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During boreal summer the upper troposphere/lower stratosphere (UTLS) in theNorthern Hemisphere shows a distinct maximum in water vapour (H2O)mixing ratios and a coincident minimum in ozone (O3) mixing ratios, bothconfined within the Asian monsoon anticyclone (AMA). This well-known featurehas been related to transport processes emerging above the convective systemsduring the Asian summer monsoon (ASM), further modified by the dynamics ofthe AMA. We compare the ability of chemistry–climate models (CCMs) toreproduce the climatological characteristics and variability of H2O,O3, and temperature in the UTLS during the boreal summer with MIPASsatellite observations and ERA-Interim reanalyses. By using a multiple linearregression model the main driving factors, the strength of the ASM, thequasi-biennial oscillation (QBO), and the El Niño–Southern Oscillation(ENSO), are separated. The regression patterns related to ENSO show acoherent, zonally asymmetric signal for temperatures and H2O mixingratios for ERA-Interim and the CCMs and suggest a weakening of the ASM duringENSO warm events. The QBO modulation of the lower-stratospheric temperaturenear the Equator is well represented as a zonally symmetric pattern in theCCMs. Changes in H2O and O3 mixing ratios are consistent with theQBO-induced temperature and circulation anomalies but less zonally symmetricthan the temperature pattern. Regarding the ASM, the results of theregression analysis show for ERA-Interim and the CCMs enhanced H2O andreduced O3 mixing ratios within the AMA for stronger ASM seasons. TheCCM results can further confirm earlier studies which emphasize theimportance of the Tibetan Plateau/southern slope of the Himalayas as the mainsource region for H2O in the AMA. The results suggest that H2O istransported towards higher latitudes at the north-eastern edge of the AMArather thantowards low equatorial latitudes to be fed into the tropicalpipe

Interannual variability of the boreal summer tropical UTLS in observations and CCMVal-2 simulations