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An atmospheric river (AR) is a narrow corridor or filament of concentrated moisture (hydrosphere) in the atmosphere. Other names for this phenomenon are tropical plume, tropical connection, moisture plume, water vapor surge, and cloud band. [1] [2]
Atmospheric rivers consist of narrow bands of enhanced water vapor transport, typically along the boundaries between large areas of divergent surface air flow, including some frontal zones in association with extratropical cyclones that form over the oceans. [3] [4] [5] [6] Pineapple Express storms are the most commonly represented and recognized type of atmospheric rivers; the name is due to the warm water vapor plumes originating over the Hawaiian tropics that follow various paths towards western North America, arriving at latitudes from California and the Pacific Northwest to British Columbia and even southeast Alaska. [7] [8]
In some parts of the world, changes in atmospheric humidity and heat caused by climate change are expected to increase the intensity and frequency of extreme weather and flood events caused by atmospheric rivers. This is expected to be especially prominent in the Western United States and Canada. [9]
The term was originally coined by researchers Reginald Newell and Yong Zhu of the Massachusetts Institute of Technology in the early 1990s to reflect the narrowness of the moisture plumes involved. [3] [5] [10] Atmospheric rivers are typically several thousand kilometers long and only a few hundred kilometers wide, and a single one can carry a greater flux of water than Earth's largest river, the Amazon River. [4] There are typically 3–5 of these narrow plumes present within a hemisphere at any given time. These have been increasing [11] in intensity slightly over the past century.
In the current research field of atmospheric rivers, the length and width factors described above in conjunction with an integrated water vapor depth greater than 2.0 cm are used as standards to categorize atmospheric river events. [8] [12] [13] [14]
A January 2019 article in Geophysical Research Letters described them as "long, meandering plumes of water vapor often originating over the tropical oceans that bring sustained, heavy precipitation to the west coasts of North America and northern Europe." [15]
As data modeling techniques progress, integrated water vapor transport (IVT) is becoming a more common data type used to interpret atmospheric rivers. Its strength lies in its ability to show the transportation of water vapor over multiple time steps instead of a stagnant measurement of water vapor depth in a specific air column (IWV). In addition, IVT is more directly attributed to orographic precipitation, a key factor in the production of intense rainfall and subsequent flooding. [14]
Cat | Strength | Impact | Max. IVT [lower-alpha 1] |
---|---|---|---|
1 | Weak | Primarily beneficial | ≥250–500 |
2 | Moderate | Mostly beneficial, also hazardous | ≥500–750 |
3 | Strong | Balance of beneficial and hazardous | ≥750–1000 |
4 | Extreme | Mostly hazardous, also beneficial | ≥1000–1250 |
5 | Exceptional | Primarily hazardous | ≥1250 |
Notes
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The Center for Western Weather and Water Extremes (CW3E) at the Scripps Institution of Oceanography released a five-level scale in February 2019 to categorize atmospheric rivers, ranging from "weak" to "exceptional" in strength, or "beneficial" to "hazardous" in impact. The scale was developed by F. Martin Ralph, director of CW3E, who collaborated with Jonathan Rutz from the National Weather Service and other experts. [17] The scale considers both the amount of water vapor transported and the duration of the event. Atmospheric rivers receive a preliminary rank according to the 3-hour average maximum vertically integrated water vapor transport. Those lasting less than 24 hours are demoted by one rank, while those lasting longer than 48 hours are increased by one rank. [16]
Examples of different atmospheric river categories include the following historical storms: [17] [18]
Typically, the Oregon coast averages one Cat 4 atmospheric river (AR) each year; Washington state averages one Cat 4 AR every two years; the San Francisco Bay Area averages one Cat 4 AR every three years; and southern California, which typically experiences one Cat 2 or Cat 3 AR each year, averages one Cat 4 AR every ten years. [18]
Usage: In practice, the AR scale can be used to refer to "conditions" without reference to the word "category", as in this excerpt from the CW3E Scripps Twitter feed: "Late-season atmospheric river to bring precipitation to the high elevations over northern California, western Oregon, and Washington this weekend, with AR 3 conditions forecast over southern Oregon." [19]
Atmospheric rivers have a central role in the global water cycle. On any given day, atmospheric rivers account for over 90% of the global meridional (north-south) water vapor transport, yet they cover less than 10% of any given extratropical line of latitude. [4] Atmospheric rivers are also known to contribute to about 22% of total global runoff. [20]
