August 18, 2023

August 18, 2023

Studying the Salty Source of Atmospheric Rivers Helps Predict Extreme Rainfall in California

Dr. Ray Schmitt



min read

Image credit: NOAA

In the winter of 2022–23, California was hit by an unusual sequence of heavy rainfall events commonly called “atmospheric rivers.” That’s a great bit of shorthand that’s been adopted by everyone from the USDA to mainstream media outlets to describe these sustained, drenching rains.

But while the term “atmospheric river” vividly conveys the impact of this phenomenon, it gives no hint of its cause. Is it possible to trace the origins of these atmospheric rivers? More importantly, by studying those origins, can we infer when the next one might occur? And can we develop a set of indicators that could tell us which areas will most likely be impacted by a developing atmospheric river (AR)?

(Spoiler alert: Based on our research at SalientPredictions, the answer to all of these questions is yes.)

Riding the Pineapple Express

Satellite images of the atmosphere’s water vapor content clearly illustrate the course of the atmospheric rivers that soaked California. That water vapor originated in the far western tropical Pacific (Figure 1), where the atmosphere holds more water vapor than at higher latitudes. This is due to the higher temperatures in the tropics, and the strong temperature dependence of the vapor pressure of water. We are all familiar with the higher humidity of summer compared to the dry air of winter.

There’s always a water balance between local evaporation and precipitation — and consequently the import or export of water. In the tropics there is a great deal of local recycling and thus a high standing stock of water vapor available for export. Figure 1 shows that cyclonic dips of the jet stream have dug into the tropical moisture and are carrying it past Hawaii and on to California. (That’s why the term “Pineapple Express” is often used to describe these rainfall events.) The quantities of water that such wind patterns carry actually are comparable to the flows of rivers.  

Figure 1. Satellite microwave image of the atmosphere’s water vapor content in the Pacific from atypical atmospheric river in 2019.  The Special Sensor Microwave Imager/Sounder (SSMIS) provides an estimate of the total water vapor content of the atmosphere. Image credit: NOAA.

The Connection between SSS and Atmospheric Rivers

Our previous research has shown that anomalously high rainfall events on land must have an oceanic origin. That’s because no watershed floods simply by recycling its own water. Moreover, when water is exported from a region of the ocean, that invariably causes an increase in sea surface salinity(SSS). By contrast, when water is imported into an oceanic region, SSS decreases.

The quantities of rainwater that fell on California were clearly large enough to have significantly impacted SSS in the region where the atmospheric river originated. Figures 2.a and 2.b illustrate this deviation from the climatological average of the SSS. Fig 2.a shows the Pacific on March 1, 2020, during a dry winter for California. Fig 2.b is from March 1, 2023, which coincided with the sequence of strong atmospheric rivers that recently hit California.

Figure 2.a. Sea Surface Salinity (SSS) anomaly from March 1, 2020, a dry year for California. Note the negative SSS anomalies (blue = fresher) to the north and east of Papua New Guinea. Image credit: Salient Predictions, Inc.
Figure 2.b. SSS anomaly for March 1, 2023, a very wet year for California due to many atmospheric rivers. Note how much saltier (red = salty) it is to the north and east of Papua New Guinea, which satellite SSMIS images show to be a common origin region for atmospheric rivers hitting California. Image credit: Salient Predictions, Inc.

Of course, variations in SSS are only an indicator of how the atmosphere carries water to or from a specific region of the ocean. But there also have to be troughs in the jet stream that dig into the tropical moisture reservoir and carry water to California. Understanding those troughs is essential to developing prospective forecast models.

Navigating Atmospheric River Forecasts

The Salient forecasting engine uses many oceanic and atmospheric variables to predict precipitation. And our forecast model did well in anticipating at long lead the atmospheric rivers that swamped California in 2023. Figure 3.a shows the anomaly in precipitation for the west that we forecast in early February for late March. Figure 3.b shows the probability distribution of our forecast compared to the European Centre for Medium-Range Weather Forecasts (ECMWF) for one site. Salient’s forecast picked up the high rainfall, while the ECMWF forecast was for dryer than normal conditions at the same site.

Figure 3.a.  Atmospheric River (AR) forecast 30 days ahead. Green represents above normal rainfall while brown represents below normal rainfall. The Salient forecast picked up the AR of late March in this early February forecast. Image credit: Salient Predictions, Inc.
Figure 3.b. Superior statistics at a site in the Tahoe National Forest for the AR forecast compared to ECMWF, which  forecast drier than normal conditions for March 2023. The Salient AI model did the best at anticipating the high rainfall. Our API viewer provides the probability distributions of the forecasts for any select site.

At Salient we have a special skill for long-range precipitation forecasts. These can be especially helpful to farmers, water managers, and hydropower operators. Reach out to us if your business could benefit from better forecasts of extreme precipitation events, from floods to droughts, involving rivers both on land and in the atmosphere.


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