scholarly journals Structure, Propagation, and Mixing of Energetic Baroclinic Tides in Mamala Bay, Oahu, Hawaii

2006 ◽  
Vol 36 (6) ◽  
pp. 997-1018 ◽  
Author(s):  
Matthew H. Alford ◽  
Michael C. Gregg ◽  
Mark A. Merrifield
Keyword(s):  
Energy Flux ◽  
Dissipation Rate ◽  
Spring Tide ◽  
Ocean Model ◽  
Turbulent Dissipation ◽  
Submarine Ridge ◽  
Vertical Displacements ◽  
Diapycnal Diffusivity ◽  
Baroclinic Currents ◽  
Spatial Maps

Abstract Large semidiurnal vertical displacements (≈100 m) and strong baroclinic currents (≈0.5 m s−1; several times as large as barotropic currents) dominate motions in Mamala Bay, outside the mouth of Pearl Harbor, Hawaii. During September 2002, the authors sought to characterize them with a 2-month McLane moored profiler deployment and a 4-day intensive survey with a towed CTD/ADCP and the Research Vessel (R/V) Revelle hydrographic sonar. Spatial maps and time series of turbulent dissipation rate ε, diapycnal diffusivity Kρ, isopycnal displacement η, velocity u, energy E, and energy flux F are presented. Dissipation rate peaks in the lower 150 m during rising isopycnals and high strain and shows a factor-of-50 spring–neap modulation. The largest Kρ values, in the western bay near a submarine ridge, exceed 10−3 m2 s−1. The M2 phases of η and u increase toward the west, implying a westward phase velocity cp ≈ 1 m s−1 and horizontal wavelength ≈60 km, consistent with theoretical mode-1 values. These phases vary strongly (≈±45°) in time relative to astronomical forcing, implying remotely generated signals. Energy and energy flux peak 1–3 days after spring tide, supporting this interpretation. The group velocity, computed as the ratio F/E, is near ≈1 m s−1, also in agreement with theoretical mode-1 values. Spatial maps of energy flux agree well with results from the Princeton Ocean Model, indicating converging fluxes in the western bay from waves generated to the east and west. The observations indicate a time-varying interference pattern between these waves that is modulated by background stratification between their sources and Mamala Bay.

scholarly journals Internal Tides and Mixing in a Submarine Canyon with Time-Varying Stratification

2012 ◽  
Vol 42 (12) ◽  
pp. 2121-2142 ◽  
Author(s):  
Zhongxiang Zhao ◽  
Matthew H. Alford ◽  
Ren-Chieh Lien ◽  
Michael C. Gregg ◽  
Glenn S. Carter
Keyword(s):  
Energy Flux ◽  
Spring Tide ◽  
Internal Tide ◽  
Submarine Canyon ◽  
Internal Tides ◽  
Time Variability ◽  
Thermocline Depth ◽  
Turbulent Dissipation ◽  
Submarine Ridge ◽  
Lee Waves

Abstract The time variability of the energetics and turbulent dissipation of internal tides in the upper Monterey Submarine Canyon (MSC) is examined with three moored profilers and five ADCP moorings spanning February–April 2009. Highly resolved time series of velocity, energy, and energy flux are all dominated by the semidiurnal internal tide and show pronounced spring-neap cycles. However, the onset of springtime upwelling winds significantly alters the stratification during the record, causing the thermocline depth to shoal from about 100 to 40 m. The time-variable stratification must be accounted for because it significantly affects the energy, energy flux, the vertical modal structures, and the energy distribution among the modes. The internal tide changes from a partly horizontally standing wave to a more freely propagating wave when the thermocline shoals, suggesting more reflection from up canyon early in the observational record. Turbulence, computed from Thorpe scales, is greatest in the bottom 50–150 m and shows a spring-neap cycle. Depth-integrated dissipation is 3 times greater toward the end of the record, reaching 60 mW m−2 during the last spring tide. Dissipation near a submarine ridge is strongly tidally modulated, reaching 10−5 W kg−1 (10–15-m overturns) during spring tide and appears to be due to breaking lee waves. However, the phasing of the breaking is also affected by the changing stratification, occurring when isopycnals are deflected downward early in the record and upward toward the end.

