“A DANA is an acronym, of depression isolated at high levels, that is, a system of low pressures that is formed at high levels and is isolated from the general atmospheric circulation . At high levels we understand about 9,000 meters of altitude ”, explains Francisco Martín, meteorologist and coordinator of the RAM ( Magazine of the Amateur of Meteorology ). “Little by little, this drop, without reflection, in principle, on the surface it loses altitude and is reflected at other levels, at 5,000 meters, at 3,000 meters, until it is even reflected on the surface.” DANAS do not always produce adverse phenomena such as heavy rains .
To better understand what a DANA is and how it works, Martín makes a simile with an orchestra of musicians and its director. If the person who conducts the orchestra has among his ranks brilliant musicians, the concert he will give will be wonderful. Now let’s imagine that DANA is the one conducting the orchestra. If you can recruit excellent quality ingredients, as if they were brilliant musicians, the result will be “glorious”. On the contrary, if the ingredients available are of poor quality, the result will also be bad, that is, it will not produce adverse phenomena. “It is a system capable, in some moments, of putting in the same place and at the same moment a set of atmospheric ingredients to generate heavy rains. Sometimes it succeeds and other times it doesn’t ”, explains the expert. And he emphasizes that this can also be done by a front or a storm. “ To produce heavy rains, torrential rains, you don’t need the presence of a DANA . It can be a storm, it can be a cold front that arrives and acts as a conductor that unites all those ingredients to give intense rains ”.
How is a DANA produced?
For a DANA to be formed, a series of factors must be present. In Martín’s words, there must be top-notch ingredients or what is the same “top-quality gasoline .” And what is considered “premium gasoline?
First, you have to give a lot of humidity to withstand the intense, torrential and generalized rains. In addition, the air has to generate updrafts or downdrafts , which is known in meteorology as instability. It is essential that there is a “match” that ignites the gasoline, a trigger mechanism that can be a DANA, a front, a storm, a convergence line … or another atmospheric phenomenon. Finally, if there are winds that vary with height, known as wind shear or shear , the result can be tremendous. An example of wind variation with height would be uplift winds at the surface and southwest or west winds at height. “When this type of wind appears in our latitudes, potentially damaging storms organize. By organizing, they are more durable, more intense and deeper ”, explains Francisco Martín.
The DANAS are more frequent in late summer and early autumn , but they can occur at any time of the year, as long as the ingredients that we have already mentioned are given. They can occur anywhere, although they are more likely to occur in eastern and mountainous areas.
DANAS, rainfall and global warming
Are the DANAS more intense now than before? Francisco Martín thinks that DANAS are not more intense now than before, but that better ingredients are given, better quality gasoline, as he prefers to call them for his training. “This better quality gasoline is due, in part, to a warming world .”
The expert explains that the air is currently retaining more humidity , a consequence of the increase in temperature it is experiencing. Increased humidity generates more intense precipitation, releases latent heat when clouds and precipitation form, and this results in torrential rains now being more intense than in years past .
But can it be categorically said that intense atmospheric events are due to global warming? José Miguel Viñas, Meteored meteorologist, refers to what the IPCC has recently published, that is, the changes in the recently observed climate are generalized, occur throughout the world, are fast, increasingly intense and without precedents in thousands of years. The IPCC leaves out of question the fact that human activities are causing this change, making extreme events such as heat waves, torrential rains and droughts more frequent and of greater intensity.
“This current warm phase obviously entails a series of changes in meteorological patterns on a large scale, but we are still far from knowing in great detail how these changes are manifesting ,” says Viñas.
What is known, as the expert points out, is that Hadley cells are expanding to higher latitudes . These cells are cells that are used in a simplified atmospheric model to understand the atmospheric circulation of the Earth. This phenomenon is occurring mainly in the northern hemisphere. “Due to the way the Earth is warming, more convection is taking place in the tropics. And that is not the region of the world most affected by the rise in temperature. This is causing the Hadley cells to be expanding both in height and latitudinally towards the two northern and southern areas of the Earth. This upsets the entire mechanism in which the atmospheric circulation is organized, between the different latitudinal bands of the Earth ”, says the meteorologist. Viñas explains that the expansion of Hadley cells towards the north is producing changes in the jet stream , the subtropical jet and the polar jet. The subtropical jet is moving further north, and events such as the unusual and subtropical storm Filomena fit in well.
On the other hand, but also related to global warming, there is an accelerated loss of ice in the Arctic which in turn weakens the polar vortex. According to IPCC data, since 1979, 40% of the Arctic ice pack has melted. The polar vortex is the mass of cold air that is found over the polar caps. According to Viñas, this weakening also affects the jet stream . “With this vortex increasingly weakened by the loss of ice, it is more common for the polar jet to show large waves, large troughs and large ridges in our geographic environment.”
For all that has been said, that the polar jet descends at times and the subtropical one rises, situations can arise in which both are. And this increases the probability of hybrid cyclones occurring in our latitudes. By hybrid cyclones we refer to polar cyclones or storms that acquire subtropical characteristics or vice versa. “These transitions have always been possible, but because of these configurations that occur now in the jet, they begin to be more likely than a few years ago,” says Viñas.
The conclusion is that there is an increasing probability of extreme high-impact hydrometeorological events . Viñas cites the storm Gloria from January 2020 and Filomena herself from January 2021 as clear consequences of the changes that are taking place in the atmospheric circulation as a result of warming. Also the DANAS that are not produced in autumn. However, it emphasizes that no immediate causal relationship can be established between these atmospheric phenomena and climate change . “Among other things because in a great catastrophe, for example, a great flood, factors that are not atmospheric come into play, such as poor urbanization in flood zones , something that is very common not only in our country but throughout the world. The final result of a meteorological event or a DANA is often amplified by those factors that are not purely atmospheric, ”explains the expert. “Despite this, an increase in the intensity of rainfall is being detected.”
