Influence of interannual variations in transport on summertime abundances of ozone over the Middle East

We used the GEOS-Chem model to investigate the impact of interannual variations in transport on summertime ozone abundances (between 1987 and 2006) in the middle troposphere over the Middle East. We found that ozone abundances fluctuated interannually by about +/- 7% (or +/- 6 ppbv from the 20-year mean of similar to 80 ppbv). In the 20-year mean, ozone transported from Asia and ozone produced locally were the dominant sources of ozone, accounting for 31% and 23%, respectively, of ozone abundances over the Middle East, with an interannual variability of +/- 30% and +/- 15%, respectively. We found that the interannual variations in the Asian and local sources were related to the strengths of the South Asian High and the Arabian anticyclone, respectively. In years when the Asian influence was weaker in the region, transport from other areas, such as North America, was enhanced. Consequently, variations in ozone transported from Asia were strongly anti-correlated with variations in ozone transported from North America, for example, with a correlation coefficient of r = -0.75. This trade-off between transport from Asia and other regions was found to be linked to the position and strength of the subtropical westerly jet over central Asia. When the westerly jet is displaced poleward, transport of ozone from Asia is enhanced and transport from North America and other regions in the Northern Hemisphere is diminished. In contrast, when the jet is displaced equatorward, transport of ozone from Asia is diminished and transport from North America and other regions in the Northern Hemisphere is enhanced. These results suggest that climate-related changes in the position of the westerly jet will have implications for the transport of pollution into the Middle East.