February 26, 2022 – Coyote Gulch

Click on the link to read the article on the NOAA website (Kai-Chih Tseng and Nat Johnson):

Guest co-author Dr. Kai-Chih Tseng is a postdoctoral researcher at Princeton University and NOAA’s Geophysical Fluid Dynamics Laboratory, an expert in climate variability and prediction, including the study of atmospheric rivers. In the summer of 2022, Dr. Tseng will begin an assistant professorship in the Department of Atmospheric Sciences at National Taiwan University.

Last December, a mind-boggling 18-foot snowfall fell in California’s Sierra Nevada Mountains! How does so much snow fall in one place in such a short time? One of the main phenomena responsible for these extreme rains and snowfalls, especially in areas like the western United States, is the atmospheric river. Like their terrestrial counterparts, atmospheric rivers carry massive amounts of water over thousands of miles. These aerial versions, however, often bring both severe disruption and great benefit from the heavy rains and mountain snows they produce. In this blog post, we’ll give you a brief overview of atmospheric rivers and (of course!) explain how they are affected by ENSO (El Niño-Southern Oscillation).

Flying Mississippis

Atmospheric rivers are long, narrow corridors of moisture-laden air that stretch from the tropics to higher latitudes. They can produce heavy rain and snowfall in a short time, especially when the air is lifted over high ground, cooling the air and condensing the moisture into droplets, like wringing a atmospheric sponge. When you see these impressively long features on satellite imagery, it’s no wonder they’re compared to rivers. In fact, an average atmospheric river carries 25 times more water than the Mississippi River!

They form when warm, moist air from low latitudes is transported poleward like a conveyor belt in front of a cold front following a powerful mid-latitude storm. Around the world, atmospheric rivers are responsible for more than 90% of the water vapor transported to mid-latitudes from the tropics and are a critical water source for many regions, such as California and Nevada. They can also be quite destructive, causing severe flooding and damaging winds, with the strongest atmospheric rivers in the western United States typically causing damage in the hundreds of millions of dollars (1).

Click the image for an animation of the atmospheric river hitting the west coast of the United States on October 23, 2021. Credit: NOAA

Several notable atmospheric rivers made landfall along the west coast of the United States during the past fall and winter. On October 24-25, 2021, an intense atmospheric river brought high winds and historic rains reaching up to a foot to the San Francisco Bay Area, providing temporary respite from a lingering drought (but clearly not enough to end it). The animation above (2) shows the narrow, river-like corridor of concentrated water vapor that resulted in this historic rain. California is no stranger to this “boom or bust” rainfall pattern. Incredibly, up to half of the annual precipitation in parts of California falls on just 5-10 wet days during the year (is it any wonder seasonal precipitation forecasting is so difficult?), and the rivers atmospherics are a major source of those few wet days (3). California is unique in extreme rainfall variability, but other western states, such as Washington and Oregon, also depend on atmospheric rivers for water supply.

So where and how often?

Average number of days each winter (December-February) with an atmospheric river occurrence over the northeast Pacific Ocean. Maps calculated with ERA5 Reanalysis data covering the period 1992-2020 and with Mundhenk et al. (2016) atmospheric river detection method.

Atmospheric rivers (see footnote 4 for how we define them) occur across much of the globe outside the tropics and in all seasons, but are more common in storm tracks near currents -jets. Their impacts on the United States are most pronounced in the winter. The figure above shows that during a typical period from December to February, atmospheric rivers near North America most often occur off the North Pacific and North Atlantic. Although their importance is emphasized in the western United States due to their large contribution to annual rain and snowfall totals, they also occur frequently in the central and eastern states of the United States, where about 10 days of winter can be expected each year with an atmospheric river occurrence.

This is the ENSO blog!

