The impact of stratospheric aerosol intervention on the North Atlantic and Quasi-Biennial Oscillations in the Geoengineering Model Intercomparison Project (GeoMIP) G6sulfur experiment

Abstract

Abstract. As part of the Geoengineering Model Intercomparison Project a numerical experiment known as G6sulfur has been designed in which temperatures under a high-forcing future scenario (SSP5-8.5) are reduced to those under a medium-forcing scenario (SSP2-4.5) using the proposed geoengineering technique of stratospheric aerosol intervention (SAI). G6sulfur involves introducing sulphate aerosol into the tropical stratosphere where it reflects incoming sunlight back to space, thus cooling the planet. Here we compare the results from six Earth-system models which have performed the G6sulfur experiment and examine how SAI affects two important modes of natural variability, the northern wintertime North Atlantic Oscillation (NAO) and the Quasi-Biennial Oscillation (QBO). Although all models show that SAI is successful in reducing global-mean temperature as designed, they are also consistent in showing that it forces an increasingly positive phase of the NAO as the injection rate increases over the course of the 21st century, exacerbating precipitation reductions over parts of southern Europe compared with SSP5-8.5. In contrast to the robust result for the NAO there is less consistency for the impact on the QBO, but the results nevertheless indicate a risk that equatorial SAI could cause the QBO to stall and become locked in a phase with permanent westerly winds in the lower stratosphere.

Stratospheric Ozone Response to Sulfate Aerosol and Solar Dimming Climate Interventions based on the G6 Geoengineering Model Intercomparison Project (GeoMIP) Simulations

“…(Figure 4, left column) for all three models, including a weakening of the subtropical jet stream and a strengthening of the 195 polar vortex compared to SSP5-8.5, consistent with what has been found in early studies (e.g., Tilmes et al, 2017;Jones et al, 2021).…”

“…WACCM6 shows a strengthening of the tropical winds and CNRM-ESM2-1 shows a weakening of the tropical winds. UKESM1-0-LL shows a strong increase of the zonal tropical winds between 20-50 hPa and a decrease above those altitudes by the end of the century which is aligned with the permanent lock-in of the QBO into a westerly phase after 2055 (Jones et al, 2021). These differences are likely connected to the differences in the heating response that are caused by different injection strategies with injections in lower altitudes…”

“…This relatively small injection amount results in a sudden increase of SAD from 2 to 10 µm 2 /cm 3 within the first year of the application (Figure 6). This happens because the aerosol microphysical scheme first produces smaller particles that grow slowly with increasing injection, resulting in the initial increase in SAD (as also discussed in Tilmes et al (2021)). After this initial increase, SAD and AOD (Figure 2a) stay constant until about 2050 along with the roughly constant injection amount.…”

“…, which may reduce some of the climate impacts, including the precipitation reduction over southern Europe in winter (Jones et al, 2021) and weakening of the monsoonal precipitation over India (Simpson et al, 2019). Therefore, solar dimming may be a closer analog to SAI approaches using less absorbing aerosols than sulfates.…”

Australian wildfires cause the largest stratospheric warming since Pinatubo and extends the lifetime of the Antarctic ozone hole

“…The rationale for such an approach is that it samples different phases within the cycles of stratospheric natural variability such as the Quasi Biennial Oscillation, which are represented well by the UKESM1 model (e.g. 71 ).…”