Raymond W. Schmitt

Estimates of Diapycnal Mixing in the Abyssal Ocean

Profiles of diapycnal eddy diffusivity to a maximum depth of 4000 meters were derived from ocean velocity and temperature microstructure data obtained in conjunction with separate experiments in the Northeast Pacific and Northeast Atlantic oceans. These profiles indicate that in the ocean interior where the internal wave field is at background intensity, the diapycnal eddy diffusivity is small (on the order of 0.1 x 10–4 meters squared per second) and independent of depth, in apparent contradiction with large-scale budget studies. Enhanced dissipation is observed in regions of elevated internal wave energy, particularly near steeply sloping boundaries (where the eddy diffusivity estimates exceed 1 x 10–4 meters squared per second). These results suggest that basin-averaged mixing rates may be dominated by processes occurring near the ocean boundaries.

Transport of Freshwater by the Oceans

Abstract The global distribution of freshwater transport in the ocean is presented, based on an integration point at Bering Strait, which connects the Pacific and Atlantic oceans via the Artic Ocean. Through Bering Strait, 0.8 × 106 m3 s−1 of relatively fresh, 32.5 psu, water flows from the Pacific into the Arctic Ocean. Baumgrtner and Reichels tabulation of the act gain of freshwater by the ocean in 5° latitude intervals is then integrated from the reference location at Bering Strait to yield the meridional freshwater transport in each ocean. Freshwater transport in the Pacific is directed northward at nearly all latitudes. In the Atlantic, the freshwater transport is directed southward at all latitudes, with a small southward freshwater transport out of the Atlantic across 35°S. Salt transport, which must be considered jointly with the freshwater transport, is northward throughout the Pacific and southward throughout the Atlantic (in the same direction as the freshwater flux) and is equal to the sal...

Evaporation Minus Precipitation and Density Fluxes for the North Atlantic

Abstract Estimates of evaporation (E) over the North Atlantic Ocean by Bunker have been combined with estimates of precipitation (P) by Dorman and Bourke to produce new annual and seasonal maps of E–P and surface density flux. Although uncertainties about precipitation algorithms and exchange coefficients still presist, it is felt that the high spatial resolution of these data set permits an estimate of E–P that is a significant improvement over previous work. The maps of E–P show considerably more detail than earlier maps, including a previously uncharted minimum with a northeast to southwest trend across the subtropical gyre. The two regions of maximal E–P display a close connection with continental air flows off Africal and North America, suggesting that the relative narrowness of the North Atlantic contributes to its status as a net evaporation basin. The zonally integrated E–P values are combined with river runoff estimates to obtain the meridional flux of freshwater, which can be compared with fluxe...

The Contribution of Salt Fingers to Vertical Mixing in the North Atlantic Tracer Release Experiment

Abstract The North Atlantic Tracer Release Experiment (NATRE) was performed in an area moderately favorable to salt fingers. However, the classic finger signature of a distinct thermohaline staircase caused by upgradient density flux was absent. This is likely because mixing by turbulence was sufficiently strong to disrupt the formation of permanent step and layer systems. Despite the lack of a staircase, optical shadowgraph profiles revealed that small-scale tilted laminae, previously observed in a salt-finger staircase, were abundant at the NATRE site. Using microstructure observations, the strength of salt-finger mixing has been diagnosed using a nondimensional parameter related to the ratio of the diffusivities for heat and buoyancy (Γ, “the dissipation ratio”). By examining the dissipation ratio in a parameter space of density ratio (Rρ) and Richardson number (Ri), the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred. While the model for turbulence descr...

Form of the Temperature-Salinity Relationship in the Central Water: Evidence for Double-Diffusive Mixing

Abstract Ingham (1966) reported that the temperature-salinity relationships in the Central Waters were much better described by a curve of constant density ratio (Rρ = αΔT/βΔS) than by a straight line. His result is quantitatively verified and a simple, but powerful, double-diffusive mechanism is proposed to explain the observed constancy of Rρ in the main thermocline. The mechanism is based on the evidence from theory, experiment and observation that the intensity of salt-finger convection is a strong function of Rρ. This dependence, plus the fact that more salt than heat is transferred by the fingers, causes any deviation from a constant Rρ to be the site of convergence or divergence of the vertical salt flux that acts to remove the perturbation in Rρ. A linear treatment of the mechanism shows that Rρ can be “diffused” with an effective diffusivity that is much greater than the diffusivities of heat or mass. A few numerical examples illustrate the predicted effects of salt fingering on the T-S relation,...

Internal Tide Reflection and Turbulent Mixing on the Continental Slope

Abstract Observations of turbulence, internal waves, and subinertial flow were made over a steep, corrugated continental slope off Virginia during May–June 1998. At semidiurnal frequencies, a convergence of low-mode, onshore energy flux is approximately balanced by a divergence of high-wavenumber offshore energy flux. This conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide. It is suggested that elevated near-bottom mixing (Kρ ∼ 10−3 m2 s−1) observed offshore of the supercritical continental slope arises from the reflection of a remotely generated, low-mode, M2 internal tide. Based on the observed turbulent kinetic energy dissipation rate ϵ, the high-wavenumber internal tide decays on time scales O(1 day). No evidence for internal lee wave generation by flow over the slopes corrugations or internal tide generation at the shelf break was found at this site.