Internal Tides and Turbulence along the 3000-m Isobath of the Hawaiian Ridge

2006 ◽  
Vol 36 (6) ◽  
pp. 1165-1183 ◽  
Author(s):  
Craig M. Lee ◽  
Thomas B. Sanford ◽  
Eric Kunze ◽  
Jonathan D. Nash ◽  
Mark A. Merrifield ◽  
...  
Keyword(s):  
Numerical Model ◽  
Dissipation Rate ◽  
Internal Tide ◽  
Internal Tides ◽  
Energy Fluxes ◽  
Density Profiles ◽  
Turbulent Dissipation ◽  
Model Average ◽  
Diapycnal Diffusivity ◽  
Hawaiian Ridge

Abstract Full-depth velocity and density profiles taken along the 3000-m isobath characterize the semidiurnal internal tide and bottom-intensified turbulence along the Hawaiian Ridge. Observations reveal baroclinic energy fluxes of 21 ± 5 kW m−1 radiating from French Frigate Shoals, 17 ± 2.5 kW m−1 from Kauai Channel west of Oahu, and 13 ± 3.5 kW m−1 from west of Nihoa Island. Weaker fluxes of 1–4 ± 2 kW m−1 radiate from the region near Necker Island and east of Nihoa Island. Observed off-ridge energy fluxes generally agree to within a factor of 2 with those produced by a tidally forced numerical model. Average turbulent diapycnal diffusivity K is (0.5–1) × 10−4 m2 s–1 above 2000 m, increasing exponentially to 20 × 10−4 m2 s–1 near the bottom. Microstructure values agree well with those inferred from a finescale internal wave-based parameterization. A linear relationship between the vertically integrated energy flux and vertically integrated turbulent dissipation rate implies that dissipative length scales for the radiating internal tide exceed 1000 km.

scholarly journals Internal Waves and Turbulence in the Antarctic Circumpolar Current

2013 ◽  
Vol 43 (2) ◽  
pp. 259-282 ◽  
Author(s):  
Stephanie Waterman ◽  
Alberto C. Naveira Garabato ◽  
Kurt L. Polzin
Keyword(s):  
Internal Wave ◽  
Wave Field ◽  
Dissipation Rate ◽  
Antarctic Circumpolar Current ◽  
Wave Radiation ◽  
Turbulent Dissipation ◽  
Internal Wave Field ◽  
Diapycnal Diffusivity ◽  
Circumpolar Current ◽  
The Antarctic

Abstract This study reports on observations of turbulent dissipation and internal wave-scale flow properties in a standing meander of the Antarctic Circumpolar Current (ACC) north of the Kerguelen Plateau. The authors characterize the intensity and spatial distribution of the observed turbulent dissipation and the derived turbulent mixing, and consider underpinning mechanisms in the context of the internal wave field and the processes governing the waves’ generation and evolution. The turbulent dissipation rate and the derived diapycnal diffusivity are highly variable with systematic depth dependence. The dissipation rate is generally enhanced in the upper 1000–1500 m of the water column, and both the dissipation rate and diapycnal diffusivity are enhanced in some places near the seafloor, commonly in regions of rough topography and in the vicinity of strong bottom flows associated with the ACC jets. Turbulent dissipation is high in regions where internal wave energy is high, consistent with the idea that interior dissipation is related to a breaking internal wave field. Elevated turbulence occurs in association with downward-propagating near-inertial waves within 1–2 km of the surface, as well as with upward-propagating, relatively high-frequency waves within 1–2 km of the seafloor. While an interpretation of these near-bottom waves as lee waves generated by ACC jets flowing over small-scale topographic roughness is supported by the qualitative match between the spatial patterns in predicted lee wave radiation and observed near-bottom dissipation, the observed dissipation is found to be only a small percentage of the energy flux predicted by theory. The mismatch suggests an alternative fate to local dissipation for a significant fraction of the radiated energy.

Energy Flux and Dissipation in Luzon Strait: Two Tales of Two Ridges

2011 ◽  
Vol 41 (11) ◽  
pp. 2211-2222 ◽  
Author(s):  
Matthew H. Alford ◽  
Jennifer A. MacKinnon ◽  
Jonathan D. Nash ◽  
Harper Simmons ◽  
Andy Pickering ◽  
...  
Keyword(s):  
Energy Flux ◽  
Spring Tide ◽  
Internal Tide ◽  
Wave Pattern ◽  
The Philippines ◽  
Luzon Strait ◽  
Net Energy ◽  
Energy Fluxes ◽  
Flux Divergence ◽  
Turbulent Dissipation

