The viscous lee wave problem and its implications for ocean modelling

Ocean circulation models employ horizontal viscosity and diffusivity to represent unresolved sub-gridscale processes such as breaking internal waves. Computational power has now advanced sufficiently to permit regional ocean circulation models to be run at sufficiently high (100m-1km) horizontal resolution to resolve a significant part of the internal wave spectrum. Here we develop theory for boundary generated internal waves in such models, and in particular, where the waves dissipate their energy. We focus specifically on the steady lee wave problem where stationary waves are generated by a large-scale flow acting across ocean bottom topography. We generalise the energy flux expressions of Bell (1975) to include the effect of horizontal viscosity and diffusivity. Applying these results for realistic parameter choices we show that in the present generation of models with O(1)m2 s −1 horizontal viscosity/diffusivity boundary-generated waves will inevitably dissipate the majority of their energy within a few hundred metres of the boundary. This dissipation is essentially spurious since it is a direct consequence of the artificially high viscosity/diffusivity used in the numerical models and hence caution is necessary in comparing model results to ocean observations. Our theory further predicts that O(0.01)m2 s −1 horizontal viscosity/diffusivity is required to satisfactorily reduce the spurious dissipation and enable a realistic representation of wave dynamics in ocean modelsThe authors acknowledge funding from the ARC Centre of Excellence for Climate System Science grant number CE1101028

Evaluation of a wind tunnel designed to investigate the response of evaporation to changes in the incoming long-wave radiation at a water surface

To investigate the sensitivity of evaporation to changing long-wave radiation we developed a new experimental facility that locates a shallow water bath at the base of an insulated wind tunnel with evaporation measured using an accurate digital balance. The new facility has the unique ability to impose variations in the incoming long-wave radiation at the water surface whilst holding the air temperature, humidity and wind speed in the wind tunnel at fixed values. The underlying scientific aim is to isolate the effect of a change in the incoming long-wave radiation on both evaporation and surface temperature. In this paper, we describe the configuration and operation of the system and outline the experimental design and approach. We then evaluate the radiative and thermodynamic properties of the new system and show that the shallow water bath naturally adopts a steady-state temperature that closely approximates the thermodynamic wet-bulb temperature. We demonstrate that the long-wave radiation and evaporation are measured with sufficient precision to support the scientific aims.</p

Observed eddy-internal wave interactions in the Southern Ocean

The physical mechanisms that remove energy from the Southern Ocean’s vigorous mesoscale eddy field are not well understood. One proposed mechanism is direct energy transfer to the internal wave field in the ocean interior, via eddy-induced straining and shearing of preexisting internal waves. The magnitude, vertical structure, and temporal variability of the rate of energy transfer between eddies and internal waves is quantified from a 14-month deployment of a mooring cluster in the Scotia Sea. Velocity and buoyancy observations are decomposed into wave and eddy components, and the energy transfer is estimated using the Reynolds-averaged energy equation. We find that eddies gain energy from the internal wave field at a rate of −2.2 ± 0.6 mW m−2, integrated from the bottom to 566 m below the surface. This result can be decomposed into a positive (eddy to wave) component, equal to 0.2 ± 0.1 mW m−2, driven by horizontal straining of internal waves, and a negative (wave to eddy) component, equal to −2.5 ± 0.6 mW m−2, driven by vertical shearing of the wave spectrum. Temporal variability of the transfer rate is much greater than the mean value. Close to topography, large energy transfers are associated with low-frequency buoyancy fluxes, the underpinning physics of which do not conform to linear wave dynamics and are thereby in need of further research. Our work suggests that eddy–internal wave interactions may play a significant role in the energy balance of the Southern Ocean mesoscale eddy and internal wave fields

Potential of pre-contrast T1 mapping as a marker of interstitial fibrosis in severe aortic stenosis

International audiencen.

Non-invasive assessment of interstitial myocardial fibrosis in pressure-overload left ventricular hypertrophy

International audiencen.

Closing the loops on Southern Ocean dynamics: From the circumpolar current to ice shelves and from bottom mixing to surface waves

A holistic review is given of the Southern Ocean dynamic system, in the context of the crucial role it plays in the global climate and the profound changes it is experiencing. The review focuses on connections between different components of the Southern Ocean dynamic system, drawing together contemporary perspectives from different research communities, with the objective of closing loops in our understanding of the complex network of feedbacks in the overall system. The review is targeted at researchers in Southern Ocean physical science with the ambition of broadening their knowledge beyond their specific field, and aims at facilitating better-informed interdisciplinary collaborations. For the purposes of this review, the Southern Ocean dynamic system is divided into four main components: large-scale circulation; cryosphere; turbulence; and gravity waves. Overviews are given of the key dynamical phenomena for each component, before describing the linkages between the components. The reviews are complemented by an overview of observed Southern Ocean trends and future climate projections. Priority research areas are identified to close remaining loops in our understanding of the Southern Ocean system

A New Mechanism for Mode Water Formation Involving Cabbeling and Frontogenetic Strain at Thermohaline Fronts. Part II: Numerical Simulations

