If there is a field of study within atmospheric dynamics whose notoriety has grown exponentially in recent decades, it is undoubtedly that of atmospheric rivers (AR).
Perhaps the reason is that atmospheric rivers play a fundamental role in a large number of factors in the weather, such as the radiative (energy) balance of the planet or its hydrological cycle. Or perhaps it is their growing tendency to be named in the weather 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 are atmospheric rivers?
Atmospheric rivers are regions of the atmosphere whose moisture content is much higher than that of neighboring regions. They are usually 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 the mid-latitudes.
Their nature allows them to function as great 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 of 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”.
Its role in rainfall
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, provide an essential amount of moisture to the atmosphere from mid-latitudes and continental, which could not be received otherwise.
Other atmospheric rivers, however, are extreme phenomena that can bring associated rainfall of more than 100 l / m² in a single day, having a negative economic and social impact on the regions that are affected by them.
At the 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 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.
What will the atmospheric rivers of tomorrow be like?
The answer to the question of what the atmospheric rivers of tomorrow will be like depends, of course, on the atmosphere in which they reside.
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 professors 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 make 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 increased – and therefore most likely will increase – by approximately 7% for each degree Celsius of humidity that the lower part of the atmosphere warms up.
This is a well-known ratio to students of thermodynamics, as it is predicted by the Clausius-Clapeyron equation, which determines the maximum amount of moisture that an air cell can hold before reaching saturation.
In addition, 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 in the form of atmospheric rivers originates: the Gulf of Mexico.
A warmer atmosphere will be a more humid atmosphere
A warmer atmosphere will be a more humid atmosphere, and we now have strong reasons to assume that this increase in humidity will carry over 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 reason, since a good part of the resources that the planet provides us depends on the climate.
Between that complex collage Among the phenomena that will constitute the climate of tomorrow, atmospheric rivers seem to play a prominent role to which we can attribute, without fear of being wrong, a good part of the water that reaches our houses, crops, reservoirs and rivers.