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Dry soils in northern Mexico may trigger episodes of simultaneous drought and heat wave hundreds of miles away in the southwestern United States, such as Arizona, New Mexico, and Texas, according to a new study. These "hot droughts" in the region increasingly persist through consecutive days and nights rather than easing up after sundown, the research also found, leaving no window for afflicted areas to recover.

Hot drought can kill crops, worsen wildfire risk, and shock workers and outdoor enthusiasts with unexpectedly high temperatures, all more than either drought or heat alone can do. Scientists involved in the study say the findings could help communities better anticipate and prepare for these stressful events in advance, such as by limiting outdoor working hours, keeping medically vulnerable individuals inside, and opening cooling centers when soils far upwind desiccate.

The study in Geophysical Research Letters.

"Hot droughts will propagate to other parts of the country and have detrimental effects on health, on infrastructure, on daily life," said Enrique Vivoni, a hydrologist at Arizona State University and senior author on the study. As climate change continues, the authors said, more places will likely experience the dry soil conditions that spur and spread hot drought. "We need systems to alert us to hot drought just like we have systems that alert us to hurricanes."

In summer 2023, Southwestern North America weathered an unusually intense hot drought. Using temperature records, rain gauge readings, and from satellite and ground-based measurements, Vivoni and Somnath Mondal, a hydroclimatologist at Northeastern University, set out to characterize the event in the context of previous hot droughts and identify the conditions that set the stage.

For this study, they defined hot drought as any period when at least two weeks of abnormally low rainfall overlapped with at least three straight days of unusually high temperatures.

Even ordinary summers in the region can get brutal, with daytime temperatures since 1980 typically sitting at around 35 to 40 degrees Celsius (95 to 104 degrees Fahrenheit) in some locations. But 2023's hot drought, the duo found, cranked up the heat by as much as 8 degrees Celsius (14 degrees Fahrenheit).

This resulted primarily from weather patterns that suppressed the transfer of atmospheric moisture from the Pacific Ocean into the North American Monsoon, which from July to September typically provides 40 to 80% of the region's annual rainfall. The weak monsoon exacerbated the drought already gripping the U.S. Southwest and Northern Mexico at the time.

"Lack of rainfall can increase heat, which can further intensify the loss of water," noted Mondal. Soil heats up and releases heat more readily when dry, since more of the sun's energy is spent warming the air and earth rather than evaporating soil moisture.

All told, the event reached nearly five times the severity of average hot drought conditions in the region over the past four decades.

Dry winds know no borders, and desert nights lose their cool

The duo also made two surprising discoveries they hadn't gone looking for.

Normally, when rain falls over northern Mexico, some of the moisture evaporates from the land surface and returns to the atmosphere, recharging rainclouds that then bring rain downwind to the southwestern U.S. The weak 2023 monsoon may have left Mexican soils too dry to start this cycle, the researchers suggested, triggering hot drought north of the border as well. The correlation between dry Mexican soils and hot drought in the southwestern U.S., on the rise since 1980, is strong enough that soil dryness in the southwestern U.S. itself appears to play a smaller role.

"In 2023, Mexico influenced Arizona's hot drought in a stronger way than the soil of Arizona itself," Mondal said, describing the first discovery. "I verified it five times to be sure I was doing the right calculation."

"We know we receive water vapor, clouds, and rain from Mexico," Vivoni said. "We didn't know we could also receive a hot drought."

The second surprise came from the strength of a hot drought at night. Previous hot drought research has mostly ignored nighttime conditions, since in a stable desert climate, most daytime heat dissipates after dark.

But in extreme cases like in 2023, so much daytime heat accumulates that it doesn't all fade away overnight. Instead, some of it hangs in the atmosphere, piling onto the heat of the next day, which adds to the following night's heat, and so on, creating a cycle that can intensify over weeks. The researchers found this occurring increasingly over the past 40 years—even in rural areas, which typically retain less heat overnight than urban zones.

As climate change makes hot droughts more intense and frequent, health risks from heatstroke and heat-related mortality also rise.

"There isn't a good understanding that in a hot drought, you need to take more precaution than if it's just a heat wave," Vivoni said. When temperatures stay high through the night, for instance, even hikers and laborers who rise early to beat the heat may be in danger.

Raising awareness about these risks could make communities safer, the researchers said. Monitoring upwind climate conditions could also provide early warnings of hot drought for downwind regions.

Looking forward, the duo would like to create models to examine the physics of how hot drought propagates downwind, rather than making inferences based on observations in upwind and downwind locations. Mondal also hopes to investigate whether the downwind transfer of hot drought occurs in other arid, monsoonal regions, such as the India–Pakistan border.

"Climate doesn't respect national borders," Vivoni said. "We're more interconnected than we thought."