Neil S. Oakey

Vertical Nitrate Fluxes in the Oligotrophic Ocean

The vertical flux of nitrate across the thermocline in the upper ocean imposes a rigorous constraint on the rate of export of organic carbon from the surface layer of the sea. This export is the primary means by which the oceans can serve as a sink for atmospheric carbon dioxide. For the oligotrophic open ocean regions, which make up more than 75% of the worlds ocean, the rate of export is currently uncertain by an order of magnitude. For most of the year, the vertical flux of nitrate is that due to vertical turbulent transport of deep water rich in nitrate into the relatively impoverished surface layer. Direct measurements of rates of turbulent kinetic energy dissipation, coupled with highly resolved vertical profiles of nitrate and density in the oligotrophic eastern Atlantic showed that the rate of transport, averaged over 2 weeks, was 0.14 (0.002 to 0.89, 95% confidence interval) millimole of nitrate per square meter per day and was statistically no different from the integrated rate of nitrate uptake as measured by incorporation of 15N-labeled nitrate. The stoichiometrically equivalent loss of carbon from the upper ocean, which is the relevant quantity for the carbon dioxide and climate question, is then fixed at 0.90 (0.01 to 5.70) millimole of carbon per square meter per day. These rates are much lower than recent estimates based on in situ changes in oxygen over annual scales; they are consistent with a biologically unproductive oligotrophic ocean.

Two Years in the Life of a Mediterranean Salt Lens

Abstract A lens of Mediterranean water (Meddy) was tracked in the eastern North Atlantic for two years with SOFAR floats. The Meddy was first found between the Canary Islands and the Azores in October 1984. It center moved in an irregular pattern, at speeds of a few cm s−1, and translated 1100 km to the south in two years. This Meddy was surveyed four times by CTD and velocity profilers, and once with the microstructure profiler EPSONDE. When observed during the first two surveys the Meddy had a core that was stably and smoothly stratified in both salinity and temperature, nearly uniform in the horizontal, and was saltier than the surrounding ocean by 0.65 psu. The Meddy was eroded from its edges, top and bottom, and lost salt and hat with an e-folding time of about one year. The salinity at the center remained at its original value during the first year and decreased during the second year. Evidence was seen for mixing by lateral intrusions, double diffusion, and turbulence; the intrusions are thought to...

Variations in Apparent Mixing Efficiency in the North Atlantic Central Water

Abstract Microstructure data from the North Atlantic Tracer Release Experiment (NATRE) are presented, providing detailed profiles of the thermal variance χ in the upper 360 m of the Canary Basin for the fall and spring seasons. The Osborn–Cox model is used to compute the diffusivity KT. The diffusivity for the depth range 240–340 m is found to be 1.0(±0.04) × 10−5 m2 s−1 in the fall and 2.2(±0.1) × 10−5 m2 s−1 in the spring, in good agreement with dye-inferred diffusivities at similar depths. Measured turbulent kinetic energy (TKE) dissipation rates were found to be contaminated by hydrodynamic noise, so the Osborn dissipation method was not used to compute Kρ. However, data segments for which the TKE dissipation rate (e) was large enough to be unaffected by noise were used to compute the “apparent mixing efficiency” Γd. The computed Γd values are used to investigate variations in apparent mixing efficiency with respect to density ratio (Rρ) and turbulence Reynolds number [e/(νN2)], in an attempt to eluci...

Evolution of a mediterranean salt lens : scalar properties

Abstract The evolution of a Mediterranean salt lens (Meddy) over a two year period is examined. Several nondimensional numbers can be used to describe the overall decay in the structure of the Moddy. Two Rossby numbers, one using the central relative vorticity and another using the radius and velocity of the azimuthal velocity maximum, did not change over the two year period. However, the Burger number N2H2/(f2L2) increased as the Meddy decayed. Another Burger number, the ratio of total kinetic energy to total available potential energy, decreased from 1.1 to 0.6 over a one year period. The rates at which the Meddy lost salt and heat are consistent with estimates of horizontal fluxes by intrusions. A horizontal diffusivity of O(5 m2 s−1) is needed if this flux by intrusions is parameterized by an eddy coefficient. Simple models of the evolution of an isolated eddy by horizontal and vertical mixing of mass and momentum are examined. These simple attempts to explain the evolution of the Meddy suggest more c...

