Nowadays, much is said about climate change and its consequences. For example, the increase in the average global temperature, the appearance of rare and extreme precipitation events, severe droughts, the melting of glaciers and ice caps, and the consequent sea level rise.
Atmospheric Rivers (ARs) are structures that transport large amounts of moisture in the atmosphere. They were equated to land rivers, such as the Amazon River, which is the most powerful in the world, hence the origin of the name. According to the meteorological glossary of the American Meteorological Society (AMS), ARs are long, narrow, and transitory corridors of intense horizontal transport of water vapor, normally found in the lower troposphere, associated with the cold fronts of extratropical cyclones. In the mid-1970s, phenomena with similar characteristics were already being talked about, but it was only in 1990 that they were named “Atmospheric Rivers”.
Prolonged droughts and floods are clear evidence of climate change that can be closely linked to ARs. The ARs are responsible for transporting large amounts of moisture from tropical and extratropical regions to higher latitudes and polar regions, representing about 90% of the northward transport of moisture. They are found in both the Northern and Southern Hemispheres, where they intensify precipitation and cause flooding and landslides; and as an important component of the hydrological cycle, represent a strong contribution to the replenishment and redistribution of water in terrestrial rivers and groundwater, in the moisture content of the soils and even in the thickness of the snow layers in mountainous or polar regions. In the polar regions, ARs are also responsible for the formation of large polynyas, which are areas where one would expect to find sea ice, but that are thawed due to the transport of moisture from the tropics and extratropics.
The Arctic Amplification refers to the Arctic warming about two or three times faster than the rest of the globe, resulting in a decrease in the extent of the Arctic ice. Arctic ice loss can influence climate in mid-latitudes. The climate of the northern hemisphere has undergone several evident changes, with reductions in the amount of annual precipitation, prolonged droughts in some regions, increase in extreme precipitation phenomena, and an increase in the frequency of tropical and extratropical cyclones, among others.
How do ARs respond to Artic sea ice loss?
To better understand this aspect, which is still poorly supported in the literature, a study was carried out at the University of California, in the United States of America. In this study, the researchers used AR detection algorithms, which estimates the amount of water vapor transported in the atmosphere, forced by atmospheric climate models that consider the loss of Arctic sea ice. The researchers modelled the Northern Hemisphere in winter, corresponding with the passage of many tropical and extratropical cyclones, thus being associated with a greater number of ARs. The frequency response of ARs to Arctic sea ice loss corresponds to a global warming of 2°C, according to this study. In terms of region variation, this research concludes that over the North Pacific the ARs extend to the northeast and occur closer to the west coast of North America. Over the North Atlantic, the ARs move to the equator. The response of ARs at midlatitudes is mainly governed by changes in wind. Above 60°N latitude, weak winds tends to reduce the frequency of ARs while increasing atmospheric moisture (due to loss of sea ice) tends to increase their frequency, resulting in relatively small changes in the ARs.
However, other phenomena associated with climate change may contribute to changes in ARs, such as changes in ocean surface temperature. The Arctic amplification has motivated an increase in the frequency of extreme events and ARs, which has modified the climate of mid-latitudes, including extreme precipitation events, more rigorous winters, prolonged droughts and desertification.
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Source: Ma, W., Chen G., Peings Y., Alviz, N., Atmospheric River Response to Arctic Sea Ice Loss in the Polar Amplification Model Intercomparison Project, Geophysical Research Letters 48:20, e2021GL094883 (2021). DOI: 10.1029/2021GL094883.
Author: Cátia Lavinia Gonçalves
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