Unlocking the secrets of the North American monsoon

Death Valley is famously the hottest and driest place in North America, recording less than two inches of rainfall a year. But earlier this month, the California desert nearly broke its single-day record with a whopping 1.46 inches. The storms that swept through on August 5 set off flash flooding that left about a thousand people stranded in Death Valley National Park. 

Meteorologists said it was a once-in-a-thousand-year storm—for now, at least—and have chalked it up to the effects of the North American monsoon, also called the Southwest monsoon.

What’s that, you say? A monsoon—in the United States? Although monsoons are most commonly associated with India, where heavy rain blankets the country each summer, this seasonal phenomenon occurs around the world.

First recorded about a century ago, the North American monsoon season runs from mid-June through September. It reaches from Mexico—which receives as much as 70 percent of its annual precipitation during monsoon season—into portions of the southwestern United States. Although its effects can be felt as far west as Death Valley, Arizona and New Mexico bear the brunt of the monsoon in the U.S., and receive about 50 percent of their annual rainfall during the season.

“Monsoon rainfall is extremely important for the desert Southwest,” says Andreas Prein, a scientist at the National Center for Atmospheric Research in Boulder, Colorado. Although the monsoon rains can be destructive, they are also life-affirming as they replenish the parched region’s water supply and bring an end to wildfire season.

As climate change makes the planet even hotter and drier, Prein and other scientists are seeking to better understand the conditions that drive the monsoon—and how to predict and plan for it.

What is the North American monsoon?

A monsoon is a seasonal change in wind patterns that delivers rainfall to a large area or continent, typically causing dry winters and wet summers. Although people often think of monsoons as a steady downpour that drenches the entire region for weeks or months on end, that’s rarely the case: Monsoons are highly variable from day to day and year to year.

“Predicting exactly what places on a given day will get monsoon rainfall is extremely challenging,” says Chris Castro, an expert on the North American monsoon at the University of Arizona. “One part of town can get a good dousing of monsoon rain—like an inch or more in a given storm—and other parts of the city can be completely dry.”

Similarly, some years are drier than others. In 2020, for example, the Southwest set a record for the driest and hottest monsoon season with just 2.97 inches of rainfall—followed by one of its wettest monsoon seasons ever in 2021 with 7.93 inches.

But even though it might not rain all day every day from mid-June until late September, Castro explains that establishing these set dates for monsoon season help prepare the public for the hazards that come with monsoon thunderstorms—including flash flooding and dust storms from the strong winds blowing over the dry land.

These storms also have implications for wildfire season, which begins in spring in the Southwest. Heavy rains can cause a debris flow—a cascading slurry of soil, rocks, water, and other materials—from areas where a recent wildfire has burned away the vegetation that would normally keep it all in place. Lightning storms early in monsoon season can also set fire to the parched landscape.

But the North American monsoon also brings an end to wildfire season as the arrival of rain drives down temperatures and adds moisture to the air and soil. When monsoon season is delayed or if it’s drier than usual, wildfires have more time to burn. But understanding why monsoons are so varied from year to year starts with understanding what causes them.

What causes the North American monsoon?

Most monsoons are understood to be caused by changing temperatures of the land and water in dry, subtropical regions. For most of the year, the wind in these areas blows out to the ocean. In the summer, however, the land heats up much faster than the ocean. Because warm air is less dense than cooler air, the air pressure over land begins to fall—triggering the winds to shift as the high-pressure system over the ocean begins to push inland.

Castro says that has long been the understanding of what causes the North American monsoon—and he argues it is still the best explanation. But among climate scientists, he says, its cause is “an active area of debate.”

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William Boos, a climate scientist at the University of California Berkeley, takes a different view of the monsoon’s cause. “It is sort of the most peculiar and smallest monsoon on the planet,” he says. While most monsoons look like a “big blob” on a radar map, he says that the North American monsoon is fairly skinny. That unusual structure is what prompted him to wonder whether there might be a different force in play—such as the Sierra Madre Mountain range in northwest Mexico.

To investigate the question, Boos and his team ran high-resolution simulations of what would happen to the monsoon if the Sierra Madres didn’t exist. They theorized it might shift or weaken the monsoon, but “we found that it just wiped out the North American monsoon.”

In November 2021, his study published in the journal Nature suggested that the North American monsoon is caused not by contrasting temperatures but by jet stream winds bouncing off the Sierra Madres. The mountains deflect much of the eastbound jet stream south to the tropics—yet some of the warm, moist air still manages to push eastward over the mountains. “What the mountains are doing is they’re providing this uplift to the jet stream that focuses the rain very strongly in this one region,” Boos says.

Castro says that even higher resolution models—down to the kilometer—would be needed to be able to say definitively whether the mountain range plays a role. Boos disagrees but acknowledges that there’s more work to be done to test the hypothesis. His team is now analyzing the last 40 years of historical data to see if it aligns with this theory.

Predicting monsoons

Having a better understanding of the North American monsoon could help in making critical decisions about how to manage the region’s water supply. In the meantime, however, scientists are already developing better tools for predicting how abundant it will be in any given season.

Prein, the author of a recent study that identified a better way to forecast monsoons, explains the models that have typically been used for seasonal forecasts are too coarse to simulate the monsoon very well because they can’t really represent the topography of the region. “What we tried to do with our study is find a way around that,” he says.

Analyzing meteorological data from the last 40 years, Prein’s team found that moisture surges in the atmosphere, not even a mile off the ground, were the best predictor for monsoonal rainfall. Then, plugging that data into existing forecasting models, the study showed that one of the models—the European Center for Medium-Range Weather Forecasts—could correctly forecast the monsoonal rain in any given historical year months in advance.

Castro says these findings are essential for water supply management. Finding out in April that the coming monsoon season will be dry, for example, will allow communities to adjust their water conservation plans. A forecast of a wet monsoon, meanwhile, will give them warning to harvest and store that monsoonal rain—especially as climate change makes it increasingly erratic.

Climate change and monsoons

As a North American monsoon expert, people ask Castro all the time whether the seasonal phenomenon will save the Southwest from drought as the climate gets hotter and drier. “I say, well, no it’s not” going to resolve the region’s water shortages.

The literature is mixed on exactly how climate change will affect the monsoon, but Castro says one thing that’s clear is that the warming of Earth’s atmosphere will allow it to hold an exponentially greater amount of water vapor. This means that if conditions are otherwise primed for rain, the sheer amount of water in the atmosphere ensures that the storm will be bigger and more intense. “The last two years have been really illustrative of this,” Castro says.

His own research backs up the theory that climate change is causing monsoonal rains to become more intense—and less frequent. In a study analyzing historical data from 1951 to 2010 in the southwestern U.S., Castro’s team found that average rainfall has declined while extreme precipitation has increased over the years.

These changes have major implications for the region, which isn’t prepared to face a once-in-a-thousand-year storm every couple of years. More intense storms can cause flooding—as seen in Death Valley—and set off massively destructive debris flows capable of shutting down highways and railway lines. It’s also hard to rely on intense monsoon rains for water because a lot of it is just going to wash off the land, Castro says.

“So you have to totally rethink, how are you going to store that water from these more intense but intermittent events?” he says, adding that it might mean investing more in rainwater harvesting or building ways to channel rainwater into ponds to recharge the groundwater.

Prein says the future of the North American monsoon in a changing climate is ultimately still up for debate. But as scientists continue to investigate the many other questions swirling around the monsoon, he expects they’ll be able to unravel some answers.

“We have new models that can help us to predict monsoon rainfall months in advance, which I think is really exciting,” he says. “I think we are on the edge to better understand climate change signals on the monsoon.”