Diapycnal Fluxes Research Articles

Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment

THE distributions of heat, salt and trace substances in the ocean thermocline depend on mixing along and across surfaces of equal density (isopycnal and diapycnal mixing, respectively). Measurements of the invasion of anthropogenic tracers, such as bomb tritium and 3He (see, for example, refs 1 and 2), have indicated that isopycnal processes dominate diapycnal mixing, and turbulence measurements have suggested that diapycnal mixing is small3,4, but it has not been possible to measure accurately the diapycnal diffusivity. Here we report such a measurement, obtained from the vertical dispersal of a patch of the inert compound SF6 released in the open ocean. The diapycnal diffusivity, averaged over hundreds of kilometres and five months, was 0.11 ± 0.02 cm2 s−1, confirming previous estimates1–4. Such a low diffusivity can support only a rather small diapycnal flux of nitrate into the euphotic zone; it justifies the neglect of diapycnal mixing in dynamic models of the thermocline25–27, and implies that heat, salt and tracers must penetrate the thermocline mostly by transport along, rather than across, density surfaces.

Ring Genesis and the Related Heat Transport. Part II: A Model Comparison

Abstract Various ocean circulation models have been compared with respect to their performance in the genesis of rings and the subsequent heat transport. Emphasis has been placed on the role of the spurious diapycnal fluxes of heat and momentum in Cartesian models, arising when the horizontal dissipation mixes through sloping isopycnals. Quasigeostrophic, isopycnal coordinate, and Cartesian primitive equation models in a two-layer periodic channel domain have been used to simulate the process of eddy detachment from an eastward-flowing jet. This jet is modeled after the Gulf Stream east of Cape Hatteras. On this jet a small sinusoidal disturbance is super-imposed, which, through the release of available potential energy, grows until it ultimately has developed into ringlike eddies. Simulations with the Cartesian primitive equation model appear to suffer from spurious diapycnal mixing of both heat and momentum. This retards the process of Rossby wave breaking and prolongs the growth of the meander, thus ca...

Simple Models of Stratification

Abstract Simple analytical and numerical models of gratified circulation are discussed. The models are driven by buoyancy and mechanical forcing, and buoyancy diffusion is parameterized in term of diapycnal mass fluxes. The interior responds in typical Sverdrup fashion, and the requirements of balanced heat and energy budgets are invoked to calculate the eastern boundary stratification parameters. This effort differs from past attempts to calculate stratification in that buoyancy flux boundary conditions or explicitly considered in the calculation of diapycnal fluxes. A major feature of the solutions is the interplay of planetary dynamics and buoyancy diffusion. This results in the most important components of the thermocline structure, namely density outcrops and interior “pinchoffs.” These are, in turn, essential to the production of thermodynamically balanced states. Some aspects of the present model resemble features found in previous numerical models.

Diapycnal mixing in the thermocline: A review

Diapycnal fluxes of momentum and heat produced by three‐dimensional turbulence play important, but poorly understood, roles in the dynamics of the main thermocline and of the equatorial undercurrent. Diverse approaches—involving inferences, measurements, and process studies—are being pursued to define these fluxes and their significance. These are reviewed to improve the coordination of future mixing studies.

Oceanic intrusions: some limitations of the Ruddick and Turner (1979) mechanism

The Ruddick and Turner (1979, Deep-Sea Research, 26, 903–913) mechanism for double-diffusive interleaving at lateral thermoclinic fronts is examined on the S-0 diagram, revealing that for the oceanographically relevant pair of properties (heat-salt rather than salt-sugar), an alternating series of diffusive and finger interfaces does not form, but rather each interface is a fingering interface. Since active oceanic intrusions do exhibit regions of diffusive stratification between the salt fingering regions, it is concluded that they must have their origins in ‘smooth’ thermoclinic fronts, that is, fronts that are significantly wider than the height of an intrusion. This has direct implications for the way in which the diapycnal fluxes associated with double-diffusive interleaving are parameterized in ocean models.

Double-Diffusive Interleaving. Part II: Finite Amplitude, Steady State Interleaving

Abstract The exponentially growing solutions of the linear stability analysis of double-diffusive interleaving are allowed to grow to finite amplitude. At this stage the double-diffusive fluxes across the intrusion boundaries must be parameterized differently to those in the growing solution because every second finger interface becomes a diffusive interface in the steady state solution. Using the wavenumbers that are set by the initial, growing intrusions from the linear stability problem, together with the changed parameterization of the double-diffusive fluxes, it is shown that a steady state is reached. The ratio of the gradients of potential temperature and salinity along any particular intrusion is the same as in the linear stability analysis. When expressed in terms of density changes, this ratio is close to 0.9 rather than the commonly assumed value of 0.5 (being the buoyancy-flux ratio of a single salt finger interface). After solving for the velocity field, the isopycnal and diapycnal fluxes of ...

On the Parameterization of Diapycnal Fluxes due to Double-Diffusive Intrusions

Abstract An attempt is made to Parameterized the large-Scale average diapycnal (cross-isopycnal) mixing that presumably occurs in the thermohaline fronts that develop when large-scale epipycnal (along-isopycnal) gradients of T and S are stirred along isopycnals by mesoscale eddies. It is assumed that double-diffusive intrusions develop at the fronts and that their thickness is given by the formula of Ruddick and Turner (1979). This, combined with a crude estimate of the frontal width and a very over-simplified model of the eddy field, leads to a formula for the average diapycnal diffusivity for salt or some neutral tracer, and suggests that the mechanism is important in weakly stratified water with a large epipycnal gradient of salinity. The diapycnal eddy diffusivities for temperature is negative for a stably stratified temperature field. However, the opposite signs of the diapycnal diffusivities for salt and heat are unlikely to lead to observable consequences on account of the dominance, in duxes acros...