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Hans Burchard and Tom P. Rippeth

Abstract

Recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability, suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing. The bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows remarkable variability on tidal, inertial, and synoptic time scales that has yet to be explained. In this paper observations from the seasonally stratified northern North Sea are presented for a time when the water column has a distinct two-layer structure. Bulk shear estimates, based on ADCP measurements, show a bulk shear vector that rotates in a clockwise direction at the local inertial period, with episodes of bulk shear spikes that have an approximately twice daily period, and occur in bursts that last for several days. To explain this observation, a simple two-layer model based on layer averaging of the one-dimensional momentum equation is developed, forced at the surface by wind stress and damped by (tidally dominated) sea bed friction. The two layers are then linked through an interfacial stress term. The model reproduces the observations, showing that the bulk shear spikes are a result of the alignment of the wind stress, tidal bed stress, and (clockwise rotating) bulk shear vectors. Velocity microstructure measurements are then used to confirm enhanced levels of mixing during a period of bulk shear spikes. A numerical study demonstrates the sensitivity of the spike generation mechanism to the local tidal conditions and the phasing and duration of wind events.

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Tom P. Rippeth, Eirwen Williams, and John H. Simpson

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A high-frequency (1.2 MHz) acoustic Doppler current profiler (ADCP) moored on the seabed has been used to observe the mean and turbulent flow components in a narrow tidally energetic channel over six tidal cycles at neap and spring tides. The Reynolds stress has been estimated from the difference in variance between the along-beam velocities of opposing acoustic beams with a correction for the sampling scheme and bin size. Shear stress was found to vary regularly with the predominantly semidiurnal tidal flow with the stresses on the spring ebb flow (up to 4.5 Pa) being generally greater than on the flood flow (<2 Pa) when the currents are weaker. The vertical structure approximated to linear stress profiles decreasing from maximum values near the bed to almost zero stresses just below the surface. The variation in the bed stress was well represented by a quadratic drag law, based on the depth-mean current, with an estimated drag coefficient of 2.6 ± 0.2 × 10−3.

The production of turbulent kinetic energy (TKE) followed a regular cycle at the M 4 frequency with maximum values exceeding 1 W m−3 near the bed during ebb flow at spring tides and decreasing with height to ∼10−3 W m−3 at 2 m below the surface. Production was generally lowest (∼10−4 W m−3) at low water slack, which was longer than high water slack, and is marked by a rapid transition from flood to ebb. During peak ebb and flood the vertical distribution of production and the eddy viscosity N z are reasonably well described by a proposed model based on the law of the wall and a steady balance between the pressure gradient and a uniform shear stress gradient.

The stress values have been incorporated into a trial dynamical balance based on the vertically integrated linearized equation of motion along the channel. The pressure gradient term is determined by two tide gauges separated by 5 km in the along-channel direction. The stress variation is in the correct phase to match the combined slope and acceleration term but is only about 60% of the magnitude required for balance. It is suggested that this discrepancy may result from an overestimation of the local pressure gradient, which may vary significantly between the tide gauges due to changes in the channel cross section.

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Tom P. Rippeth, Neil R. Fisher, and John H. Simpson

Abstract

In regions of large horizontal density gradient, tidal straining acts to produce a periodic component of stratification that interacts with turbulent mixing to control water column structure and flow. A 25-h series of measurements of the rate of dissipation of turbulent kinetic energy (ϵ) in the Liverpool Bay region of freshwater influence (ROFI) have revealed the form of this interaction and indicate substantial differences from regions where horizontal gradients are weak. In the ROFI system there is a pronounced difference between flood and ebb regimes. During the ebb the water column stratifies and strong dissipation is confined to the lower half of the water column. By contrast, during the flood, stratification is eroded with complete vertical mixing occurring at high water and high values of dissipation (3 mW m−3) extending throughout the water column. The cycle of dissipation is therefore predominantly semidiurnal in the upper layers whereas, near the bottom boundary, the principal variation is at the M 4 frequency as observed in regions of horizontal uniformity. Toward the end of the flood phase of the cycle, tidal straining produces instabilities in the water column that release additional energy for convective mixing. Confirmation of increased vertical motions throughout the water column during the late flood and at high water is provided by measurements of vertical velocity and the error velocity from a bottom-mounted acoustic Doppler current profiler.

