Mixing rates calculated from potential vorticity

The flux and the gradient of potential vorticity are used to calculate horizontal mixing coefficients (Kyy and Kyz). It is found that the mid‐latitude to polar mixing coefficients depend on the use of nongeostrophic winds. Geostrophic winds yield Kyy values which are largely negative near the polar region, implying counter‐gradient transport of potential vorticity by eddies, while nongeostrophic winds yield Kyy values which are mostly positive. Typically, large Kyy values (in excess of 106 m2 s−1) are found in the middle to upper stratosphere during winter in the northern hemisphere, the middle to upper stratosphere during spring in the southern hemisphere, and the troposphere for both northern and southern hemispheres. As an alternative procedure, Kyy values have also been calculated using the momentum balance between the flux of potential vorticity and the residual circulation. These residual circulation balanced Kyy values in the upper stratosphere are much larger than those determined from the potential vorticity fluxes and gradients. This difference may be a result of breaking gravity waves in the upper stratosphere and lower mesosphere.