Turbulence dissipation rates and nitrate supply in the upper water column on Georges Bank

Abstract Measurements of velocity microstructure in the upper water column on Georges Bank are used to contrast the summertime structure of turbulent kinetic energy dissipation rate between the central mixed area and the tidal-mixing front, and to compare vertical nitrate fluxes with frontal-zone primary production demands. In the mixed area during weak winds, the dissipation rate varies strongly over the semidiurnal tidal period in close relation to the tidal current strength, varies with the monthly/fortnightly tidal modulation, and generally increases with distance below the sea surface. Collectively, these features provide strong support for the elevated vertical mixing rates on Georges Bank being primarily due to the tides, although wind forcing also contributes significantly. In the frontal zone on northern Georges Bank, the upper-ocean dissipation rates are about an order of magnitude weaker than in the mixed area, have a more complex temporal variation during the tidal period, and also vary with the monthly/fortnightly tidal modulation. The vertical eddy flux of nitrate into the frontal euphotic zone varies over the tidal period and with the tidal modulation. Averaged over the tidal period, the estimated fluxes are about one-third of the nitrogen demand estimated from concurrent primary production measurements, supportive of an important contribution from turbulent mixing to new production in the frontal zone, but also pointing to additional processes and/ or inadequate data coverage of this complex zone. The measured dissipation rates at both the mixed and frontal sites are in approximate agreement with the turbulence levels in two 3-D numerical models for summertime tidal and mean circulation on the Bank, one with and eddy-viscosity and the other an advanced turbulence closure. The latter model has more realistic vertical turbulence distributions and indicates strong sensitivity of the turbulence levels to horizontal position in the frontal zone.

Measurements of internal wave band eddy fluxes above a sloping bottom

The boundary layer near a sloping bottom may have a major influence on the ocean’s interior density structure (due to “boundary mixing”) and on its circulation (because of the arrest of the Ekman layer by buoyancy forces). As a first attempt to measure eddy fluxes of momentum and buoyancy, in order to quantify the mixing in this region, we have carried out a S-day pilot experiment on a sloping side of Emerald Basin on the Scotian Shelf. A moored upward-looking 1.2 MHz ADCP and a thermistor chain mounted along its vertical axis returned analyzable data between 8 and 17 m above the bottom at one-minute intervals. An extensive set of microstructure profiles was also obtained. The predominantly tidal flow regime causes the bottom boundary layer thickness to vary between 3 < z < 30 m, with most high frequency activity during the upslope phase. A bottom-normal momentum flux significantly different from zero is found in the crossisobath direction only. The main contribution comes from a band near the buoyancy frequency N, possibly indicative of advective or Kelvin-Helmholtz instability. When cast in terms of mean-flow shear, the stress yields an eddy viscosity A = 9 x 10e3 m* s-l within the boundary layer and twice this value at z = 15 m, the average height of the pycnocline that caps the boundary layer. The buoyancy flux also seems to be dominated by fluctuating signals near N, but is countergradient and only significantly different from zero at a height of about 15 m. The associated restratification occurs in short periods of approximately one hour when isotherms rise rapidly. Indirect evidence for the importance of the tertiary circulation within the boundary layer is found from the gradient of stress divergence and the mean bottom-normal velocity. An approximate turbulent kinetic energy balance has been investigated, with the currents split into three parts (mean, tidal, and the high frequency part of the internal waveband (“turbulence”)). Production balances viscous dissipation within a factor of 2. Turbulent kinetic energy production by interaction between the turbulent Reynolds stress and the mean flow shear and tidal shear are of the same order of magnitude, but the buoyancy term appears to be of equal importance at the pycnocline.