Don’t worry, we haven’t forgotten ENSO’s role! Just as ENSO impacts seasonal temperature and precipitation patterns in North America, it also affects the frequency of atmospheric river falls. Over the past 30 years, El Niño has brought more frequent than normal atmospheric rivers to the west coast, while La Niña has generally brought less frequent occurrences. This winter has been fairly consistent with typical La Niña conditions, with below average atmospheric river activity in the western United States. Despite a two-week spell in December that brought atmospheric rivers and record snowfall to California, January was the second driest on record for California and Nevada.

Deviations from the average number of days each winter (December-February) with atmospheric fluvial occurrence for (top) El Niño and (bottom) La Niña from 1992 to 2020. During El Niño winters, the number of days with of atmospheric rivers is below average along the coasts of Alaska and western Canada, but above average along the west coast of the United States. La Niña tends to bring the opposite: fewer atmospheric river days along the west coast of the United States.

But how well can we predict atmospheric rivers?

As with all extreme precipitation events, accurate predictions of individual atmospheric rivers and their impacts are limited to short-term weather forecasts. However, the latest research efforts are advancing our ability to predict regional atmospheric river activity (not individual storms) over subseasonal (about 2-4 weeks ahead) and even seasonal time horizons. On sub-seasonal time scales (5), sources of atmospheric river predictability are rooted in large-scale climate patterns such as the Madden-Julian Oscillation and the Pacific-North America pattern. On a seasonal level, a recent study by guest co-author Dr. Kai-Chih Tseng indicates that one of the models participating in the North American Multi-Model Ensemble (NMME), SPEAR, can produce seasonal forecasts of the atmospheric activity of rivers. in select areas, including coastal California and Alaska, up to nine months in advance. The direction of ENSO is one of the main reasons why seasonal atmospheric predictions of rivers may be possible.

Effects of climate change

Human-caused climate change is likely to increase the atmospheric intensity of rivers. Warming oceans increase the moisture available to these powerful storms, enhancing moisture transport and the heavy rainfall they produce. While several global climate model studies support the increasing intensity of atmospheric rivers with global warming, further studies are needed to better understand how other atmospheric properties of rivers, such as size, shape, frequency, and location, will change.

The bottom line is that any increase in atmospheric river intensity will contribute to growing water resource challenges in the western United States. .

Footnotes

1. For more information on how atmospheric rivers can be destructive to the western United States, we recommend this article. Corringham et al. (2019) use a scale that divides atmospheric rivers into 5 intensity categories, similar to what we do for hurricanes and tornadoes.

Corringham, TW, Ralph, FM, Gershunov, A., Cayan, DR and Talbot, CA (2019). Atmospheric rivers cause flood damage in the western United States. Science Advances, 5(12), eaax4631. https://doi.org/10.1126/sciadv.aax4631

2. Hats off to our friends at the Seasoned Chaos blog for providing the tools to build this animation. Please check out their great complementary article on Atmospheric Rivers.

3. For more information on the connection between atmospheric rivers and California’s water resources, we recommend this article.
Dettinger, MD, Ralph, FM, Das, T., Neiman, PJ and Cayan, DR (2011). California’s Atmospheric Rivers, Floods, and Water Resources. Water, 3(2), 445–478. https://doi.org/10.3390/w3020445

4. At this point, you might be thinking, “Wait, Nat and Kai-Chih, how do I know if an atmospheric river is happening?” Good question! There is no single, objective method to identify atmospheric rivers, but several algorithms are commonly used. All of these algorithms share at least two features in common: (1) the atmospheric vapor transport must be exceptionally strong, and (2) the vapor transport must be concentrated in a long, narrow feature. In the analysis of this blog post, we analyze daily vertically summed water vapor transport fields and use the method of Mundhenk et al. (2016) to identify grid cells where an atmospheric river occurs. Le Mundhenk et al. (2016) ensures that the two criteria listed above are met.

5. For example, check out some of the excellent experimental subseasonal atmospheric forecast products provided by the Center for Western Weather and Water Extremes (CW3E) on this site.


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