Analysis of temperature retrievals from Mars Global Surveyor Thermal Emission Spectrometer data has revealed the presence of regular, eastward propagating waves in the Northern Hemisphere. A large amplitude, zonal wave 1 with a long (~20 sol) period is particularly prominent during early winter (Ls = 220--270¿). After Ls = 270¿, a weaker and more rapidly propagating (6.5 sol period) zonal wave 1 is dominant. These waves have a deep vertical structure (>40 km) correlated with the axis of the winter hemisphere westerly jet. Simulations with a Mars general circulation model suggest that the fast wave is associated with baroclinic instability due to the strong meridional temperature gradient at the surface and is consistent with surface pressure oscillations seen in Viking Lander data. By contrast, the slow wave has the appearance of a large-amplitude Rossby wave that is coupled with an inertially unstable region in the subtropics.