Mixing in a coastal environment: 2. A view from microstructure measurements

[1] During the Coastal Mixing and Optics Experiment in 1996 and 1997, an integrated dye and microstructure experiment was done to measure and compare mixing rates on the continental shelf. The results of the dye experiment are presented in the companion paper by Ledwell et al. [2004]. In this paper, we explore the results from microstructure measurements using a vertical profiling instrument. We measure temperature and velocity microstructure and, along with simultaneous measurements of salinity and temperature as well as a shipboard acoustic Doppler current profiler (ADCP), are able to estimate the vertical diffusivities of heat, mass, and momentum. In three of four dye injections performed, we were able to make a comparison of the diffusivity from both dye and microstructure measurements. Although the mixing rates were quite small (vertical diffusivity of heat, K T < 10 -5 m 2 s- ), the two techniques yielded consistent results. A comparison of the vertical diffusivities K T and K ρ (the vertical diffusivity for density) allowed us to determine a flux Richardson number of R f = 0.16 ± 0.03. R f showed little dependence on either the buoyancy frequency, N, or gradient Richardson number, R i . A clear relationship was found between the ratio of diffusivities, K m /K T and R i consistent with K m /K T = 5 R i . Turbulence levels were extremely low, with Cox numbers in one experiment of about 20 and in the other three of about 5 (i.e., K T about 20 and 5 times molecular diffusion, respectively).

Hydrographic patterns and vertical mixing in the equatorial Pacific along 150°W

The WEC88 cruise sampled along a meridional transect from 15°N to 15°S along 150°W from February 17 to March 18, 1988, with a 6-day time series at the equator. The large-scale hydrographic patterns were typical for boreal spring. Equatorial maxima in dissipation of turbulent kinetic energy ∈, and of thermal variance χ, were found between 2°N and 2°S for the top 60 m. The equatorial time series coincided with a shift from southward to northward velocity, which returned the zonal current system to the equator. This led to a decrease in temperature, and increases in salinity, nutrient, and chlorophyll concentrations in the surface layer. Vertical diffusivity as well as ∈ and χ increased with the observed intensification of the Equatorial Undercurrent. Maximum values of ∈ and χ were observed at around 55 m, and the temporal trends occurred first at depth. Turbulent heat flux out of the mixed layer was the same order of magnitude as the penetrative irradiance at that depth. Maximum vertical heat flux occurred at depth in response to large diffusivity coefficients. The Richardson number was useful in predicting the regions of enhanced mixing in the meridional transect. However, for the equatorial time series, where the Ri was less than 0.45, intensity of dissipation was not proportional to Richardson number.

Stirring by Small-Scale Vortices Caused by Patchy Mixing

Evidence is presented that lateral dispersion on scales of 1–10 km in the stratified waters of the continental shelf may be significantly enhanced by stirring by small-scale geostrophic motions caused by patches of mixed fluid adjusting in the aftermath of diapycnal mixing events. Dye-release experiments conducted during the recent Coastal Mixing and Optics (CMO) experiment provide estimates of diapycnal and lateral dispersion. Microstructure observations made during these experiments showed patchy turbulence on vertical scales of 1–10 m and horizontal scales of a few hundred meters to a few kilometers. Momentum scaling and a simple random walk formulation were used to estimate the effective lateral dispersion caused by motions resulting from lateral adjustment following episodic mixing events. It is predicted that lateral dispersion is largest when the scale of mixed patches is on the order of the internal Rossby radius of deformation, which seems to have been the case for CMO. For parameter values relevant to CMO, lower-bound estimates of the effective lateral diffusivity by this mechanism ranged from 0.1 to 1 m 2 s 1 . Revised estimates after accounting for the possibility of long-lived motions were an order of magnitude larger and ranged from 1 to 10 m 2 s 1 . The predicted dispersion is large enough to explain the observed lateral dispersion in all four CMO dye-release experiments examined.

Estimates of Dissipation in the Ocean Mixed Layer Using a Quasi-Horizontal Microstructure Profiler

Abstract Some recent measurements of the mixed layer in oceans and lakes have indicated that the rate of the dissipation of turbulent kinetic energy, e, is much higher than expected from a purely shear-driven wall layer. This enhancement has usually been attributed to wave breaking. In this study, measurements of dissipation in the open-ocean mixed layer on the continental shelf off Nova Scotia are integrated with air–sea flux estimates and directional wave spectra to further study this issue. A microstructure profiler gliding quasi-horizontally provides estimates of e starting within 2 m of the ocean surface as it slowly descends through the mixed layer. Dissipation rates were found to be enhanced relative to the wind stress production and indicated that ∼6% of the wind energy at 10 m is dissipated in the ocean mixed layer. In addition, results from this experiment demonstrate that the WAVES scaling for e, based on wind and wave parameters, is valid for the case of a simple windsea in which the swell can...