Waveguidability of the midlatitude jet

Volkmar Wirth

A strong and narrow midlatitude jet is known to act as a good waveguide that is able to duct Rossby waves in the zonal direction. The amount of zonal ducting is important, as it may lead to circumglobal Rossby waves and possibly to quasi-resonance.

Unfortunately, it is not clear how to best quantify “waveguidability”. In this work we are trying to make progress in this direction using a simple barotropic model in an idealized forced-dissipative configuration. Rossby waves are generated locally through pseudo-orographic forcing and they are dissipated through Rayleigh friction. Depending on the strength and width of the jet the wave activity is either refracted equatorwards reaching the other hemisphere (upper panel, weak waveguidability), or it remains mostly in the Northern Hemisphere (lower panel, strong waveguidability).

This setup allows us to quantify waveguidability by diagnosing the latitudinal probability density function of waviness in a sector far downstream of the forcing. We then compare the result with idealized waveguide concepts invoking either a very strong meridional gradient of potential vorticity or the existence of two turning latitudes in the framework of WKB theory.