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Lars Arneborg, Carol Janzen, Bengt Liljebladh, Tom P. Rippeth, John H. Simpson, and Anders Stigebrandt

Abstract

Two microstructure profilers, two ships, and four moorings with acoustic Doppler current profilers and conductivity–temperature loggers were used in an intensive effort to map the spatial and temporal variations of vertical mixing in the stagnant deep basin of Gullmar Fjord, Sweden. During three days in the beginning of August 2001 a continuous time series of turbulent kinetic energy dissipation profiles was obtained with one microstructure profiler at a fixed position near the deepest part of the fjord. During the same period the other microstructure profiler was used to obtain six sections of dissipation through the length of the basin. Two moorings were deployed in the fjord basin for one month from the end of July to the end of August. The mapping of dissipation rates reveals that the dissipation in the deep basin is confined to areas just inside the sill. More than 77% of the dissipation in the fjord basin happens above the sloping bottoms closest to the sill.

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Brian D. Scannell, Tom P. Rippeth, John H. Simpson, Jeff A. Polton, and Joanne E. Hopkins

Abstract

The combination of acoustic Doppler current profilers and the structure function methodology provides an attractive approach to making extended time series measurements of oceanic turbulence (the rate of turbulent kinetic energy dissipation ε) from moorings. However, this study shows that for deployments in the upper part of the water column, estimates of ε will be biased by the vertical gradient in wave orbital velocities. To remove this bias, a modified structure function methodology is developed that exploits the differing length scale dependencies of the contributions to the structure function resulting from turbulent and wave orbital motions. The success of the modified method is demonstrated through a comparison of ε estimates based on data from instruments at three depths over a 3-month period under a wide range of conditions, with appropriate scalings for wind stress and convective forcing.

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John H. Simpson, William R. Crawford, Tom P. Rippeth, Andrew R. Campbell, and Joseph V. S. Cheok

Abstract

The free-fall FLY profiler has been used to determine the variation in energy dissipation ε in the water column over a tidal cycle at mixed and stratified sites in the Irish Sea. It was found that ε exhibits a strong M4 variation with a pronounced phase lag that increases with height above the bed. In mixed conditions this M4 signal, which extends throughout the water column, is reasonably well reproduced by turbulent closure models of the vertical exchange. In the summer stratified situation, the M4, signal in ε is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed. The lowest levels of dissipation (∼10−5 W m−3), measured in the pycnocline, are significantly above the system noise level and much higher than predicted by a model using the Mellor-Yamada level 2 closure scheme (MY2.0). However, when allowance is made for the diffusion of TKE, the model (MY2.2) simulates the depth-time distribution of dissipation in the stratified case satisfactorily if the diffusivity Kq = 0.2ql. With Kq set equal to vertical eddy viscosity Nz,. which depends on the Richardson number Ri, the model underestimates dissipation in the pycnocline by two decades, which would imply the possibility of a midwater source of TKE. The observed depth-integrated dissipation is found to be consistent with estimates based on the energy lost from the tidal wave when adjustment is made for the unsampled high energy region close to the bed.

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Liam Brannigan, Yueng-Djern Lenn, Tom P. Rippeth, Elaine McDonagh, Teresa K. Chereskin, and Janet Sprintall

Abstract

Observations are used to evaluate a simple theoretical model for the generation of near-inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two-layer structure. The model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors. A climatology of stratification and shear variance in Drake Passage is presented, which shows that these assumptions are most applicable to summer, fall, and spring but are not highly applicable to winter. Temperature, salinity, and velocity data from a high spatial resolution cruise in Drake Passage show that the model does not predict all large changes in shear variance; the model is most effective at predicting changes in shear squared when it arises owing to near-inertial wind-driven currents without requiring a rotating resonant wind stress. The model is also more effective where there is a uniform mixed layer above a strongly stratified transition layer. Rotary spectral and statistical analysis of an additional 242 Drake Passage transects from 1999 to 2011 confirmed the presence of this shear-spiking mechanism, particularly in summer, spring, and fall when stratification is stronger.