Abstract Internal tide generation, propagation, and dissipation are investigated in Luzon Strait, a system of two quasi-parallel ridges situated between Taiwan and the Philippines. Two profiling moorings deployed for about 20 days and a set of nineteen 36-h lowered ADCP–CTD time series stations allowed separate measurement of diurnal and semidiurnal internal tide signals. Measurements were concentrated on a northern line, where the ridge spacing was approximately equal to the mode-1 wavelength for semidiurnal motions, and a southern line, where the spacing was approximately two-thirds that. The authors contrast the two sites to emphasize the potential importance of resonance between generation sites. Throughout Luzon Strait, baroclinic energy, energy fluxes, and turbulent dissipation were some of the strongest ever measured. Peak-to-peak baroclinic velocity and vertical displacements often exceeded 2 m s−1 and 300 m, respectively. Energy fluxes exceeding 60 kW m−1 were measured at spring tide at the western end of the southern line. On the northern line, where the western ridge generates appreciable eastward-moving signals, net energy flux between the ridges was much smaller, exhibiting a nearly standing wave pattern. Overturns tens to hundreds of meters high were observed at almost all stations. Associated dissipation was elevated in the bottom 500–1000 m but was strongest by far atop the western ridge on the northern line, where >500-m overturns resulted in dissipation exceeding 2 × 10−6 W kg−1 (implying diapycnal diffusivity Kρ > 0.2 m2 s−1). Integrated dissipation at this location is comparable to conversion and flux divergence terms in the energy budget. The authors speculate that resonance between the two ridges may partly explain the energetic motions and heightened dissipation.

Diapycnal Mixing in the Subthermocline of the Mariana Ridge from High-Resolution Seismic Images

2021 ◽  
Vol 51 (4) ◽  
pp. 1283-1300
Author(s):  
Qunshu Tang ◽  
Zhiyou Jing ◽  
Jianmin Lin ◽  
Jie Sun
Keyword(s):  
High Resolution ◽  
Dissipation Rate ◽  
Large Scale ◽  
Internal Tide ◽  
Ocean Model ◽  
Small Scale ◽  
Far Field ◽  
Continuous Map ◽  
Diapycnal Diffusivity ◽  
Seismic Images

AbstractThe Mariana Ridge is one of the prominent mixing hotspots of the open ocean. The high-resolution underway marine seismic reflection technique provides an improved understanding of the spatiotemporal continuous map of ocean turbulent mixing. Using this novel technique, this study quantifies the diapycnal diffusivity of the subthermocline (300–1200-m depth) turbulence around the Mariana Ridge. The autotracked wave fields on seismic images allow us to derive the dissipation rate ε and diapycnal diffusivity Kρ based on the Batchelor model, which relates the horizontal slope spectra with +1/3 slope to the inertial convective turbulence regime. Diffusivity is locally intensified around the seamounts exceeding 10−3 m2 s−1 and gradually decreases to 10−5–10−4 m2 s−1 in ~60-km range, a distance that may be associated with the internal tide beam emanating paths. The overall pattern suggests a large portion of the energy dissipates locally and a significant portion dissipates in the far field. Empirical diffusivity models Kρ(x) and Kρ(z), varying with the distance from seamounts and the height above seafloor, respectively, are constructed for potential use in ocean model parameterization. Geographic distributions of both the vertically averaged dissipation rate and diffusivity show tight relationships with the topography. Additionally, a strong agreement of the dissipation results between seismic observation and numerical simulation is found for the first time. Such an agreement confirms the suitability of the seismic method in turbulence quantification and suggests the energy cascade from large-scale tides to small-scale turbulence via possible mechanisms of local direct tidal dissipation, near-local wave–wave interactions, and far-field radiating and breaking.

scholarly journals Global Patterns of Diapycnal Mixing from Measurements of the Turbulent Dissipation Rate

2014 ◽  
Vol 44 (7) ◽  
pp. 1854-1872 ◽  
Author(s):  
Amy F. Waterhouse ◽  
Jennifer A. MacKinnon ◽  
Jonathan D. Nash ◽  
Matthew H. Alford ◽  
Eric Kunze ◽  
...  
Keyword(s):  
Internal Wave ◽  
Wave Field ◽  
Dissipation Rate ◽  
Wave Generation ◽  
Wave Power ◽  
Turbulent Dissipation ◽  
Turbulent Dissipation Rate ◽  
Diapycnal Diffusivity ◽  
Internal Wave Generation ◽  
O 10

Abstract The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.

scholarly journals Estimation of Basin-scale turbulence distribution in the North Pacific Ocean using CTD-attached thermistor measurements

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Yasutaka Goto ◽  
Ichiro Yasuda ◽  
Maki Nagasawa ◽  
Shinya Kouketsu ◽  
Toshiya Nakano
Keyword(s):  
North Pacific ◽  
Dissipation Rate ◽  
North Pacific Ocean ◽  
Fast Response ◽  
Ocean Model ◽  
Turbulent Dissipation ◽  
The North ◽  
Basin Scale ◽  
Scale Turbulence ◽  
The North Pacific

