The large-scale circulation during air quality hazards in Bergen, Norway

In this study, we assessed the large-scale circulation anomalies leading to wintertime air pollution episodes in Bergen, Norway. Bergen is an example of a city, where a strong interplay between large-scale and local circulation features is relevant. Certain large-scale circulation regimes that are different from the usually assumed large-scale stagnation, lead to local air pollution episodes. We assessed these circulation regimes and their predictability. For this, we modified and applied a previously developed atmospheric circulation proxy for the identification of air pollution episodes. Use of this proxy on data from a high-resolution atmospheric general circulation model showed a good reproduction of the total number of potentially polluted days per month and their inter-monthly variability. We also found a link between the persistence of the flow above the Bergen valley and the occurrence and severity of the local air pollution episodes. Analysis of the large-scale circulation over the North Atlantic-European region, with respect to air pollution in Bergen, revealed that the persistence in the meteorological conditions connected to the air pollution episodes is not necessarily caused by large-scale anomalies of the atmospheric circulation over the Norwegian west coast. It is rather connected to anomalies further upstream as far away as Greenland.