A wave‐activity view of the relation between the Mediterranean storm track and the North Atlantic Oscillation in winter

The North Atlantic storm track with its dominant mode of variability, the North Atlantic Oscillation (NAO), is believed to impact the Mediterranean storm track and thereby vastly extend its influence downstream. To study such impact, a particular formulation of wave activity suitable for travelling Rossby waves is exploited to compare the upper‐tropospheric branch of Rossby‐wave propagation in the positive and negative phases of the NAO. The analysis is carried out for composites of the critical months and the NAO events defined based on the monthly and daily indices of the NAO, respectively. Quantitative measures including the outward wave activity flux through the boundaries of the Mediterranean domain are used to assess the impact. Overall, the impact of the North Atlantic storm track on the Mediterranean storm track is shown to be noticeably greater in the negative NAO. Further, the existence of a wave source region in the central Mediterranean region, which is stronger in the positive NAO, is established.