They are also the major cause of extreme precipitation events that cause severe flooding in many mid-latitude, westerly coastal regions of the world, including the West Coast of North America, [21] [22] [23] [12] Western Europe, [24] [25] [26] the west coast of North Africa, [5] the Iberian Peninsula, Iran [27] and New Zealand. [20] Equally, the absence of atmospheric rivers has been linked with the occurrence of droughts in several parts of the world, including South Africa, Spain and Portugal. [20]
The inconsistency of California's rainfall is due to the variability in strength and quantity of these storms, which can produce strenuous effects on California's water budget. The factors described above make California a perfect case study to show the importance of proper water management and prediction of these storms. [8] The significance that atmospheric rivers have for the control of coastal water budgets juxtaposed against their creation of detrimental floods can be constructed and studied by looking at California and the surrounding coastal region of the western United States. In this region atmospheric rivers have contributed 30–50% of total annual rainfall according to a 2013 study. [28] The Fourth National Climate Assessment (NCA) report, released by the U.S. Global Change Research Program (USGCRP) on November 23, 2018 [29] confirmed that along the U.S. western coast, landfalling atmospheric rivers "account for 30%–40% of precipitation and snowpack. These landfalling atmospheric rivers "are associated with severe flooding events in California and other western states." [7] [12] [30]
The USGCRP team of thirteen federal agencies—the DOA, DOC, DOD, DOE, HHS, DOI, DOS, DOT, EPA, NASA, NSF, Smithsonian Institution, and the USAID—with the assistance of "1,000 people, including 300 leading scientists, roughly half from outside the government" reported that, "As the world warms, the "landfalling atmospheric rivers on the West Coast are likely to increase" in "frequency and severity" because of "increasing evaporation and higher atmospheric water vapor levels in the atmosphere." [7] [29] [31] [32] [33]
Based on the North American Regional Reanalysis (NARR) analyses, a team led by National Oceanic and Atmospheric Administration's (NOAA) Paul J. Neiman, concluded in 2011 that landfalling ARs were "responsible for nearly all the annual peak daily flow (APDF)s in western Washington" from 1998 through 2009. [34]
The front cover of the NCA4 report features a natural-color NASA image of conditions over the northeastern Pacific on February 20, 2017. The report said that this AR brought a "stunning" end to the American West's 5-year drought with "some parts of California received nearly twice as much rain in a single deluge as normally falls in the preceding 5 months (October–February)". NASA Earth Observatory's Jesse Allen created the front cover visualization with the Visible Infrared Imaging Radiometer Suite (VIIRS) data on the Suomi National Polar-orbiting Partnership (NPP) satellite. [35]
According to a May 14, 2019 article in San Jose, California's The Mercury News , atmospheric rivers, "giant conveyor belts of water in the sky", cause the moisture-rich "Pineapple Express" storm systems that come from the Pacific Ocean several times annually and account for about 50 percent of California's annual precipitation. [36] [37] University of California at San Diego's Center for Western Weather and Water Extremes's director Marty Ralph, who is one of the United States' experts on atmospheric river storms and has been active in AR research for many years, said that, atmospheric rivers are more common in winter. For example, from October 2018 to spring 2019, there were 47 atmospheric rivers, 12 of which were rated strong or extreme, in Washington, Oregon and California. The rare May 2019 atmospheric rivers, classified as Category 1 and Category 2, are beneficial in terms of preventing seasonal wildfires but the "swings between heavy rain and raging wildfires" are raising questions about moving from "understanding that the climate is changing to understanding what to do about it." [38]
Atmospheric rivers have caused an average of $1.1 billion in damage annually, much of it occurring in Sonoma County, California, according to a December 2019 study by the Scripps Institution on Oceanography at UC San Diego and the US Army Corps of Engineers, [39] which analyzed data from the National Flood Insurance Program and the National Weather Service. Just twenty counties suffered almost 70% of the damage, the study found, and that one of the main factors in the scale of damage appeared to be the number of properties located in a flood plain. These counties were: [37]
According to a January 22, 2019 article in Geophysical Research Letters , the Fraser River Basin (FRB), a "snow-dominated watershed" [Note 1] in British Columbia, is exposed to landfalling ARs, originating over the tropical Pacific Ocean that bring "sustained, heavy precipitation" throughout the winter months. [15] The authors predict that based on their modelling "extreme rainfall events resulting from atmospheric rivers may lead to peak annual floods of historic proportions, and of unprecedented frequency, by the late 21st century in the Fraser River Basin." [15]
In November 2021, massive flooding in the Fraser River Basin near Vancouver was attributed to a series of atmospheric rivers and was an event roughly 1/10th of a similar average event from approximately 100 years previous. [40]
