If you’ve never lived or been to the US Southwest, you may see it as a desert that’s always hot and dry. But the region experiences a late summer monsoon that produces thunderstorms and severe weather like India’s famous summer deluge.
And this year, it rained a lot.
July 2021 was the wettest month since record-keeping began at the Tucson, Arizona, airport in 1895, with 8.06 inches (205 millimeters) of rain—the equivalent of 70% of what the city receives in an average year. This year’s monsoon is the third warmest ever in Tucson, with 12.80 inches (325 millimeters) of rain.
In 2020 it was the complete opposite: Tucson had a “non-soon” drought with less than 2 inches of rain. These conditions and record high temperatures fueled Arizona’s biggest wildfire season in a decade, including the Bighorn Fire, which destroyed more than 60% of the forest in the Catalina Mountains north of Tucson.
Our monsoon system affects about 20 million people in the southwest. As researchers studying water and climate, we examine the prediction of monsoons, which are becoming more complex due to climate change. Understanding monsoon is important to educate communities about their benefits and risks and how to stay safe from impacts like flash floods.
from dry to wet
The word monsoon comes from the Arabic word Mausim or Ritu. Its most traditional use is to describe a large-scale wind change over the Indian subcontinent from the ocean that coincides with intense summer rains there. But the monsoon also occurs in Africa, Australia and South America, as well as Mexico and the southwestern US.
Monsoon circulation moves warm, moist air inland from the ocean, causing rainfall during the summer season. In the southwest, this pattern begins when an area of high pressure, called a monsoon ridge, forms over the mountainous regions of Mexico and moves toward the western US.
In May and June, when the center of the ridge is directly overhead, the southwest is very hot and dry. Monsoon rains begin when warm, moist air moves over the area on the southern side of the ridge. The monsoon in Arizona officially begins on June 15 and ends on September 30, with most rainfall typically occurring in July and August.
Monsoons have been important to the southwestern ecosystem for thousands of years. Many species have evolved and adapted to take advantage of the monsoon rains. The first storm signals the milkweed plants to bloom, attracting butterflies to lay their eggs. The Great Plains Toad and Red-Spotted Tadpole begin their reproductive cycle in rain-filled puddles. Cactus fruits and insects provide food for hummingbirds, white-winged pigeons and many other birds and animals.
flood in the desert
Monsoon storms occur when the mountains are cloudy during the day, with rain in the afternoon and evening. They pose a unique and serious threat to the desert environment.
Flash flooding occurs when dry soil is unable to absorb short-term, high-intensity rain quickly enough. Washes and arroyos – drainage channels that dry up except in heavy rain storms – can turn into raging currents within minutes, strong enough to carry cars and people away.
Strong thunderstorms can generate microbursts – strong surface winds that move near the force of the storm. They can also trigger dust storms known as hubs—huge walls of dust a mile or more high that reduce visibility to nearly zero.
Dry, strong thunderstorms, which herald the onset of the monsoon, can start and spread wildfires. One of these storms ignited the infamous Yarnell Hill Fire in June 2013, which killed 19 firefighters. Monsoon rains can trigger mud and debris flows at the fire burn mark, leading to early wildfire damage.
The atmospheric circulation pattern in July and August 2021 was particularly favorable for an active monsoon and severe weather over the southwest. Most of southern Arizona received torrential rains over several days and weeks. the cause of these storms flash floodhandjob high windshandjob dust stormshandjob mud and debris flows And heavy power. emergency responders carried out Nearly 100 Swift-Water Rescues in Tucson. Forecasters in Phoenix issued more than 100 flash flood warnings in August.
This year’s record monsoon also benefited. This replenished the local water supply throughout Arizona, which was a . is in severe prolonged drought. In the Tucson Basin, the monsoon produced continuous flows into tributaries of the Santa Cruz River, which helped recharge groundwater. Water reserves rose 5% in reservoirs managed by the Salt River Project, which supplies water to more than 2 million people in central Arizona, at a time when record lows are occurring elsewhere in the West.
Monsoon rains also brought the Sonoran Desert back to life, including the areas where the 2020 bighorn fires killed thousands of saguaros.
future of monsoon
Predicting monsoon and how it might change is challenging. High-resolution atmospheric models that explicitly simulate individual thunderstorms, including our own regional modeling system at the University of Arizona, have greatly improved daily weather forecasts in recent decades. But it is still almost impossible to predict when and where storms will occur on a given day.
It is also essentially impossible to predict months in advance how intense the monsoon rains will be. This year, long range forecasts The wet trend didn’t start until mid to late June.. Climate change is making monsoon rains more extreme and variable, driven by hot summers and characterized by less frequent but more intense storms.
If recent years are any indication, our region is already experiencing these effects, with record heat waves, large and catastrophic wildfires, and a monsoon that is basically non-existent in a year, then The next produces record rainfall and severe weather. Such changes are increasing people’s exposure to weather and climate extremes in the Southwest.
The bigger concern is whether a more extreme and erratic monsoon will lead to an increase in the boundary points of failure – for example, flood control infrastructure that collapses from intense rainfall, or a forest fire so devastating that the forest cannot recover. Can. It is important to clearly understand this type of risk in order to build a more resilient and sustainable future for the Southwest.
[Over 110,000 readers rely on The Conversation’s newsletter to understand the world. Sign up today.]