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OSNAP is an international programme designed to provide a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic. The OSNAP observing system consists of two legs spanning the subpolar North Atlantic from southern Labrador to Greenland to Scotland. 

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Our involvement in OSNAP is in understanding the fluid dynamics relating variations in circulation and forcing to the subpolar North Atlantic to the overturning circulation downstream. This work involves the use of an adjoint model to explore sensitivity of the overturning to upstream forcing and circulation anomalies. This is part of the UK-OSNAP consortium funded by NERC

 

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The ocean is full of turbulent mesoscale eddies, the ocean’s analogue of the weather systems but on a much smaller spatial scale. Together with James Maddison and various collaborators, we have been developing a new GEOMETRIC approach to representing the unresolved turbulent eddies in ocean climate models.

This approach gives us three highly desirable properties:

  • It reproduces the correct dimensional growth rate for the Eady model of baroclinic instability, for which there is an exact mathematical solution.
  • It does a good job of modelling the eddy diffusivity across four orders of magnitude for the nonlinear, fully turbulent Eady problem.
    Comparison of diagnosed and predicted eddy diffusivities from fully nonlinear numerical calculations (Bachman et al., 2017).
  • Eddy saturation, the surprising insensitivity of the Antarctic Circumpolar Current to surface wind stress, is captured as an emergent property.

We are now working towards implementing our new parameterisation in the NEMO ocean model.