On bottom drag, horizontal eddy viscosity, and energy dissipation in models and observations of oceanic mesoscale eddies

We utilize numerical simulations of forced-dissipated two-layer geostrophic turbulence, in-situ observations, and satellite observations, to study the energy dissipation of oceanic mesoscale eddies. We argue that both bottom drag and horizontal eddy viscosity are required to yield model eddies that compare well with observations. Moderately strong large-scale friction acting in the bottom layer only is required to produce realistically surface intensified eddy kinetic energy. This is true whether the bottom drag is linear (1) or quadratic (2) in the flow. When the turbulence model is run at high resolution and only a spectral filter is used for small-scale dissipation, very little energy dissipation takes place in the upper layer, and the forward cascades at small horizontal scales are much smaller than those seen (3) in satellite altimetry data. Inclusion of a horizontal eddy viscosity of order 50 m^2 s^-1 in the model generates the forward cascades at small scales, as seen in satellite observations, and energy dissipation in the upper layer, consistent with in-situ microstructure observations. Eddy viscosities of this order are in agreement with values inferred from in-situ current meter data at the POLYMODE Local Dynamics Experiment (Polzin et al., this session), and from a nearly global analysis of altimetric data. The viscosities may represent interactions between mesoscale eddies and internal waves. We argue that the nondimensional eddy viscosity increases with increasing latitude, and that this may explain differences in the spectral kinetic energy fluxes that arise with changing latitude in the satellite observations.

(1) Arbic, B.K., and Flierl, G.R., Journal of Physical Oceanography, volume 34, 2257-2273, 2004.

(2) Arbic, B.K., and Scott, R.B., paper under review.

(3) Scott, R.B., and Wang, Journal of Physical Oceanography, volume 35, 1650-1666, 2005.