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W. D. Smyth, J. N. Moum, and D. R. Caldwell

Abstract

The time evolution of mixing in turbulent overturns is investigated using a combination of direct numerical simulations (DNS) and microstructure profiles obtained during two field experiments. The focus is on the flux coefficient Γ, the ratio of the turbulent buoyancy flux to the turbulent kinetic energy dissipation rate ϵ. In observational oceanography, a constant value Γ = 0.2 is often used to infer the buoyancy flux and the turbulent diffusivity from measured ϵ. In the simulations, the value of Γ changes by more than an order of magnitude over the life of a turbulent overturn, suggesting that the use of a constant value for Γ is an oversimplification. To account for the time dependence of Γ in the interpretation of ocean turbulence data, a way to assess the evolutionary stage at which a given turbulent event was sampled is required. The ratio of the Ozmidov scale L O to the Thorpe scale L T is found to increase monotonically with time in the simulated flows, and therefore may provide the needed time indicator. From the DNS results, a simple parameterization of Γ in terms of L O/L T is found. Applied to observational data, this parameterization leads to a 50%–60% increase in median estimates of turbulent diffusivity, suggesting a potential reassessment of turbulent diffusivity in weakly and intermittently turbulent regimes such as the ocean interior.

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W. D. Smyth, P. O. Zavialov, and J. N. Moum

Abstract

Measurements of velocity, hydrography, surface meteorology, and microstructure were made through several squall events during a westerly wind burst that occurred in the Western Pacific warm pool in December 1992. Sustained wind forcing generated a weakly stratified turbulent surface layer that extended to the top of the main thermocline. Following each rain event, freshwater formed a statically stable layer in the upper 4–12 m. The subsequent evolution of the mixing profile was strongly depth-dependent. Turbulence increased dramatically in the fresh layer adjacent to the surface but decreased in the underlying layer. The factor by which turbulence decreased following a given squall was strongly correlated with the net rainfall. The observed timescale for the decay of the turbulence was about 0.7 buoyancy periods, similar to decay times observed near the surface after sunrise. However, these decay times are significantly larger than those estimated indirectly (as the ratio of dissipation rate to turbulent kinetic energy) from turbulent patches in the thermocline. To account for the discrepancy, the authors hypothesize that turbulence production continues to act during the observed decay process, partially counteracting the effect of dissipation.

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D. Hebert, J. N. Moum, and D. R. Caldwell

Abstract

In spite of the effects of several form of temporal variability that tend to mask geographical patterns in turbulence intensity, our evidence indicates that the turbulence is enhanced above the equatorial undercurrent in comparison to latitudes north and south of it. This evidence consists of three meridional transects of micro-structure observations across the equator (at 140°W in 1984 and 1987. and at 110°W in 1987) along with an equatorial station at 140°W and a longitudinal transect along the equator from 140°W to 110°W. All three meridional transects show a peak in averaged estimates of the turbulent kinetic energy dissipation rates, ε, at the equator, although in 1984 the peak was not significant at the 95% level. The major sources a temporal variability were the diurnal buoyancy flux variation and the wind stress variations, which had a typical period of a few days. After the diurnal variability is removed by averaging, it can be shown that, for similar wind stress, ε is larger over the undercurrent than away from it. Examination of the 16-m, 1-hour averaged ε, in terms of the vertical shear of horizontal velocity and the stratification (determined over similar space and time scales), indicated a tendency of this mean ε to vary with the Richardson number, Ri, when Ri<1. However, closer examination showed that the dependence of ε on Ri varied with depth. Therefore, a simple parameterization for mixing rates on Ri is not valid for all depth.

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J. N. Moum, J. M. Klymak, J. D. Nash, A. Perlin, and W. D. Smyth

Abstract

Winter stratification on Oregon’s continental shelf often produces a near-bottom layer of dense fluid that acts as an internal waveguide upon which nonlinear internal waves propagate. Shipboard profiling and bottom lander observations capture disturbances that exhibit properties of internal solitary waves, bores, and gravity currents. Wavelike pulses are highly turbulent (instantaneous bed stresses are 1 N m−2), resuspending bottom sediments into the water column and raising them 30+ m above the seafloor. The wave cross-shelf transport of fluid often counters the time-averaged Ekman transport in the bottom boundary layer. In the nonlinear internal waves that were observed, the kinetic energy is roughly equal to the available potential energy and is O(0.1) megajoules per meter of coastline. The energy transported by these waves includes a nonlinear advection term 〈uE〉 that is negligible in linear internal waves. Unlike linear internal waves, the pressure–velocity energy flux 〈up〉 includes important contributions from nonhydrostatic effects and surface displacement. It is found that, statistically, 〈uE〉 ≃ 2〈up〉. Vertical profiles through these waves of elevation indicate that up(z) is more important in transporting energy near the seafloor while uE(z) dominates farther from the bottom. With the wave speed c estimated from weakly nonlinear wave theory, it is verified experimentally that the total energy transported by the waves is 〈up〉 + 〈uE〉 ≃ cE〉. The high but intermittent energy flux by the waves is, in an averaged sense, O(100) watts per meter of coastline. This is similar to independent estimates of the shoreward energy flux in the semidiurnal internal tide at the shelf break.

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W. D. Smyth, S. J. Warner, J. N. Moum, H. T. Pham, and S. Sarkar

Abstract

Factors thought to influence deep cycle turbulence in the equatorial Pacific are examined statistically for their predictive capacity using a 13-yr moored record that includes microstructure measurements of the turbulent kinetic energy dissipation rate. Wind stress and mean current shear are found to be most predictive of the dissipation rate. Those variables, together with the solar buoyancy flux and the diurnal mixed layer thickness, are combined to make a pair of useful parameterizations. The uncertainty in these predictions is typically 50% greater than the uncertainty in present-day in situ measurements. To illustrate the use of these parameterizations, the record of deep cycle turbulence, measured directly since 2005, is extended back to 1990 based on historical mooring data. The extended record is used to refine our understanding of the seasonal variation of deep cycle turbulence.

