  Saturn's northern summertime hemisphere was mapped by JWST/MIRI (4.9-27.9
$\mu$m) in November 2022, tracing the seasonal evolution of temperatures,
aerosols, and chemical species in the five years since the end of the Cassini
mission. The spectral region between reflected sunlight and thermal emission
(5.1-6.8 $\mu$m) is mapped for the first time, enabling retrievals of
phosphine, ammonia, and water, alongside a system of two aerosol layers (an
upper tropospheric haze $p<0.3$ bars, and a deeper cloud layer at 1-2 bars).
Ammonia displays substantial equatorial enrichment, suggesting similar
dynamical processes to those found in Jupiter's equatorial zone. Saturn's North
Polar Stratospheric Vortex has warmed since 2017, entrained by westward winds
at $p<10$ mbar, and exhibits localised enhancements in several hydrocarbons.
The strongest latitudinal temperature gradients are co-located with the peaks
of the zonal winds, implying wind decay with altitude. Reflectivity contrasts
at 5-6 $\mu$m compare favourably with albedo contrasts observed by Hubble, and
several discrete vortices are observed. A warm equatorial stratospheric band in
2022 is not consistent with a 15-year repeatability for the equatorial
oscillation. A stacked system of windshear zones dominates Saturn's equatorial
stratosphere, and implies a westward equatorial jet near 1-5 mbar at this
epoch. Lower stratospheric temperatures, and local minima in the distributions
of several hydrocarbons, imply low-latitude upwelling and a reversal of
Saturn's interhemispheric circulation since equinox. Latitudinal distributions
of stratospheric ethylene, benzene, methyl and carbon dioxide are presented for
the first time, and we report the first detection of propane bands in the 8-11
$\mu$m region.
