A New Quantitative Framework for Mediterranean Cyclogenesis: Local Wave Activity and Cross-Basin Dynamics

The North Atlantic large-scale circulation is known to affect Mediterranean cyclonic activity, representing a unique case of interacting storm tracks. Variability in the North Atlantic, such as extratropical storm activity, jet stream position and weather regimes, modulates the flux of potential vorticity downstream. This plays a crucial role in the life cycle of Mediterranean cyclones by influencing baroclinic growth and triggering instability over the basin.

In this work, we use the framework of Finite Amplitude Local Wave Activity (FALWA; Huang & Nakamura, 2016) to deconstruct and quantify the role of the North Atlantic circulation in driving Mediterranean cyclonic activity. FALWA is a diagnostic that keeps track of the wave activity ”stored” within circulation undulations, relative to a zonalized flow. It obeys an exact conservation relation; thus, its local rate of change is either due to a flux convergence, or to non conservative source-sink terms. This allows for a closed mechanistic budget analysis of the response, differentiating between horizontal advection by the mean flow, barotropic and baroclinic processes, and diabatic forcing. To the best of our knowledge, the method has not yet been applied to the Mediterranean.

Using reanalysis, we examine how extrabasin sources of wave activity influence the paths, intensity and associated rainfall patterns of storms in the Mediterranean. We find that different combinations of advective and baroclinic sources result in distinct cyclone trajectories, impacting precipitation at both 10-day and monthly scales. We are currently producing preliminary results from the CMIP6 ensmeble and exploring the relevance of this mechanism to the projected drying trend in the Mediterranean.