  Saturn's polar stratosphere exhibits the seasonal growth and dissipation of
broad, warm, vortices poleward of $\sim75^\circ$ latitude, which are strongest
in the summer and absent in winter. The longevity of the exploration of the
Saturn system by Cassini allows the use of infrared spectroscopy to trace the
formation of the North Polar Stratospheric Vortex (NPSV), a region of enhanced
temperatures and elevated hydrocarbon abundances at millibar pressures. We
constrain the timescales of stratospheric vortex formation and dissipation in
both hemispheres. Although the NPSV formed during late northern spring, by the
end of Cassini's reconnaissance (shortly after northern summer solstice), it
still did not display the contrasts in temperature and composition that were
evident at the south pole during southern summer. The newly-formed NPSV was
bounded by a strengthening stratospheric thermal gradient near $78^\circ$N. The
emergent boundary was hexagonal, suggesting that the Rossby wave responsible
for Saturn's long-lived polar hexagon - which was previously expected to be
trapped in the troposphere - can influence the stratospheric temperatures some
300 km above Saturn's clouds.
