Impacts of the QBO and ENSO on upper‐tropospheric Rossby‐wave activity associated with the North Atlantic and Mediterranean storm tracks

The differences in North Atlantic and Mediterranean storm tracks between the phases of the Quasi‐Biennial Oscillation (QBO) and El Niño–Southern Oscillation (ENSO) are investigated using a form of wave‐activity diagnostic suitable for transient eddies. Using the JRA‐55 reanalysis data for the 60‐year period 1958–2017, various monthly‐mean diagnostics for the wave‐activity flux and its divergence are presented, with focus on the 300‐hPa level as representing the upper‐tropospheric activity. This is carried out for the phases of the QBO and ENSO separately, as well as for their joint distributions, during early and late winter, corresponding to November–December and January–March periods, respectively. Results show that, in both early and late winter, the sole impact of the QBO manifests itself in making the Mediterranean storm track stronger during the easterly phase of the QBO. There is a significant contrast between the wave‐emission areas of the North Atlantic and Mediterranean storm tracks, in that the latter acts as a local source. For the North Atlantic storm track, the phase of ENSO determines the location of wave reception. Overall, among the joint distributions, the most marked impact is that of ENSO under the westerly phase of the QBO, followed by that of the QBO under La Niña in late winter. Comparison with the signature of the North Atlantic Oscillation indicates that the stratospheric pathway for both QBO and ENSO impacts is likely to be stronger in late winter. The Quasi‐Biennial Oscillation (QBO) and El Niño–Southern Oscillation (ENSO) are two main modes of low‐frequency variability with global impacts over a wide range of time‐scales. Among such impacts are those related to the North Atlantic and Mediterranean storm tracks, which shape the weather and climate across large areas of the Northern Hemisphere. The two phases of the QBO and ENSO, as well as their joint distributions, are compared in terms of the characteristics of large‐scale wave propagation.