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. We analyse the influence of the El Nino Southern Oscillation (ENSO) onthe atmospheric circulation and the mean ozone distribution in the tropicaland subtropical UTLS region. In particular, we focus on the impact of ENSO onthe onset of the Asian summer monsoon (ASM) anticyclone. Using theMultivariate ENSO Index (MEI), we define climatologies (composites) ofatmospheric circulation and composition in the months following El Ninoand La Nina (boreal) winters and investigate how ENSO-related flowanomalies propagate into spring and summer. To quantify differences in thedivergent and non-divergent parts of the flow, the velocity potential (VP)and the stream function (SF) are respectively calculated from the ERA-Interimreanalysis in the vicinity of the tropical tropopause at potentialtemperature level θ=380  K. While VP quantifies the well-known ENSOanomalies of the Walker circulation, SF can be used to study the impact ofENSO on the formation of the ASM anticyclone, which turns out to be slightlyweaker after El Nino winters than after La Nina winters. In addition,stratospheric intrusions around the eastern flank of the anticyclone into thetropical tropopause layer (TTL) are weaker in the months after strong ElNino events due to more zonally symmetric subtropical jets than after LaNina winters. By using satellite (MLS) and in situ (SHADOZ) observationsand model simulations (CLaMS) of ozone, we discuss ENSO-induced differencesaround the tropical tropopause. Ozone composites show more zonally symmetricfeatures with less in-mixed ozone from the stratosphere into the TTL duringand after strong El Nino events and even during the formation of the ASManticyclone. These isentropic anomalies are overlaid with the well-knownanomalies of the faster (slower) Hadley and Brewer–Dobson circulations afterEl Nino (La Nina) winter. The duration and intensity of El Nino-related anomalies maybe reinforced through late summer and autumn if the El Nino conditionslast until the following winter.

El Niño Southern Oscillation influence on the Asian summer monsoon anticyclone