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The summer monsoon anticyclones are the dominant climatological features of the Northern Hemispheric (NH) summertime circulation in the upper troposphere and lower stratosphere (UTLS). However, the response of these anticyclones to the increased levels of CO2 remains highly uncertain, as does the impact on the distribution of UTLS ozone and other tracers. This study examines the response of the NH summertime monsoon anticyclones and UTLS ozone to the abrupt increase in CO2 forcing using output from a suite of coupled ocean–atmosphere general circulation model simulations. These models show an equatorward shift of the Asian summer monsoon anticyclone, a weakening of the North American summer monsoon anticyclone, and a stronger westerly flow penetrating deep into the tropics above the Pacific Ocean and North America. We use additional idealized experiments from atmosphere‐only general circulation models with prescribed SSTs and sea ice concentration to isolate the direct atmospheric radiative effects from the indirect effect of SST warming on the UTLS monsoon anticyclones. Comparison between atmosphere‐only and coupled ocean–atmosphere experiments shows that SST warming is the principal mechanism producing UTLS monsoonal circulation changes. The 4×CO2 experiments result in a significant reduction up to 40%–50% of the UTLS ozone in the northern tropics, which could have an impact on radiative balance near the surface. Key Points: Models simulate an equatorward shift of the Asian and disappearance of the North American monsoon anticyclones in 4 × CO2 simulationsChanges in the upper troposphere and lower stratosphere anticyclones are dominated by the effect of indirect SST warming by CO2Circulation changes due to 4 × CO2 result in a significant reduction of ozone in the northern tropics at 100 hPa [ABSTRACT FROM AUTHOR]

Abstract: The interannual variability of tropical lower stratosphere ozone and its connections to sea surface temperatures in the equatorial Pacific are examined using a combination of chemistry climate model simulations, satellite observations, and reanalyses. The model simulations and observations show large differences in the magnitude of interannual variability in ozone between northern tropic (NT; EQ‐18° N) and southern tropic (EQ‐18° S) during boreal summer but small differences in winter. The interannual variability during boreal summer is highly correlated with summer sea surface temperatures in the eastern and central Pacific Ocean and El Niño–Southern Oscillation (ENSO) events. Larger variability in NT ozone is primarily due to meridional advection, connected to the changes in the onset date and strength of the Asian summer monsoon anticyclone. The Asian summer monsoon anticyclone forms earlier in a season and tends to be stronger during cold (La Niña) events leading to more isentropic transport of ozone from the extratropics into the NT, with the reverse for warm (El Niño) events. [ABSTRACT FROM AUTHOR]