Influence of long-term changes in solar irradiance forcing on the Southern Annular Mode

The Southern Annular Mode (SAM) is the leading mode of climate variability in the extratropical Southern Hemisphere, with major regional climate impacts. Observations, reconstructions, and historical climate simulations all show positive trends in the SAM since the 1960s; however, earlier trends in palaeoclimate SAM reconstructions cannot be reconciled with last millennium simulations. There are also large differences in the magnitude of solar irradiance change between various solar reconstructions, although most last millennium climate simulations have relied on a low-amplitude solar-forcing scenario. Here we investigate the sensitivity of the SAM to solar irradiance variations using simulations with a range of constant solar-forcing values and last millennium transient simulations with varying amplitude solar-forcing scenarios. We find the mean SAM state can be significantly altered by solar irradiance changes and that transient last millennium simulations using a high-amplitude solar scenario have an improved and significant agreement with proxy-based SAM reconstructions. Our findings suggest that the effects of solar forcing on high-latitude climate may not be adequately incorporated in most last millennium simulations due to solar irradiance changes that are too small and/or the absence of interactive atmospheric chemistry in the global climate models used for these palaeoclimate simulations.