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Glenn S. Carter
and
Michael C. Gregg

Abstract

A microstructure survey near the head of Monterey Submarine Canyon, the first in a canyon, confirmed earlier inferences that coastal submarine canyons are sites of intense mixing. The data collected during two weeks in August 1997 showed turbulent kinetic energy dissipation and diapycnal diffusivity up to 103 times higher than in the open ocean. Dissipation and diapycnal diffusivity within 10 km of the canyon head were among the highest observed anywhere (ε = 1.1 × 10−6 W kg−1; K ρ = 1.0 × 10−2 m2 s−1). Mixing occurred mainly in an on-axis stratified turbulent layer, with thickness and intensity increasing from neap to spring tide. Strain spectra showed a gentler than k −1 z rolloff, suggesting that critical reflection and scattering may push energy into high wavenumbers. Dissipation dependence on shear appears to be much weaker in the canyon than in the open ocean, with indications that the dependence maybe as low as ε ∝ S . Coastal canyons may account for a small but significant fraction of the internal tide energy budget. A crude estimate of global dissipation in canyons is 58 GW, ≈15% of the estimated global M 2 internal tide dissipation.

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Glenn S. Carter
and
Michael C. Gregg

Abstract

Near-diurnal internal waves were observed in velocity and shear measurements from a shipboard survey along a 35-km section of the Kaena Ridge, northwest of Oahu. Individual waves with upward phase propagation could be traced for almost 4 days even though the ship transited approximately 20 km. Depth–time maps of shear were dominated by near-diurnal waves, despite the fact that Kaena Ridge is a site of considerable M 2 barotropic-to-baroclinic conversion. Guided by recent numerical and observational studies, it was found that a frequency of ½M 2 (i.e., 24.84-h period) was consistent with these waves. Nonlinear processes are able to transfer energy within the internal wave spectrum. Bicoherence analysis, which can distinguish between nonlinearly coupled waves and waves that have been independently excited, suggested that the ½M 2 waves were nonlinearly coupled with the dominant M 2 internal tide only between 525- and 595-m depth. This narrow depth range corresponded to an observed M 2 characteristic emanating from the northern edge of the ridge. The observations occurred in close proximity to the internal tide generation region, implying a rapid transfer of energy between frequencies. Strong nonlinear interactions seem a likely mechanism. Nonlinear transfers such as these could complicate attempts to close local single-constituent tidal energy budgets.

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Rob A. Hall
and
Glenn S. Carter

Abstract

The M 2 internal tide in Monterey Submarine Canyon is simulated using a modified version of the Princeton Ocean Model. Most of the internal tide energy entering the canyon is generated to the south, on Sur Slope and at the head of Carmel Canyon. The internal tide is topographically steered around the large canyon meanders. Depth-integrated baroclinic energy fluxes are up canyon and largest near the canyon axis, up to 1.5 kW m−1 at the mouth of the upper canyon and increasing to over 4 kW m−1 around Monterey and San Gregorio Meanders. The up-canyon energy flux is bottom intensified, suggesting that topographic focusing occurs. Net along-canyon energy flux decreases almost monotonically from 9 MW at the canyon mouth to 1 MW at Gooseneck Meander, implying that high levels of internal tide dissipation occur. The depth-integrated energy flux across the 200-m isobath is order 10 W m−1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head, where internal tide energy escapes onto the shelf. Reducing the size of the model domain to exclude remote areas of high barotropic-to-baroclinic energy conversion decreases the depth-integrated energy flux in the upper canyon by 20%. However, quantifying the role of remote internal tide generation sites is complicated by a pressure perturbation feedback between baroclinic energy flux and barotropic-to-baroclinic energy conversion.

