  We investigate the characteristics of equatorial waves associated with the
maintenance of superrotation in the stratosphere of a Titan general circulation
model. A variety of equatorial waves are present in the model atmosphere,
including equatorial Kelvin waves, equatorial Rossby waves, and mixed
Rossby-gravity waves. In the upper stratosphere, acceleration of superrotation
is strongest around solstice and is due to interaction between equatorial
Kelvin waves and Rossby-type waves in winter-hemisphere mid-latitudes. The
existence of this 'Rossby-Kelvin'-type wave appears to depend on strong
meridional shear of the background zonal wind that occurs in the upper
stratosphere at times away from the equinoxes. In the lower stratosphere,
acceleration of superrotation occurs throughout the year and is partially
induced by equatorial Rossby waves, which we speculate are generated by
quasigeostrophic barotropic instability. Acceleration of superrotation is
generally due to waves with phase speeds close to the zonal velocity of the
mean flow. Consequently, they have short vertical wavelengths which are close
to the model's vertical grid scale, and therefore are likely to be not properly
represented. We suggest this may be a common issue amongst Titan GCMs which
should be addressed by future model development.
