Scale selection of mixed Rossby-gravity waves through wave-mean flow interactions

Mixed Rossby-gravity (MRG) waves contribute significantly to tropical variability in the upper troposphere and the stratosphere. Studies based on reanalysis data suggest that the scale of MRG waves in the two regions is different, with the troposphere dominated by synoptic-scale MRG waves, while the MRG waves in the stratosphere have planetary scales. Using the recently discovered excitation mechanism of the MRG waves by wave-mean flow interactions, we investigate whether this mechanism can explain the different scale selection in the troposphere and the stratosphere.

We carry out high-accuracy numerical simulations with a spherical shallow water model (TIGAR) that includes the MRG wave as a subset of prognostic variables. This framework allows to identify wave-mean flow interactions as the main driver of MRG scale selection in comparison with excitation driven by external forcing and wave-wave interactions. Simulations with idealized background zonal wind field and profiles derived from ERA5 reanalysis from 1 and 200 hPa show that the jet position is a decisive factor for the MRG scale selection. Particularly, when the jet is located closer to the equator, as in boreal winter in the troposphere, synoptic-scale MRG waves are excited. In the case of jets embedded well in extratropics (40-50°), such as in the upper stratosphere, wave-mean flow interactions generate MRG waves with planetary scales. These results explain the MRG scale selection in the upper troposphere and the stratosphere by a single mechanism and highlight the importance of representing accurately wave-mean flow interactions in climate models.