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Supplemental Information

Key Points

• Global warming reduces ozone in the tropical lower stratosphere, an effect typically attributed to strengthening stratospheric upwelling

• Yet, global warming also deepens the troposphere, which erodes the ozone layer and reduces transport of ozone into the lower stratosphere

• Along with strengthening upwelling, tropospheric expansion contributes at leading order to reductions in tropical lower stratospheric ozone

Abstract

In response to global warming, ozone is predicted to increase aloft due to stratospheric cooling but decrease in the tropical lower stratosphere. The ozone reductions have been primarily attributed to a strengthening Brewer-Dobson circulation, which upwells ozone-poor air. Yet, this paper finds that strengthening upwelling only explains part of the reduction. The reduction is also driven by tropospheric expansion under global warming, which erodes the ozone layer from below, the low ozone anomalies from which are advected upwards. Strengthening upwelling and tropospheric expansion are correlated under global warming, making it challenging to disentangle their relative contributions. Therefore, chemistry-climate model output is used to validate an idealized model of ozone photochemistry and transport with a tropopause lower boundary condition. In our idealized decomposition, strengthening upwelling and tropospheric expansion both contribute at leading order to reducing tropical ozone. Tropospheric expansion drives bottom-heavy reductions in ozone, which decay in magnitude into the mid-stratosphere.