On “Too Fast” Baroclinic Planetary Waves in the General Circulation

Abstract

Recent altimetric observations of the ocean surface reveal signatures of long planetary waves near the annual frequency band. Comparisons of the observed wave speeds and those predicted by standard linear theory suggest that the latter is inadequate as it yields different westward speeds than those measured; in the extratropical latitudes the predicted speeds are typically slower than the observations. Here the problem of long, baroclinic wave propagation in a forced, stratified ocean is considered theoretically with a view toward explaining these observations of ?too fast? planetary waves. From a quasigeostrophic analysis, it is argued that baroclinic waves in a sheared environment are accelerated to the west via their interactions with both the mean advective field and the mean potential vorticity field. Conditions under which the ratio of actual to linear phase speeds matches the observed ratio are computed and found to be typical of the open ocean. Extensions of these ideas to continuously stratified quasigeostrophic and layered planetary geostrophic systems are discussed.