The website and Twitter feed for Severe Weather Europe are reporting that weather models continue to forecast a "Medicane" to hit Greece this weekend. According to the latest weather information the Medicane, which will be named Zorbas, will form in the Ionian sea on Friday, Sept 28th. For many of you, the term Medicane may be new. Here is what you need to know about them, and why it is technically incorrect to describe them as "Mediterranean hurricanes."
The term Medicane is combination of the words "Mediterranean + Hurricane." But is it really like a hurricane? Not really. According to Sarah Fecht's outstanding blog at Columbia University,
Medicanes have a lot in common with tropical storms, with strong winds spinning around a core and torrential rainfall. In 2014, Medicane Qendresa hit Malta with sustained winds of up to 70 miles per hour and gusts of up to 95mph. Nevertheless, the waters of the Mediterranean aren’t extensive or warm enough to sustain the strength needed to call these storms legitimate hurricanes. In addition, says Yochanan Kushnir, who studies climate variability at Columbia’s Lamont-Doherty Earth Observatory, medicanes’ physics and dynamics are different from those of hurricanes.
One of the primary differences is that Medicanes are more common during the Fall to Winter season where as hurricane are more likely during the Summer to early Fall. They form over relatively warm waters but do not require the same degree of warmth. According to a website on Medicanes published by Grup de Meteorologia, Medicanes have developed in sea surface temperatures as cool as 59 degrees F. A typical hurricane requires sea surface temperatures of at least 80 degrees F. Medicanes also move west to east while hurricanes typically move east to west except oddball storms like Hurricane Ophelia in 2017 that formed in the Atlantic and targeted Ireland.
Another major difference between a hurricane and a Medicane is that they are somewhat cold-core systems according to Yochanan Kushnir, a scientist that studies climate variability at Columbia University. Hurricanes (and their tropical brethren) are warm-core systems. Ok, Dr. Shepherd you went all "meteorological jargony" on us. What does all of that mean? Hurricanes derive their energy from water that evaporates from the ocean and ultimately condenses into cloud droplets. The core or a hurricane can be 10-18 degrees Celsius warmer than the surrounding area. But how does that happen? According to NASA's Earth Observatory website,
(Dr. Joanne) Simpson solved an important part of the “warm core” mystery when she applied her “hot-tower hypothesis” to hurricanes. Hurricane winds spray warm water off the tops of ocean waves, and it easily evaporates into water vapor. The heat of the warm ocean water is latent—hidden—in the water vapor. Simpson proposed that a few of the huge cloud towers in the eyewall are active at any one time and that inside these “hot towers” the warm air rises, and water vapor condenses. During condensation, the heat that was latent in the water vapor is released into the upper part of the hurricane eyewall, shedding some of the high energy air into the eye on the way. This process maintains the essential warm core.
Cold core systems like many of the storms that affect the United States, particularly in Fall and Winter, have a cold pool residing in the upper level of the atmosphere (see graphic below). Such storms derive their energy from differences in the temperature of the air. Hurricane Sandy actually transitioned from a warm-core hurricane to a cold-core extratropical system. A recent peer-review study in the journal Climate Dynamics notes in its abstract that Medicanes "develop in those areas of the Mediterranean region where intrusions of cold air in the upper troposphere can produce configurations of thermodynamical disequilibrium of the atmosphere similar to those associated with the formation of tropical cyclones." This is a fancy way of saying suggest there are elements of a cold-core and tropical environment in place to provide the energetics for these types of storms. It should be clear that some some scientists debate whether there is really a strong distinction between Medicaines and tropical or subtropical cyclones.
Medicane Zorbas is projected to bring hurricane force winds and dangerous flooding to parts of southern Greece this weekend. Though such storms only happen once or twice per year, they can present a significant hazard to society.
