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

Abstract. Observations of ozonesonde measurements of the NDACC/SHADOZ program and humidity profiles from the daily Météeo-France radiosondes at Réunion Island (21.1° S, 55.5° E) from November 2013 to April 2016 are analyzed to identify the origin of wet upper tropospheric air masses with low ozone mixing ratio observed above the island, located in the South West Indian Ocean (SWIO). A seasonal variability in hydration events in the upper troposphere was found and linked to the convective activity within the SWIO basin. In the upper troposphere, ozone mixing ratios were lower (mean of 57 ppbv) in humid air masses (RH > 50 %) compared to the background mean ozone mixing ratio (73.8 ppbv). A convective signature was identified in the ozone profile dataset by studying the probability of occurrence of different ozone thresholds. It was found that ozone mixing ratios lower than 45 to 50 ppbv had a local maximum of occurrence between 10 and 13 km in altitude, indicative of the mean level of convective outflow. Combining FLEXPART Lagrangian backtrajectories with METEOSAT 7 infrared brightness temperature products, we established the origin of convective influence on the upper troposphere above Réunion island. It has been found that the upper troposphere above Réunion island is impacted by convective outflows in austral summer. Most of the time, deep convection is not observed in the direct vicinity of the island, but more than a thousand of kilometers away from the island, in the tropics, either from tropical storms or the Inter Tropical Convective Zone. In November and December, the air masses above Réunion Island originate, on average, from Central Africa and the Mozambique channel. During January, February the source region is the North-east of Mozambique and Madagascar. Those results improve our understanding of the impact of the ITCZ and tropical cyclones hydration of the upper troposphere in the subtropics in the SWIO.