Jochen Kerkmann (EUMETSAT) and Scott Bachmeier (CIMSS)Jump to images
According to Wikipedia, Mediterranean tropical cyclones are rare weather phenomena. These systems are the subject of some debate within meteorological circles, whether they closely fit the definition of tropical cyclones, or subtropical cyclones. Their origins are typically non-tropical, and develop over open waters under strong, initially cold-core cyclones (cut-off lows), similar to (sub)tropical cyclones in the Atlantic Basin. Cold air aloft appears to be the main trigger for instability in the development of these systems. If a "hurricane season" were ever to be defined for the Mediterranean region, it would extend from June to May the following year, based upon occurrences so far. Thus, the formation of Mediterranean (sub)tropical cyclones are possible, during anytime of the year, but with a maximum frequency in autumn (September-December) when both cut-off lows and warm surface water occur at the same time.
According to Robert Hart (Florida State University), tropical and extratropical cyclones have, historically, been viewed as two discrete, mutual exclusive cyclone groups. However, warm SSTs, increased surface fluxes, enhanced convection and enhanced latent heat release can blur that once-perceived fine line between tropical and extratropical cyclones. While it is well known that symmetric, non-frontal warm-core tropical cyclones can become asymmetric, frontal, cold-core extratropical cyclones, it is less well known that, in contrast, cold-core, extratropical cyclones can convert to warm-core, tropical cyclones. This often occurs when a cold-core, cut-off low remains stationary for several days over an area with relatively high SSTs (but not necessarily 26°C).
Additionally, and also according to Wikipedia, meteorological literature documents a total of 50 "Mediterranean Hurricanes" that have occurred in: September 1947, September 1969, September 1973, August 1976, January 1982, September 1983, December 1984, December 1985, January 1991, October 1994, January 1995, September 1996, twice in October 1996, September 1997, March 1999, twice in September 2003, October 2003, August 2005, September 2005, twice in October 2005, December 2005, August 2006, twice in September 2006, March 2007, twice in October 2007, June 2008, August 2008, September 2008, December 2008, January 2009, May 2009, twice in September 2009, October 2009, October 2010, November 2010, November 2011 and twice in February 2012. However, 2 of the storms formed over the Black Sea, in 2005, and again in January 2012.
The tropical storm of November 2011 started over the period 4-6 November 2011, when an extratropical system slowly transformed into a subtropical low over the warm waters of the Mediterranean Sea (see Airmass RGB Animation, 3 Nov 01:00 UTC - 7 Nov 08:00 UTC, MPG, 8421 KB). The storm was then given the identification Invest 99L, by the United States Naval Research Laboratory (NRL). As the storm slowly moved westwards, it caused flooding in Spain and the Balearic Islands (see HRV loop of 6 November 2011, 07:15-16:00 UTC, MPG, 4945 KB). As the storm further continued its westward movement, it slowly became organised, and convection began to increase. On 7 November 2011, NOAA began watching the subtropical area of low pressure, now located in the Gulf of Lions and which NOAA had earlier identified as INVEST 99L, as the storm organised itself into a subtropical disturbance. Later that day, the subtropical disturbance transformed and strengthened, into a tropical depression off the coast of France. The storm was then given the identification 01M/99L by NOAA. Late on November 7, the storm was upgraded to tropical storm status as it strengthened significantly. At that time, the Satellite Services Division and NESDIS both classified the storm as Tropical Storm 01M. On 8 November 2011, the storm continued to strengthen as it came closer to France. At peak intensity, the storm had a minimum low pressure of 991 hPa. Wind speeds were estimated to have reached 45 knots according to various satellite analysis techniques (see also Metop-A ASCAT wind product, PNG, 324 KB). On November 9, however, the storm made landfall in south-eastern France, near Hyères, where it dissipated completely shortly thereafter.
Overall, the tropical storm caused severe flooding, in parts of Spain, Italy and France. From 6 to 8 November 2011, the storm produced a total of 600 mm of rain in about 72 hours over southwestern Europe. 11 people died from the storm; 6 being Italian and 5 French.
The animations below show several sequences of Meteosat-8 HRV images at 5-minute intervals (rapid scans). The development of the tropical storm over the western Mediterranean Sea during the period 6-9 November can be easily followed. The rapid scan HRV images have also been used to derive an approximate storm track (PNG, 869 KB), based on the low level circulation centres that are sometimes visible when there are enough gaps in the high level clouds.
Some other interesting tropical storm cases over the Mediterranean Sea and the North Atlantic are given in the following list:
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