What drives atmospheric rivers? Da Yang explains how these “rivers in the sky” gain and lose momentum, and how researchers are studying their physical properties to improve forecasts and reduce risks.
By Ula Chrobak, Stanford Doerr School of Sustainability
In the winter of 1862, an estimated 37 inches of rain fell in Sacramento over two months. The newly elected governor, Leland Stanford, had to travel to his inauguration by rowboat, and the city remained flooded for months afterward.
Atmospheric rivers likely caused that historic downpour. These narrow plumes of vapor and strong winds carry moisture from near the tropics to the poles, producing heavy rain and gusts. While they can be a welcome relief from drought and wildfire season, they can also bring damage and risk. Rain dumped over California by a series of atmospheric rivers in early 2023 led to billions of dollars in losses and at least 21 deaths. In recent days, meteorologists have warned of flood risks and travel hazards from the latest parade of atmospheric rivers, which are expected to bring a “firehose of moisture” to the West Coast.
To help communities prepare for future atmospheric rivers, scientists are trying to better understand how they work. Da Yang, an assistant professor of geophysics at the Stanford Doerr School of Sustainability, is applying a new mathematical framework to describe the physics of atmospheric rivers, which may ultimately help forecast them further in advance with greater precision.
Here are five key facts to know about this meteorological phenomenon.
Atmospheric rivers carry moisture thousands of miles.
Atmospheric rivers originate from cyclones forming outside the tropics. When a north-south gradient in air temperature combines with strong wind shear, the cyclone picks up a massive amount of moisture and forms a long band of vapor.
These rivers in the sky are the primary way water vapor moves from lower latitudes, near the equator, toward the poles, explains Yang. On average, they transport water at a rate more than two times that of the Amazon River. In California, they deliver up to half of the state’s annual precipitation.
Continuing north of mid-latitude regions like California, atmospheric rivers also influence polar regions, said Yang. Upon reaching ice sheets, they can either add to snowfall or accelerate melting if they bring rain.
New math describes atmospheric river physics.
Previous research focused on monitoring atmospheric rivers and improving forecasts, which is critical for preparing populations for storms. After moving to California for his PhD at Caltech, Yang grew curious about the torrential winter storms and what drives them. Scientists did not fully understand what drives their development or causes them to move eastward, he said. Questions remained unanswered about how atmospheric rivers grow from “weak, small disturbances to expansive rainstorms,” Yang said, and what determines how many atmospheric rivers form in a year.
Puzzling over these questions, Yang and colleagues created a new framework that simplifies the math of atmospheric rivers. The formula traditionally used “is such a bulky equation, it’s really hard to derive another equation to describe atmospheric rivers’ temporal evolution,” he said.
The new mathematical framework, published in November 2024 in the journal Nature Communications, is called vapor kinetic energy. Co-authored by Yang with his former postdoctoral scholar Hing Ong, the equation neatly captures the forces behind how atmospheric rivers move, as well as how they gain and lose momentum. In the past year, some researchers have begun using the framework to analyze the inner workings of atmospheric rivers and potentially improve predictions.
Air moving from high to low pressure strengthens atmospheric rivers.
The new framework describes how atmospheric rivers strengthen and weaken. “At any instance, we are able to pinpoint which physical process is making atmospheric rivers grow in intensity, and which physical process is trying to dissipate atmospheric rivers,” said Yang.
The moisture plumes are strengthened by conversion of potential energy to kinetic energy. It’s akin to a kid sitting at the top of a slide: When she is at the top, her potential energy is high due to her height above the ground. As she slides down, that potential energy becomes kinetic energy.
In the same way, when air moves from high to low pressure, potential energy turns to kinetic, which accelerates wind speeds.
Meanwhile, condensation and turbulence act as atmospheric speed bumps. Condensation takes away water vapor and the energy it holds, reducing the river’s intensity. Turbulence, the same chaotic winds that make air travel bumpy, also hits the brakes on atmospheric rivers.
Climate change could intensify atmospheric rivers.
The warmer the atmosphere is, the more moisture it can hold. This means that climate change will likely supercharge these systems, Yang explained.
Scientists project atmospheric rivers will become stronger and wider as the planet warms, according to the most recent National Climate Assessment, elevating the risk of floods across the Western U.S. Already, these “rivers in the sky” have grown warmer along the Pacific Coast over the past several decades, and carried larger amounts of moisture into the region because of rising temperatures in the Pacific Ocean.
Stanford researchers including civil and environmental engineering professor Jack Baker have found that back-to-back atmospheric rivers, which are likely to become more common with climate change, cause up to four times more economic damage than would be expected if they struck individually.
Other factors also influence the formation, trajectory, and intensity of atmospheric rivers. For instance, Yang and other researchers are working to understand how these weather systems interact with multi-year fluctuations in sea surface temperatures known as El Niño and La Niña.
Improving forecasts can reduce risks.
Because Yang and Ong’s vapor kinetic energy framework describes the physical processes of atmospheric rivers, it can help researchers improve weather and climate models. They can clarify why one model outperforms another at predicting the strength of an oncoming atmospheric river two weeks out. “We now have a new tool to compare the contribution of individual processes to these two model simulations” and learn how to improve simulations of atmospheric rivers, Yang said.
Scientists hope to improve atmospheric river forecasting to the point where they can make predictions one month, even two months, out. This sort of timeframe could enable better water management and reduce hazards and destruction – and perhaps the need for rowboats during storms.