While a large body of research has shown the impacts of the atmospheric rivers on weather-related natural disasters over the western U.S. and Europe, little is known about their mechanisms and contribution to flooding in the Middle East. However, a rare atmospheric river was found responsible for the record floods of March 2019 in Iran that damaged one-third of the country's infrastructures and killed 76 people. [27] This AR was named Dena, after the peak of the Zagros Mountains, which played a crucial role in precipitation formation. AR Dena started its long, 9000 km journey from the Atlantic Ocean and travelled across North Africa before its final landfall over the Zagros Mountains. Specific synoptic weather conditions, including tropical-extratropical interactions of the atmospheric jets, and anomalously warm sea-surface temperatures in all surrounding basins provided the necessary ingredients for formation of this AR. Water transport by AR Dena was equivalent to more than 150 times the aggregated flow of the four major rivers in the region (Tigris, Euphrates, Karun and Karkheh). The intense rains made the 2018-2019 rainy season the wettest in the past half century, a sharp contrast with the prior year, which was the driest over the same period. Thus, this event is a compelling example of rapid dry-to-wet transitions and intensification of extremes, potentially resulting from the climate change.
In Australia, northwest cloud bands are sometimes associated atmospheric rivers that originate in the Indian Ocean and cause heavy rainfall in northwestern, central, and southeastern parts of the country. They are more frequent when temperatures in the eastern Indian Ocean near Australia are warmer than those in the western Indian Ocean (i.e. a negative Indian Ocean Dipole). [41] [42] Atmospheric Rivers also form in the waters to the east and south of Australia and are most common during the warmer months. [43]
According to an article in Geophysical Research Letters by Lavers and Villarini, 8 of the 10 highest daily precipitation records between the period of 1979-2011 have been associated with atmospheric rivers events in areas of Britain, France and Norway. [44]
According to a 2011 Eos magazine article [Note 2] by 1998, the spatiotemporal coverage of water vapor data over oceans had vastly improved through the use of "microwave remote sensing from polar-orbiting satellites", such as the special sensor microwave/imager (SSM/I). This led to greatly increased attention to the "prevalence and role" of atmospheric rivers ARs. Prior to the use of these satellites and sensors, scientists were mainly dependent on weather balloons and other related technologies that did not adequately cover oceans. SSM/I and similar technologies, provide "frequent global measurements of Integrated Water Vapor (IWV) over the Earth's oceans." [45] [46]
Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds. Jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including opposite to the direction of the remainder of the jet.
Extreme weather or extreme climate events includes unexpected, unusual, severe, or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Often, extreme events are based on a location's recorded weather history and defined as lying in the most unusual ten percent. The main types of extreme weather include heat waves, cold waves and tropical cyclones. The effects of extreme weather events are seen in rising economic costs, loss of human lives, droughts, floods, landslides and changes in ecosystems.
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.
La Niña is an oceanic and atmospheric phenomenon that is the colder counterpart of El Niño, as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern. The name La Niña originates from Spanish for "the girl", by analogy to El Niño, meaning "the boy". In the past, it was also called an anti-El Niño and El Viejo, meaning "the old man."
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.
El Niño–Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea temperature is known as El Niño and the cooling phase as La Niña. The Southern Oscillation is the accompanying atmospheric component, coupled with the sea temperature change: El Niño is accompanied by high air surface pressure in the tropical western Pacific and La Niña with low air surface pressure there. The two periods last several months each and typically occur every few years with varying intensity per period.
The Madden–Julian oscillation (MJO) is the largest element of the intraseasonal variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection. Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s, through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall.
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A typhoon is a mature tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere. This region is referred to as the Northwestern Pacific Basin, and is the most active tropical cyclone basin on Earth, accounting for almost one-third of the world's annual tropical cyclones. For organizational purposes, the northern Pacific Ocean is divided into three regions: the eastern, central, and western. The Regional Specialized Meteorological Center (RSMC) for tropical cyclone forecasts is in Japan, with other tropical cyclone warning centers for the northwest Pacific in Hawaii, the Philippines, and Hong Kong. Although the RSMC names each system, the main name list itself is coordinated among 18 countries that have territories threatened by typhoons each year.
