Could synoptic-scale waves drive the 25-day cycle of circulation during austral summer?

Abstract:

The recent discovery of a 25-day cycle in precipitation and circulation in the storm track region during austral summer, may provide new potential for predictability on intra-seasonal timescales. Investigations of the "Baroclinic Annular Mode" by Thompson et al have shown a robust period of ~25-days in eddy-kinetic energy, heat-flux and precipitation. Furthermore, a complete latitude-by-latitude budget of finite-amplitude wave activity (FAWA) reveals that low-level poleward heat flux gives rise to a similar periodicity in the waves themselves.

The mean state of the summer circulation provides a waveguide that supports a small number of discrete modes. These modes, corresponding with zonal wavenumbers 4, 5 and 6, tend to be associated with short synoptic timescales. So the question remains: what mechanism generates a 25-day timescale in the atmosphere? Using a series of GCM simulations we test a theory that interference between discrete wave modes gives rise to this unexpected timescale. This new wave interaction paradigm is based on the idea that waves with the same zonal wavenumber, but different vertical structures, can interfere constructively and modulate the poleward heat flux.