The Stories Behind the Science: How Does the Ocean’s Saltiness Affect Tropical Storms?

Image courtesy of NASA

An image of super typhoon Haiyan approaching the Philippines in 2013.

Pounding winds, rising water, flooded homes. While many of us have seen the images of hurricanes and other tropical storms on TV, both Karthik Balaguru and Greg Foltz experienced them in person. Although he now lives in Washington State, Balaguru faced them as a child in India as well as during his graduate work in Texas. Foltz lives in Miami, Florida, a place that's no stranger to storms.

Both remember the fear and concern that came with big storms. But the storms sparked something else in them too: curiosity. Balaguru studies the interactions between the ocean and climate at the Department of Energy's Pacific Northwest National Laboratory, while Foltz researches oceanography for the National Oceanic and Atmospheric Administration. Both scientists explore the factors that cause tropical storms — especially those in the volatile Pacific Ocean — to get stronger or weaker. With support from the DOE's Office of Science and in collaboration with researchers Ruby Leung and Kerry Emanuel, they tackled a question no one else had explored: How does the ocean's saltiness affect tropical storms' strength in the context of climate change?

Most scientists didn't think the ocean's saltiness — salinity — was all that important in storm development. After all, heat is the engine that drives storms' strength. Hurricanes, cyclones, and typhoons — different names for strong tropical storms depending on the region — gather their energy from heat at the ocean's surface. That's why hurricanes slow down and weaken when they reach land.

But Balaguru and Foltz suspected other scientists were missing something important.

"The impact of salinity is not very straightforward," said Balaguru. "I can see why people would not look at it more closely."

As wind in a storm churns the ocean, it brings up cooler subsurface ocean water. By making the surface cooler, the mixing minimizes the energy the storm can draw up. As a result, the process is a self-braking mechanism for storms, especially big ones.

But changes in salinity throw a wrench in this process. When it rains, freshwater from the rain makes surface water less salty. Because freshwater is lighter than saltwater, it resists getting mixed in and down. That prevents cool water from coming to the surface and minimizes the self-braking mechanism. There's a greater potential for storms to get stronger and stronger.

"Nobody's considered salinity, at least not on these timescales," said Foltz, referring to climate change. "It was kind of an obvious thing to do."

This process is particularly influential in the western Pacific Ocean, which has some of the least salty surface water of the tropical oceans. It's also the most active tropical storm breeding ground in the world.

"[People in the western Pacific are] very vulnerable there because of sea level rise," said Foltz. In 2013, super typhoon Haiyan killed 6,000 people in the Philippines. "If there is an impact on tropical cyclones of salinity, that's a good place to look."

The biggest challenge was sorting out the relationships among salinity, ocean temperatures, and storm strength. Fortunately, Balaguru and Foltz had previously developed a method for separating out these effects. They used that method to analyze 35 years of historical data and estimate future typhoon behavior. Plugging data on ocean temperature, salinity, and air temperature into a model, they calculated how cold the water was after the typhoon had passed (their wakes). The colder the wake, the weaker the storm. But were the wakes in less salty areas warmer — and therefore producing stronger storms — than the ones in more salty areas?

The answer was a definitive yes — at least for the strongest of storms. Salinity had minor effects on weaker storms. But for super typhoons with wind speeds of 150 miles an hour or more, the effects were huge.

"It was a little bit like a Eureka moment for us," said Balaguru. "The magnitude of the effect was what really caught us by surprise."

As salinity changed over time, so did the storms. According to the study, from 1958 to 2013, increased rainfall from climate change made the ocean less salty. The areas with the biggest decreases in salinity also experienced increasingly strong storms.

When they ran predictions of future storms, the results were even more dramatic. Other scientists estimated climate change could weaken tropical cyclones because the warmer water would increase surface cooling due to the mixing mechanism. But Balaguru and Foltz's analysis showed increased rainfall from climate change could make surface water less salty. That could actually amp up storms' intensities.

"Salinity nearly cancels the effect of temperature," said Balaguru. "The only thing that salinity can do is enhance the intensity of cyclones."

The results were so different from reigning views that the researchers not only double-checked their calculations, but ran them through 16 separate climate models.

While the storms experienced in Balaguru's childhood may have scared him back then, the research they inspired is helping him and other scientists better understand past and future storms. As he said, "There is still room for a lot of discovery when it comes to tropical cyclones."

The Stories Behind the Science series takes a look at the process and drama of the research behind a selection of major studies supported by the Department of Energy's Office of Science.

The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit https://science.energy.gov.

Shannon Brescher Shea is a senior writer/editor in the Office of Science, [email protected].