Equatorial Waves and Superrotation in the Stratosphere of a Titan General Circulation Model

TL;DR

This study examines how equatorial waves contribute to superrotation in Titan's stratosphere using a general circulation model, highlighting wave interactions and their dependence on seasonal and vertical factors.

Contribution

It identifies specific wave types involved in superrotation and discusses their seasonal variability and the limitations of current model vertical resolution.

Findings

Superrotation is strongest around solstice due to wave interactions.

Equatorial Kelvin and Rossby waves play key roles in superrotation.

Vertical wavelength issues may affect wave representation in models.

Abstract

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.