Unlock the secrets of the North American monsoon
Death Valley is famous the hottest and driest place in North America, recording less than two inches of precipitation per year. But earlier this month, the California desert nearly broke its one-day record with a whopping 1.46 inches. The storms that swept through on August 5 caused flash floods that left about a thousand people stranded in Death Valley National Park.
Meteorologists said it was a once-in-a-thousand-year storm — at least for now — and attributed it to the effects of the North American monsoon, also called southwest monsoon.
What is it, you say? A monsoon — in the United States? Although monsoons are most often associated with India, where heavy rains covers the country every summerthis seasonal phenomenon occurs all over the world.
First recorded about a century ago, the North American monsoon season runs from mid-June through September. He comes from Mexico, which receives up to 70% of its annual rainfall during the monsoon season – in parts 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 United States and receive about 50 percent of their annual precipitation during the season.
“Monsoon rains are extremely important for the southwest desert,” says Andreas Prein, a scientist at the National Center for Atmospheric Research in Boulder, Colorado. Although monsoon rains can be destructive, they are also vital as they replenish the parched region’s water supply and end the forest fire season.
As climate change makes the planet even hotter and drier, Prein and other scientists seek 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 which distributes precipitation over a wide area or continent, usually causing dry winters and wet summers. Although people often think of monsoons as a steady downpour that floods the whole region for weeks or months, this is rarely the case: monsoons are highly variable from day to day and from year to year. the other.
“Predicting exactly which places will receive monsoon rains on any given day is extremely difficult,” says Chris Castro, a North American monsoon specialist at the University of Arizona. “Part of the city can get a fair amount of monsoon rain – like an inch or more in any given storm – and other parts of the city can be completely dry.”
Similarly, some years are drier than others. In 2020, for example, the South West set record for driest and hottest monsoon season with only 2.97 inches of precipitation, followed by one of its wettest monsoon seasons ever in 2021 with 7.93 inches.
But even if it doesn’t rain all day every day from mid-June to the end of September, Castro says setting these fixed dates for the monsoon season helps prepare the public for the dangers that come with the monsoon storms, including flash floods and dust storms from high winds blowing over land.
These storms also affect the wildfire season, which starts in spring in the Southwest. Heavy rain may cause debris flow– a cascading suspension of dirt, rocks, water and other materials – from areas where a recent wildfire has burned away the vegetation that would normally hold everything in place. Thunderstorms at the start of the monsoon season can also set the parched landscape on fire.
But the North American monsoon also ends the wildfire season with the arrival of rain lowers temperatures and adds moisture to the air and soil. When the monsoon season is delayed or 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?
It is believed that most monsoons are caused by changing land and water temperature in dry and subtropical regions. For most of the year, the wind in these regions blows towards the ocean. In summer, however, the land warms up much faster than the ocean. Because the warm air is less dense than the cooler air, the air pressure above the land begins to drop, triggering the change in winds as the high pressure system above the he ocean begins to push inland.
Castro says that’s long been the understanding of what causes the North American monsoon — and he argues it’s still the best explanation. But among climate scientists, he says, its cause is “an area of active debate.”
Guillaume Boos, a climatologist at the University of California at Berkeley, has a different view on the cause of the monsoon. “It’s kind of the most peculiar and smallest monsoon on the planet,” he says. While most monsoons look like a “fat blob” on a radar map, he says the North American monsoon is pretty lean. This unusual structure is what prompted him to wonder if there might be a different force at play, like the Sierra Madre mountain range in northwest Mexico.
To investigate the matter, Boos and his team run high-resolution simulations of what would happen to the monsoon if the Sierra Madres did not exist. They speculated that it might displace or weaken the monsoon, but “we found that it just knocked out the North American monsoon.”
In November 2021, his study published in the journal Nature suggested that the North American monsoon is not caused by contrasting temperatures, but by jet stream winds bouncing off the Sierra Madres. The mountains deflect much of the eastbound jet stream south toward the tropics, but some of the warm, moist air still manages to push east over the mountains. “What the mountains do is they provide this uplift to the jet stream that concentrates the rain very heavily in that one area,” Boos says.
Castro says even higher resolution models — down to the kilometer — would be needed to be able to say definitively if the mountain range is playing a role. Boos disagrees but acknowledges that there is still work to be done to test the hypothesis. His team is currently analyzing the past 40 years of historical data to see if it fits this theory.
Predict the monsoons
Having a better understanding of the North American monsoon could help make critical decisions about how to manage the region’s water supply. In the meantime, however, scientists are already developing better tools to predict its abundance in any given season.
Prein, the author of a recent study that identified a better way to forecast monsoons, explains that 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.
By analyzing weather data from the past 40 years, Prein’s team found that surges of humidity in the atmosphere, less than a kilometer above the ground, were the best predictor of monsoon rainfall. Then, by integrating this data into existing forecasting models, the study showed that one of the models – the European Center for Medium-Range Weather Forecasts – could correctly forecast monsoon rains in a 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 warn them to harvest and store that monsoon 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 if the seasonal phenomenon will save the Southwest from drought as the climate gets hotter and drier. “I say, well, no, it’s not” will solve the region’s water shortages.
The literature is mixed on exactly how climate change will affect the monsoon, but Castro says one thing is clear, and that is that the warming of Earth’s atmosphere will allow it to retain an exponentially greater amount of carbon dioxide vapor. water. This means that if conditions are otherwise primed for rain, the amount of water in the atmosphere ensures that the storm will be larger and more intense. “The last two years have been really illustrative of that,” Castro says.
His own research supports the theory that climate change is causing the monsoon rains become more intense and less frequent. In a study analyzing historical data from 1951 to 2010 in the southwestern United States, Castro’s team found that average precipitation has decreased while extreme precipitation has increased over the years.
These changes have major implications for the region, which is not prepared for a storm every two years. More intense storms can cause flooding – as seen in Death Valley – and trigger massively destructive debris flows capable of closing highways and railways. It’s also hard to rely on intense monsoon rains for water, because much of it will simply wash away the land, Castro says.
“So you have to totally rethink, how are you going to store that water from those more intense but intermittent events?” he says, adding that this could mean investing more in rainwater harvesting or building ways to channel rainwater into ponds to recharge 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 will be able to tease out some answers.
“We have new models that can help us predict monsoon rains months in advance, which I think is really exciting,” he says. “I think we are on the verge of better understanding the signals of climate change on the monsoon.”