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Raymond W. Schmitt

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, showing that a constant Rρ is the basic state of a fluid in which some salt fingering is taking place. The model suggests that the large scale T-S relation may be controlled as much by the details of the microscale diffusive processes as by the large-scale atmospheric forcing.

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Raymond W. Schmitt

Abstract

Over a century before Melvin Stern discovered salt fingers, W. Stanley Jevons performed the first salt finger experiment in an attempt to model cirrus clouds. Remarkably, he seemed to realize that a more rapid diffusion of heat relative to solute played a role in the experiments. However, he oversimplified the physics and incorrectly assumed that the “interfiltration of minute, thread-like streams” was a general result of superposing heavy fluid over light fluid. Interestingly, Lord Rayleigh became aware of these experiments more than two decades later. Here newly discovered evidence is presented that Rayleigh repeated the Jevons experiments at the Cavendish laboratory in Cambridge in April 1880. The results led him to initiate the study of buoyancy effects in fluids by formulating several stability problems for a stratified, but nondiffusive, fluid. He thus discovered the expression for the buoyancy frequency of internal waves and the convective phenomenon now known as the Rayleigh–Taylor instability. His neglect of diffusion meant that he missed an opportunity to discover double-diffusive convection; though given the limited knowledge of fluid physics at the time, this is understandable. The historic record shows a tortuous intellectual path in which observations of clouds led to an inappropriate experimental demonstration of salt fingers that inappropriately motivated the theoretical discovery of the frequency of internal waves, which was ignored until well into the next century.

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Raymond W. Schmitt

Abstract

An equation for the conservation of density ratio on isopycnal surfaces is derived. It is shown that a vertical shear can modify the density ratio if salinity (and temperature) gradients exist along isopycnals. The mixing processes of turbulence, salt fingering and lateral isopycnal mixing are evaluated for possible balances with the shear term in the “R p, = constant” Central Waters. It is found that turbulence cannot provide a balance and that isopycnal mixing can balance only with a particular vertical variation in the mixing rate. Salt firm however, provide a simple balance between the shear term and the vertical variation of flux divergence, since beat and salt are transported at different rates. The “conservation of density ratio” should provide a useful constraint on mixing estimates from hydrographic data in both double-diffusive and non-double-diffusive regimes and could be applicable to modelling studies of the temperature-salinity relation in the thermocline.

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Raymond W. Schmitt

Abstract

Solutions are given for a rich variety of salt finger planforms, including triangular and asymmetric modes. Preliminary laboratory experiments failed to realize the triangular mode. However, Osborn has recently discovered asymmetric “salt fountains” in near-surface microstructure data. The fastest growing asymmetric finger solution given here has scales consistent with Osborn's observations, though a selection mechanism for the asymmetry remains to be identified.

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Jubao Zhang and Raymond W. Schmitt

Abstract

The impact of salt fingers on the thermohaline circulation in a single hemisphere basin under mixed boundary conditions is investigated through scaling analysis and numerical experiments. By assuming that the internal density ratio is determined by the surface horizontal density ratio, the effect of a double-diffusive parameterization on the vertical diffusivity of density (K ρ) is estimated. Salt fingers reduce the magnitude of K ρ, with the extent of the reduction dependent on the magnitude of the density ratio. The reduced diapycnal mixing leads to a diminished thermohaline circulation and modifies the stability criteria for the thermal/haline mode transition under mixed boundary conditions. Quasi-equilibrium numerical experiments with the GFDL Modular Ocean Model produce results consistent with the scaling analysis in the reduction of the magnitude of the thermohaline circulation and the change in the critical freshwater forcing required for the existence of the stable thermal mode. Sensitivity experiments are also conducted on the variables in the salt finger parameterization and found to be consistent with the scaling analysis. These results indicate that salt fingers make the thermohaline circulation more susceptible to transition to the haline mode (haline catastrophe), so should not be ignored in long-term climate prediction models.

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Julian J. Schanze and Raymond W. Schmitt

Abstract

Owing to the larger thermal expansion coefficient at higher temperatures, more buoyancy is put into the ocean by heating than is removed by cooling at low temperatures. The authors show that, even with globally balanced thermal and haline surface forcing at the ocean surface, there is a negative density flux and hence a positive buoyancy flux. As shown by McDougall and Garrett, this must be compensated by interior densification on mixing due to the nonlinearity of the equation of state (cabbeling). Three issues that arise from this are addressed: the estimation of the annual input of density forcing, the effects of the seasonal cycle, and the total cabbeling potential of the ocean upon complete mixing. The annual expansion through surface density forcing in a steady-state ocean driven by balanced evaporation–precipitation–runoff (EPR) and net radiative budget at the surface Q net is estimated as 74 000 m3 s−1 (0.07 Sv; 1 Sv ≡ 106 m3 s−1), which would be equivalent to a sea level rise of 6.3 mm yr−1. This is equivalent to approximately 3 times the estimated rate of sea level rise or 450% of the average Mississippi River discharge. When seasonal variations are included, this density forcing increases by 35% relative to the time-mean case to 101 000 m3 s−1 (0.1 Sv). Likely bounds are established on these numbers by using different Q net and EPR datasets and the estimates are found to be robust to a factor of ~2. These values compare well with the cabbeling-induced contraction inferred from independent thermal dissipation rate estimates. The potential sea level decrease upon complete vertical mixing of the ocean is estimated as 230 mm. When horizontal mixing is included, the sea level drop is estimated as 300 mm.

