When the Atlantic hurricane season begins to quiet down in late October and November, it’s time to cast an eye toward the Mediterranean Sea for “medicanes”--a nickname for storms that develop tropical characteristics just off the coast of southern Europe. Medicanes aren’t considered full-fledged tropical systems, since the waters of the Mediterranean aren’t extensive or warm enough to sustain a true hurricane. However, it’s quite possible for an existing center of low pressure in the Mediterranean to briefly take on tropical characteristics, including a symmetric structure and a small core of warm air. Such was the case with a low that intensified last weekend while traveling from near Malta (south of Italy) toward the Greek island of Crete. As it pushed east, this medicane sent high surf west toward Malta
and east toward Israel
The storm swept across Crete on Monday with high winds and heavy rain. On Crete’s northwest coast, the city of Chania (Souda Air Base) recorded 3.07 inches of rain and peak wind gusts of 60 mph
on Monday, October 31. Sustained winds topped out at just 35 mph. Crete’s largest city, Heraklion, on the island’s northeast coast, got 0.71 inches of rain
, with top wind gusts of 46 mph. On Friday, a ship reported a wind gust to 57 mph, noted Capital Weather Gang
. The system probably hit its peak intensity while over the open Mediterranean west of Crete during the weekend. As reported by weather.com
, sea surface temperatures in the region were only about 22 - 24°C (72 - 75°F). That’s as much as 2°C above average for this time of year, but well below the usual 26°C benchmark for tropical development. Still, it appears the system briefly took on the symmetric warm-core features typical of a tropical storm.Figure 1.
MODIS satellite image of the medicane approaching Crete taken on Sunday afternoon, October 30, 2016. John Knaff (CIRA/RAMMB/Colorado State University) produced this large-scale black-and-white satellite loop
. Image credit: NASA.Figure 2.
Image from a color-enhanced infrared satellite loop
of the medicane centered just south of Greece’s Peloponnese peninsula at 0345Z (05:45 a.m. local time) Monday, October 31, 2016. Image credit: Scott Bachmeier, CIMMS/SSEC/University of Wisconsin-Madison.Where to watch for medicanes
This week’s storm occurred in one of two “hot spots” where medicanes are most likely to develop, according to a long-term climatology
published in 2014 in Climate Dynamics
. Interpolating from long-term atmospheric data for the period 1948-2011, the authors estimated that medicanes occur once or twice per year, on average, but with much year-to-year variability. None of the world’s meteorological agencies are tasked with monitoring medicanes, so countless such storms have no doubt gone unrecognized
, especially before routine satellite monitoring began. The most favored area for development is in the western Mediterranean, especially between Spain and the islands of Sardinia and Corsica. The other hot spot is the Ionian Sea, especially just southeast of Malta and the Italian peninsula, where this week’s storm cranked up. In both areas, medicanes become more likely in autumn, peak in winter, and decrease in spring, according to the study, although the western area has a broader “season.”
Since the waters of the Mediterranean aren’t warm enough to support conventional hurricane development, medicanes rely on colder air aloft, typically brought in as part of an upper-level low that decays over the Mediterranean. Wind shear relaxes as the upper low decays, and the contrast between the cold air aloft and the relatively warm sea surface temperatures can stimulate the formation of showers and thunderstorms. These, in turn, may congeal around a weak surface low and help give it a symmetric, warm-core structure--and sometimes even a cloud-free, eye-like feature. Often a medicane’s warm core will be enveloped within broader cold-core features, which makes it more akin to a hybrid or subtropical storm than a tropical storm.
The last Mediterranean storm to get this much notice occurred in the first week of November 2014
. It was dubbed Qendresa by the Free University of Berlin, whose vortex-naming practices have become the default choice for medicanes. Qendresa produced wind gusts as high as 96 mph on the north coast of Malta. Winds at the Luqa, Malta, airport looked suspiciously like what one would observe with a tropical storm passing overhead--a double peak with a near-calm in between, with the pressure falling to 984 mb during the calm. Wunderground member Zivipotty
, a meteorology student in Hungary who has analyzed Qendresa
, believes it was primarily subtropical in nature, as it weakened rapidly once it became detached from its parent frontal system. In early November 2011, another noteworthy storm named Rolf
took shape in the western Mediterranean. Rolf was the only medicane to be officially monitored by NOAA, whose Satellite Analysis Branch named it 01M and tracked it for two days
. Zivipotty found Rolf
to be more tropical in nature than Qendresa at its peak. Figure 3.
Winds at the 250-mb level (about 34,000 feet above sea level) were very weak (less than 20 knots on the legend at right, or 23 mph) above the medicane, which is shown here as a 1013-millibar low southwest of Greece at 12Z (2:00 pm local time) Sunday, October 30, 2016. Image credit: tropicaltidbits.com
Near-surface winds derived from the space-borne European Advanced Scatterometer (ASCAT) show speeds topping 50 knots (58 mph) around the center of the medicane as it approached Crete on Monday, October 31. Image credit: ASCAT and NOAA/NESDIS
.Could bona fide hurricanes develop in the Mediterranean later this century?
According to research published in 2007, an increase in ocean temperatures of 3°C (5.4°F) in the Mediterranean by the end of the century could lead to hurricanes forming there. Miguel Angel Gaertner of the University of Castilla-La Mancha in Toledo, Spain, ran 9 different climate models with resolutions of about 50 km and found that some (but not all) of the models simulated hurricanes in the Mediterranean in September by the end of the century, when sea surface temperatures there could reach 30°C (86°F). Though the Mediterranean could start seeing hurricanes by the end of the century, these storms should be rare and relatively short-lived for three reasons:
1) The Mediterranean is quite far north and is subject to strong wind shear from jet stream activity.
2) The waters are shallow, and have relatively low heat content. There is no deep warm water current like the Gulf Stream.
3) The Mediterranean has a lot of large islands and peninsulas poking into it, increasing the chances that a tropical storm would weaken when it encountered land.Wikipedia
has an excellent page on Mediterranean tropical-like cyclones (medicanes).Figure 5
. Infrared image from Japan’s Himawari-8 satellite of Invest 90W and invest 99W in the Northwest Pacific at 1630Z (12:30 pm EDT) Wednesday, November 2, 2016. Image credit: CIMMS/SSEC/University of Wisconsin-Madison
.A potential tropical threat to the Philippines next week
Two large but loosely organized tropical waves in the Northwest Pacific are expected to gather strength, and one of them may pose a significant threat to the Philippines in a week or so. The westernmost of the two waves, Invest 90W, will have supportive conditions for growth as it moves west over the next few days: very warm sea surface temperatures of 29 - 30°C, a very moist atmosphere (mid-level relative humidities around 80%), and fairly light wind shear of around 10 - 15 knots. By early next week, the GFS, European, and UKMET models all project Invest 90W to reach typhoon strength. On its westward track, 90W could be approaching the Philippines by the middle of next week, perhaps still strengthening. The other wave, Invest 99W, should track further northwest, possibly gaining typhoon strength while over the open Northwest Pacific.
Meanwhile, the National Hurricane Center is keeping an eye on two systems with limited potential for development. A nontropical low well northeast of the Lesser Antilles may take on subtropical characteristics late this week as it heads toward the central North Atlantic, while a disturbance west of Costa Rica could become a tropical cyclone as it heads toward the northeast Pacific. NHC gives both systems a 20% chance of development over the next five days.
We’ll be back with a new post on Friday.
Bob Henson and Jeff Masters