Frictional decay of abyssal boundary currents

A theory is presented to explain the observed longevity of abyssal boundary currents flowing along sloping topography. Typically such currents are many Rossby radii wide, and their energy is dominantly potential, residing in the broad upturn of isopycnals near the slope. The rate of decay of energy, on the other hand, is governed by the much smaller kinetic energy of the flow absorbed by the bottom boundary layer. The spin-down time is thus increased by a (possibly large) factor of PE/KE times that required to dissipate the kinetic energy alone. The ratio PE/KE is calculated from data on two sections across the Deep Western Boundary Current in the North Atlantic, and is found to be 10 and 41 in those instances, consistent with the slow spin-down of the current in that region. The change in cross-sectional shape of the current during spin-down is predicted using a 1½-layer model. It is found that the upper tip of the current moves down the slope with a self-preserving shape, while the lower edge becomes thicker and broader. The along-slope transport of the current remains constant, even as the energy decreases. The spin-down time may be interpreted as that required for the Ekman transport to drain away the isopycnal displacement which defines the flow

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Total exchange flow, entrainment, and diffusive salt flux in estuaries

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 1205-1220, doi:10.1175/JPO-D-16-0258.1.The linkage among total exchange flow, entrainment, and diffusive salt flux in estuaries is derived analytically using salinity coordinates, revealing the simple but important relationship between total exchange flow and mixing. Mixing is defined and quantified in this paper as the dissipation of salinity variance. The method uses the conservation of volume and salt to quantify and distinguish the diahaline transport of volume (i.e., entrainment) and diahaline diffusive salt flux. A numerical model of the Hudson estuary is used as an example of the application of the method in a realistic estuary with a persistent but temporally variable exchange flow. A notable finding of this analysis is that the total exchange flow and diahaline salt flux are out of phase with respect to the spring–neap cycle. Total exchange flow reaches its maximum near minimum neap tide, but diahaline salt transport reaches its maximum during the maximum spring tide. This phase shift explains the strong temporal variation of stratification and estuarine salt content through the spring–neap cycle. In addition to quantifying temporal variation, the method reveals the spatial variation of total exchange flow, entrainment, and diffusive salt flux through the estuary. For instance, the analysis of the Hudson estuary indicates that diffusive salt flux is intensified in the wider cross sections. The method also provides a simple means of quantifying numerical mixing in ocean models because it provides an estimate of the total dissipation of salinity variance, which is the sum of mixing due to the turbulence closure and numerical mixing.T. Wang was supported by the Open Research Fund of State Key Laboratory of Estuarine and Coastal Research (Grant SKLEC-KF201509), the Fundamental Research Funds for the Central Universities (Grant 2017B03514), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDA11010203). W. R. Geyer was supported by NSF Grant OCE 0926427 and ONR Grant N00014-16-1-2948. P. MacCready was supported by NSF Grant OCE-1634148.2017-09-1

Estuarine exchange flow is related to mixing through the salinity variance budget

Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 48 (2018): 1375-1384, doi:10.1175/JPO-D-17-0266.1.The relationship between net mixing and the estuarine exchange flow may be quantified using a salinity variance budget. Here “mixing” is defined as the rate of destruction of volume-integrated salinity variance, and the exchange flow is quantified using the total exchange flow. These concepts are explored using an idealized 3D model estuary. It is shown that in steady state (e.g., averaging over the spring–neap cycle) the volume-integrated mixing is approximately given by Mixing ≅ SinSoutQr, where Sin and Sout are the representative salinities of in- and outflowing layers at the mouth and Qr is the river volume flux. This relationship provides an extension of the familiar Knudsen relation, in which the exchange flow is diagnosed based on knowledge of these same three quantities, quantitatively linking mixing to the exchange flow.The work was supported by the National Science Foundation through Grants OCE-1736242 to PM and OCE-1736539 to WRG and by the German Research Foundation through Grants TRR 181 and GRK 2000 to HB

Observations of Flow and Mixing in Juan de Fuca Canyon

Juan de Fuca Canyon, Washington, which cuts across the continental shelf from the mouth of the Strait of Juan de Fuca to the shelf break, is a likely conduit for deep (below shelf break) Pacific water to enter the Salish Sea. This is important to the Salish Sea ecosystem because the deeper Pacific water has lower pH and dissolved oxygen. Despite its potential importance to Ocean Acidification in the Salish Sea, very few direct observations have been made in the Canyon. Here we report breaking internal lee waves, strong mixing and hydraulic control associated with wind-driven up-canyon flow near the shelf break in Juan de Fuca Canyon. Unlike the flow above the canyon rim, which shows a tidal modulation typical on continental shelves, the flow within the canyon is consistently up-canyon during our observations, with isopycnals tilted consistent with a geostrophic along-canyon momentum balance. As the flow encounters a sill near the canyon entrance at the shelf break, it accelerates significantly and undergoes elevated mixing on the upstream and downstream sides of the sill. On the downstream side, a strong lee wave response is seen, with displacements of O(100 m) and overturns tens of meters high. The resulting diffusivity is sufficient to substantially modify coastal water masses as they transit the canyon