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Daniel T. Georgi and Raymond W. Schmitt

Abstract

Continuous conductivity-temperature-depth-dissolved-oxygen (CTD) data are used to investigate the spatial distribution of fine and microstructure between the Azores and Flemish Cap. The CTD data are used to calculate a conductivity-microstructure Cox number. This indicator summarizes microstructure variance from the 0.08–2 m vertical wavelength range. The CTD data are also used to calculate the finestructure-temperature Cox number. Finally, the fine and microstructure data are combined to calculate lateral flux and flux divergence for the waters east of the Atlantic Current.

The distribution of the conductivity Cox number indicates that vertical mixing is more intense above the base of the main thermocline (5°C isotherm) than below it, and that mixing is more prominent near the North Atlantic Current than farther east. Stations near the front indicate elevated conductivity Cox numbers associated with intrusive features. The hydrographic sections and the finestructure data reveal the presence of intrustive features along the entire section, particularly at the depth of the mid-thermocline oxygen mininum. The finestructure variance exceeds the variance expected from internal-wave straining by a factor of 2–8.

Lateral fluxes and eddy diffusivities are calculated from the finestructure data with the model proposed by Joyce (1977). The calculated lateral fluxes and eddy diffusivities, O(103 m2 s−1), indicate that considerable mixing is occurring at the depth of the oxygen minimum. As the microstructure data indicate that vertical mixing weakens to the east and the finestructure intensities decrease to the east, we conclude that there is a net flux divergence at the level of the oxygen minimum. This flux divergence is consistent with the water-mass modifications required to convert. Gulf Stream water into the water found in and to the east of the North Atlantic Current.

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Samuel J. Levang and Raymond W. Schmitt

Abstract

The global water cycle is predicted to intensify under various greenhouse gas emissions scenarios. Here the nature and strength of the expected changes for the ocean in the coming century are assessed by examining the output of several CMIP5 model runs for the periods 1990–2000 and 2090–2100 and comparing them to a dataset built from modern observations. Key elements of the water cycle, such as the atmospheric vapor transport, the evaporation minus precipitation over the ocean, and the surface salinity, show significant changes over the coming century. The intensification of the water cycle leads to increased salinity contrasts in the ocean, both within and between basins. Regional projections for several areas important to large-scale ocean circulation are presented, including the export of atmospheric moisture across the tropical Americas from Atlantic to Pacific Ocean, the freshwater gain of high-latitude deep water formation sites, and the basin averaged evaporation minus precipitation with implications for interbasin mass transports.

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Samuel J. Levang and Raymond W. Schmitt

ABSTRACT

Regional connectivity is important to the global climate salinity response, particularly because salinity anomalies do not have a damping feedback with atmospheric freshwater fluxes and may therefore be advected over long distances by ocean circulation, resulting in nonlocal influences. Climate model intercomparison experiments such as CMIP5 exhibit large uncertainty in some aspects of the salinity response, hypothesized here to be a result of ocean dynamics. We use two types of Lagrangian particle tracking experiments to investigate pathways of exchange for salinity anomalies. The first uses forward trajectories to estimate average transport time scales between water cycle regimes. The second uses reverse trajectories and a freshwater accumulation method to quantitatively identify remote influences in the salinity response. Additionally, we compare velocity fields with both resolved and parameterized eddies to understand the impact of eddy stirring on intergyre exchange. These experiments show that surface anomalies are readily exchanged within the ocean gyres by the mean circulation, but intergyre exchange is slower and largely eddy driven. These dynamics are used to analyze the North Atlantic salinity response to climate warming and water cycle intensification, where the system is broadly forced with fresh surface anomalies in the subpolar gyre and salty surface anomalies in the subtropical gyres. Under these competing forcings, strong intergyre eddy fluxes carry anomalously salty subtropical water into the subpolar gyre which balances out much of the local freshwater input.

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Louis St. Laurent and Raymond W. Schmitt

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 (R ρ) and Richardson number (Ri), the signal of salt fingers was discerned even under conditions where turbulent mixing also occurred. While the model for turbulence describes most dissipation occurring when Ri < 1, dissipation at larger Ri is better described by the salt-finger model. Based on the results of the parameter space analysis, a method is proposed for estimating the salt-finger enhancement of the diapycnal haline diffusivity (k s) over the thermal diffusivity (k θ). During April 1992 at the NATRE site, it was found that k θ = (0.08 ± 0.01) cm2 s−1 and k s = (0.13 ± 0.01) cm2 s−1 for the neutral density surface local to the tracer release isopycnal (σ θ ∼ 26.75 kg m−3, z ∼ 300 m). The flux divergence of buoyancy was also computed, giving the diapycnal advection w∗ = −(1.7 ± 1.2) m yr−1. Moreover, divergence of vertical buoyancy flux was dominated by the haline component. For comparison, the tracer release method gave a diffusivity of k s = (0.12 ± 0.02) cm2 s−1 (May–November 1992) and a diapycnal velocity of w∗ = −(3 ± 1) m yr−1 (May 1992–November 1994) at this site. The above numbers are contrasted to diffusivity estimates derived from turbulence theory alone. Best agreement between tracer-inferred mixing rates and microstructure based estimates is achieved when the salt-finger enhancement of k s is taken into account.

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