Jorge Eiras Barca and Iago Algarra Cajide
If there is a field study within the atmospheric dynamics whose notoriety has experienced an exponential growth in the last decades is, without a doubt, that of the atmospheric rivers (AR, for its acronym in English).
The reason perhaps it is that atmospheric rivers play a prominent role in a large number of factors of the climate such as the radiative (energy) balance of the planet or its hydrological cycle. Or perhaps it is their growing tendency to be named in meteorological bulletins when, in the company of a tropical storm or explosive cyclogenesis, they sometimes bring more than 70 l / m² of precipitation in a few hours.
What What are atmospheric rivers?
The atmospheric rivers are regions of the atmosphere whose moisture content is much higher than that of neighboring regions. They tend to be very long and (relatively) narrow regions – thousands of km long versus a few hundred km wide – and usually accompany the cold fronts so characteristic of mid-latitudes.
Their nature allows them to function as large highways that distribute moisture – and with it, energy in the form of latent heat – from the humid and warm subtropical and tropical regions to the rest of the planet.
These formations are, therefore, essential for the maintenance of the good health of our hydrological cycle, and an indispensable mechanism for the radiative balance of the planet. Their elongated shape and the enormous amount of water they carry (greater than the flow of the Mississippi River) have inspired the characteristic and attractive name of "atmospheric rivers".
Their role in rainfall
The Atmospheric rivers present a huge variability between them. No two are the same. Most are events of moderate intensity, and are therefore considered beneficial. Among other things, contribute an indispensable amount of moisture to the atmosphere of mid-latitudes and continental, which could not be received in any other way.
Other atmospheric rivers, however, are extreme phenomena that can be associated rainfall exceeding 100 l / m² in a single day, having a negative economic and social impact on the regions that are affected by them.
At a global level, these are common phenomena. There are usually about three or four simultaneously for each hemisphere, usually located on the great oceanic corridors. Its high season is the winter corresponding to each hemisphere, when the atmosphere is less humid, but much more dynamic than that of the summer season.
The western coasts of the great continents, including the Iberian Atlantic coast, are the regions hot of arrival of atmospheric rivers . Those arriving in Spain transport a high percentage of rain from the Gulf of Mexico. In winter the Iberian Peninsula usually receives 3 or 4 a month.
Another active region of the Spanish-speaking world is the coast of Chile, where the atmospheric rivers of the Pacific usually generate significant rainfall in their interaction with the Andes mountain range.  a) Satellite composition (05 November 2020) of precipitable water in mm where an RA is observed crossing the Atlantic Ocean until it reaches the coast of Iceland. b) Three-dimensional view of an AR simulated with the WRF meteorological model. c) and d) Anomalous humidity regions associated with RA and its increasing progression in the period 1979-2016
Sources: CIMSS / Eiras-Barca et al. (2016) / Algarra et al., (2020)
What will the atmospheric rivers of tomorrow be like?
The answer to the question of how the atmospheric rivers of tomorrow will be depends, as is logical, how is the atmosphere they reside in.
Most prospective analyzes predict a warmer atmosphere, and with different dynamics. In this context, it is considered that atmospheric rivers will tend to be more frequent, and also more intense, although with great differences between the different regions of the planet.
In a recent study led by teachers Luis Gimeno and Raquel Nieto from the University of Vigo, and carried out in collaboration with the University of Lisbon and the University of Illinois, we have analyzed the variation in moisture content during the last decades in strategic regions for the phenomenon. This helps us to carry out a robust projection and determine what tomorrow will be like in a context of global warming.
In the article, published in Nature Communications it is shown that the moisture content has been increased – and therefore, very likely, will increase – by approximately 7% for each degree centigrade of humidity that the lower part of the atmosphere warms.
This is a well-known ratio to thermodynamicists, as it is predicted by the Clausius-Clapeyron equation which determines the maximum amount of moisture that an air cell can contain before reaching saturation.
Furthermore, we have shown that, of all the regions of the planet, the clearest signal in this regard is observed precisely in the region where most of the humidity that reaches Europe originates in the form of atmospheric rivers: the Gulf of Mexico.
The warmer will be a more humid atmosphere, and we now have strong reasons to assume that this increase in humidity will move at a similar rate to atmospheric rivers.
The amount of humidity that we will receive in the future from the subtropical regions will be greater, and also the probability of extreme precipitations, not very convenient for the correct use of water as a resource, and dangerous.
The effort of the scientific community to try to understand, predict and anticipate the climate of the future is great, and not without This is because a good part of the resources provided by the planet depend on the climate.
Among this complex collage of phenomena that will constitute the climate of tomorrow, atmospheric rivers seem to play a prominent role in that we can attribute, without fear of being wrong, a good part of the water that reaches our houses, crops, reservoirs and rivers.
The article Atmospheric rivers, the air highways that regulate the climate has been written in Notebook of Scientific Culture .