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Tom P. Rippeth, John H. Simpson, Eirwen Williams, and Mark E. Inall

Abstract

Simultaneous measurements of the rates of turbulent kinetic energy (TKE) dissipation (ε) and production (P) have been made over a period of 24 h at a tidally energetic site in the northern Irish Sea in water of 25-m depth. Some ε profiles from ∼5 m below the surface to 15 cm above the seabed were obtained using a fast light yo-yo (FLY) microstructure profiler, while P profiles were determined from a bottom-mounted high-frequency acoustic Doppler current profiler (ADCP) using the variance method. In homogeneous flow of the kind observed, the turbulence regime should approximate to local equilibrium so that, with no buoyancy forces involved, ε and P are expected to covary with mean values that are equal. The results show a close tracking of ε and P for most of the observational period. For the second tidal cycle, when there was no significant surface wave activity, a mean ratio of ε/P ≃ 0.63 ± 0.17 was obtained. Although this is a significant deviation from unity, it is within the range of uncertainty previously reported for the ε measurements. A marked phase lag of between 5 and 20 min between the maximum P and the maximum ε is interpreted using a simple model in terms of the decay rate of TKE. Consideration of inherent instrument noise has enabled an estimate of the lowest P threshold measurable using the variance technique. For the chosen averaging parameters a value of P min ∼ 7 × 10−5 W m−3 is estimated. Two other significant differences between the two sets of measurements are attributed to errors in the stress estimate. The first is a bias in the estimate of stress resulting from a combination of instrument tilt (1°–3.5°) and surface wave activity. The second are anomalously high stress estimates, covering nearly one-half of the water column at times, which are thought to be due to instrument noise associated with the large wave orbital velocities.

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Yueng-Djern Lenn, Tom P. Rippeth, Chris P. Old, Sheldon Bacon, Igor Polyakov, Vladimir Ivanov, and Jens Hölemann

Abstract

Vertical mixing in the bottom boundary layer and pycnocline of the Laptev Sea is evaluated from a rapidly sampled 12-h time series of microstructure temperature, conductivity, and shear observations collected under 100% sea ice during October 2008. The bottom boundary turbulent kinetic energy dissipation was observed to be enhanced (ϵ ∼ 10−4 W m−3) beyond background levels (ϵ ∼ 10−6 W m−3), extending up to 10 m above the seabed when simulated tidal currents were directed on slope. Upward heat fluxes into the halocline-class waters along the Laptev Sea seabed peaked at ∼4–8 W m−2, averaging out to ∼2 W m−2 over the 12-h sampling period. In the Laptev Sea pycnocline, an isolated 2-h episode of intense dissipation (ϵ ∼ 10−3 W m−3) and vertical diffusivities was observed that was not due to a localized wind event. Observations from an acoustic Doppler current meter moored in the central Laptev Sea near the M 2 critical latitude are consistent with a previous model in which mixing episodes are driven by an enhancement of the pycnocline shear resulting from the alignment of the rotating pycnocline shear vector with the under-ice stress vector. Upward cross-pycnocline heat fluxes from the Arctic halocline peaked at ∼54 W m−2, resulting in a 12-h average of ∼12 W m−2. These results highlight the intermittent nature of Arctic shelf sea mixing processes and how these processes can impact the transformation of Arctic Ocean water masses. The observations also clearly demonstrate that absence or presence of sea ice profoundly affects the availability of near-inertial kinetic energy to drive vertical mixing on the Arctic shelves.

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Natasha S. Lucas, Alan L. M. Grant, Tom P. Rippeth, Jeff A. Polton, Matthew R. Palmer, Liam Brannigan, and Stephen E. Belcher

Abstract

Understanding the processes that control the evolution of the ocean surface boundary layer (OSBL) is a prerequisite for obtaining accurate simulations of air–sea fluxes of heat and trace gases. Observations of the rate of dissipation of turbulent kinetic energy (ε), temperature, salinity, current structure, and wave field over a period of 9.5 days in the northeast Atlantic during the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) are presented. The focus of this study is a storm that passed over the observational area during this period. The profiles of ε in the OSBL are consistent with profiles from large-eddy simulation (LES) of Langmuir turbulence. In the transition layer (TL), at the base of the OSBL, ε was found to vary periodically at the local inertial frequency. A simple bulk model of the OSBL and a parameterization of shear driven turbulence in the TL are developed. The parameterization of ε is based on assumptions about the momentum balance of the OSBL and shear across the TL. The predicted rate of deepening, heat budget, and the inertial currents in the OSBL were in good agreement with the observations, as is the agreement between the observed value of ε and that predicted using the parameterization. A previous study reported spikes of elevated dissipation related to enhanced wind shear alignment at the base of the OSBL after this storm. The spikes in dissipation are not predicted by this new parameterization, implying that they are not an important source of dissipation during the storm.

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