${\rm SDiff}(S^2)$ and the orbit method

The group of area preserving diffeomorphisms of the two sphere, ${\rm SDiff}(S^2)$, is one of the simplest examples of an infinite dimensional Lie group. It plays a key role in incompressible hydrodynamics and it recently appeared in general relativity as a subgroup of two closely related, newly defined symmetry groups. We investigate its representation theory using the method of coadjoint orbits. We describe the Casimir functions and the Cartan algebra. Then we evaluate the trace of a simple ${\rm SDiff}(S^2)$ operator using the Atiyah-Bott fixed point formula. The trace is divergent but we show that it has well-defined truncations related to the structure of ${\rm SDiff}(S^2)$. Finally, we relate our results back to the recent appearances of ${\rm SDiff}(S^2)$ in black hole physics.