Salinity variance mixing in the Salish Sea is controlled by river flow

A salinity variance framework is used to study mixing in the Salish Sea, a large fjordal estuary. Output from a realistic numerical model is used to create salinity variance budgets for individual basins within the Salish Sea for 2017-2019. The salinity variance budgets quantify the mixing in each basin and diagnose the numerical mixing, which is found to contribute about one-third of the total mixing in the model. Whidbey Basin has the most intense mixing, due to its shallow depth and large river flow. Unlike in most other estuarine systems previously studied using the salinity variance method, mixing in the Salish Sea is controlled by the river flow and does not exhibit a pronounced spring-neap cycle. A mixedness\u27\u27 analysis is used to determine when mixed water is expelled from the estuary. The river flow is correlated with mixed water removal, but the coupling is not as tight as with the mixing. Because the mixing is so highly correlated with the river flow, a steady-state approximation can be used to predict the mixing in the Salish Sea and Puget Sound with good accuracy, even without any temporal averaging. Over a long time average, the mixing in Puget Sound is directly related to the exchange flow salt transport

The dynamics of pressure and form drag on a sloping headland: Internal waves versus eddies

Topographically generated eddies and internal waves have traditionally been studied separately even though bathymetry that creates both phenomena is abundant in coastal regions. Here a numerical model is used to understand the dynamics of eddy and wave generation as tidal currents flow past Three Tree Point, a 1 km long, 200 m deep, sloping headland in Puget Sound, WA. Bottom pressure anomalies due to vertical perturbations of the sea surface and isopycnals are used to calculate form drag in different regions of the topography to assess the relative importance of eddies versus internal waves. In regions where internal waves dominate, sea surface and isopycnal perturbations tend to work together to create drag, whereas in regions dominated by eddies, sea surface, and isopycnal perturbations tend to counteract each other. Both phenomena are found to produce similar amounts of form drag even though the bottom pressure anomalies from the eddy have much larger magnitudes than those created by the internal waves. Topography like Three Tree Point is common in high latitude, coastal regions, and therefore the findings here have implications for understanding how coastal topography removes energy from tidal currents.Keywords: tides, internal lee waves, topography, form drag, internal waves, eddie

LiveOcean: a daily forecast model of biogeochemistry in Washington marine waters

LiveOcean is a daily forecast model of ocean conditions for the coastal waters of Washington, Oregon, and Vancouver Island, as well as the Salish Sea. It is forced with realistic tides, winds, rivers, and ocean conditions. The model simulates biogeochemical properties including phytoplankton, nitrate, dissolved oxygen, dissolved inorganic carbon, and alkalinity, up to 3 days in the future. It is used for the prediction of ocean acidification events in coastal estuaries, and for harmful algal bloom events on coastal beaches. I will describe the model construction, comparisons with observations, uses, and future developments

Measurement of Tidal Form Drag Using Seafloor Pressure Sensors

As currents flow over rough topography, the pressure difference between the up-and downstream sides results in form drag-a force that opposes the flow. Measuring form drag is valuable because it can be used to estimate the loss of energy from currents as they interact with topography. An array of bottom pressure sensors was used to measure the tidal form drag on a sloping ridge in 200 m of water that forms a 1-km headland at the surface in Puget Sound, Washington. The form drag per unit length of the ridge reached 1 x 10⁴ N m⁻¹ during peak flood tides. The tidally averaged power removed from the tidal currents by form drag was 0.2 W m⁻², which is 30 times larger than power losses to friction. Form drag is best parameterized by a linear wave drag law as opposed to a bluff body drag law because the flow is stratified and both internal waves and eddies are generated on the sloping topography. Maximum turbulent kinetic energy dissipation rates of 5 x 10⁻⁵ W kg⁻¹ were measured with a microstructure profiler and are estimated to account for 25%-50% of energy lost from the tides. This study is among the first to measure form drag directly using bottom pressure sensors. The measurement and analysis techniques presented here are suitable for periodically reversing flows because they require the removal of a time-mean signal. The advantage of this technique is that it delivers a continuous record of form drag and is much less ship intensive compared to previous methods for estimation of the bottom pressure field.Keywords: Stratified flow, Rough topography, Lee Waves, Ocean, Nonlinear internal waves, Headland, Conversion, Dissipation, Slope, Generatio