According to a recent research, scientists have discovered that atmospheric rivers have shifted towards the poles by approximately 6°–10° over the course of the last forty years. This radical shift has had a major impact on the weather conditions by altering rainfall distribution, intensity of rainfall, snowfall, and storm events, especially in mid- and high-latitude regions around the globe.
About the Research
The research was conducted by a team of scientists at the University of California, Santa Barbara, led by Zhe Li and Qinghua Ding. It was published in the Science Advances in October, 2024. During the research, they analysed satellite-based reanalysis data and multiple climate models, and investigated the changing latitudinal positions of atmospheric rivers in both north and south hemispheres. Their findings suggested that these rivers shifted around six to ten degrees polewards on an average during boreal winters since 1979. This finding has profound implications. The shift has led to a marked decline in atmospheric river (AR) activity in lower latitudes (approximately 30°N and 30°S) including regions like California and Southern Brazil and a corresponding increase around 50°N and 50°S, such as in British Columbia, Alaska, and Northern Europe. This change influences global climate by altering moisture transport, leading to more frequent extreme weather events such as floods in some regions and drought in others.
The scientists noted that this poleward migration is not just the result of long-term warming trends but also of changes in the Pacific Ocean’s surface temperature patterns, particularly the increased frequency and persistence of La Nina events over the past two decades. These ocean-atmosphere interactions influence large-scale circulation patterns, steering atmospheric rivers towards higher latitudes.
The researchers further asserted that there has been drastic increment in the levels of global warming across the globe over the past few decades. As a result, extreme weather phenomena, such as heat waves, extreme precipitation, flash droughts, flash floods and winter storms, have become increasingly frequent and severe worldwide.
Atmospheric Rivers
Atmospheric rivers are the long, narrow bands of concentrated water vapour that flow in the direction of air in the atmosphere. The intensity and size of these rivers vary greatly. Generally, extending over 1,000 miles in length and around 375 miles in width, these rivers in the sky transport immense amounts of moisture from tropical and subtropical oceans towards mid-latitude landmasses. They impact the snowfall and rainfall greatly, causing heavy rainfall and storms along the West Coasts of continents such as North America, South America, and Europe.
Do You Know?
An average atmospheric river carries as much water vapour (7.5 to 15 times) as the amount of water that flows out of the Mississippi river.
Formation and Shape of Atmospheric Rivers The supply of moisture from the warm tropical regions leads to the formation of atmospheric rivers. But these atmospheric rivers are distinct from each other. As the jet stream is highly dynamic and sensitive to temperature gradients, the ARs move towards the poles in different forms. Some ARs stretch across oceans and continents, bending and breaking as they interact with mountains or pressure systems. Their shifting shapes affect not only when rain falls but also how intense it will be. That is, their shape changes during the movement towards the poles.
Common Tendency of Atmospheric Rivers Most ARs are concentrated in the extratropical zones, lying in both hemispheres, between 30° to 50° latitudes. These belts include regions like the Western United States, Southern Australia, Southern Chile, Parts of Chine, and Southern Europe.
As a clear tendency has been observed—reduced AR activity around 30°N/S and increased intensity near 50°N/S—locations such as Alaska, British Columbia, and Scandinavia now face more extreme rainfall, while subtropical zones such as California and Southern Brazil are witnessing increased aridity.
Do You Know?
The ‘Pineapple Express’ is a notable atmospheric river that brings water vapour to the West Coast of North America, originating from the tropical Pacific close to Hawaii and causing heavy rainfall in Oregon, British Columbia, some parts of California, and Washington.
Causes of the Poleward Shift of the Atmospheric Rivers
In the eastern tropical Pacific, the surface temperatures of the sea got altered which is the major cause of the poleward shift of the ARs. In this region of the Pacific, there has been a long-term cooling tendency in the waters, particularly since 2000. Resultantly, atmospheric circulation is affected around the world. This cooling tendency is attributed to La Nina conditions, which drive these rivers to the Poles.
The atmospheric rivers are moving towards the poles owing to a chain of processes interlinked with each other.
La Nina Conditions During La Nina phases, the surface temperatures of the sea in the eastern tropical Pacific cool down, strengthening the Walker circulation, an atmospheric loop that increases convection in the Western Pacific. As a result of this powerful circulation, the tropical rainfall belt expands, together with alterations in the patterns of atmospheric current, causing wind patterns and high-pressure anomalies steering atmospheric rivers away from the equator.
[The Walker circulation refers to the huge loops of air that sometimes rise and sometimes fall over the Tropics, thereby influencing rainfall or snowfall.]
El Nino Conditions On the contrary, during El Nino events, the central and eastern Pacific become warmer, weakening the Walker circulation, and tend to keep ARs closer to the equator. However, the relative dominance of La Nina-like conditions over the last two decades has tilted the balance towards a poleward AR trajectory.
According to long-term assessments by the Intergovernmental Panel on Climate Change (IPCC), global average temperatures have risen about 1.1 °C since pre-industrial times. This warming has influenced atmospheric circulation, particularly the jet streams, causing them to shift towards the poles. As a result, ARs are also moving polewards.
Consequences of Poleward Shift of the ARs
The changing location of ARs is producing a range of climatic consequences:
- In Subtropics (30°N/S), fewer ARs are reaching areas like California, Southern Brazil, and North Africa. This is leading to reduced annual rainfall, water scarcity, wildfires, and agricultural stress.
- In Mid and High Latitudes (50°N/S and above), regions are now experiencing heavier rainfall, flooding, landslides, and storm surges, affecting places like Alaska, northern Europe, and parts of eastern Russia.
- In Polar regions, the ARs reaching the Artic contribute to the accelerated sea ice melting, warming of permafrost, and loss of habitat for polar species. These feedback loops further intensify global warming.
Many regions that depend on ARs for over half of their annual water supply, such as New Zealand, Portugal, the UK, southeastern US, and Chile may face erratic water availability, with implications for drinking water, agriculture, and energy generation.
A study published in Communications Earth & Environment journal in May 2023 issue revealed that 70 per cent of India’s major floods between 1985 and 2020 were linked to ARs, including 2013 Uttarakhand and 2018 Kerala floods. India witnessed 596 major AR events over 95 per cent occurring between June and September, with their frequency and intensity rising in recent decades due to global warming.
Impacts of ARs in the Future
Until now, the shifts in the atmospheric rivers have been the consequences of natural processes. However, global warming due to reckless human activities may also be one of the factors responsible for the shifts. Owing to global warming, the atmospheric rivers will be more frequent and intense in future. This is so because greater amount of moisture can be held by warmer atmosphere. Frequent and severe atmospheric rivers may increase the risk of floods, posing new challenges on agriculture, infrastructure, and disaster management in places not accustomed to heavy downpour.
However, it is difficult to predict future changes, as the natural swings between La Nina and El Nino cannot be forecast properly. It remains complex due to uncertainties in jet stream dynamics, and the interaction between natural and anthropogenic factors. Additionally, climate models often underestimate internal variability, which limits the accuracy of long-term AR forecasts.
Conclusion
The poleward shift of atmospheric rivers highlights the urgent need for advanced climate models and flexible strategies for water management that can be applicable across the globe. Weather forecasts need to be improved upon so that frequency, intensity, and direction of atmospheric rivers can be known beforehand to adapt to the climatic changes. Further research must be done to determine impacts of these shifts comprehensively. This will enable the people to find out the ways of dealing with different precipitation patterns and weather phenomena.
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