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Manikandan Mathur
,
Glenn S. Carter
, and
Thomas Peacock

Abstract

An established analytical technique for modeling internal tide generation by barotropic flow over bottom topography in the ocean is the Green function–based approach. To date, however, for realistic ocean studies this method has relied on the WKB approximation. In this paper, the complete Green function method, without the WKB approximation, is developed and tested, and in the process, the accuracy of the WKB approximation for realistic ridge geometries and ocean stratifications is considered. For isolated Gaussian topography, the complete Green function approach is shown to be accurate via close agreement with the results of numerical simulations for a wide range of height ratios and criticality; in contrast, the WKB approach is found to be inaccurate for small height ratios in the subcritical regime and all tall topography that impinges on the pycnocline. Two ocean systems are studied, the Kaena and Wyville Thomson Ridges, for which there is again excellent agreement between the complete Green function approach and numerical simulations, and the WKB approximate solutions have substantial errors. This study concludes that the complete Green function approach, which is typically only modestly more computationally expensive than the WKB approach, should be the go-to analytical method to model internal tide generation for realistic ocean ridge scenarios.

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Colette G. Kerry
,
Brian S. Powell
, and
Glenn S. Carter

Abstract

The baroclinic tides are a crucial source of mixing energy into the deep ocean; however, the incoherent portion of the spectrum is not well examined because it is difficult to observe. This study estimates the coherent and incoherent M2 internal tide energy fluxes in the Philippine Sea using a primitive equation model that resolves the M2 barotropic and baroclinic tides and the time-evolving atmospherically forced eddying circulation. A time-mean, incoherent, internal tide energy flux of 25% of the coherent energy flux is found to emanate eastward into the Philippine Sea from the Luzon Strait and a time-mean incoherent portion of 30% of the coherent energy flux propagates westward into the South China Sea (SCS). The incoherent internal tide energy results from baroclinic tide generation and propagation variability. Quantifying the incoherent portion estimates the energy missing from altimeter-derived or line-integral acoustic measurements and places short-lived, in situ observations in the context of variability.

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Colette G. Kerry
,
Brian S. Powell
, and
Glenn S. Carter

Abstract

This study uses a primitive equation model to estimate the time-varying M2 internal tide dissipation in the Philippine Sea in the presence of the subtidal circulation. The time-mean diapycnal diffusivity due to the M2 internal tide is estimated to be 4.0–4.8 × 10−4 m2 s−1 at the Luzon Strait and 2–9 × 10−5 m2 s−1 in the Philippine Sea basin. The variability in internal tides and their interactions with the subtidal ocean circulation results in significant spatial and temporal variability in the energy available for mixing. The subtidal circulation influences internal-tide-induced mixing in two ways: by introducing variability in internal tide generation and by increased dissipation of baroclinic energy associated with greater velocity shear. Close to the generation site, mixing is dominated by high-mode internal tide dissipation, while in the far field the influence of the mesoscale energy on internal tide dissipation is significant, resulting in increased dissipation. This study presents model-based estimates of the important and relatively unknown effect of mesoscale circulation on internal-tide-induced mixing away from internal tide generation sites in a region of high eddy kinetic energy.

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Colette G. Kerry
,
Brian S. Powell
, and
Glenn S. Carter

Abstract

This study examines the effects of the subtidal circulation on the generation and propagation of the M2 internal tide in the Philippine Sea using a primitive equation model. Barotropic to baroclinic conversion at the Luzon Strait is found to vary due to the background circulation changes over the generation site and the changing influence of remotely generated internal tides from the Mariana Arc. The varying effect of remotely generated waves results from both changing generation energy levels at the Mariana Arc and variability in the propagation of the internal tides across the Philippine Sea. The magnitude and direction of the depth-integrated baroclinic energy fluxes vary temporally, due to a combination of changing generation, propagation, and dissipation. Spatial patterns of internal tide propagation near the Luzon Strait are influenced by the locations of mesoscale eddies to the east and west of the strait. The results provide insight into the mechanisms of variability of the baroclinic tides and highlight the importance of considering both the remotely generated internal tides and the subtidal dynamics to estimate internal tide energetics.

