New measures of mixing by eddies in the atmosphere and ocean

Two new measures of eddy diffusivity will be described and the results of their application to atmospheric and oceanic flows will be presented. Both measures aim to quantify regional (latitude and longitude) variations in the eddy diffusivity and to ellucidate the dynamical processes leading to the eddy mixing.

The first diagnostic is a modification to the Nakamura effective diffusivity. Calculations on patches of the ocean of size 10 degrees by 10 degrees using this diagnostic highlight the role of topography and of wave-mean flow interactions in determining the mixing structure.

The second diagnostic, the Lyapunov Diffusivity, combines the tracer-based effective diffusivity with the particle-based Lyapunov exponent. Application to consider isentropic mixing in the upper troposphere/lower stratosphere shows there to be significant longitudinal variation to the strength of mixing at the Northern subtropical jet, with the strongest barrier over Asia and the Western Pacific, a weaker barrier over the Western Atlantic, and mixing regions at the jet exits over the Eastern Pacific and Atlantic. Again, the role of wave-mean flow interactions in determining this mixing structure will be highlighted.

There is scope for both diagnostics to be applied to a range of atmospheric and oceanic problems where the velocity field is known or can be estimated and a measure of eddy diffusivity is required.