Summertime tropospheric-ozone variability over the Mediterranean basin observed with IASI

Abstract. The Mediterranean basin is one of the most sensitive regions in the world regarding climate change and air quality. This is partly due to the singular dynamical situation of the Mediterranean basin that leads to tropospheric-ozone concentrations that are among the highest over the Northern Hemisphere. Six years of summertime tropospheric ozone observed by the Infrared Atmospheric Sounding Interferometer (IASI) instrument from 2007 to 2012 have been analysed to document the variability of ozone over this region. The satellite observations have been examined together with meteorological analyses (from ECMWF) to understand the processes driving this variability. Our work confirmed the presence of a steep west–east ozone gradient in the lower troposphere with the highest concentrations observed over the eastern part of the Mediterranean basin. This gradient is mainly explained by diabatic convection over the Persian Gulf during the Indian monsoon season, which induces an important subsidence of ozone-rich air masses from the upper to the lower troposphere over the central and the eastern Mediterranean basin. IASI observations of ozone concentrations at a 3 km height show a clear summertime maximum in July that is well correlated to the maximum of downward transport of ozone-rich air masses from the upper troposphere. Even if this feature is robust over the six analysed years, we have also investigated monthly ozone anomalies – one positive (June 2008) and one negative (June and July 2009) – using daily IASI observations. We show that the relative position and the strength of the meteorological systems (Azores anticyclone and Middle Eastern depression) present over the Mediterranean are key factors in explaining both the variability and the anomalies of ozone in the lower troposphere in this region.