DANA against a changing climate – Alfa y Omega

Low separation or “cold collapse”, or isolated depression at high levels (DANA) of the atmosphere as means of calling a natural and well-known physiological phenomenon in subtropical latitudes. Winds at altitude blow mainly from the west, creating highways where they reach the highest speed, creating strong currents known as jet streams. These fairways are usually located in subpolar latitudes but are very meandering, so their winds often reach our latitudes. Their work is essential: when they bend to the north they carry warm air from the south and when they bend to the south they carry cold air from the north. Sometimes the strong winds reach as far as North Africa and the curvature becomes so obvious that the highway breaks and creates a roundabout of cold winds rotating counterclockwise. It separates it from the main road and creates a drop out or a pocket of cold air over some area of ​​the Mediterranean or North Africa.

Cold air at altitude is dense and tends to sink; but at lower levels it is hot and, in the areas near the sea, also very humid. This generates intense instability in the vertical, a lot of energy that could be released. This situation organizes strong storms, a natural way for the atmosphere to balance forces, mixing warm, moist air with cold, dry air. As a result, DANA always has severe storms associated with heavy rain.

There are places, such as the Mediterranean coast of the Iberian Peninsula, which also have a sea that, mostly in autumn, is usually warmer than the air, which contributes to vertical instability, which ‘ inject a lot of moisture. This is why autumn is the most common season for DANA-related heavy rains across the region. In addition, in the Spanish Levante, when the wind at low levels blows from the sea, it carries large amounts of moist air and the interaction with the orography and, in addition to the DANA at its peak, they are the perfect cocktail to generate torrential rains. It is under these conditions that the highest daily rainfall values ​​in all of Spain – and even in Europe – have historically been recorded, and that is why the inhabitants know the severity of the “cold fall.”

On the other hand, it is known that carbon dioxide emissions due to the burning of fossil fuels are driving the Earth’s global climate to conditions similar to those at the end of the Mesozoic era. At that time, the planet was extremely hot, with an average global temperature that was more than 10ºC higher than today, and it was also very humid. These conditions fostered a lush biosphere that captured, through photosynthesis, carbon in its organic matter and allowed the planet to gradually cool. However, intensive use of fossil fuels reverses this process in a few decades, causing global warming to occur more quickly. climate changes more intense and devastating regional events.

The most recent research indicates that, at a global level, DANAs have become more frequent in recent decades and that this situation will become more prominent in the decades to come. Global warming means that high westerly winds are slowly moving towards the pole. At the same time, too much heat is rising in the polar ice, as the hottest region. So the jet streams become more intense, especially in their curvature, to deepen the exchange of air parcels and restore balance. Therefore, the breakdown and generation of DANA becomes more frequent.

On the other hand, excess heat also accumulates in the seas and oceans. In recent years, meteorologists have been closely watching the Mediterranean Sea, which is warming at a faster rate than the oceans. In the past summers its surface temperature has warmed more than 4 ºC compared to the norm. A warmer sea brings much more water vapor into an atmosphere that is itself warmer and able to hold more moisture. All of this, when a storm like the one associated with DANA forms, causes more precipitation. This has already been seen on the Mediterranean coast in the Iberian Peninsula. There are studies that show that when heavy rains occur in an area, there will be more and more rain throughout the area. The catastrophic event on October 29, which surprised us with its severity, left the intensity record, which had not been recorded before, of 180 liters per square meter in one hour in Turis. Moreover, this episode, which is still recorded in the memory of everyone, is not isolated, but is part of a series of similar events in other areas of the Mediterranean Sea.

So, while we await scientific studies on what climate change means, the above makes us think that we may not face another catastrophic flood, like so many others . Perhaps it is time to acknowledge that the risk associated with flooding in this region is significant, that its frequency will increase in the near future and that many communities are vulnerable if adequate adaptation and prevention measures are not put in place. action There is no doubt that adopting decisions based on the best available scientific evidence, as societies organized in a democratic State, is the most effective way to protect the population.