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J. N. Moum, D. Hebert, C. A. Paulson, D. R. Caldwell, M. J. McPhaden, and H. Peters

Abstract

Appearing in this issue of the Journal of Physical Oceanography are three papers that present new observations of a distinct, narrow band, and diurnally varying signal in temperature records obtained in the low Richardson number shear flow above the core of the equatorial undercurrent. Moored data suggest that the intrinsic frequency of the signal is near the local buoyancy frequency, while towed data indicate that the horizontal wavelength in the zonal direction is 150–250 m. Coincident microstructure profiling shows that this signal is associated with bursts of turbulent mixing, it seems that this narrowband signal represents the signature of instabilities that ultimately cause the turbulence observed in the equatorial thermocline. Common problems in interpreting the physics behind the signature are discussed here.

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J. N. Moum, D. R. Caldwell, J. D. Nash, and G. D. Gunderson

Abstract

Observations of mixing over the continental slope using a towed body reveal a great lateral extent (several kilometers) of continuously turbulent fluid within a few hundred meters of the boundary at depth 1600 m. The largest turbulent dissipation rates were observed over a 5 km horizontal region near a slope critical to the M 2 internal tide. Over a submarine landslide perpendicular to the continental slope, enhanced mixing extended at least 600 m above the boundary, increasing toward the bottom. The resulting vertical divergence of the heat flux near the bottom implies that fluid there must be replenished.

Intermediate nepheloid layers detected optically contained fluid with θS properties distinct from their surroundings. It is suggested that intermediate nepheloid layers are interior signitures of the boundary layer detachment required by the near-bottom flux divergance.

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J. N. Moum, A. Perlin, J. M. Klymak, M. D. Levine, T. Boyd, and P. M. Kosro

Abstract

Closely spaced vertical profiles through the bottom boundary layer over a sloping continental shelf during relaxation from coastal upwelling reveal structure that is consistent with convectively driven mixing. Parcels of fluid were observed adjacent to the bottom that were warm (by several millikelvin) relative to fluid immediately above. On average, the vertical gradient of potential temperature in the superadiabatic (statically unstable) bottom layer was found to be −1.7 × 10−4 K m−1, or 6.0 × 10−5 kg m−4 in potential density. Turbulent dissipation rates (ε) increased toward the bottom but were relatively constant over the dimensionless depth range 0.4–1.0z/D (where D is the mixed layer height). The Rayleigh number Ra associated with buoyancy anomalies in the bottom mixed layer is estimated to be approximately 1011, much larger than the value of approximately 103 required to initiate convection in simple laboratory or numerical experiments. An evaluation of the data in which the bottom boundary layer was unstably stratified indicates that the greater the buoyancy anomaly is, the greater the turbulent dissipation rate in the neutral layer away from the bottom will be. The vertical structures of averaged profiles of potential density, potential temperature, and turbulent dissipation rate versus nondimensional depth are similar to their distinctive structure in the upper ocean during convection. Nearby moored observations indicate that periods of static instability near the bottom follow events of northward flow and local fluid warming by lateral advection. The rate of local fluid warming is consistent with several estimates of offshore buoyancy transport near the bottom. It is suggested that the concentration of offshore Ekman transport near the bottom of the Ekman layer when the flow atop the layer is northward can provide the differential transport of buoyant bottom fluid when the density in the bottom boundary layer decreases up the slope.

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T. M. Dillon, J. N. Moum, T. K. Chereskin, and D. R. Caldwell

Abstract

The conventional view of equatorial dynamics requires that the zonal equatorial wind stress be balanced, in the mean, by the vertical integral of “large-scale” terms, such as the zonal pressure gradient, mesoscale eddy flux, and mean advection, over the upper few hundred meters. It is usually presumed that the surface wind stress is communicated to the interior by turbulent processes. Turbulent kinetic energy dissipation rates measured at 140°W during the TROPIC HEAT I experiment and a production rate–dissipation rate balance argument have been used to calculate the zonal turbulent stress at 30 to 90 m depth. The calculated turbulent stress at 30 m depth amounts to only 20% of the wind stress and decreases exponentially with depth below 30 m. Typical large-scale estimates of the zonal pressure gradient, mesoscale eddy flux, and advection have a depth scale larger than the turbulent stress, and are inconsistent with the vertical divergence of the stress as estimated from the dissipation rate measurements. It is concluded that either 1) the measured estimates of dissipation rate are too small, 2) the actual large-scale zonal pressure gradient, mesoscale eddy flux, and advection during our observation period were highly atypical and had a very shallow depth scale, 3) some process other than the simple diffusion of momentum through shear instabilities is transporting the momentum, or 4) the assumption of a production-dissipation balance in the turbulent kinetic energy budget is incorrect. The first two possibilities are unlikely.

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J. N. Moum, D. R. Caldwell, C. A. Paulson, T. V. Chereskin, and L. A. Regier

Abstract

A 3°N to 3°S transect of the equator at 140°15'W was made in November 1984. Vertical profiles of temperature, conductivity and turbulent dissipation were obtained at approximately 1 km intervals. Contrary to previous results, we found no obvious peak in dissipation either at the equator or clearly associated with the Equatorial Undercurrent. A thermistor chain towed behind the ship indicated the rich (and previously unseen) variability of the hydrophysical field of the equatorial ocean. Some of this variability (especially, internal waves) is intimately linked to mixing processes.

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