Abstract

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. 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 global climate models.

Highlights

  • The evolution of climate over the last millennium provides a unique setting for determining how modes of climate variability respond to natural and anthropogenic forcing

  • 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 global climate models

  • We explore changes in solar forcing on the Southern Annular Mode (SAM) using solar constant and last millennium transient simulations that cover large amplitude solar changes and find that the SAM index significantly decreases with a decrease in solar forcing

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Summary

Introduction

The evolution of climate over the last millennium provides a unique setting for determining how modes of climate variability respond to natural and anthropogenic forcing. The temporal evolution of external forcings (such as atmospheric greenhouse gas concentrations, volcanic eruptions and changes in solar irradiance) is reasonably well understood over the last millennium (Schmidt et al, 2011; Schmidt et al, 2012; Jungclaus et al, 2017), allowing their effects on climate to be explored using global climate models. Such simulations have primarily been compared to proxy-based reconstructions of global or hemispheric mean temperature (Neukom et al., 2018; Neukom et al, 2019), or analysed for changes in tropical or Northern Hemisphere modes of climate variability (Ortega et al, 2015; Otto-Bliesner et al, 2016).

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