Search Results

You are looking at 21 - 30 of 48 items for

  • Author or Editor: J. N. Moum x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
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.

Full access
D. R. Caldwell, T. M. Dillon, and J. N. Moum

Abstract

Evaluation of the Rapid-Sampling Vertical Profiler, which was developed for sampling the hydrophysical fields in the upper ocean from a moving vessel, shows that the instrument is useful for near-microscale measurements of temperature and salinity and also for turbulent kinetic energy dissipation measurements with airfoil probes. A depth of 200 meters is reached from a ship moving at 6 knots. The vertical resolution is 3 cm, the temperature resolution is 0.5 millidegrees, the salinity resolution is 0.6 parts per million, and the sigma-t resolution is 0.0004. (The estimates given for resolution are 99% confidence limits on series of 3-cm samples.) The instrument falls with a speed uniform within 20% in an orientation within a few degrees of vertical. Vibrations within the dissipation range of turbulence are sufficiently small to permit the measurement of turbulence with airfoil probes in many regions of the ocean.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Open access
D. R. Caldwell, R-C. Lien, J. N. Moum, and M. C. Gregg

Abstract

Although the process of restratification of the ocean surface layer at the equator following nighttime convection is similar in many ways to the process at midlatitudes, there are important differences. A composite day calculated from 15 days of consistent conditions at 140°W on the equator was compared with midlatitude observations by Brainerd and Gregg. In the depth range of 20–40 m, 1) minimum nighttime stratification was similar [N 2 of 1.2–3.2 (× 10−6 s−2) vs 0.4–1.7 (× 10−6 s−2)], 2) maximum daytime stratification was significantly larger, as might be expected from the greater surface heat input [N 2 of 8–21 (× 10−6 s−2 vs 3–7 (× 10−6 s−2)], and 3) minimum nighttime shear was similar [shear-squared was 1.4–4.6 (× 10−6 s−2) vs 0.8–1.9 (× 10−6 s−2)], but the maximum daytime shear was much larger [shear-squared of 24–41 (× 10−6 s−2) vs 3–7 (× 10−6 s−2)].

For much of the surface layer, the dominant identifiable cause of restratification in both cases was the divergence of the penetrating shortwave radiation, although at the equator the divergence of turbulent flux was important from 10 to 25 m. In both cases the divergence of vertical fluxes accounted for only 60%–70% of the restratification; relaxation of lateral gradients was probably the source for the rest. At the equator, the shear in the upper 40 m was restored in the daytime by turbulent transport of momentum injected by the wind.

In the region convectively mixed at night, turbulence decayed exponentially in the daytime in both cases, the e-folding time, τ ε, being 1.7 ± 0.2 h at the equator, 1.5 h in midlatitude. A dimensionless decay time, N τ ε, was 7.2–9.3 compared with 6.0 in the midlatitude case. In both cases the vertical scale of the turbulence was controlled by the Ozmidov scale, and the turbulence remained active throughout the day.

At the equator “deep-cycle” nighttime turbulence was generated in the always-stratified water at depth 60–80 m never reached by nighttime convection. Neither shear nor stratification varied significantly diurnally. The decay of this turbulence was similar to that above in that its vertical scale was controlled by the Ozmidov scale and remained active throughout the day, but the e-folding timescale was much longer, 3.5 h (N τ ε = 66–96). For the turbulence to persist this long, turbulence production must be a large proportion of ε.

Full access
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.

Full access