Isentropic averaging

The equations of motion, thermodynamics and scalar concentration are averaged separately over infra-grid scales (comprising roughly ocean microstructure scales) and sub-grid scales (encompassing 10–100 km scales), the latter average being carried out on constant potential temperature surfaces. (Since variation of salinity has been neglected, potential temperature is synonymous with entropy.) These methods of averaging are used to lend precision to statements about oceanic turbulent diffusion: that infra-grid scales are primarily responsible for dientropic diffusion; and that sub-grid scales are responsible for along-isentropic diffusion of passive scalars. Equivalent sets of averaged equations expressed either in isentropic coordinates or quasi-Cartesian coordinates can be obtained. Diffusion tensors for potential temperature express only infra-grid effects. For other passive tracers, diffusion caused by sub-grid scales of motion is also felt, whose effects are shown by scale analysis to be oriented principally along infra-grid averaged isentropes

Wind-driven mid-ocean baroclinic gyres over topography: A circulation equation extending the Sverdrup relation

What is the circulation driven by wind stress in a stratified ocean above topography? This question was answered by Sverdrup (1947) for vertically integrated transport over flat topography only. By applying the ideas and methods of Rhines and Young (1982a,b), a modified form of the Sverdrup transport relation can be derived for the case of stratification and topography in certain circumstances. This circulation equation is, in quasigeostrophic form,J(Ψ, βy + χf0h′T/H) = − χf0−1gAH J(βy, h′T) + z · ∇xτ/&rho0,where most symbols have their usual meanings, while χ is a parameter no larger than 1 that depends on stratification, bottom friction and horizontal diffusivity. The effect of topography is attenuated (χ is reduced) by strong stratification, strong bottom friction, or weak horizontal diffusivity. The circulation equation applies strictly to uniform bottom slope or other topographies obeying ∇2h′T = 0, though it approximately holds for ∇2h′T ≃ 0, a criterion for which is that the scale of bottom topography greatly exceeds the baroclinic Rossby radius of deformation. It holds only above deep closed circulations. It is remarkable for the form of the characteristic lines for transport, βy + χf0h′T/H = constant, and the extra forcing term on the right, which depends on topography.Examples are given of two-layer flows driven by wind-stress curl over east-west and north-south sloping topography. The determination of the boundary of the deep gyre is an implicit nonlinear problem. The solution for the case of east-west slope illustrates the general method for solving such a problem

Some effects of bottom topography on baroclinic stability

The effects of Fourier components of cross-stream topographic slope are included in the classical linear theory of quasigeostrophic baroclinic instability. The effect of uniformly sloping topography is reviewed, emphasis being placed on the possibility of destabilization through interaction between long topographic baroclinic Rossby waves and short thermal baroclinic Rossby waves with the same downstream wave number and phase speed...

Meridional heat transport across the Antarctic Circumpolar Current by the Antarctic Bottom Water overturning cell

The heat transported by the lower limb of the Southern Ocean meridional overturning circulation is commonly held to be negligible in comparison with that transported by eddies higher in the water column. We use output from one of the first global high resolution models to have a reasonably realistic export of Antarctic Bottom Water, the OCCAM one twelfth degree model. The heat fluxed southward by the deep overturning cell using the annual mean field for 1994 at 56S is 0.033 PW, but the 5-day mean fields give a larger heat flux (0.048 and 0.061 PW depending on calculation method). This is more than 30% of previous estimates of the total heat flux. Eddies and other transients add considerably to the heat flux. These results imply that this component of meridional heat flux may not be negligible as has been supposed

Asymptotic regimes in mixed-layer deepening

The model equation for the mixed layer proposed by Niiler (1975) combines the Kraus-Turner turbulent erosion prescription with the Pollard-Rhines-Thompson treatment of induced shear-flow deepening and limiting by Coriolis forces. We show here that both are special cases which emerge asymptotically from the model equation. Numerical solutions show the dynamics to pass through four distinct regimes, in the case of wind-mixing of an initially resting fluid

The all-Atlantic temperature-salinity-pressure relation and patched potential density

The relation between temperature, salinity, and pressure in the Atlantic Ocean is examined. Most of the Atlantic resolves itself into three two-dimensional manifolds of three-dimensional thermodynamic space: a northern, more saline, branch, and a southern, fresher, branch, each quite independent of pressure, and between them a bridge, on which density is uniform at constant pressure. The properties of the branches are crucial to the construction of joint potential density surfaces, patched together at 1000 db intervals. By resolving more finely in pressure (illustrated with 200 db spacing), a finer system of patched potential density surfaces can be obtained, and indeed the continuous limit can be taken. This limit gives a form of orthobaric density, regionally differentiated because it is based on the duplicate regional branches. A mapping can be devised, using the properties of the bridge waters, that links the southern and northern forms of orthobaric density across the boundary between their respective regions of validity. The parallel of patched potential density surfaces to orthobaric density surfaces permits the use of measures developed for the latter to estimate quantitative measures of the material nature (or otherwise) of the former. Simply put, within the waters of the respective branches the patched isopycnals, or orthobaric isopycnals, are very nearly material, limited only by inherent irreversible mixing processes. However, where these isopycnals cross the bridge waters, significant, reversible, material exchange across them may occur.A difficulty may be encountered with coarsely resolved, regionally differentiated, patched potential density. This is that there exist ranges of density which cannot be consistently linked across the regional boundary. A solution for the difficulty, suggested by the continuousform (regionally differentiated orthobaric density), is proposed

Swimming pools in remote indigenous communities : some basic information for planning a pool

"The four reports included here provide the basic information any community considering building a pool should have at its fingertips. In the first report, epidemiologist and medical practitioner, Dr Carmen Audera reviews the potential health benefits and risks of providing swimming pools in remote communities. A CRC-funded summer student project allowed Andrew Peart and Cassandra Szoeke to systematically gather information from Indigenous communities with pools about the benefits, risks, logistics and costs of installing and maintaining a swimming pool. They also gathered information from communities without pools about where people swim, how this is managed and whether there are associated risks. Centre for Appropriate Technology staff member and engineer, Jonathan Duddles, compiled the necessary information about construction and maintenance options and finally another CRC-funded summer student, Nigel Vivian, worked with CAT engineer, Bob Lloyd, to examine the feasibility of monitoring pool water for chemical and microbiological hazards." (Foreword