Buoyancy Forcing by Turbulence above Rough Topography in the Abyssal Brazil Basin

Abstract Observations of turbulent dissipation above rough bathymetry in the abyssal Brazil Basin are presented. Relative to regions with smooth bathymetry, dissipation is markedly enhanced above rough topography of the Mid-Atlantic Ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors. Furthermore, mixing levels in rough areas are modulated by the spring–neap tidal cycle. Internal waves generated by barotropic tidal flow over topography are the likely mechanism for supplying the energy needed to support the observed turbulent dissipation. A model of the spatial and temporal patterns in the turbulent dissipation rate is used to constrain the diapycnal advection in an inverse calculation for the circulation in an area of rough bathymetry. This inverse model uses both beta-spiral and integrated forms of the advective budgets for heat, mass, and vorticity, and contains sufficient information to resolve the full three-dimensional flow. The inverse model solution re...

The growth rate of super-critical salt fingers

Abstract An expression for the growth rate of super-critical salt fingers is derived from similarity solutions to the Boussinesq governing equations. The fastest growing fingers have an e-folding time of about one Brunt-Vaisala period in the ‘central waters’ of the worlds oceans, with faster growth rates realized in water masses with larger salinity gradients, such as below the Mediterranean outflow. This rapid growth rate supports the notion that salt fingers must be nearly ubiquitous in the main thermo-halocline of the ocean. The flux ratio and wavenumber of the fastest growing fingers are also computed, with the explicit dependence on Prandtl number and diffusivity ratio retained. The wave number of the fastest growing fingers agrees with the oceanic observations of Magnell (Journal of Physical Oceanography, 6, 511–523, 1976), and the flux ratio agrees well with the experimental data of Turner (Deep-Sea Research, 14, 599–611, 1967) for heat and salt, as well as with the data of Stern and Turner (Deep-Sea Research, 16, 497–511, 1969) and Lambert and Demenkow (Journal of Fluid Mechanics, 54, 627–640, 1972) for sugar and salt. The good agreement in systems having two orders of magnitude difference in Prandtl number and diffusivity ratio suggests that time-dependent effects must play an important role in the dynamics of salt finger convection.

Near‐boundary mixing above the flanks of a midlatitude seamount

Fine-scale velocity and density profile data with concurrent turbulent velocity and temperature dissipation estimates obtained above the flanks of Fieberling Guyot, a seamount in the eastern North Pacific Ocean, are examined for evidence of near-bottom boundary mixing. Fine-scale shear and strain spectral levels were elevated over the flanks of the seamount in a 500-m-thick stratified layer above the bottom. The velocity shear was horizontally isotropic, clockwise and counterclockwise-with-depth shear spectral levels were comparable, and no significant correlation between shear and strain was observed. Above the steepest bottom slopes near the seamount summit rim, excess vertical strain relative to shear was observed (as compared to the background internal wave field), suggesting the presence of high-frequency internal waves. These signals may have been the product of wave reflections from the steep flanks of the seamount and/or wave generation from tidal currents flowing over the rough bottom. Associated with the enhanced shears and strains were more frequent occurrences of low 10-m Richardson number events, increased overturning scales, and larger estimated turbulent eddy diffusivity relative to observations 15 km or more from the seamount. In particular, turbulent diffusivity estimates increased from O(0.1×10−4 m2 s−1) in the ocean interior to 1–5×10−4 m2 s−1 within 500 m vertically (1–3 km horizontally) of the seamount flank. A simple geometric scaling argument suggests that boundary mixing of this intensity has relevance to the large-scale circulation at abyssal depths where a large fraction of the ocean waters is in close proximity to the bottom.

C-SALT: An investigation of the thermohaline staircase in the western tropical North Atlantic

Abstract The Caribbean-Sheets and Layers Transect (C-SALT) field program focused on the “staircase” layering found in the main thermicline when temperature and salinity distributions are favorable for the formation of salt fingers. Large- and small-scale surveys, microstructure studies and temporal monitoring of the layers were carried out in spring and autumn of 1985 in an area east of Barbados in the tropical Atlantic. The staircase investigated contained about 10 well-mixed layers, 5–30 m thick, separated by thinner interfaces with sharp changes in temperature and salinity. The depth range of the layers coincided with a minimum density ratio. The area of strong steps was 0.5× 10 6 km 2 in the spring and 0.6 × 10 6 km 2 in the autumn. Individual layers were laterally coherent over scales of 200–400 km and retained their identity for the eight months between surveys. Layers became warmer, saltier and denser to the north and west, with a heat/salt density ratio of 0.85. This ratio is close to that expected for a vertically divergent salt finger flux. This direct indication of water mass conversion provides striking evidence that salt fingers maintain the staircase and make a major contribution to mixing in the thermocline of the region.