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.  

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.

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.

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.