A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean. In the Indian Ocean, South Pacific, or (rarely) South Atlantic, comparable storms are referred to simply as "tropical cyclones", and such storms in the Indian Ocean can also be called "severe cyclonic storms".
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All types of floods can occur in California, though 90 per cent of them are caused by river flooding in lowland areas. Such flooding generally occurs as a result of excessive rainfall, excessive snowmelt, excessive runoff, levee failure, poor planning or built infrastructure, or a combination of these factors. Below is a list of flood events that were of significant impact to California.
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.
Climate change can affect tropical cyclones in a variety of ways: an intensification of rainfall and wind speed, a decrease in overall frequency, an increase in the frequency of very intense storms and a poleward extension of where the cyclones reach maximum intensity are among the possible consequences of human-induced climate change. Tropical cyclones use warm, moist air as their source of energy or "fuel". As climate change is warming ocean temperatures, there is potentially more of this fuel available. Between 1979 and 2017, there was a global increase in the proportion of tropical cyclones of Category 3 and higher on the Saffir–Simpson scale. The trend was most clear in the North Atlantic and in the Southern Indian Ocean. In the North Pacific, tropical cyclones have been moving poleward into colder waters and there was no increase in intensity over this period. With 2 °C (3.6 °F) warming, a greater percentage (+13%) of tropical cyclones are expected to reach Category 4 and 5 strength. A 2019 study indicates that climate change has been driving the observed trend of rapid intensification of tropical cyclones in the Atlantic basin. Rapidly intensifying cyclones are hard to forecast and therefore pose additional risk to coastal communities.
Hurricane Joanne was one of four tropical cyclones to bring gale-force winds to the Southwestern United States in the 20th century. A tropical depression developed on September 30, 1972. It then moved west northwest and intensified into a hurricane on October 1. Hurricane Joanne peaked as a Category 2 hurricane, as measured by the modern Saffir-Simpson hurricane wind scale (SSHWS), October 2. Joanne then slowed and began to re-curve. Joanne made landfall along the northern portion of the Baja California Peninsula as a tropical storm. The tropical storm moved inland over Sonora on October 6 and was believed to have survived into Arizona as a tropical storm. In Arizona, many roads were closed and some water rescues had to be performed due to a prolonged period of heavy rains. One person was reportedly killed while another was electrocuted. A few weeks after the hurricane, Arizona would sustain additional flooding and eight additional deaths.
The "Ridiculously Resilient Ridge", sometimes shortened to "Triple R" or "RRR", is the nickname given to a persistent anticyclone that occurred over the far northeastern Pacific Ocean, contributing to the 2011–2017 California drought. The "Ridiculously Resilient Ridge" nickname was originally coined in December 2013 by Daniel Swain on the Weather West Blog, but has since been used widely in popular media as well as in peer-reviewed scientific literature.
Tiffany Shaw is a geophysical scientist from Canada. She is currently an associate professor at the University of Chicago. She is known for her extensive contributions to the geophysical and atmospheric sciences.
Caroline C. Ummenhofer is a physical oceanographer at the Woods Hole Oceanographic Institution where she studies extreme weather events with a particular focus on the Indian Ocean. Ummenhofer makes an effort to connect her discoveries about predicting extreme weather events and precipitation to helping the nations affected.
The effects of climate change on the water cycle are profound and have been described as an "intensification" or an overall "strengthening" of the water cycle. This effect has been observed since at least 1980. One example is the intensification of heavy precipitation events. This has important knock-on effects on the availability of freshwater resources, as well as other water reservoirs such as oceans, ice sheets, atmosphere and land surface. The water cycle is essential to life on earth and plays a large role in the global climate and the ocean circulation. The warming of the earth is expected to cause changes in the water cycle for various reasons. For example, warmer atmosphere can contain more water vapor which has effects on evaporation and rainfall. Oceans play a large role as well, since they absorb 93% of heat. The increase in ocean heat content since 1971 has a big effect on the ocean as well as the cycle. To avoid further, or more extreme, changes to the water cycle, greenhouse gas emissions must be reduced.
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