Abstract
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.
Highlights
Kalabokas et al (2013) show that high ozone concentrations in the lower troposphere are related to anticyclonic events, which reinforce subsidence and the Etesian advection of potentially ozone-enriched air masses coming from Europe
Note that potential vorticity at lower altitudes is less reliable for such an analysis due to the nonconservative character of PV in the troposphere induced by diabatic processes (Holton, 1992)
Infrared Atmospheric Sounding Interferometer (IASI) observations with more than 200 000 ozone profiles per summer over the Mediterranean basin provide a unique data set to investigate the intraseasonal variability of ozone in this region
Summary
Lower-tropospheric ozone (O3) is a pollutant harmful to both human health and vegetation (Levy et al, 2001; Fuhrer, 2009). A lower-tropospheric north– south circulation, referred to as Etesian winds, occurs over the central Mediterranean basin between these two meteorological systems (east of Greece; Ziv et al, 2004) These meteorological conditions favour (1) the horizontal transport of polluted air masses, with potentially high ozone concentrations, from eastern continental Europe to the Mediterranean Sea (Kalabokas et al, 2008; Richards et al, 2013) and (2) the vertical downward transport of ozone-enriched air masses from the upper troposphere and the lower stratosphere. Ozone observations derived from IASI measurements using the approach developed by Eremenko et al (2008) are used to document the spatiotemporal variability of lower- and upper-tropospheric ozone over the Mediterranean basin during summertime.
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