Flow Regimes and Vacillation Cycles in the Presence of Topography

An idealized model which resolves the interactions between the mean meridional overturning circulation, zonal jet streams and baroclinically unstable eddies shows three main kinds of equilibrated jet streams- a thermally driven subtropical jet, an eddy driven jet, and a merged thermally-eddy driven jet. These regimes differ in the location and variability of the jet stream, as well as in the structure, zonal wavenumber and phase speed of the dominant modes. The presence of topography in this model, depending on its shape and location, affects regime structures and transitions. Focusing on a subtropical regime for which we add topography in the midlatitudes leads to vacillation cycles between the subtropical and merged jet regimes. These vacillation cycles are characterized initially by surface winds strengthening over the topography, generating strong waves which through a positive feedback with the zonal jet leads the jet to migrate poleward, eventually weakening once reaching the midlatitudes. We will examine this vacillation mechanism, as well as present a similar mechanism that occurs in the case of isolated topography which has implications to quasi-stationary wave growth in the atmosphere.