Effects of Stratospheric Sulfate Injection on Tropopause Height

The spatial and temporal structure of the tropopause plays a significant role in the exchange between the troposphere and the stratosphere. Studies of tropopause height during the last decades have shown an upward trend, both in reanalysis and climate models, which would continue in global warming scenarios. This trend has been related to the cooling of the tropical lower stratosphere and plays a role in the broadening of the tropics. We have computed the geopotential height for the tropopause using GLENS (Stratospheric Aerosol Geoengineering Large Ensemble Project) control and geoengineering simulations to see if sulfate injection would compensate for the positive trend. The goal of the geoengineering simulations in GLENS was to maintain not only global mean surface temperature but also interhemispheric and equator-to-pole surface temperature gradients at 2020 values under an RCP8.5 greenhouse gas scenario. Our results show an upward trend in the RCP8.5 runs, but show a downward trend of the same magnitude for the geoengineering simulations in the middle and low latitudes. Thus, trying to maintain global temperature using the GLENS simulated sulfate injection scheme would lead to a very low annual mean tropical tropopause (1 km lower in 2100 than in 2020), whereas for the rest of the globe the descent would be smaller as latitude increases. This would lead to an equatorward shift of the subtropical jets and to an increase of the moisture flux into the lower stratosphere.