The importance of polar and singular waveguides for the occurrence of Rossby wave resonance

The occurrence of extreme weather has recently been associated with the mechanism of Rossby wave resonance along a circumglobal jet. Resonance is possible to the extent that the jet acts as a zonal waveguide. Recently, a method was introduced to diagnose this mechanism in the framework of the linear barotropic model through numerically solving a judiciously designed model configuration. In that method, any wave activity leaving the jet region is dissipated in sponges and, hence, discarded from further consideration.

The present work goes a step further by explicitly accounting for polar and singular waveguides, which occur through wave reflection off the pole or off a critical level. In the absence of damping, these reflective boundaries generate additional resonant cavities and allow higher meridional modes to participate in the resonance. These higher meridional modes imply resonance at multiple zonal wavenumbers, in stark contrast with the earlier results. However, when a small amount of damping is included, any wave activity is strongly dissipated before these reflecting surfaces are encountered. Consequently, the impact of the polar and the singular waveguides vanishes, and the resonant behavior reduces to that from the original diagnostic. It is concluded that the impact of reflecting surfaces beyond the jet region proper is unlikely to be of practical importance for diagnosing the Rossby wave resonance along a circumglobal midlatitude jet.