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Colette G. Kerry
,
Brian S. Powell
, and
Glenn S. Carter

Abstract

This study investigates the impact of remotely generated internal tides on model estimates of barotropic to baroclinic tidal conversion for two generation sites bounding the Philippine Sea: the Luzon Strait and the Mariana Island Arc. A primitive equation model is used to characterize the internal tides generated by the principal semidiurnal tide (M 2) over a domain encompassing the two generation sites. Energetic internal tides are generated at the Luzon Strait where nearly 17 GW of barotropic tide energy is converted to baroclinic energy, of which 44% (4.78 GW) is radiated eastward into the Philippine Sea. From the Mariana Arc, baroclinic energy propagates westward into the Philippine Sea as a result of 3.82 GW of barotropic to baroclinic energy conversion. Simulations that focus on each generation site without influence of the other are performed, and comparisons show that remotely generated internal tides have a significant effect on local conversion at the two sites. Total conversion is greater in the absence of remotely generated internal tides at both sites: 11% greater at the Luzon Strait and 65% greater at the Mariana Arc. The first three modes of the remotely generated internal tides traverse the basin and alter the amplitude and phase of bottom pressure. The arrival of the remote internal tides varies significantly with changing stratification and mesoscale circulation. The results suggest that an important source of variability in local conversion around the globe is due to remotely generated internal tides.

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Glenn S. Carter
,
Michael C. Gregg
, and
Mark A. Merrifield

Abstract

Microstructure observations over a small seamount on the Kaena Ridge, Hawaii, showed asymmetry in the along- and across-ridge directions. The ∼400-m-high seamount is on the southern edge of the ridge (centered at 21°43′49″N, 158°38′48″W), 42 km northwest of Oahu. A 1-km-resolution numerical simulation shows that the flow within the depth range of the seamount tends to be accelerated around the seamount rather than going up and over it. The flow patterns, however, are more complicated than for an isolated seamount because of the influence of the ∼3000-m-high Kaena Ridge. Comparison with the numerical simulations indicates that the across-ridge asymmetry, in which dissipation on the north-northeastern side of the seamount was higher and more concentrated toward the bed than on the south-southwestern side, is consistent with an M 2 tidal beam generated at the northern edge of the ridge. The along-ridge asymmetry, with higher dissipation on the east-southeastern flank than on the west-northwestern flank, is in qualitative agreement with M 2 shear variance from the model simulation. The average observed dissipation rate over the seamount was ε = 6.2 × 10−8 W kg−1, and diapycnal diffusivity was K ρ = 1.3 × 10−3 m2 s−1. Dissipation measurements following the 1000-m isobath south-southwest of the seamount suggest along-ridge internal tide generation caused by topographic steering that creates an along-ridge current over critical topography northwest of the seamount.

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Eric Kunze
,
Leslie K. Rosenfeld
,
Glenn S. Carter
, and
Michael C. Gregg

Abstract

Velocity, temperature, and salinity profile surveying in Monterey Submarine Canyon during spring tide reveals an internal wave field almost an order of magnitude more energetic than that in the open ocean. Semidiurnal fluctuations and their harmonics dominate, near-inertial motions are absent. The ratio of horizontal kinetic to available potential energy is less than one in much of the canyon, inconsistent with hydrostatic internal waves. The excess potential energy may be due to isopycnal displacements induced by barotropic tide flow over the sloping bottom. Removal of the expected barotropic contribution raises the energy ratio to 2.04–2.10, in line with the semidiurnal internal wave value of 2.13. Finescale shear and strain are also elevated. Finescale parameterizations for turbulent eddy diffusivities, which have proven successful in the open ocean, underestimate upper-canyon microstructure estimates of 100 × 10−4 m2 s−1 by a factor of 30. Energy fluxes and near-bottom velocities are strongly steered by the sinuous canyon topography. A vertically integrated influx of 5 kW m−1 at the mouth diminishes to ±1 kW m−1 toward the shallow end of the canyon. Both sinks and sources of internal wave energy are indicated by energy-flux convergences and divergences along the canyon axis. Along-axis energy-flux convergences are consistent with microstructure dissipation rates ϵ. The high diapycnal eddy diffusivities may drive strong nutrient fluxes to enhance bioproductivity.

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