Linking air stagnation in Europe with the large-scale atmospheric circulation

Air stagnation refers to a period when a stable air mass becomes settled over a region and remains quasi-stationary for an extended amount of time. Weak winds in the lower- to mid-troposphere and the absence of precipitation during air stagnation prohibit the ventilation and washout of particles so pollutants can accumulate near the surface. This allows for such pollutants to reach levels harmful to humans, and poses severe health risks. Understanding the development of stagnant conditions is therefore crucial for studying poor air quality and its societal impact. 

Here, the linear relationship between European air stagnation and the large-scale circulation is explored across all seasons and during the 1979--2018 period. Dynamical based indices identifying atmospheric blocking, Rossby wave breaking, subtropical ridges, and the North Atlantic eddy-driven and subtropical jets are used to describe the large-scale circulation as predictors in a statistical model of air stagnation variability. It is found that the large-scale circulation can explain approximately 60% of the variance in monthly air stagnation in five distinct regions within Europe. The variance explained by the model does not vary strongly across regions and seasons. However, the dynamical indices most related to air stagnation do depend on region and season. The blocking and Rossby wave breaking predictors tend to be the most important for describing air stagnation variability in northern regions whereas ridges and the subtropical jet are more important to the south. The demonstrated correspondence between air stagnation and the large-scale circulation can be used to assess the representation of air stagnation in climate models, which is key for understanding how air quality and its associated health risks may change in the future.