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 theor
y suggest that the latter is inadequate as it yields different westwar
d speeds than those measured; in the extratropical latitudes the predi
cted speeds are typically slower than the observations. Here the probl
em of long, baroclinic wave propagation in a forced, stratified ocean
is considered theoretically with a view toward explaining these observ
ations of ''too fast'' planetary waves; From a quasigeostrophic analys
is, it is argued that baroclinic waves in a sheared environment are ac
celerated to the west via their interactions with both the mean advect
ive field and the mean potential vorticity field. Conditions under whi
ch the ratio of actual to linear phase speeds matches the observed rat
io 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.