More Frequent Hurricanes Raise Risk to U.S. East and Gulf Coasts

Researchers typically simulate hurricanes in models that represent Earth’s climate. But those simulations can quickly grow computationally expensive. The models must have sufficiently high resolution and, to generate the numbers needed to make trustworthy projections, researchers would have to perform many simulations of storms.

Balaguru and his coauthors instead developed a unique model known as RAFT. The advantage of RAFT lies in its ability to unravel the many factors that shape hurricanes, from the winds that steer storms to the warm waters from which they draw power. Meant to be combined with climate models, it can also generate many simulated storms, which makes robust statistical analyses possible.

This ability allowed Balaguru’s team to examine the relative significance of each contributing factor behind the changes. The resulting study, said Balaguru, marks the first time such an analysis has been carried out. 

“We saw that storm frequency near the coast was changing,” said Balaguru. “But why? Is it because the storms are getting stronger? Is it because they’re heading more in that direction? Our approach helped us isolate the key variables at play and determine which was most important.” 

RAFT generated hundreds of hurricane tracks: the paths that hurricanes take after they form over ocean waters. The results showed that, if current warming trends continue, storms would both strengthen and more frequently reach the East and Gulf coasts. 

In ramping up the frequency of landfalling hurricanes, Balaguru’s team found that one factor stood more dominant than all others: wind. 

What’s Making Hurricanes Worse?
The new study’s authors uncovered a previously undescribed mechanism behind the rising coastal hurricane frequency. It begins with relatively stronger warming in the Eastern Pacific Ocean, where the sea surface is typically cooler.

As greenhouse gas emissions continue to climb, they trap more and more heat within the Earth system. This warms both land and sea surfaces, although not uniformly. Most climate models project larger sea surface warming in the Eastern Pacific than the Central Pacific.

As the sea surface warms, more water evaporates from the ocean surface and moistens the air above. Because moist air is less dense, it rises higher in the atmosphere where the moisture condenses to form clouds and releases heat. This process is known as convection.

This convection generates a pattern of planetary-scale waves in the atmosphere known as Rossby waves. Though this process begins in the Eastern Pacific, the influence of Rossby waves sprawls great distances. In this case, circulation changes induced by these waves are such that air moves in a clockwise direction close to the surface over the Gulf of Mexico. But higher in the atmosphere, air moves in the opposite direction.  In other words, these Rossby waves produce atmospheric circulation patterns like cyclones and anticyclones—PNNL scientists have uncovered such patterns and their effects before. 

These waves ultimately trigger changes in the winds in the upper and lower troposphere—winds that steer hurricanes. Take a look at the third animation below. Notice in the upper atmosphere the counterclockwise motion of air induced by the Rossby wave. This motion is what pulls and pushes hurricanes toward the East and Gulf coasts. These changes in the steering winds that determine the path of a hurricane have the added effect of weakening wind shear, making landfalling storms even stronger.

Earlier research into the steering winds that guide hurricanes have made similar projections. But this work marks the first time that scientists have proposed a physical mechanism behind the changes that make coastal storms more damaging in a warmer future.

“Many studies have shed light on how hurricanes will change in the future,” said atmospheric scientist and study author Ruby Leung. “By identifying a mechanism that connects the various hurricane changes together, this study offers a more unified perspective underscoring the enhanced coastal hurricane risks in the future.”

Some uncertainties remain, according to the study’s authors. Better understanding cyclogenesis—the process by which hurricanes form—could help paint a more detailed and accurate picture of hurricane behavior. Further study and better modeling of sea surface warming, too, could lead to more robust and detailed projections. 

In their effort to better understand the processes that govern hurricane changes, the team plans to explore how global warming affects tropical cyclones in other coastal regions.