OS12B-01 10:20h
Boundary Intensification of Vertical Velocity in a Beta-Plane Basin
The buoyancy-driven circulation of simple two-layer models on the beta-plane is studied in order to determine the role of beta in determining the magnitude and structure of the vertical motions forced in response to surface heating and cooling. Both analytical and numerical approaches are used to describe the change in circulation pattern and strength as a consequence of the planetary vorticity gradient. The physics are quasi-geostrophic at lowest order but sensitive to small non quasi-geostrophic mass fluxes across the boundary of the basin. The height of the interface between the two layers serves as a analogue of temperature and the vertical velocity at the interface consists of a cross-isopycnal velocity, modeled in terms of a relaxation to a prescribed interface height, as well as an adiabatic representation of eddy thickness fluxes represented as lateral diffusion of interface displacement. In the numerical model the lateral eddy diffusion of heat is explicitly represented by a resolved eddy field. In the plausibly more realistic case when the lateral diffusion of buoyancy dominates the diffusion of momentum, the major vertical velocities occur at the boundary of the basin as in earlier f-plane studies. The effect of the planetary vorticity gradient is to intensify the sinking on the western wall and enhance the magnitude of that sinking with respect to the f-plane models. The vertical mass flux in the Sverdrup interior exactly balances the vertical flux in the region of the strong horizontal transport of the western boundary current leaving the net flux to occur in a very narrow region near the western boundary tucked well within the western boundary current. On the other hand, if the lateral diffusion of heat is arbitrarily and unrealistically eliminated the vertical mass flux is forced to occur in the interior. The circulation pattern is extremely sensitive to small net inflows or outflows across the basin perimeter. The cross- basin flux determines the interface height on the basin's eastern boundary and affects the circulation pattern across the entire basin.
OS12B-02 10:35h
Ageostrophic frontal secondary circulation with confluence and mixing
Fronts are important sites for biological productivity and vertical exchanges of water masses in the upper ocean. The vertical velocity due to ageostrophic secondary circulation at fronts is a key factor for these processes. The ageostrophic flow near open ocean fronts has been previously diagnosed in observations using the quasi-geostrophic (QG) and balanced model Omega equations that assume inviscid adiabatic flow conditions. This study investigates two dimensional ageostrophic secondary circulation due to variable mixing and confluence ($\alpha$), using modified QG and SG equations. The two-dimensional modified SG equations used here differ from previous studies in that the confluence advected by along frontal ageostrophic flow is included. With these approaches, scalings for idealized fronts and the Azores front diagnostics are re-examined. The resulting ageostrophic frontal circulations indicate a strong dependency on the Burger number, $NH/fL$, where $H$ and $L$ are the vertical and horizontal scale of front, $N$ is the buoyancy frequency, and $f$ is the Coriolis parameter. The relative importance between deformation rate and vertical mixing can be parameterized by $\alpha H^2/A_v$, where $A_v$ is the vertical viscosity. The re-analyzed SeaSoar sections at Azores front with these equations indicate that vertical mixing intensifies the near surface secondary circulation.
OS12B-03 10:50h
Ocean Bioturbation
Ocean mixing is thought to control the climatically important oceanic overturning circulation. Here we argue the marine biosphere, by a mechanism like the bioturbation occurring in marine sediments, mixes the oceans as effectively as the winds and tides. Over-fishing is thus suggested as a mechanism of global climate change.