Submesoscale-resolving numerical simulations are used to investigate a mechanism for sustained mode water formation via cabbeling at thermohaline fronts subject to a confluent strain flow. The simulations serve to further elucidate the mechanism and refine the predictions of the analytical model of Thomas and Shakespeare. Unlike other proposed mechanisms involving air-sea fluxes, the cabbeling mechanism, in addition to driving significant mode water formation, uniquely determines the thermohaline properties of the mode water given knowledge of the source water masses on either side of the front. The process of mode water formation in the simulations is as follows: Confluent flow associated with idealized mesoscale eddies forces water horizontally toward the front. The frontogenetic circulation draws this water near adiabatically from the full depth of the thermohaline front up to the surface 25 m, where resolved submesoscale instabilities drive intense mixing across the thermohaline front, creating the mode water. The mode water is denser than the surrounding stratified fluid and sinks to fill its neutral buoyancy layer at depth. This layer gradually expands up to the surface, and eddies composed entirely of this mode water detach from the front and accumulate in the diffluent regions of the domain. The process continues until the source water masses are exhausted. The temperature-salinity (T-S) relation of the resulting mode water is biased to the properties of the source water that has the larger isopycnal T-S anomaly. This mechanism has the potential to drive O(1) Sv (1 Sv [10(6) m(3) s(-1)) mode water formation and may be important in determining the properties of mode water in the global oceans

What Controls Near‐Surface Relative Humidity Over the Ocean?

Abstract Observations and models show that near‐surface relative humidity is nearly constant at ∼80% over the ocean in the current climate, and almost invariant in the global mean in projected future climates. Here, this behavior is investigated through the development of a simple theoretical model for near‐surface relative humidity by considering the moisture balance above a uniform ocean surface. The relative humidity is predicted to depend on only the near‐surface wind speed, air‐surface temperature difference, surface wetness and large‐scale moisture convergence. Although developed in the context of moist over‐ocean convection, the theory is able to determine the relative humidity in a suite of idealized simulations over both wet and dry surfaces with a root‐mean‐square error of less than 3%. The theory also predicts the climatology of relative humidity over the ocean with a root‐mean‐square error of less than 3%. The theory thus provides a theoretical basis for investigating changes in relative humidity over the ocean, water vapor feedbacks and the water cycle in current and future climates

A New Mechanism for Mode Water Formation involving Cabbeling and Frontogenetic Strain at Thermohaline Fronts

A simple analytical model is used to elucidate a potential mechanism for steady-state mode water formation at a thermohaline front that involves frontogenesis, submesoscale lateral mixing, and cabbeling. This mechanism is motivated in part by recent observations of an extremely sharp, density-compensated front at the North Wall of the Gulf Stream. Here, the intergyre, along-isopycnal, salinity-temperature difference is compressed into a span of a few kilometers, making the flow susceptible to cabbeling. The sharpness of the front is caused by frontogenetic strain, which is presumably balanced by submesoscale lateral mixing processes. The balance is studied with the simple model, and a scaling is derived for the amount of water mass transformation resulting from the ensuing cabbeling. The transformation scales with the strain rate, equilibrated width of the front, and the square of the isopycnal temperature contrast across the front. At the major ocean fronts where mode waters are found, this isopycnal temperature contrast decreases with increasing density near the isopycnal layers where mode waters reside. This implies that cabbeling should result in a convergent diapycnal mass flux into mode water density classes. The scaling for the transformation suggests that at these fronts the process could generate 0.01-1 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) of mode water. These formation rates, while smaller than mode water formation by air-sea fluxes, should be independent of season and thus could fill select isopycnal layers continuously and play an important role in the dynamics of mode waters on interannual time scales

On stratified flow over a topographic ridge in a rotating annulus

Interactions between the rotating stratified oceanic and atmospheric flows and topography play a fundamental role in Earth's climate. Here we use laboratory experiments in a differentially-heated rotating annulus to explore stratified flow-topography interactions in a dynamical regime of strong background geostrophic turbulence. A localised small-scale topographic ridge is differentially-rotated at a range of angular velocities around the base of the annulus to impose a relative velocity between the stratified fluid and the small ridge. Considering the idealised setup of the laboratory configuration, the experiments exhibit rich dynamics that include, but are not limited to, lee waves, internal bores, baroclinic instabilities, boundary currents, large-scale gyres, blocking, and geostrophic eddies. Despite the complicated nature of the circulations, several bulk properties and features of the system are able to be characterised globally by relatively simple parameters. We find that the most useful parameter for describing the flows is the internal Froude number (Frn), which is the ratio between the imposed ridge velocity and the internal wave phase speed in the stratified fluid. Standing features that are predominantly barotropic and stationary relative to the ridge are only able to occur when the imposed ridge velocity is less than the internal wave phase speed (|Frn|1). This implies that the flow-topography interactions leading to stationary dynamics are primarily internal stratified processes, which steer the flow and shape the background geostrophic turbulence such that it can support standing barotropic features.</p