Rivers in the Sky

A recently discovered, large-scale cloud and storm pattern has serious connections to severe weather and climate change.

May 23, 2022

There are things in the world of weather and climate that we do not yet understand, or perhaps even know about. That surmise derives from a list of ways in which new technologies in different ages have allowed clearer visions of the workings of the climate system, and led to new understanding.

A brief and very incomplete historical tour of such technology/discovery linkages could begin with Benjamin Franklin’s first description of the Gulf Stream in the 1700s. His understanding of this major current were made possible by observations by many during repeated crossings of the Atlantic by sailing ships, the technological marvel of their age.

A basic understanding of the role of atmospheric pressure gradients in driving weather was first developed in the early 1900s, but it took technological breakthroughs in instrumentation, remote sensing and communications to collect and map those gradients and many other variables in real time and make the modern weather prediction system possible.

The strength and extent of jet streams, those high elevation, high speed currents of air that determine the pathway taken by storms and can turn thunderstorms into tornadoes, was not understood until high elevation aviation became commonplace in the middle of the twentieth century.

Enhanced satellite remote sensing of sea surface temperatures globally now allow us to map the Gulf Stream and measure the state of the El Niño/La Niña system in real time.

So maybe there are other discoveries to be made as new technologies allow a clearer vision of the global climate system.

To support that idea, here is a recent example.

We now often hear about “Atmospheric Rivers” and their connection to extreme weather events, yet the definition of this phenomenon is little more than 20 years old, and again has been made possible by improved detection technologies. We can now track the several Rivers that can be occurring at any one time across the globe, although we are usually made aware of them only when one ends with a waterfall somewhere on land.

So what are these Rivers in the air? How do they form, what do they do, and the question to which we always return, will they change in frequency and intensity as the climate system warms? As with other major discoveries, detection and description tend to precede understanding and prediction, so it is easier to see them than it is to describe how they form or predict their occurrence.

What they are and what they do

Now that we can see Atmospheric Rivers (more on how below), it is fairly straightforward to describe them. They are streams of concentrated water vapor that move tremendous amounts of moisture at high speed over very long distances. They form above the surface of warm tropical ocean waters where near-surface winds drive them towards the poles, but not necessarily directly (see the image below). These Rivers can be very long (up to thousands of miles) and relatively narrow (relative to length anyway – up to 300 miles across), creating the appearance of a conveyor belt of heat and moisture.

The amount of water transported by these Rivers is immense. Estimates of just how much water include 7 to 15 times that of the liquid flow in the Mississippi River, or twice the flow of the Amazon, or greater than any liquid river on Earth. Atmospheric Rivers may contain up to 90% of all the moisture moving poleward at any one time and the storms they deliver may contribute more than half of all stream runoff in coastal continental areas.

Such deluges can help relieve drought and extinguish fires, or lead to catastrophic floods, depending on the year, season and condition of the land receiving the waterfall. There is now a rating system for Atmospheric Rivers (from 1 to 5), like the one used for hurricanes, based on the rate of water vapor transport.

How We Know They Are Out There

One indicator of the scientific interest in a topic is the number of papers published. Before 2004, fewer than 10 scientific papers per year mentioned Atmospheric Rivers. By 2015, that number had risen to around 200. This increase is tied to the launch of a better satellite system that allows us to see the Rivers more clearly. How better?

Water and other materials emit very small amounts of microwave radiation. Yes, these are the same wavelengths your microwave oven uses to heat things, but your oven uses electrical energy to create that radiation. Water in all its forms always emits an identifiable “fingerprint” of microwave radiation at different wavelengths. A passive satellite sensor, if sensitive enough, can gather information across those wavelengths and separate the water signal from emissions of other surfaces.

So here is where the improved technology comes in. There has been a long series of remote sensing satellites, starting in 1981, that have measured upwelling microwave emissions. Before 2003, the best sensor sampled 7 different kinds and wavelengths of microwave emissions. In 2003, the latest update (called the Special Sensor Microwave Imager/Sounder or SSMIS for short), increased the number of wavelength/types of microwave emissions that could be sampled from 7 to 24. It also reduced the “noise” created by the instruments, enhancing the readable signal.

Think of this in terms of improving the resolution on your cell phone camera, or your TV screen. Objects that were blurry or invisible at lower resolution suddenly spring into view. This is essentially what happened with the improved resolution of the SSMIS sensor. This more than 3-fold increase in the number and kind of wavelengths sampled, and the reduced noise in the sensors, allowed a greatly increased sensitivity and resolution in estimates of water vapor content in the atmosphere. Bingo, Atmospheric Rivers, previously invisible, sprang into view.

Impacts

Discovery of Atmospheric Rivers and the link to extreme weather events has led to a host of news articles describing the Rivers and the havoc they can wreak.

