The unusual factors behind the extraordinary heat across the southern US

The South is no stranger to heat, but the temperatures and humidity right now are testing even the hardiest denizens of Dixie as the hot weather stretches into a third week.

The heat wave along the Gulf Coast, stretching over Arkansas, Mississippi, Louisiana, Oklahoma, New Mexico, Texas, and Mexico, is straining the limits of infrastructure and human survival.

According to the National Weather Service, extreme heat is the number one weather-related killer in the US. The recent weather has already proven dangerous and deadly in places like Texas, which set an all-time energy demand record this week as millions across the state switched on fans and air conditioners to cope with triple-digit temperatures. The heat in Arkansas followed tornadoes, high winds, and hail that knocked out power for 62,000 customers last week.

The recent sweltering weather stands out for its timing, its severity, and its duration. Parts of Louisiana issued excessive heat warnings earlier in the year than they’ve ever done before.

The 100-degree-plus temperatures are a function of normal summer weather on top of unusual atmospheric patterns and global temperature cycles aligning in their hot phases. The southern US also has some unique features that are enhancing the heat. All the while, the planet as a whole is warming up.

Heat waves are also baking other parts of the world, and many places have already broken temperature records before summer is in full swing. Some of the drivers behind this weather are going to gather strength through the year, so 2023 is poised to become the hottest year on record for the planet.

How the jet stream is fueling the extreme heat

Heat waves are typically defined as sustained temperatures above the 95th percentile for a region. That means that the temperatures that count as a heat wave are lower in Alaska than they are in Arkansas. “Keep in mind, it’s always hot in south Louisiana in the summer,” Barry Keim, Louisiana’s state climatologist and a professor at Louisiana State University, told Vox.

Stretches of excessive heat usually follow a formula: A high-pressure system — where atmospheric pressure above an area increases — creates a column of air that heats up and dries out as it sinks. This drives clouds away, allowing the sun to heat up the ground unobstructed, with the sinking air acting as a heat dome holding the hot air in place over several days.

But there are several other variables amplifying this formula right now. One of them is the jet stream, or rather, jet streams. These are bands of air currents blowing west to east at 275 miles per hour or more, gusting 4 to 8 miles above the Earth’s surface. They usually form circles around the planet and act as boundaries between warm and cold air in the sky. In the Northern Hemisphere, the main streams are the polar jet, ensconcing cold air at the North Pole, and the subtropical jet, holding warm equatorial air at bay. However, the streams can meander and blur together. “If you look at any particular day, it’s not always clear that there’s two distinct streams,” Rachel White, an atmospheric scientist at the University of British Columbia, told Vox.

These undulations are known as Rossby waves. Right now, the jet streams appear to be following a roller coaster track over North America rather than neat, straight lines. “It’s more ‘wave’ than ‘jet’ right now,” White said.

In particular, there appears to be a northward bulge in the jet stream extending over the southern US. That’s holding a high-pressure system in place. “When that pattern persists over the same region for a period of days, that’s how you get a heat wave,” Steve Seman, an assistant teaching professor at Pennsylvania State University, said.

In this satellite image, you can see a patch of clear air over the southern US, bounded by a ridge of clouds:

Climate Reanalyzer/NOAA

On the other side of that hump, the weather is much cooler, as seen in this temperature map:

NOAA/National Weather Service

“Under the ridge, where we are, we’ve had the heat. So in that sense, the jet stream is responsible for our weather, but at the same time responsible for the cooler weather on either side,” John Nielsen-Gammon, the Texas state climatologist and a professor of atmospheric sciences at Texas A&M University, told Vox. “If we just had high pressure extending across all of the southern US, we wouldn’t have gotten the high temperatures that we saw, because the more localized ridge produces a stronger sinking of air and also allows for the clockwise recirculation of air.”

But right along that ridge, there’s a lot of rollicking weather. The rushing jet stream can draw air and moisture upward, fueling convective storms. “Those areas have been very active with storm activity: Tornadoes, high winds, wind damage, and hail have taken place,” Keim said. “Every place to the south of that boundary is basking in very high heat and humidity levels.”

