ABSTRACT: Medicanes, the “Hurricanes” of the Mediterranean: Physical Mechanisms and Future Changes in the Context of Anthropogenic Climate Change

Abstract:

Medicanes or “mediterranean hurricanes” are extreme cyclonic windstorms that from a physical point of view operate much as tropical cyclones. Although with typical wind intensities far below those registered in their tropical analogues, these Mediterranean cyclones pose serious threat to the affected islands and coastal regions and can adversely affect open sea activities. Some examples of medicanes that evolved in the Balearic region will be presented through the analysis of available surface and remote sensing observations. We will also review the physical mechanisms responsible for medicane intensification, rooted in the air-sea interaction, and provide contrasting elements with respect to the more common baroclinic cyclones characteristic of our latitudes. In particular, the wind-induced surface-heat exchange (WISHE) mechanism that considers the energy cycle of a mature tropical cyclone as analogous to a Carnot heat engine, can be applied to predict the potential intensity of the storm. The probability of genesis remains particularly difficult, although an empirical genesis index (GENIX) that depends on the kinematic and thermodynamic attributes of the environment, has been formulated for the tropical regions and successfully tested for medicanes.

With an average frequency of only 2-3 events per year and given the lack of systematic, multidecadal databases, an objective evaluation of the long-term risk of medicane-induced winds in the current climate is impractical with standard methods. Also, there is increasing concern on the way these extreme phenomena could change in frequency, intensity or regional variability as a result of human influences on climate. Attempts to evaluate the medicane risk and its possible future changes have been undertaken by our group based on two main perspectives. The first method consists of detecting and tracking symmetric warm-core cyclonic disturbances generated in nested climatic simulations. Owing to the high computational cost, this technique is limited by the horizontal resolutions used, which do not usually allow the RCMs to resolve the vast majority of medicanes as we know them today (i.e. small-scale vortices with very detailed inner region dynamics), and by the production of too few climate realizations as to permit an adequate and complete sampling of the probability distribution function of storms.

The second approach involves a statistical-deterministic method, originally devised for the tropical cyclone problem but which has been adapted for the dynamics of mid latitudes. It  generates thousands of synthetic tracks of medicanes along with their radial distributions of winds; these synthetic storms are compatible with the “climates” provided by 30 CMIP5 models in both historical and RCP85 simulations for a recent (1986-2005) and a future (2081-2100) period, respectively. The present-to-future multimodel mean changes in medicane risk will be analyzed, with special attention to robust patterns in terms of consensus among individual models on the sign of change. Downscaled fields obtained using the historical ERA-interim and NCEP-ncar reanalyses will be used as reference.

The results reveal that the future change in the number of medicanes is unclear (on average the total frequency of storms does not vary) but it is found a profound redistribution of events depending on the lifetime maximum wind: we project a higher number of moderate and violent medicanes at the expense of “ordinary” storms. Spatially, the method yields an increased occurrence of medicanes in the western Mediterranean and Black Sea that is balanced by a reduction of storm tracks in contiguous areas, particularly in the central Mediterranean; however, future extreme events (winds >60 kt) become more probable in all subbasins. As the production of wind energy—and thus the power of destruction—of these storms is proportional to the wind speed cubed, we see our findings as a cause of future concern for exposed Mediterranean societies. By applying oceanic simulation tools, we are currently working in the quantitative assessment of the future impacts of medicanes in terms of storm surges and wind-waves.