Hurricane Laura: 2020’s “extremely active” hurricane season, explained

Hurricane Laura has weakened to a tropical storm after making landfall Wednesday at Category 4 strength, bringing damaging rains and winds to the Gulf Coast and leaving at least six people dead in the US.

Even before the storm reached US shores, it caused devastation in the Caribbean, killing people in Haiti, the Dominican Republic, and Cuba. It left hundreds of thousands without power and more than a million without clean drinking water.

In the US, the storm has left behind expansive flooding, with water reaching up to 9 feet high. In Lake Charles, Louisiana, it triggered a chemical fire. Researchers expect Laura and its aftermath to be a multibillion-dollar disaster. And it’s hammering the United States as the country faces the ongoing Covid-19 pandemic.

Laura is one of more than a dozen named Atlantic storms in what forecasters have described as an “extremely active” 2020 season. It follows hurricanes like Hanna and Isaias and gained steam alongside Marco. These storms brought torrential rainfall and spawned tornadoes. They have proven to be costly and deadly.

Forecasters saw some of this coming, but even the experts had to revise their estimates upward for the number of storms this year. Warmer water and calm atmospheric conditions in the spring and summer over the North Atlantic Ocean have proven ideal conditions, not just to birth tropical cyclones, but to help them rapidly intensify.

The peak of hurricane season still lies ahead, so even more tropical storms are likely to rise this year, some of which may become major hurricanes that do major damage.

Scientists have learned a great deal about what elements portend a boisterous hurricane season, and are now grappling with what to expect in the coming years as the planet warms.

What has made 2020’s hurricane season so intense

Back in May, the National Oceanic and Atmospheric Administration anticipated a busier than normal hurricane season in the Atlantic. The agency forecast 13 to 19 named storms, of which six to 10 would become hurricanes. Of those, three to six would become major hurricanes.

This is higher than an average season, which produces 12 named storms, six hurricanes, and three major hurricanes.

But NOAA then updated its forecast earlier this month after Atlantic storms formed at a record-setting clip. Usually only two named storms form by early August, but this year saw nine in the same time frame. In fact, three named storms formed before the official June 1 start of the hurricane season. The count is now 13, and the updated forecast anticipates between 19 and 25 named storms, which means several more are likely on the way.

“This is one of the most active seasonal forecasts that NOAA has produced in its 22-year history of hurricane outlooks,” said Secretary of Commerce Wilbur Ross in a May 21 statement.

Tropical storms in a given year are named in alphabetical order. But the World Meteorological Organization only came up with 21 names this year. If there are more storms, meteorologists will start using names from the Greek alphabet.

There were only 21 names selected for the 2020 Atlantic storms, but there may be 25 named storms this season.

So what were the warning signs?

Aaron Piña, a deputy program scientist at NASA headquarters, told Vox a key part of hurricane development this year was the temperature of the sea surface in the Atlantic Ocean. In order to form a hurricane, surface water needs to be at least 26°C or 79°F.

The 2020 season began with warmer-than-average sea surface temperatures in the Atlantic and the Caribbean Sea. Those waters continued to warm up as the summer months set temperature records across many parts of the Northern Hemisphere. As this water heats up, more energy is available for hurricanes to spin up, grow, and strengthen.

But it’s not the only factor. Major storms like hurricanes also require a fair amount of calm in the air to form. Shifts in wind speed and direction with altitude, known as wind shear, can prevent storms from forming or rip apart weather systems before they blossom into major storms. But this year, the air over the North Atlantic was more placid, with less wind shear than is typical.

That’s in part due to the phase of El Niño—Southern Oscillation, or ENSO, a periodic warming and cooling pattern in the Pacific Ocean that can influence weather around the world. During an El Niño year, trade winds over the eastern Pacific Ocean weaken, allowing the surface of the eastern Pacific to warm up. In the other phase of the cycle known as a La Niña, which occurs over two to seven years, the opposite happens.

“In an El Niño [year], what we would see in the Atlantic basin is a lot more what we call wind shear, which kind of doesn’t allow tropical cyclones to strengthen or even develop,” Piña said. But in 2020, the ENSO was in its neutral phase, leading to more stable air over the Atlantic.

That’s not to say wind shear hasn’t affected hurricanes after they’ve formed. “Hurricane Marco ended up totally just getting decapitated by that wind shear,” Piña said, describing how the storm, which was poised to strike the Gulf Coast alongside Laura, suddenly weakened before landfall.

The humidity of the air matters too. As air heats up, it can hold onto more moisture, so the storms that do form end up dishing out more rain. Air absorbs about 7 percent more water for every degree Celsius the air warms, roughly. However, tropical storms need cooler temperatures in the upper atmosphere. That creates a temperature gradient from the sea to the sky and a path for all the heat energy stored in the water to dissipate. Therefore wet air close to the sea surface and cooler air above the clouds help foment major storms.