OS12B-04 11:05h
The power consumed by mixing processes in the deep-ocean interior
In the traditional view, the meridional overturning circulation of the ocean is assumed to close through diabatic processes in the ocean interior. In this scenario, mixing in the ocean interior acts to transfer heat from the temperate waters of the upper ocean to the coldest waters of the abyss. The upwelling of warmed abyssal waters balances the production of deep water occurring at high latitudes. The mechanical energy needed for this scenario has been estimated at 2 TW. In an alternate view, the meridional overturning cell is closed with only minimal mixing in the ocean interior. This adiabatic closure occurs largely through upwelling in the Southern Ocean. In this view, mixing is required for only the densest waters of the global ocean. A recent estimate suggests that only 0.5 TW of mechanical energy is needed for mixing these densest waters. We present an alternative estimate of the power consumed by mixing. Unlike the previous estimates, our calculation is independent of an assumed production rate of deep water. Instead, we start from estimates of the mixing rate, and assume a steady-state turbulent energy balance. We use global climatological data to estimate the power consumption by mixing in control volumes distinguished by their neutral density. Average diffusivity values, estimated in basin-scale hydrographic inversions, are also used for these calculations. We estimate the power associated with mixing between 30 S and 47 N as 1.4 TW, partitioned as 0.5 TW in ventilated (thermocline) waters, 0.5 TW in deep waters, and 0.4 TW in bottom waters. An additional 1 TW is estimated for power consumed in the Southern Ocean (latitudes south of 30 S), mostly associated with the enhanced mixing rate of Antarctic Bottom Water. Thus, we conclude that 2 TW is likely a lower bound for the energy consumed by mixing in the ocean interior, with roughly 3 TW of power required as the mechanical energy input.
OS12B-05 11:20h
Assessing the Hydraulic Criticality of Deep Ocean Overflows
Two methods for assessing the hydraulic criticality of a modelled or observed deep overflow are discussed. The methods should be of use in determining the position of the control section, which is needed to establish the transport relation helpful for long-term monitoring from upstream. Both approaches are based on a multiple streamtube idealization in which the observed flow at a particular section is divided up into subsections (streamtubes). There are no restrictions on the bottom topography or potential vorticity distribution. The first criteria involves evauation of a generalized Jacobian condition based on the conservation laws for each streamtube; the second involves direct calculation of the long-wave phase speeds. We also comment on the significance of the local Froude number F of the flow and argue that F must pass through unity across a section of hydraulic control. These criteria are applied to some numerically modelled flows and are used in the companion presentation (Girton, et al.) to evaluate the hydraulic criticality of the Faroe Bank Channel.
OS12B-06 11:35h
Is the Faroe Bank Channel a hydraulically-controlled overflow?
The overflow of dense water from the Nordic Seas through the Faroe Bank Channel (FBC) has attributes suggesting hydraulic control, including an asymmetry across the sill reminiscent of flow over a dam. However, because of the influence of the earth's rotation, as well as the presence of continuous gradients in velocity and density, the standard approach of looking for a Froude number ($v/\sqrt{g'd}$) of unity to diagnose criticality is not adequate. Of primary importance is the nature and speed of information-carrying waves---the flow is subcritical if any of these waves can travel upstream, supercritical if no waves can travel upstream, and critical if the fastest waves are arrested by the flow. We present a comparison of several different techniques for assessing the hydraulic criticality of overflows applied to data from a set of velocity and density sections across the FBC. These include: 1) modifications to the (non-rotating) local Froude number to account for shear and stratification in the flow; 2) rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross-section; and 3) direct computation of shallow water wave speeds from the observed overflow structure, using a newly-developed generalized hydraulic condition and multiple-streamtube approach. Two of these three methods give similar answers, suggesting the location of control to be 60-100 km downstream of the sill and not at the sill itself. We discuss the implications of these results for hydraulic predictions of overflow transport and variability, as well as reasons for the failure of the parabolic model.
OS12B-07 11:50h
Transmission of Equatorial Waves Through a Strait
Numerical and analytical solutions are presented for the transmission of equatorial waves through a narrow strait penetrating a meridional barrier. For narrow straits close to the equator, interference between reflected Kelvin waves within the strait is shown to control the transmission of both Kelvin and Rossby waves. Resonant conditions typical of short strait lengths allows for significant energy transmission even through narrow straits.
OS12B-08 12:05h
The Effects of Potential Vorticity on Flows Through a Gap
Mathematical solutions for constant potential vorticity critically controlled flow through passages in the deep ocean are complicated and not available in simple form. Thus two simple formulas for volume flux are developed here by fitting them to constant potential vorticity numerical values. The simplest agrees to better than 6 % and one slightly more complicated formula agrees within 1.4 %. They are up to 24 % less than zero potential vorticity formulas which have been applied to ocean passages. The better formula is used to make new predictions of flux magnitude through nine ocean passages that have current meter measurements. The range of the revisions compared to zero potential vorticity predictions is a few percent. For further improvement between prediction and observation, other factors such as realistic bottom topography, friction, mixing, waves and eddies must be included.