Solar forcing of climate: model results

Abstract

The role of stratospheric ozone changes in determining the climate response to solar forcing is investigated using a version of the Unified Model from the UK Meteorological Office which includes a mixed-layer ocean of constant depth (HadSM3). Two experiments have been performed, both of which include a wavelength-dependent reduction in total solar irradiance (TSI) of 7.5 W m −2 (0.55%). The second experiment includes, in addition, an estimate of the accompanying stratospheric ozone changes. A large change in TSI is used (approximately a factor of two greater than the `best-guess' change between present mean levels and the Maunder Minimum) to demonstrate the sensitivity of the climate system to this forcing. Results show that in the annual mean, the temperature response of the model is enhanced by the inclusion of the ozone changes, by approximately 15–20%. We compare results from our TSI and ozone experiment to those of Shindell et al. [Science 294 (2001) 2149] who performed a similar study with the GISS GCM. Temperature changes are greater in our simulation, as expected from the larger magnitude forcing, however the circulation response is very different: our results do not resemble the Arctic Oscillation, whilst those of Shindell et al. [loc. cit.] project strongly onto this leading mode of variability. The lack of a fully resolved stratosphere in our model is a potential reason for this distinction. To test this possibility, we repeated our combined irradiance and ozone experiment using identical model formulations, but with different vertical extents: the first extends to a height of 5 hPa, the second to 0.01 hPa. Both simulations produce a relatively weak surface pressure response to solar forcing that does not strongly resemble the Arctic Oscillation.