Another big factor here is that the world is in a strong El Niño this year. This is the warm phase of the Pacific Ocean’s temperature cycle, where the sea surface heats up and starts to spread along the equator. That warmer water in turn shifts weather patterns, bringing more rain to some areas, drought to others, and generally more hot weather around the world.

In the southern US, El Niño usually makes the subtropical jet stream more active in the winter, bringing cooler and wetter than average weather to the Gulf Coast. “Since this El Niño has already kicked in, that subtropical jet that we know is normally hyperactive in the winter, we’re actually seeing this hyperactivity taking place in the summer when normally we don’t even notice [it],” Keim said.

Confusingly, wobbly jet streams can induce the opposite effect, too. Rather than letting warm air encroach northward, waves can let cold air drift southward. In 2021, waves in the polar jet stream spilled chilly Arctic air over the US, leading to a cold snap in Texas. “It’s sort of the other side of the same coin,” White said.

Scientists are still trying to sort out how jet streams have changed over time, and they don’t all agree on whether climate change is playing a role. “That’s the million-dollar question,” White said. “My perspective is that there isn’t a strong scientific consensus.”

One proposed mechanism is that the Arctic is warming up to four times as fast as the rest of the planet, and as the temperature gradient between the North Pole and regions farther south gets shallower, the polar jet stream gets wavier. But the ordinary chaos of weather can also induce these waves, and since much of our understanding of jet streams is drawn from satellites, there isn’t a very long record of historical observations with which to compare. “The jury is still out on that, I think,” Seman said.

Warm waters means hot, sticky air

One of the other remarkable aspects of the heat wave across the South is the high humidity. Meteorologists track the combination of heat and humidity with the heat index, which reflects how the weather feels on the human body and how difficult it is to cool off by sweating. When the heat index gets too high, the weather can quickly become dangerous to many people.

NOAA

Gulf Coasters are of course well acquainted with humidity, but there are factors making the current damp spell unusual as well.

The Atlantic Ocean is seeing record-high temperatures this year, helping heat up water in the Gulf of Mexico. And since the 1970s, the gulf has been warming about twice as fast as the rest of the world’s oceans. Hot water holds on to less oxygen, which can suffocate fish. High temperatures in the Gulf of Mexico have already been blamed for a fish kill in Texas this year.

Hotter water also evaporates more readily. “​​That ‘pump’ off the Gulf of Mexico is becoming even stronger, raising the humidity levels,” Keim said. “By virtue of that, our heat index values … go up to 113 [degrees Fahrenheit]. We’re getting into a very dangerous range.”

Further inland, the hot air is drawing water out of soil and vegetation. “We’re already seeing some crops starting to wilt from the heat and lack of moisture,” said Nielsen-Gammon.

The heat wave isn’t just manifesting at the upper end of the temperature range; it’s also pushing up minimum temperatures so that it stays warm overnight and into the early morning. When temperatures linger into the 90s after the sun sets, people get little relief from the heat. It can disrupt sleep patterns and worsen underlying health problems.

“There may be more danger than a typical heat event, due to the longevity of near-record or record high nighttime lows and elevated heat index readings,” the National Weather Service’s Weather Prediction Center reported on Friday. Heat waves that occur earlier in the season tend to be more deadly because people are less acclimated to hot weather. Older adults, people with preexisting health problems, and people without access to cooling, like those in prisons, face especially high risks from high temperatures.

These effects are all further enhanced in dense cities like Houston and New Orleans where asphalt and concrete soak up the sun, creating heat islands that get hotter than their rural surroundings.

On top of all this, the climate is changing. Average temperatures are rising, increasing the likelihood and severity of heat waves in much of the world. “Climate change looms large and hangs over all of this,” Keim said.

The heat wave across the South may break over the next week, but it is setting the stage for even more scorching weather. The current heat wave “will probably further increase the chances of hot temperatures because it’s accelerated the normal drying out of soils that takes place across Texas during June, July and August,” Nielsen-Gammon said. So drink plenty of water, stay in the shade, and take frequent breaks as you brace for the heat that lies ahead.

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