Oceanic eddies, fluctuations on scales on the order of one km to hundreds of km, derive their energy primarily from baroclinic instability processes. It is common to parameterize this loss of available potential energy of the balanced flow in ocean models by an additional eddy-driven advection velocity, or, equivalently a skew diffusivity. In addition, eddies also mix tracers along isopycnals, which is implemented as along-isopycnal diffusion with isopycnal diffusivity. Eddy diffusivities currently are specified in coarse ocean models without connection to the energy budget, and, more fundamentally, it is unclear to what extent, where and on what scales the downgradient eddy-diffusion model is appropriate at all. Rotational components of the eddy fluxes associated with the advective terms in the eddy variance equation are generally large, so that production and dissipation of eddy energy do not balance locally.
Our main questions are:
- What is the spatial and temporal variation of eddy-mean flow interactions and scale dependence of eddy diffusivities on a global scale and in one key observational region of the ocean, the Benguela upwelling Current?
- What are the limits and validities of the eddy diffusion model and consequences for energy-consistent parameterizations of eddy-mean flow interactions across a range of eddy scales?
To address these questions we will use Lagrangian particle statistics in both eddying ocean models and observations to assess the spatial and temporal variability of eddy diffusivities, eddy-mean flow interactions, and submesoscale processes, and to quantify dispersion regimes on different scales to assess the eddy-diffusion model and get insight into energy cascades. In-situ observations and a Lagrangian drifter experiment at the upwelling front in the Benguela current system will serve as the major observational reference to study the lifetime of meso- and submesoscale features, as well as their dissipation in the interior ocean. Results from the analyses of observations and eddying ocean models will be used to construct energy-consistent parameterizations that use prognostic equations of eddy kinetic energy, taking non-localness into account.
Li, Q., Lee, S., & Griesel, A. (2016). Eddy Fluxes and Jet-Scale Overturning Circulations in the Indo–Western Pacific Southern Ocean. Journal of Physical Oceanography, 46(10), 2943-2959.