  The incredible longevity of Cassini's orbital mission at Saturn has provided
the most comprehensive exploration of a seasonal giant planet to date. This
review explores Saturn's changing global temperatures, composition, and aerosol
properties between northern spring and summer solstice (2015-2017), extending
our previous review of Cassini's remote sensing investigations (2004-14,
Fletcher et al., 2018) to the grand finale. The result is an unprecedented
record of Saturn's climate that spans almost half a Saturnian year, which can
be used to test the seasonal predictions of radiative climate models, neutral
and ion photochemistry models, and atmospheric circulation models. Hemispheric
asymmetries in tropospheric and stratospheric temperatures were observed to
reverse from northern winter to northern summer; spatial distributions of
hydrocarbons and para-hydrogen shifted in response to atmospheric dynamics
(e.g., seasonally-reversing Hadley cells, polar stratospheric vortex formation,
equatorial stratospheric oscillations, and inter-hemispheric transport); and
upper tropospheric and stratospheric aerosols exhibited changes in optical
thickness that modulated Saturn's visible colours (from blue hues to a golden
appearance in the north near solstice), reflectivity, and near-infrared
emission. Numerical simulations of radiative balance and photochemistry do a
good job in reproducing the observed seasonal change and phase lags, but
discrepancies between models and observations still persist, indicating a
crucial role for atmospheric dynamics and the need to couple chemical and
radiative schemes to the next generation of circulation models. With Cassini's
demise, an extended study of Saturn's seasons, from northern summer to autumn,
will require the capabilities of ground- and space-based observatories, as we
eagerly await the next orbital explorer at Saturn.
