  This article is devoted to stationary solutions of Euler's equation on a
rotating sphere, and to their relevance to the dynamics of stratospheric flows
in the atmosphere of the outer planets of our solar system and in polar regions
of the Earth. For the Euler equation, under appropriate conditions, rigidity
results are established, ensuring that the solutions are either zonal or
rotated zonal solutions. A natural analogue of Arnold's stability criterion is
proved. In both cases, the lowest mode Rossby-Haurwitz stationary solutions
(more precisely, those whose stream functions belong to the sum of the first
two eigenspaces of the Laplace-Beltrami operator) appear as limiting cases. We
study the stability properties of these critical stationary solutions. Results
on the local and global bifurcation of non-zonal stationary solutions from
classical Rossby-Haurwitz waves are also obtained. Finally, we show that
stationary solutions of the Euler equation on a rotating sphere are building
blocks for travelling-wave solutions of the 3D system that describes the
leading order dynamics of stratospheric planetary flows, capturing the
characteristic decrease of density and increase of temperature with height in
this region of the atmosphere.
