The Bjerknes Centre is a collaboration on climate research, between the University of Bergen, NORCE, the Institute of Marine Research, Nansen Environmental and Remote Sensing Centre.

Seminar: Scale Dependent Horizontal Eddy Diffusivity

Aleksi Nummelin from NORCE Norwegian Research Centre AS will give a seminar talk on March 25

Aleksi Nummelin
Aleksi Nummelin


Short biography:

My main research interest are high latitude climate and ocean dynamics. In terms of the high latitude climate, I am trying to understand why there is such a large model spread in future projections of high latitude climate. In terms of ocean dynamics I work with the MicroInverse method, estimating and parameterizing the ocean lateral sub-gridscale diffusivity.




Oceanic tracer distributions are modified by ocean motions of all scales, but only the large scale motions are resolved by the present day Earth System Models. In these models the unresolved sub-gridscale tracer transport is generally parameterized through diffusive closures with a scale-independent constant diffusion coefficient. However, evidence from observations and basic theory suggest that diffusivity should be scale dependent. We use a recently developed MicroInverse method to diagnose diffusivities from sea surface temperature and sea level observations, as well as from idealized model simulations. We compare these results to existing parameterizations of stirring by shear and strain in the large scale currents, as well as mixing by sub-gridscale velocity fluctuations. We find that the spatial pattern of diffusivity is linked to the kinetic energy of the flow, and that the large scale stirring explains most of the diagnosed diffusivity below 100 km scale, while the small scale mixing only becomes important at larger scales. We also find that between 25-100 km resolution the diffusivity scales as the length scale (grid size) to the power of 4/3 in the tropics and close to the boundary currents, while the scaling is closer to 2/3 in the gyre interiors. Our results clearly demonstrate that the tracer diffusion coefficient in ocean models should vary spatially, and depend on the model grid-size. We also emphasize the need to parameterize shear driven sub-gridscale tracer transport in ocean models, and we suggest strategies for developing statistical parameterizations based on the MicroInverse diffusivities.


Arranged date for the seminar talk: Mar 25, 2019 at  14:15, BCCR seminar room 4020