Abstract. The atmospheric response to the 11-year solar cycle is separated into the
contributions from changes in direct radiative heating and photolysis rates
using specially designed sensitivity simulations with the UM-UKCA (Unified
Model coupled to the United Kingdom Chemistry and
Aerosol model) chemistry–climate model. We perform a number of idealised time-slice
experiments under perpetual solar maximum (SMAX) and minimum conditions
(SMIN), and we find that contributions from changes in direct heating and
photolysis rates are both important for determining the stratospheric
shortwave heating, temperature and ozone responses to the amplitude of the
11-year solar cycle. The combined effects of the processes are found to be
largely additive in the tropics but nonadditive in the Southern Hemisphere
(SH) high latitudes during the dynamically active season. Our results
indicate that, in contrast to the original mechanism proposed in the
literature, the solar-induced changes in the horizontal shortwave heating
rate gradients not only in autumn/early winter but throughout the
dynamically active season are important for modulating the dynamical
response to changes in solar forcing. In spring, these gradients are
strongly influenced by the shortwave heating anomalies at higher southern
latitudes, which are closely linked to the concurrent changes in ozone. In
addition, our simulations indicate differences in the winter SH dynamical
responses between the experiments. We suggest a couple of potential drivers
of the simulated differences, i.e. the role of enhanced zonally asymmetric
ozone heating brought about by the increased solar-induced ozone levels
under SMAX and/or sensitivity of the polar dynamical response to the
altitude of the anomalous radiative tendencies. All in all, our results
suggest that solar-induced changes in ozone, both in the
tropics/mid-latitudes and the polar regions, are important for modulating
the SH dynamical response to the 11-year solar cycle. In addition, the
markedly nonadditive character of the SH polar vortex response simulated in
austral spring highlights the need for consistent model implementation of
the solar cycle forcing in both the radiative heating and photolysis
schemes.