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. Observations of ozonesonde measurements of the NDACC/SHADOZ (Network for the Detection of Atmospheric Composition Change and the Southern Hemisphere ADditional OZonesondes)program and humidity profiles from the daily Météo-Franceradiosondes at Réunion island (21.1∘ S, 55.5∘ E)from November 2013 to April 2016 were analyzed to identify the origin of wetupper-tropospheric air masses with low ozone mixing ratio observed above theisland, located in the southwest Indian Ocean (SWIO). A seasonalvariability in hydration events in the upper troposphere was found andlinked to the convective activity within the SWIO basin. In the uppertroposphere, ozone mixing ratios were lower (mean of 57 ppbv) in humid airmasses (RH &gt; 50 %) compared to the background mean ozone mixingratio (73.8 ppbv). A convective signature was identified in the ozoneprofile dataset by studying the probability of occurrence of different ozonethresholds. It was found that ozone mixing ratios lower than 45 to 50 ppbvhad a local maximum of occurrence between 10 and 13 km in altitude,indicative of the mean level of convective outflow. Combining FLEXPARTLagrangian back trajectories with METEOSAT-7 infrared brightness temperatureproducts, we established the origin of convective influence on the uppertroposphere above Réunion island. It has been found that the uppertroposphere above Réunion island is impacted by convective outflows inaustral summer. Most of the time, deep convection is not observed in thedirect vicinity of the island, but it is observed more than 1000 km awayfrom the island, in the tropics, either from tropical storms or the Intertropical Convection Zone (ITCZ). In November and December, the air massesabove Réunion island originate, on average, from central Africa and theMozambique Channel. During January and February the source region is thenortheast of Mozambique and Madagascar. Those results improve ourunderstanding of the impact of the ITCZ and tropical cyclones on thehydration of the upper troposphere in the subtropics in the SWIO.

atmospheric chemistry and physics

Impact of convection on the upper-tropospheric composition (water vapor and ozone) over a subtropical site (Réunion island; 21.1° S, 55.5° E) in the Indian Ocean