The flash flood that killed thousands of people in Libya this week followed a "medicane" – a rare but destructive weather phenomenon that scientists believe will intensify in a warming world.
The term is an amalgamation of the words "Mediterranean" and "hurricane". Used by scientists and weather forecasters, it is less well-known to the wider public.
Medicanes, which tend to form over parts of the Mediterranean Sea near the North African coast, are similar to hurricanes and typhoons, although they can develop over cooler waters.
On satellite imagery they look like a swirling mass of storm clouds with an eye in the middle.
Fierce winds and approximately 170 millimetres of rain were unleashed by Storm Daniel when it made landfall in Libya on Sunday.
Scientists say the storm bore all the hallmarks of a medicane.
"We are confident that climate change is supercharging the rainfall associated with such storms," says professor Liz Stephens from the University of Reading.
I don't think it is possible to fathom the destructive power of a dam break, this video is truly terrifying.— Prof. Liz Stephens (@liz_stephens) September 12, 2023
In this situation, the water management infrastructure has added to the risk from #StormDaniel's exceptional rainfall... https://t.co/oUOCerQYN2
The Mediterranean cyclones are usually smaller and weaker than their tropical equivalents and have a smaller space in which to develop.
However their peak strength is usually the equivalent of a Category 1 hurricane on the Saffir-Simpson scale, encompassing speeds of 119 to153 kilometres per hour.
Medicanes tend to form in the autumn when the sea is warm, usually in the western Mediterranean and the region between the Ionian Sea and the North African coast.
A layer of colder air from higher altitudes forms convections with warmer air rising from the sea that converge around a centre of low pressure.
Rare but deadly
US National Oceanic and Atmospheric Administration says that medicanes form once or twice per year on average.
While hurricanes move from east to west, medicanes tend to go from west to east.
Before striking Libya, Storm Daniel also struck Bulgaria, Greece and Turkey last week.
Between 2016 and 2018, three medicanes occurred off Greece, while in 2019 Spanish weather services identified one between the Balearic Islands and the Algerian coast.
A medicane packing winds of up to 120 kilometres per hour, dubbed Ianos, hit Greece in September 2020, killing three people in the city of Karditsa and triggering floods, landslides and power cuts.
The Italian island of Sicily was also struck in 2021.
We have a #medicane in the making. Storm #Daniel in the Mediterranean now has tropical characteristics heading south and will make landfall tonight near Benghazi, Libya. The dark shades of purple are winds 50 mph+ pic.twitter.com/3J3ZTnCEDU— Jeff Berardelli (@WeatherProf) September 9, 2023
In 2020, French weather service Meteo France said it was difficult to work out climate signals from medicanes due to their rarity.
While scientists are increasingly able to unpick the likely effect of climate change on the probability of an extreme weather event happening and its intensity, no such attribution study has yet been carried out on Storm Daniel.
In general, experts say the warming of sea surface temperatures, driven by human-induced climate change, is going to make extreme storms more intense.
Oceans have absorbed 90 percent of the excess heat produced by human activity since the dawn of the industrial age, according to scientists.
Spanish researchers said the Mediterranean reached its highest temperature on record in July as Europe baked under a series of heatwaves.
The surface waters of the eastern Mediterranean and Atlantic are two to three degrees Celsius warmer than usual, which would have turbocharged Daniel.
"The fact that Daniel could form into a medicane is likely a result of warmer sea surface temperatures and hence man-made climate change," said climate scientist Karsten Haustein of Leipzig University.