AbstractA recently developed technique for microstructure measurement based on a fast-response thermistor mounted on a conductivity-temperature-depth equipment was used on eight cruises to obtain 438 profiles. Thus, the spatial distribution of turbulent dissipation rates across the North Pacific sea floor was illustrated, and was found out to be related to results obtained using tide-induced energy dissipation and density stratification. The observed turbulence distribution was then compared with the dissipation rate based on a high-resolution numerical ocean model with tidal forcing, and discrepancies and similarities between the observed and modelled distributions were described. The turbulence intensity from observation showed that the numerical model was overestimated, and could be refined by comparing it with the observed basin-scale dissipation rate. This new method makes turbulence observations much easier and wider, significantly improving our knowledge regarding ocean mixing.

scholarly journals The Response of Turbidity Maximum to Peak River Discharge in a Macrotidal Estuary

Water ◽  
2021 ◽  
Vol 13 (1) ◽  
pp. 106
Author(s):  
Yuhan Yan ◽  
Dehai Song ◽  
Xianwen Bao ◽  
Nan Wang
Keyword(s):  
River Discharge ◽  
Recovery Time ◽  
Morphological Evolution ◽  
Spring Tide ◽  
River Estuary ◽  
Sediment Concentration ◽  
Ocean Model ◽  
Turbidity Maximum ◽  
Sediment Supply ◽  
Three Dimensional Model

The Ou River, a medium-sized river in the southeastern China, is examined to study the estuarine turbidity maximum (ETM) response to rapidly varied river discharge, i.e., peak river discharge (PRD). This study analyzes the difference in ETM and sediment transport mechanisms between low-discharge and PRD during neap and spring tides by using the Finite-Volume Community Ocean Model. The three-dimensional model is validated by in-situ measurements from 23 April to 22 May 2007. In the Ou River Estuary (ORE), ETM is generally induced by the convergence between river runoff and density-driven flow. The position of ETM for neap and spring tides is similar, but the suspended sediment concentration during spring tide is stronger than that during neap tide. The sediment source of ETM is mainly derived from the resuspension of the seabed. PRD, compared with low-discharge, can dilute the ETM, but cause more sediment to be resuspended from the seabed. The ETM is more seaward during PRD. After PRD, the larger the peak discharge, the longer the recovery time will be. Moreover, the river sediment supply helps shorten ETM recovery time. Mechanisms for this ETM during a PRD can contribute to studies of morphological evolution and pollutant flushing.

scholarly journals Spring Mixing: Turbulence and Internal Waves during Restratification on the New England Shelf

2005 ◽  
Vol 35 (12) ◽  
pp. 2425-2443 ◽  
Author(s):  
J. A. MacKinnon ◽  
M. C. Gregg
Keyword(s):  
New England ◽  
Internal Waves ◽  
Mixed Layer ◽  
Wave Interaction ◽  
Heat Fluxes ◽  
Surface Mixed Layer ◽  
Turbulent Dissipation ◽  
Surface Heat Fluxes ◽  
Bottom Boundary Layers ◽  
Diapycnal Diffusivity

Abstract Integrated observations are presented of water property evolution and turbulent microstructure during the spring restratification period of April and May 1997 on the New England continental shelf. Turbulence is shown to be related to surface mixed layer entrainment and shear from low-mode near-inertial internal waves. The largest turbulent diapycnal diffusivity and associated buoyancy fluxes were found at the bottom of an actively entraining and highly variable wind-driven surface mixed layer. Away from surface and bottom boundary layers, turbulence was systematically correlated with internal wave shear, though the nature of that relationship underwent a regime shift as the stratification strengthened. During the first week, while stratification was weak, the largest turbulent dissipation away from boundaries was coincident with shear from mode-1 near-inertial waves generated by passing storms. Wave-induced Richardson numbers well below 0.25 and density overturning scales of several meters were observed. Turbulent dissipation rates in the region of peak shear were consistent in magnitude with several dimensional scalings. The associated average diapycnal diffusivity exceeded 10−3 m2 s−1. As stratification tripled, Richardson numbers from low-mode internal waves were no longer critical, though turbulence was still consistently elevated in patches of wave shear. Kinematically, dissipation during this period was consistent with the turbulence parameterization proposed by MacKinnon and Gregg, based on a reinterpretation of wave–wave interaction theory. The observed growth of temperature gradients was, in turn, consistent with a simple one-dimensional model that vertically distributed surface heat fluxes commensurate with calculated turbulent diffusivities.

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