Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D1: Mini-Symposium: Geophysical Turbulence Induced by Flow over Topography IIInvited
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Chair: Jody Klymak, University of Victoria Room: A105 |
Sunday, November 20, 2016 2:57PM - 3:23PM |
D1.00001: Turbulence during the generation of internal tides at obstacles Invited Speaker: Sutanu Sarkar Three-dimensional simulations are performed to resolve turbulent processes in tidally-forced flow over different types of two-dimensional obstacles. Our objective is to explore the dependence of the spatial and temporal distribution of turbulence on the obstacle geometry. Fine-scale variability in the flow is associated with features such as critical slope boundary layers, upslope thermal bores, breaking lee waves, downslope jets, internal wave beams and wave-wave interactions. Turbulence in these flow features is maintained through both convective and shear instabilities, and the mixing efficiency depends on the type of instability that is operative. The spatio-temporal pattern of turbulent kinetic energy and dissipation depends strongly on local slope criticality, the overall aspect ratio of the obstacle, and the appropriately defined excursion number. Implications of the simulation results for simple parameterizations of the turbulent dissipation and mixing are discussed. The work presented here was done with Masoud Jalali. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:49PM |
D1.00002: The Role of Non-Propagating Form Drag in Mixing the Ocean Invited Speaker: Kurt Polzin Topographically induced internal waves are common features in the atmosphere and ocean. In the former, they give rise to the so-called "mountain-waves" which form as winds blow over rough topography and give rise to severe downslope windstorms. In the ocean, boundary forced waves are related to flow of tides, inertial oscillations and geostrophic flows over rough topography. Depending upon length scale, stratification, latitude and frequency, the hydrodynamic response can be bottom trapped or freely upward propagating. Robust quantitative constraints on ocean mixing are few. They come from a restricted number of control volume budgets for abyssal upwelling and observations of diapycnal dispersion from anthropogenic tracer releases. This talk will summarize control volume budgets and tracer observations from the South Scotia Sea, the Brazil Basin, the Gulf of Mexico and the Western North Atlantic and discuss extant observations of fine- and microstructure that relate to the mechanisms of mixing. There are significant disparities between the fine- and microstructure observations and large scale budgets in all but the Brazil Basin. There, mixing is regarded as being associated with internal tide generation and near boundary breaking. Elsewhere, sub-inertial flow over finite amplitude topography is inferred to be the significant process. Rather than discussing the efficacy of quasi-stationary lee waves in mixing, we discuss closing the large scale budgets in the oceanic allegory of downslope windstorms: flow over finite amplitude topography giving rise to hydrualic like effects that can be summarized as converting non-propagating form drag into mixing. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:15PM |
D1.00003: Internal tide convergence and mixing in a submarine canyon Invited Speaker: Amy Waterhouse Observations from Eel Canyon, located on the north coast of California, show that elevated turbulence in the full water column arises from the convergence of remotely-generated internal wave energy. The incoming semidiurnal and bottom-trapped diurnal internal tides generate complex interference patterns. The semidiurnal internal tide sets up a partly standing wave within the canyon due to reflection at the canyon head, dissipating all of its energy within the canyon. Dissipation in the near-bottom is associated with the diurnal trapped tide, while midwater isopycnal shear and strain is associated with the semidiurnal tide. Dissipation is elevated up to 600 m off the bottom, in contrast to observations over flat continental shelf where dissipation occurs closer to the topography. Slope canyons are sinks for internal wave energy and may have important influences on the global distribution of tidally-driven mixing. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:41PM |
D1.00004: Transition from interior to boundary turbulence: implications for the buoyancy flux divergence. Invited Speaker: Ali Mashayek I will discuss results from a cascade of nested simulations of flow over rough topography in the Southern Ocean with focus on transition from turbulence induced from breaking of upward propagating internal waves in the interior and above rough topography to that induced by boundary trapped modes in vicinity of the ocean floor. I will show that this transition leads to a change in sign of the buoyancy flux divergence, which implies a change from interior downwelling to intense upwelling in the boundary layer. I will show that in the Drake Passage region, the latter dominates the former leading to a net upwelling. I will finish by highlighting the potentially important implications of the results for the abyssal branch of the global ocean meridional overturning circulation. [Preview Abstract] |
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