Separating the role of direct radiative heating and photolysis in modulating the atmospheric response to the 11-year solar cycle forcing

Abstract. The atmospheric response to the 11-year solar cycle forcing is separated into the contributions from changes in direct radiative heating and photolysis rates using specially designed sensitivity simulations with the UM-UKCA chemistry-climate model. We find that contributions from changes in direct heating and photolysis rates are important for determining the shortwave heating, temperature and ozone responses to the solar cycle forcing. The combined effects of the processes are found to be largely additive in the tropics but non-additive in the high latitudes, in particular in the Southern Hemisphere (SH) during the dynamically active season. We find marked differences in the changes in magnitude and vertical structure of shortwave heating rates gradients across the SH in austral winter, thereby highlighting a potential sensitivity of the polar dynamical response to the altitude of the anomalous radiative tendencies. In addition, 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. 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. 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 non-additive 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.