A powerful atmospheric river (AR) hit California (USA) in February 2024, which brought about massive rainfall (around 8 trillion gallons) in the region. It also brought about hurricane-force winds. It was the second AR within a week that flew over California, also known as ‘Pineapple Express’. Approx 203 mm of rainfall occurred in two days in Los Angeles (LA), California due to this AR. The rainfall amount to more than half of average rainfall in LA, i.e., 362 mm. Since December 2022, as many as 11 ARs passed over California, resulting in unusually wet winters in 2024.
The term ‘Pineapple Express’ refers to ARs that are formed near Hawaii (known for cultivation of pineapples) and flow towards the western coastal region of North America i.e., USA and Canada. Owing to this river, around 5 inches of rainfall can occur per day in California.
What is AR?
An AR can be defined as an extraordinary meteorological phenomenon which involves an elongated, narrow section of the Earth’s atmosphere that transports water vapour from the tropics, regions around the equator, to the poles of the Earth (i.e., higher latitudes). This water vapour is in extremely concentrated form. Owing to these rivers, huge amounts of moisture is deposited over a vast stretch of atmosphere. Upon reaching land, these rivers discharge their moisture in the form of rain and snow.
How ARs Develop
For AR to develop, certain conditions must be met:
Strong low-level winds These winds act as conduits for transporting water vapour across distances. Jet streams in both the northern and southern hemispheres serve as their pathways, with speeds that can reach up to 442 km/h.
High moisture levels Adequate moisture is essential for precipitation to occur.
Orographic lift This process happens when an air mass moves from a lower to a higher elevation, such as over a mountain. As the air mass ascends, it cools rapidly, increasing the humidity and leading to cloud formation and, under the right conditions, precipitation.
Categories of ARs
According to the United States Geological Survey, high-intensity ARs can be as destructive as hurricanes, and have a rating system, but their ratings can be beneficial, hazardous, or both. The ratings range from Category 1 to Category 5, with the higher numbers showing an increasing level of hazard.
Category 1 (weak) AR is a milder and briefer weather event with primarily beneficial effects, such as 24 hours of modest rainfall.
Category 2 (moderate) AR is a moderate storm with mostly beneficial effects, but also somewhat hazardous.
Category 3 (strong) AR is more powerful and longer lasting, with a balance of beneficial and hazardous impacts, which could bring 5–10 inches of rain over 36 hours, enough to help fill up reservoirs but also pushing some rivers close to flood stage.
Category 4 (extreme) AR is mostly hazardous, though also with some beneficial aspects. This type of AR could dump enough heavy rain over several days to bring many rivers to flood stage.
Category 5 (exceptional) AR is basically hazardous, which could last over 100 hours over just as occurred over the Central California coast during the 1996–97, causing over US$ 100 billion in damage.
Some Characteristics of ARs
- ARs pick up moisture from the warming oceans in the tropics.
- While travelling towards North, strong winds drive them, which make them cause heavy rainfall in the regions that they pass through. That is, the ARs dump the moisture over these regions in the form of heavy rain or snow.
- Usually, their width is 250 to 375 miles, while their length can extend up to over a thousand miles.
- They mainly occur on the east coast rather than on the west coast.
- Their appearance is like that of a narrow ribbon or band of moisture that usually stretches from the equator to higher latitudes.
- They are more prevalent during the winter season of the northern/southern hemispheres. At this time, the hemispheres are dominated by extra-tropical cyclones.
- The ARs have varied sizes and strengths. However, the AR of average size can hold the same amount of water vapour as the average flow of water found at the mouth of the Mississippi river, the second-longest river in the continent of North America.
- The ARs that are remarkably strong can carry approximately 15 times the average amount of water vapour.
- When ARs reach the Earth’s surface, they flow against mountains. Upon interacting with the heat, the water vapour in these rivers condenses, leading to rainfall or snowfall.
- While flowing over land surface, these rivers may result in heavy and frequent downpour, drastic increment in the height of the waves, and cyclone-force winds. The conditions resemble that of hurricanes.
- They are a vital source of rainfall. However, at times, they can lead to landslides, mudslides, and flash flooding, which can be devastating both for the public and the environment.
Location and Scale of the ARs
Usually, the ARs are developed over the tropical or sub-tropical regions, where the ocean water gets warm and evaporates into the atmosphere. This water vapour is then carried by strong winds over long distances in the atmosphere.
It is the low-level jet stream where these rivers are formed. The low-level jet stream is a narrow air current that flows fast.
