Guest blog by Davide Faranda, M. Carmen Alvarez Castro

Mediterranean cyclones or “medicanes”

Windstorms, extreme precipitations and instant floods seems to strike the Mediterranean area with increasing frequency. These events occur simultaneously (compound extreme events) during powerful tropical-like Mediterranean cyclones, also called medicanes. Medicanes are mesoscale cyclones with a diameter usually smaller than 300 km, with a rounded structure and a cloudless area at the center. The mesoscale cyclones are frequently associated with wind, heavy precipitation and changes in temperature, generating high risk situations such as flash floods and large-scale floods with significant impacts on human life and built environment. In the month of October 2016, a strong mesoscale cyclone passed over Sicily and Malta causing flash-floods and numerous damages on Malta. In October 2015 another mesoscale cyclone has passed over Corsica with flash-floods and wind gusts of 150km/h to finally vanish over the south of France where the intense precipitations caused important damages and several life losses. The first challenge we face in the study of medicanes is their detection in historical observations: false positives can be detected from extratropical cyclones that have no tropical structures.

A short term scientific mission

Within the COST action DAMOCLES, we have decided to tackle this problem.  A short term scientific mission (STSM) has been set-up to create an European research platform on medicanes involving the French Laboratoire des Sciences du Climat et de l’Environnement –IPSL-CNRS (Davide Faranda) and the Italian Centro Euro-Mediterraneo sui Cambiamenti Climatici (M. Carmen Alvarez Castro, S Gualdi). Within the STSM, D Faranda visited CMCC for ten days in March 2019.

Cornelia Medicane Meteosat image 04/10/1996. Source: Eumetsat

We have identified 110 cyclones with tropical characteristics.

The STSM began by a reconstruction of a robust medicanes archive using documentary sources such as the EUMETSAT satellite archives starting 1979 and visually inspect all the extratropical cyclones experiencing tropical transitions in the period 1979-2018.  With this technique, we have identified 110 cyclones with tropical characteristics.

As a first analysis we can characterize the occurrence of medicanes in seasons and their regional distribution. Most of the medicanes occur in autumn. Indeed, this is the season that corresponds better to the two requirements of having at the same time hot sea-surface temperature and the intrusion of Atlantic perturbation in the Mediterranean basin. These favorable conditions persist over winter but mostly only over the eastern basin. Here the presence of mid-tropospheric cold air often causes cyclogenesis. On a few occasions, the difference of temperature between sea and mid troposphere is enough to trigger convection around the core of the perturbation.  In spring and summer, the conditions are not favorable for medicanes development. The rare events detected are found over the west Mediterranean basin.

This is a hot research subject at the frontier between atmospheric physics, ocean dynamics and thermodynamic.

The simulation of medicanes is difficult. Their mesoscale structure and the convective phenomena associated require a fine grid for a correct representation of their structure. Regional climate simulations (10 to 25 km horizontal resolution) have been used to perform a first assessment of the probability of observing medicanes in the future under different emission scenarios. In the second part of the STSM we assessed whether the large-scale atmospheric circulation could change the frequency of occurrence. Preliminary results show a decrease of the large-scale circulation patterns favoring medicanes in all the seasons except summer.  Is this enough to state that we are likely to observe a decrease of these events in the future? No, indeed the warming expected of the Mediterranean Sea could favor convective storms that can organize into tropical cyclones without the need of large-scale circulation drivers. This is a hot research subject at the frontier between atmospheric physics, ocean dynamics and thermodynamics that we will continue to tackle in the DAMOCLES – COST action

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