A new approach to parameterizing ocean eddies: energetics, conservation and flow stability

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

The ocean is populated by a vigorous eddy field. These eddies are the oceanic analogue of weather systems in the atmosphere, but occur on a much smaller spatial scale due to the different densities of seawater and air, and differences in the vertical structure of the ocean and atmosphere. Ocean eddies also evolve on longer time scales, typically months, and can effect the global circulation on time scales up to millennia. Hence modelling the global circulation of the ocean is a much more challenging task than the atmosphere due to the greater range of spatial and temporal scales that must be captured. One consequence is that ocean climate models usually 'paramaterise' ocean eddies, that is, rather than simulate the eddies directly, the indirect effect of the eddies on the larger-scale circulation is represented by modifying the equations that are solved by the ocean model. This is a challenging task and eddy parameterisations are a source of great uncertainty in ocean climate projections. The aim of this project is to developed improved knowledge of how ocean eddies influence the large-scale ocean circulation and to developed improved eddy parameterisations. In particular, we aim to develop schemes that avoid doing things that we know are simply incorrect, such as introducing spurious sources of energy. By ensuring that our eddy parameterisations are consistent with fundamental physical principles, such as conservation of energy and angular momentum (an example of which loosely equates to the amount of water circulating around Antarctica), we believe that we can provide significant constraints on what ocean eddies can and cannot do. The new eddy parameterisations will be tested on a state-of-the-art computational ocean model.

Publications

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Klocker A (2014) Advection of baroclinic eddies by depth mean flow in Geophysical Research Letters

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Maddison J (2012) A Framework for Parameterizing Eddy Potential Vorticity Fluxes in Journal of Physical Oceanography

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Maddison J (2014) Rapid development and adjoining of transient finite element models in Computer Methods in Applied Mechanics and Engineering

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Maddison J (2013) The Eliassen-Palm flux tensor in Journal of Fluid Mechanics

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Marshall D (2013) Rossby rip currents in Geophysical Research Letters

 
Description A new framework has been developed for investigating and modelling ocean eddies in climate models. Ocean eddies are the analogue of atmospheric weather systems but occur on smaller spatial scales than are captured by the current generation of climate models, yet they have a huge impact on both the mean state of the ocean and its adjustment to climate forcing. It is therefore necessary to develop models of how these ocean eddies impact on the global circulation and structure of the ocean. Our new framework ensures that such models do not violate fundamental physical laws such as conservation of momentum and energy. It also provide a novel way of describing how eddies impact on the large-scale circulation of the oceans.
Exploitation Route Ocean turbulence is a complex and technical topic with limited immediate non-academic impacts. These impacts are likely to follow downstream from the improved climate models that will result from our work. However, we have been showing movies from our high-resolution turbulence simulations to Physics undergraduates at Oxford during lectures on fluid dynamics and this module has led to increased numbers of applicants for Oceanography and Climate PhDs over the past couple of years. The new framework has already attracted the interest of model developers at the Los Alamos National Laboratory. We are collaborating with the Met Office on the representation and impact of eddies in the Southern Ocean in their next-generation climate models and the present work will help develop improved parameterisations in the future.
Sectors Environment