ARs commonly occur in South Atlantic Oceans, extra-tropical North Pacific or Atlantic Ocean, and South-Eastern Pacific Ocean. They often flow towards the west coasts of North and South America. Besides, they pass over Antarctica, Greenland, and the south-central United States.
How ARs are Measured
A metric known as vertical Integrated Vapour Transport (IVT) is used to identify and measure the ARs in satellite data. The integrated water vapour content present in the atmosphere is measured with the help of satellite microwave radiometers. While analysing satellite or model data, scientists try to spot the corridors having a length of over 2,000 km and width below 1,000 km, and that have vertically integrated precipitable water of minimum 2 cm.
Importance of ARs
- All ARs are not destructive. Many of them are mild systems, which cause desired rainfall or snowfall, adding to the water supply. This, in turn, enhances various economic activities, including fisheries, transportation, agriculture, etc.
- Some ARs play an important role in bringing about annual precipitation in some parts of the world.
- There can be a rise in the occurrence and force of ARs due to climate change in several parts of the world, mainly in mid-latitudes. Being an integral part of the Earth’s water cycle, these systems have their impact on the environment. We need to study their impact, which will facilitate preparedness and management. Thus, flood control, water resources management, and other public policies need to be reconsidered.
Impact of ARs
ARs can significantly impact ecosystems and the global freshwater supply. They play a crucial role in ensuring water security for communities and maintaining the balance of the water cycle.
Some of the major impacts of the AR are as follows:
Heavy rainfall and flooding When ARs make landfall, they often bring intense rainfall, which can lead to severe flooding. This is especially problematic in areas with poor drainage or where rivers are already near capacity.
Landslides The heavy rains associated with ARs can saturate soil, increasing the risk of landslides and debris flows, particularly in hilly or mountainous areas.
Drought relief In some regions, atmospheric rivers can bring much-needed moisture during drought conditions. Since only about 1.2 per cent of the Earth’s freshwater is readily available as surface water, this redistribution is vital. While this can alleviate drought, it can also create a sudden and overwhelming influx of water that can lead to flooding.
Snowpack and water supply In mountainous regions, atmospheric rivers can contribute to significant snowfall, which can be beneficial for the water supply as the snow melts. However, if the snowfall is too intense, it can also contribute to avalanche risks and changes in the timing of snowmelt.
Economic impact Flooding and landslides caused by ARs can lead to significant economic losses, including damage to infrastructure, property, and agriculture. The cost of cleanup and repairs can be substantial.
Public safety The impacts of ARs on transportation, utilities, and emergency services can pose risks to public safety. Flooding can disrupt transportation networks, while landslides can block roads and cause accidents.
Ecological effects Flooding can affect local ecosystems by altering habitats, changing water flow patterns, and impacting wildlife. For example, excessive water can erode riverbanks and affect fish spawning grounds.
Impact of ARs in India
Though we cannot see ARs with our naked eyes, they have become more intense over India, leading to heavy downpours and flooding. It was between 1985 and 2020 that the impact of ARs was apparent in India. ARs were responsible for causing around three-fourths of total flooding in the country. Heavy rainfall and precipitation occurred in the southern parts of Western Ghats and Eastern Ghats. AR is one of the factors induced by climate change that increases the intensity of rainfall.
In the past several years, heavy rainfall during the monsoon and humidity with heat in summer season has been drastically intensified in the subcontinent. Owing to global warming, there has been more rainfall during the monsoon as well as irregular precipitation patterns across the subcontinent. A large number of regions now receive ‘extremely heavy’ and ‘very heavy rainfall’ in July, in the country. In 2023, the number of towns receiving ‘extremely heavy rainfall’ was 205, while the number of towns receiving ‘very heavy rainfall’ was 1,113, which was quite high.
According to the Indian Institute of Tropical Meteorology, at the time of monsoon, greater amount of moisture is carried towards the Indian subcontinent. Consequently, more flash floods and landslides occur within the country. Since the 1950s, an increase in the maximum and minimum IVT of AR has been recorded in India, which signifies that ARs have been intensified. ARs were reported to be the most frequent in peninsular India and the Indo-Gangetic Plain. As a result, most devastating floods occurred here.
Due to ARs, the microclimate of a place can be considerably altered, as more water vapour is in the air. This accelerates wind speeds and brings about heavy downpours.
Conclusion
It is the need of the hour to study and examine ARs so that disaster preparedness, water resource management, and weather forecasting can be performed effectively. With the advancement in technology and research, scientists could record and predict these weather phenomena. This can lessen the adverse effects of extreme weather conditions.
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