Major storm events result when these fast moving ribbons of very humid air reach landfall. As they are forced to rise over elevated land, the air cools as it rises. Cooler air can hold less moisture, so the water vapor condenses and falls as rain or snow. The technical term for this is orographic lift, which can also create a rain shadow effect on the leeward or downward side.

Awareness of these Atmospheric Rivers has made it possible to reach back in time and identify major storms caused and the amount of damage that resulted. One report found that damage resulting from Atmospheric Rivers hitting the western U.S. from 1978 to 2017 totaled $42.6 billion, or 84% of all storm related losses.

Poleward movement of Atmospheric Rivers can also have direct effects on other critical parts of the climate system. A recent paper in the journal Nature identifies their role in the disintegration of ice shelves in Antarctica. The authors report that “the most intense atmospheric rivers induce extremes in temperature, surface melt, sea-ice disintegration, or large swells that destabilize the ice shelves…” Collapse of the Larsen A and B ice shelves during the summers of 1995 and 2002, as well as 60% of major iceberg calving events from 2000-2020, were linked to Atmospheric Rivers.

How they form and trying to predict them

If we can see these Rivers and have some information about atmospheric conditions around them, can we describe how they form? The litmus test of this understanding would be the ability to predict accurately when and where they will occur.

A general understanding of the causes of Atmospheric Rivers suggests that they tend to form at the point of contact between large, stable high pressure areas and intense low pressure areas associated with major storms. Those storms tend to move along the path carved by the jet stream, perhaps adding extra energy to the River and aiding persistence over such long distances.

High and low pressure systems spin in opposite directions, so where they meet, they can reinforce wind speeds at the boundary. This analogy comes to mind. Ice skating kids play a game called “crack the whip” where a line of skaters forms. At high speed, the lead skater stops and pulls the next skater around. This goes on all down the line until the last skater is propelled at very high speed toward a possibly injurious landing! Is the intense storm in the second figure above pulling the River toward a possibly injurious landing on the coast in its path?

While we can visualize and to some extent describe causes of Atmospheric Rivers, predicting their occurrence is another matter altogether. As a newly identified phenomenon, the ability to predict when and where they will occur is still limited.

One study suggests that Rivers shift to the north in the summer, but that this seasonal cycle can be offset by the state of the El Niño/La Niña system. Another site provides predictions for the occurrence of Rivers but only goes out 180 hours into the future, and comes with a serious disclaimer that this is an experimental site and should not be used for planning.

Effects of climate change

We now know (but only for the last 20 years or so) that Atmospheric Rivers are a major source of extreme weather in coastal areas. How will climate change affect the frequency and intensity of these storm-makers?

A recent review in the journal Nature concludes that a warmer atmosphere will mean more moisture in Atmospheric Rivers, but that much is yet to be learned, including interactions with atmospheric circulation changes in response to warming. Additional research on the mechanisms driving River formation is urged.

One NASA-funded modeling study suggests that there is a direct link between warming and the predicted frequency and intensity of Atmospheric Rivers. A second modeling study projects fewer but larger Rivers, and greater volume overall. A USGS supported study charted 70 years of rainfall records for California, concluding that there is a long-term trend of increasing water vapor transport to the coast of California associated with warming of surface waters of Pacific Ocean.

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So it appears that we still have much to learn about the formation and prediction of Atmospheric Rivers. This is yet another key component of the rapidly changing weather/climate system that bears close watching - now that we can see it more clearly.

Sources

The first image is from:

https://www.nesdis.noaa.gov/news/goes-west-views-atmospheric-river-the-pacific-ocean

One telling of the story of Benjamin Franklin and the Gulf Stream is here:

https://www.smithsonianmag.com/smart-news/benjamin-franklin-was-first-chart-gulf-stream-180963066/

Derivation of the first “primitive” or simplest equations describing the role of pressure gradients in driving weather is attributed to Vilhelm Bjerknes:

https://en.wikipedia.org/wiki/Vilhelm_Bjerknes

Discovery of the jet stream is described here:

https://en.wikipedia.org/wiki/Jet_stream

Images of global sea surface temperature can be found here:

https://earthobservatory.nasa.gov/images/3513/global-sea-surface-temperature-from-amsr-e

and the state of the El Niño/La Niña system is reported monthly here:

https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml

The double image of the long Atmospheric River is from: https://earthobservatory.nasa.gov/blogs/earthmatters/2017/10/27/research-roundup-atmospheric-rivers/

Background information on Atmospheric Rivers is here:

https://en.wikipedia.org/wiki/Atmospheric_river

Estimates of the amount of water vapor transported by Atmospheric Rivers are from:

https://www.usgs.gov/news/featured-story/rivers-sky-6-facts-you-should-know-about-atmospheric-rivers - This source includes figure 3 which is from NOAA