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MacKinnon, JA, Alford MH, Voet G, Fitzmorris K, Johnston TMS, Siegelman M, Merrifield S, Merrifield M.  2019.  Eddy wake generation from broadband currents near Palau. Journal of Geophysical Research.   10.1029/2019jc014945  
Pratt, JL, Voet G, Pacini A, Tan S, Alford MH, Carter GS, Girton JB, Menemenlis D.  2019.  Pacific abyssal transport and mixing: through the Samoan Passage vs. around the Manihiki Plateau. Journal of Physical Oceanography. 49(6):1577-1592.   10.1175/jpo-d-18-0124.1   Abstract

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 hy- draulic 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

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) and find that squeezing modulates diapycnal tracer dispersion by factors between 0.5 and 3.

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.

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.

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.

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(8):2389-2401.   10.1175/JPO-D-16-0063.1   Abstract

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 circu- lation. 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 temper- ature 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 = 106 m3 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 (±0.4 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 × 10-3 °C yr-1 over the past two decades, as observed both in moored and shipboard hydrographic observations. While the warming reflects the recently observed in- creasing 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.

Voet, G, Girton JB, Alford MH, Carter GS, Klymak JM, Mickett JB.  2015.  Pathways, Volume Transport and Mixing of Abyssal Water in the Samoan Passage. Journal of Physical Oceanography. 45(2):562-588.   10.1175/JPO-D-14-0096.1   Abstract

The flow of dense water through the Samoan Passage accounts for the major part of the bottom water renewal in the North Pacific and is thus an important element of the Pacific meridional overturning circulation. A recent set of highly resolved measurements used CTD/LADCP, a microstructure profiler, and moorings to constrain the complex pathways and variability of the abyssal flow. Volume transport estimates for the dense northward current at several sections across the passage, calculated using direct velocity measurements from LADCPs, range from 3.9 × 106 to 6.0 × 106 ± 1 × 106 m3 s-1. The deep channel to the east and shallower pathways to the west carried about equal amounts of this volume transport, with the densest water flowing along the main eastern channel. Turbulent dissipation rates estimated from Thorpe scales and direct microstructure agree to within a factor of 2 and provide a region-averaged value of O(10-8) W kg-1 for layers colder than 0.8°C. Associated diapycnal diffusivities and downward turbulent heat fluxes are about 5 × 10-3 m2 s-1 and O(10) W m-2, respectively. However, heat budgets suggest heat fluxes 2–6 times greater. In the vicinity of one of the major sills of the passage, highly resolved Thorpe-inferred diffusivity and heat flux were over 10 times larger than the region-averaged values, suggesting the mismatch is likely due to undersampled mixing hotspots.

Alford, MH, Girton JB, Voet G, Carter GS, Mickett JB, Klymak JM.  2013.  Turbulent mixing and hydraulic control of abyssal water in the Samoan Passage. Geophysical Research Letters. 40(17):4668–4674.   10.1002/grl.50684   Abstract

We report the first direct turbulence observations in the Samoan Passage (SP), a 40 km wide notch in the South Pacific bathymetry through which flows most of the water supplying the North Pacific abyssal circulation. The observed turbulence is 1000 to 10,000 times typical abyssal levels —strong enough to completely mix away the densest water entering the passage—confirming inferences from previous coarser temperature and salinity sections. Accompanying towed measurements of velocity and temperature with horizontal resolution of about 250 m indicate the dominant processes responsible for the turbulence. Specifically, the flow accelerates substantially at the primary sill within the passage, reaching speeds as great as 0.55 m s−1. A strong hydraulic response is seen, with layers first rising to clear the sill and then plunging hundreds of meters downward. Turbulence results from high shear at the interface above the densest fluid as it descends and from hydraulic jumps that form downstream of the sill. In addition to the primary sill, other locations along the multiple interconnected channels through the Samoan Passage also have an effect on the mixing of the dense water. In fact, quite different hydraulic responses and turbulence levels are observed at seafloor features separated laterally by a few kilometers, suggesting that abyssal mixing depends sensitively on bathymetric details on small scales.

