NOVEMBER 2001: Transatlantic transport of pollution and the North Atlantic Oscillation (NAO)

Relationship between the North Atlantic Oscillation (NAO) index (blue line) and the transatlantic transport of North American ozone pollution to Europe (red line). The NAO index is the difference in normalized sea-level pressure between a station in Iceland and a station in the Azores. It is widely used as a climate forecasting tool for seasonal temperature and precipitation in Europe. We find here that it can also serve to forecast the transatlantic transport of North American pollution to Europe. The Figure shows the time series for 1993-1997 of the observed NAO index and the surface ozone enhancement at Mace Head from transatlantic transport of North American anthropogenic pollution as simulated by the GEOS-CHEM model. Mace Head, on the west coast of Ireland, is a long-term observing station for surface ozone; the GEOS-CHEM model provides a good simulation of the variability of ozone at that site and shows that North American pollution events there are forerunners of events over continental Europe. The strong correlation between North American ozone pollution at Mace Head and the NAO index is evident from the Figure, and is driven by the stronger westerly winds associated with positive values of the index. General circulation models predict a decline in the NAO index over the next decades, implying a decrease in the transatlantic transport of North American pollution to Europe. This work was done by Qinbin Li and a full account is given in Li et al. [2001].