Hurricanes are moving slower, making them more deadly, study says

Tropical cyclones have grown more sluggish since the mid-20th century, a new study says.

A study published in the journal Nature on Wednesday, June 6, says that hurricanes are lingering in one place for longer.

The study determined this by focusing on the “translation speed.”

The translation speed measures how quickly a storm is moving over an area, for example from the Florida Keys to the Florida Panhandle.

To analyze the changes in translation speeds, the researchers tapped into a global data set on past tropical storms. The data include estimates of the latitude and longitude of each named storm’s center at six-hour intervals.

The researchers were able to measure how quickly the storm moved across the landscape. They were then able to calculate the average speeds of the storms from year to year.

The study found that between 1949 and 2016, tropical cyclone translation speeds declined 10 percent worldwide.

While more sluggish storms may sound less dangerous, these slower-moving storms may actually be more deadly, according to the study.

For instance, Hurricane Harvey crawled over Texas in August 2017 dumped more than 30 inches of rain in two days and nearly 50 inches over four days in some areas.

A Harris County report released on Monday, June 4, found that Harvey’s rainfall exceeded every known flooding event in American history since 1899.

Harvey demonstrates the devastating impacts that lingering hurricanes can have on impacted areas. These lingering storms will likely occur more often as the Earth’s atmosphere warms.

The study said that as the Earth’s atmosphere warms, the atmospheric circulation changes. This causes a general weakening of summertime tropical circulation.

Tropical cyclones are carried along within their ambient environmental wind. Therefore, the translation speed of tropical cyclones will likely slow down with warming, the study said.

Steering winds largely determine the movement of hurricanes.

The steering winds draw power from the temperature differences between the tropics and the poles. Climate change has caused that temperature difference to decline.

There has been some evidence that steering winds in the tropics during the warm season may indeed get slightly weaker due to climate change, AccuWeather Senior Meteorologist Brett Anderson said.

The weaker steering winds move hurricanes more slowly.

“The result of this is a growing trend of tropical storms and hurricanes lingering over a particular region for a longer period of time, which in turns allows for a longer duration of heavy rainfall,” Anderson said.

In addition to circulation changes, warming causes increases in atmospheric water-vapor capacity. This is generally expected to increase precipitation rates, the study said.

Rain rates are expected to increase at the center of tropical cyclones with increasing global temperatures.

The latest research suggests that regions that are normally wet, which includes regions that are often in the paths of hurricanes, will become wetter through the end of this century. This is due to an increase in available water vapor resulting from the warming, according to Anderson.

The study highlights why heavy rainfall from a hurricane is more concerning than the wind of a hurricane, according to AccuWeather Hurricane Expert Dan Kottlowski.

"More people die and more damage is done due to water, including storm surge flooding, flash flooding and prolonged flooding, from hurricanes," Kottlowski said.

The study does not answer the question of what is causing the slowdown. The new study is based on an analysis of observations, so it does not directly answer that question.

"The study does point out that slower-moving hurricanes cause more rainfall over land. We have seen that over the years, so this is nothing new," Kottlowski said. "Why the hurricanes are not moving as fast as previous years is not so clear."

There is no way to tell if a hurricane will be a slower tracker over land before it forms. Once a hurricane forms and becomes embedded within a steering flow, the environment around the hurricane strongly influences how it will move during its trek across the ocean and possibly onto land, according to Kottlowski.

Most hurricanes and tropical storms are steered across the Atlantic, and sometimes into land, by the Bermuda Azores high pressure area. The area expands and contracts generally in an east to west fashion during the hurricane season.

The orientation and strength of that high typically determines where and how a hurricane tracks.

"Last year, the Bermuda Azores high was generally stronger than normal and nosed westward into the northern Gulf of Mexico more frequently than normal. This westward extension caused more storms to track farther to the west," Kottlowski said.

Most hurricanes tend to recurve northward across the western Atlantic due to the rotation of the Earth before reaching the Caribbean or the United States.

But last year, many storms and hurricanes tracked well into the Caribbean before turning northward, putting more land in harm's way.

"There is no way to tell whether the orientation and strength of the Bermuda Azores high is changing due to climate change," Kottlowski said.

The orientation and strength of the high pressure area is influenced by many factors, including sea surface temperature to the north and the upper-level wind flow across the northern and central Atlantic.

"The hypothesis that the warming of the climate is causing the slowdown cannot be proved at this point. That does not dismiss the study," Kottlowski said.

The study highlights the need for continued research into the connections between tropical cyclones and climate, saying it is essential to understanding and predicting risk.

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