According to NOAA, this mix of ingredients has produced above-average tropical storm and hurricane activity in the Atlantic Ocean.

One curveball in the storms this season was the massive Saharan dust cloud that floated over the Atlantic in June. In addition to dimming skies and seeding the ocean with nutrients, this dust also suppresses hurricanes. The dry, fast-moving Saharan dust robs the air of critical stability and moisture those storms need. But because of the Covid-19 pandemic, researchers have had to cancel some of their experiments and field campaigns aimed at studying this interaction this year.

Even with so much tropical storm activity so far, it’s important to remember the season has yet to enter its peak. There may yet be more hurricanes through the rest of hurricane season, which ends in November. Piña noted the factors that fueled the storms so far are still in place, or have amped up further. The oceanic and environmental conditions this time of the year are becoming much more favorable for storms to initiate and strengthen.

Scientists are figuring out how climate change will affect future hurricanes

Outside of the unusual conditions preceding the 2020 Atlantic hurricane season, there are also longer-term factors at play, namely climate change. As humans continue to spew greenhouse gases from fossil fuels into the atmosphere, the planet retains more heat and warms up.

This can have important consequences for hurricanes, as their formation and behavior depends so much on the amount of heat in the air and water, as well as on where it’s distributed. But hurricanes are not heat waves; they’re much more complicated, so there are many important nuances in the effects of climate change on hurricane patterns.

Fundamentally, more energy trapped by greenhouse gases means more energy hurricanes can use, which will push the distribution of the hurricanes that do occur toward greater extremes. In other words, hurricanes will get stronger.

“As the climate warms, we expect that the upper bound of how intense a hurricane can get … goes up at a certain rate with warming and that’s been known for 33 years,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. “More recently it’s also been understood that there is a cap on the rate of intensification, and that goes up faster.”

Rapid intensification has been a major component of Hurricane Laura. It surged from below hurricane strength with 65 mph winds early Tuesday to Category 4 strength on Wednesday. NOAA defines rapid intensification as a gain of 35 mph or more in wind speed over 24 hours, but Emanuel noted intensification is a continuum, and warming will likely push tropical storms toward the faster end, including storms both above and below NOAA’s threshold.

However, increases in the pace and the range of hurricane intensity don’t just depend on the amount of heat the planet accumulates. Differences in temperature between ocean systems in the tropics are also critical.

“If the Atlantic is warmer than other parts of the tropical oceans, specifically the Indian and Pacific Oceans, that’s going to have big implications both for the number and intensity of hurricanes,” Emanuel said. “If the whole world warms up, on the other hand, that also affects the intensity of hurricanes, but not nearly as fast per degree of warming.”

Climate change could also amplify elements of storm surges caused by hurricanes. These events are caused by winds pushing coastal water inland. As sea levels rise due to melting ice and warming oceans, the severity of storm surges will get worse.

“In the case of surges, I would say that the number one thing that makes me worry about surges is plain old sea level rise,” Emanuel said. The more intense storms that would form in such a scenario could increase surge levels, but they also have a smaller inner core, which would tend to generate a weaker surge. Whether the sum of sea level rise plus changes in storm behavior would result in more or less severe storm surges is not yet clear.

Scientists have already observed some trends in hurricane activity but are still teasing out the signals of climate change. One recent study found evidence that the movement of tropical cyclones like hurricanes is slowing down. Another study reported tropical cyclones have grown more intense between 1979 and 2017.

But Emanuel noted the scale of some of these changes may have also been influenced by local factors, like a reduction of sulfur air pollution over the Atlantic since the 1980s. Global climate change may be a factor here, but it’s not the only factor.

On the other hand, there are other aspects of hurricanes that have a more direct link to climate change. Extreme rainfall events have increased in recent decades and tropical storms are a major source of these downpours. “Just the fact that it’s warmer means that a given hurricane is going to produce more rain, and it goes up pretty fast with temperature,” Emanuel said.

Hurricane Harvey in 2017 was an example of some of these trends coming together. The Category 4 storm slowed down over Houston, Texas, and doused the city in more than 50 inches of rain.

For humans, this heavy rainfall may be the most important trend in hurricanes, because the flooding caused by hurricanes tends to be the deadliest and most destructive element of the storm. That was borne out in both Harvey and Hurricane Katrina in 2005, two storms that left behind severe flooding and are tied as the costliest hurricanes on record.

Despite these looming threats, coastal development is continuing in areas vulnerable to hurricanes. About 40 percent of the US population lives in a coastal county and populations are growing in many of these areas, bringing with them oil refineries, ports, and high-priced condominiums near the water. That means storms will do far more damage when they do occur.

It will take some time to tease out the specific climate signals in storms like Laura and discern how bad they would have been without the warming the world has already experienced. But the expectation is that the impacts of climate change already underway, and those in store, are worsening the toll of hurricanes.

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