Voet, G, Quadfasel D.  2010.  Entrainment in the Denmark Strait overflow plume by meso-scale eddies. Ocean Science. 6:301–310.   10.5194/os-6-301-2010   Abstract

The entrainment of buoyant ambient water into the overflow plume of Denmark Strait and the associated downstream warming of the plume are estimated using time series of currents and temperature from moored instrumentation and classical hydrographic data. Warming rates are highest (0.4–0.5 K/100 km) within the first 200 km of the sill, and decrease to 0.05–0.1 K/100 km further downstream. Stirring by mesoscale eddies causes lateral heat fluxes that explain the 0.1 K/100 km warming, but in the first 200 km from the sill also vertical diapycnal fluxes, probably caused by breaking internal waves, must contribute to the entrainment.

Fer, I, Voet G, Seim KS, Rudels B, Latarius K.  2010.  Intense mixing of the Faroe Bank Channel overflow. Geophysical Research Letters. 37(2):L02604.   10.1029/2009GL041924   Abstract

The continuous, swift flow of cold water across the sill of the Faroe Bank Channel, the deepest passage from the Nordic Seas to the North Atlantic Ocean, forms a bottom-attached dense plume (overflow). The amount and distribution of entrainment and mixing that the overflow encounters during its descent influence the ventilation of the deep North Atlantic, however, remain poorly known due to lack of direct measurements. Using the first direct turbulence measurements, we describe the dynamic properties and mixing of the overflow plume as it descends toward the Iceland Basin. The vigorously turbulent plume is associated with intense mixing and enhanced turbulent dissipation near the bottom and at the plume-ambient interface, but with a quiescent core. Our measurements show a pronounced transverse circulation consistent with rotating plume dynamics, a strong lateral variability in entrainment velocity, and a vertical structure composed of order 100 m thick stratified interface and comparably thick well-mixed bottom boundary layer with significant transport and entrainment.

Voet, G, Quadfasel D, Mork KA, Søiland H.  2010.  The mid-depth circulation of the Nordic Seas derived from profiling float observations. Tellus A. 62(4):516–529.   10.1111/j.1600-0870.2010.00444.x   Abstract

The trajectories of 61 profiling Argo floats deployed at mid-depth in the Nordic Seas—the Greenland, Lofoten and Norwegian Basins and the Iceland Plateau—between 2001 and 2009 are analysed to determine the pattern, strength and variability of the regional circulation. The mid-depth circulation is strongly coupled with the structure of the bottom topography of the four major basins and of the Nordic Seas as a whole. It is cyclonic, both on the large-scale and on the basin scale, with weak flow (<1 cm s−1) in the interior of the basins and somewhat stronger flow (up to 5 cm s−1) at their rims. Only few floats moved from one basin to another, indicating that the internal recirculation within the basins is by far dominating the larger-scale exchanges. The seasonal variability of the mid-depth flow ranges from less than 1 cm s−1 over the Iceland Plateau to more than 4 cm s−1 in the Greenland Basin. These velocities translate into internal gyre transports of up to 15 ± 10 × 106 m3 s−1, several times the overall exchange between the Nordic Seas and the subpolar North Atlantic. The seasonal variability of the Greenland Basin and the Norwegian Basin can be adequately modelled using the barotropic vorticity equation, with the wind and bottom friction as the only forcing mechanisms. For the Lofoten Basin and the Iceland Plateau less than 50% of the variance can be explained by the wind.

Dickson, RR, Dye S, Jónsson S, Köhl A, Macrander A, Marnela M, Meincke J, Olsen S, Rudels B, Valdimarsson H, Voet G.  2008.  The overflow flux west of Iceland: Variability, origins and forcing. Arctic–Subarctic Ocean Fluxes. ( Dickson RR, Meincke J, Rhines P, Eds.).:443–474.: Springer Abstract