Strange weather phenomenon causes flash floods in Spain

On October 29, some areas in Spain experienced a year’s worth of rain in just a few hours, causing severe flooding that destroyed a series of towns and left thousands of people stranded. In some places, rainfall reached 50.8 cm (500 liters/m2). The cause of this disaster was a phenomenon formed in the Mediterranean called Depresión Aislada en Niveles Altos (DANA), which means isolated low pressure at high altitudes. This is the most severe DANA recorded in the 21st century, according to the Spanish Meteorological Agency (Aemet).

What is DANA?

DANA occurs when a warm air mass meets stagnant cold at an altitude of about 9,000 m. In the upper atmosphere, there is a very strong wind that surrounds the Earth like a belt. Sometimes, the wind begins to fluctuate, moving undulating more like a snake than a belt. That oscillation can become “stuck”, allowing cold air to stay in place.

On October 29, in southeastern Spain, DANA hovered in the same area for more than 12 hours, making that day the day with the most intense weather event and is expected to last with less intensity until until November 3. DANA uses warm water as fuel, according to meteorologist Mar Gómez. DANA encountered water temperatures of 22 degrees Celsius off the coast of Valencia when the normal temperature for this time of year is 21 degrees Celsius. The temperature contrast combined with moisture and energy from the Mediterranean Sea, the region The sea is very warm after the summer months, resulting in strong storms and pouring rain.

“Winds may not be as intense as those in tropical storms, but in terms of intensity and rainfall, they can even surpass them. These types of events can cause as much damage to life and property as tropical storms.” normal zone,” said Jorge Olcina, director of the Meteorological Laboratory at the University of Alicante.

Iago Pérez, a geoscientist at Oxford University, describes DANA as one of the most dangerous meteorological phenomena in Spain because “they release huge amounts of water in a very short time”. DANA only formed over Spain, but similar weather patterns, called extratropical cyclones, can form in the Atlantic Ocean off Uruguay and Argentina.

Impact of climate change

Both Gómez and Olcina said the severity of DANA this week was directly related to climate change. However, Pérez said that linking the phenomenon to global warming requires deeper analysis.

The Mediterranean Sea is one of the ocean basins that has warmed the most in recent decades. It acts as a “moisture and energy conveyor belt,” according to Olcina. Since the 1980s, the average temperature of the Mediterranean has increased by 1.5 degrees Celsius, nearly double the increase in air temperature in the region over the same period. Since 2020, the summer in the Iberian Peninsula has recorded record temperatures and this year, the sea surface temperature has exceeded the mark of 29 degrees Celsius.

This warming changes the timing of DANA because the Mediterranean now begins to warm in May and remains warm through November. In the mid-1980s and 1990s, this phenomenon often occurred during the month of May. 9 and 10. Currently, it is estimated that the number of DANA formations per year has increased by 15 – 20% compared to 6 decades ago.

For researchers, the event provided important lessons, including the need to improve early warning. Climate change is likely to fuel more frequent intense rainfall events. That highlights the urgent need to adapt flood defense systems and rehabilitate vulnerable areas to reduce the risk associated with increased extreme weather, according to Olcina.