Did solar forcing influence the Southern Annular Mode over the Last Millennium?

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation, is a primary mode of climate variability in the Southern Hemisphere and has major climatic influences for Australia. A positive trend in the SAM can be seen in recent decades in both observational records and climate simulations, and proxy reconstructions of the SAM over the last millennium show that the SAM is currently at its most positive value compared to the last 1000 years. However, it is difficult to reconcile proxy-based reconstructions for the SAM and climate model simulations of the past millennium, as these climate simulations fail to capture the structure, magnitude, and variability seen in reconstructions for the SAM.

Here we investigate the sensitivity of the SAM to variations in solar forcing in order to understand how changes in solar irradiance may have affected the SAM over the last millennium. We explore changes in the SAM using a fully coupled climate system model with a range of constant solar forcing values that correspond to strong solar irradiance (i.e., between 1358 and 1368 W/m2) and extreme solar irradiance (i.e., 1350, 1372, and 1400 W/m2) changes. We find that a solar constant of 1358 W/m2, which corresponds to a ~7 W/m2 reduction compared to modern-day values, results in a significant negative shift on the SAM index. In addition, we explore variations in the SAM using transient climate simulations for the last millennium forced with strong solar irradiance only, in an attempt to determine if the SAM responds significantly to large (but plausible) changes in solar irradiance, and to compare to our solar constant experiments and SAM reconstructions. Investigations into the influence of solar forcing on the SAM throughout the last millennium may help reconcile differences in proxy reconstructions and climate model simulations, and enhance our understanding of the natural variability of the SAM and how it may respond in the future.