Changes to the atmospheric circulation under SAI relative to the target baseline (i.e. after controlling for temperature-drive changes) are expected due to (1) differences in the spatial and temporal forcing patterns between aerosol SW forcing and GHGs, and (2) lower-stratospheric heating from aerosol LW absorption. The latter is expected to drive a poleward shift of the mid-latitude westerlies and a strengthening of the polar vortices. We note that this uncertainty spans a large range of phenomena, and that there is no clear demarcation between our three atmospheric circulation-related uncertainties.
Metric
A detectable increase in extreme winter weather in mid-latitudes due to the extra-tropical circulation response to SAI, on a 50-year timescale at 0.5°C cooling, relative to a reference world with the same global mean temperature but neither higher CO2 nor SAI.
Uncertainty
Changes are sensitive to latitude of injection, and generally largest for equatorial injection scenarios. The processes driving these changes (particularly surface and lower stratospheric meridional temperature gradients) are fairly well understood, but the quantitative predictions are limited, in part due to a lack of multi-model assessments.
Decision relevance
Consequences include changes to winter surface climate in the Northern Hemisphere mid-latitudes (Bednarz et al., 2023), and circulation-induced effects on Antarctic ice sheets (Goddard et al., 2023; McCusker et al., 2015).