindo, Lucas ; Menemenlis, Dimitris ; Molod, Andrea ; Nelson, Arin D. ; Nyadjro, Ebenezer ; O’Rourke, Amanda K. ; Shriver, Jay F. ; Siqueira, Leo ; Small, R. Justin ; Strobach, Ehud
    High-frequency precipitation variance is calculated in 12 different free-running (non-data-assimilative) coupled high resolution atmosphere–ocean model simulations, an assimilative coupled atmosphere–ocean weather forecast model, and an assimilative reanalysis. The results are compared with results from satellite estimates of precipitation and rain gauge observations. An analysis of irregular sub-daily fluctuations, which was applied by Covey et al. (Geophys Res Lett 45:12514–12522, 2018. to satellite products and low-resolution climate models, is applied here to rain gauges and higher-resolution models. In contrast to lower-resolution climate simulations, which Covey et al. (2018) found to be lacking with respect to variance in irregular sub-daily fluctuations, the highest-resolution simulations examined here display an irregular sub-daily fluctuation variance that lies closer to that found in satellite products. Most of the simulations used here cannot be analyzed via the Covey et al. (2018) technique, because they do not output precipitation at sub-daily intervals. Thus the remainder of the paper focuses on frequency power spectral density of precipitation and on cumulative distribution functions over time scales (2–100 days) that are still relatively “high-frequency” in the context of climate modeling. Refined atmospheric or oceanic model grid spacing is generally found to increase high-frequency precipitation variance in simulations, approaching the values derived from observations. Mesoscale-eddy-rich ocean simulations significantly increase precipitation variance only when the atmosphere grid spacing is sufficiently fine (< 0.5°). Despite the improvements noted above, all of the simulations examined here suffer from the “drizzle effect”, in which precipitation is not temporally intermittent to the extent found in observations.
  • 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
    Detecting change in the Indonesian Seas
    (Frontiers Media, 2019-06-04) Sprintall, Janet ; Gordon, Arnold L. ; Wijffels, Susan E. ; Feng, Ming ; Hu, Shijian ; Koch-Larrouy, Ariane ; Phillips, Helen E. ; Nugroho, Dwiyoga ; Napitu, Asmi ; Pujiana, Kandaga ; Susanto, R. Dwi ; Sloyan, Bernadette M. ; Yuan, Dongliang ; Riama, Nelly Florida ; Siswanto, Siswanto ; Kuswardani, Anastasia ; Arifin, Zainal ; Wahyudi, A’an J. ; Zhou, Hui ; Nagai, Taira ; Ansong, Joseph ; Bourdalle-Badié, Romain ; Chanut, Jerome ; Lyard, Florent ; Arbic, Brian K. ; Ramdhani, Andri ; Setiawan, Agus
    The Indonesian seas play a fundamental role in the coupled ocean and climate system with the Indonesian Throughflow (ITF) providing the only tropical pathway connecting the global oceans. Pacific warm pool waters passing through the Indonesian seas are cooled and freshened by strong air-sea fluxes and mixing from internal tides to form a unique water mass that can be tracked across the Indian Ocean basin and beyond. The Indonesian seas lie at the climatological center of the atmospheric deep convection associated with the ascending branch of the Walker Circulation. Regional SST variations cause changes in the surface winds that can shift the center of atmospheric deep convection, subsequently altering the precipitation and ocean circulation patterns within the entire Indo-Pacific region. Recent multi-decadal changes in the wind and buoyancy forcing over the tropical Indo-Pacific have directly affected the vertical profile, strength, and the heat and freshwater transports of the ITF. These changes influence the large-scale sea level, SST, precipitation and wind patterns. Observing long-term changes in mass, heat and freshwater within the Indonesian seas is central to understanding the variability and predictability of the global coupled climate system. Although substantial progress has been made over the past decade in measuring and modeling the physical and biogeochemical variability within the Indonesian seas, large uncertainties remain. A comprehensive strategy is needed for measuring the temporal and spatial scales of variability that govern the various water mass transport streams of the ITF, its connection with the circulation and heat and freshwater inventories and associated air-sea fluxes of the regional and global oceans. This white paper puts forward the design of an observational array using multi-platforms combined with high-resolution models aimed at increasing our quantitative understanding of water mass transformation rates and advection within the Indonesian seas and their impacts on the air-sea climate system. Introduction
  • 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
    Internal lee wave closures : parameter sensitivity and comparison to observations
    (John Wiley & Sons, 2015-12-17) Trossman, David S. ; Waterman, Stephanie N. ; Polzin, Kurt L. ; Arbic, Brian K. ; Garner, Stephen T. ; Naveira Garabato, Alberto C. ; Sheen, Katy L.
    This paper examines two internal lee wave closures that have been used together with ocean models to predict the time-averaged global energy conversion rate into lee waves and dissipation rate associated with lee waves and topographic blocking: the Garner (2005) scheme and the Bell (1975) theory. The closure predictions in two Southern Ocean regions where geostrophic flows dominate over tides are examined and compared to microstructure profiler observations of the turbulent kinetic energy dissipation rate, where the latter are assumed to reflect the dissipation associated with topographic blocking and generated lee wave energy. It is shown that when applied to these Southern Ocean regions, the two closures differ most in their treatment of topographic blocking. For several reasons, pointwise validation of the closures is not possible using existing observations, but horizontally averaged comparisons between closure predictions and observations are made. When anisotropy of the underlying topography is accounted for, the two horizontally averaged closure predictions near the seafloor are approximately equal. The dissipation associated with topographic blocking is predicted by the Garner (2005) scheme to account for the majority of the depth-integrated dissipation over the bottom 1000 m of the water column, where the horizontally averaged predictions lie well within the spatial variability of the horizontally averaged observations. Simplifications made by the Garner (2005) scheme that are inappropriate for the oceanic context, together with imperfect observational information, can partially account for the prediction-observation disagreement, particularly in the upper water column.