New Measurements Suggest Tropical Cyclones May Influence Global Climate

New research from scientists at UC San Diego’s Scripps Institution of Oceanography finds tropical cyclones cause ocean turbulence that extends deeper than previously thought, causing mixing that transfers heat from the surface to waters nearly 300 meters (roughly 1,000 feet) down.

The researchers suggest that the ocean warming caused by tropical cyclones goes deep enough to persist for months or years and travel far from its point of origin, potentially altering the broader patterns of ocean circulation that partly regulate Earth’s climate.

The study, published June 20 in the Proceedings of the National Academy of Sciences, draws its conclusions from direct measurements of ocean turbulence and temperature before and after tropical cyclones. During 60 days aboard a research vessel in the Western Pacific Ocean near the Philippines, scientists observed that the turbulence caused by tropical cyclones persisted for at least three weeks and reverberated down to at least 300 meters (roughly 1,000 feet).

This research, funded by the Office of Naval Research, settles a 20-year-long debate on whether tropical cyclones have the power to shape permanent features of ocean circulation and thereby influence long-term climate variability, said Noel Gutiérrez Brizuela, the study’s first author and a physical oceanography PhD candidate at Scripps.

“People usually think of the background climate as what sets the factors that allow for cyclones to exist,” said Gutiérrez Brizuela. “These results complicate that picture. Climate does determine the likelihood of cyclones but cyclones also seem to play a role shaping the background climate.”

Tropical cyclones – called typhoons in the western North Pacific and hurricanes in the North Atlantic or central-eastern North Pacific – are rotating storms with powerful winds that form around areas of low pressure and are fueled by warm ocean surface temperatures around 27 degrees Celsius (80 degrees Fahrenheit) or warmer. Because climate change is raising air and ocean temperatures, cyclones have become more intense, though whether they will become more frequent remains an area of uncertainty.  

When the strong winds of a tropical cyclone whip up the ocean surface – warmer than the water beneath it because of its direct contact with air and exposure to sunlight – the waves and turbulence cause mixing between the warm uppermost layer and the deeper, cooler waters beneath it. Under calm conditions, the ocean is stratified, with warmer, less-dense water sitting atop colder, denser water without exchanging much heat. But the cyclone-driven turbulence stirs the heat from the surface deeper into the sea. 

At some point, the extra heat that tropical cyclones send into the ocean is released back into the atmosphere. This release of thermal energy into the atmosphere alters the climate wherever it occurs. If a cyclone’s winds only mix up the ocean enough to transfer extra heat to a depth of 100 meters (roughly 330 feet) or so, then any climate effects are likely to remain local and short lived. But if the turbulence reverberates into even deeper, colder waters, the extra heat it transports can instead circulate for years without being released back into the atmosphere, potentially heading north near Japan or east all the way to the eastern tropical Pacific off the coast of Ecuador, according to Gutiérrez Brizuela.

Until now, research largely based on satellite observations assumed that the turbulence and subsequent heat transfer from cyclones stayed in the ocean’s first 100 meters (roughly 330 feet). Consequently, the predominant assumption was that any climate after-effects from cyclones would remain local and thus didn’t need to be factored into the big picture of how much heat gets added to the Pacific Ocean each year or how that heat is distributed.

The issue with these assumptions, said Gutiérrez Brizuela, was that they weren’t based on actual measurements of what happens in the ocean after a tropical cyclone. After spending nearly two months collecting data in the western North Pacific near the Philippines in the fall of 2018, Gutiérrez Brizuela realized that he and his colleagues were uniquely positioned to tackle the fate of the heat added to the ocean by tropical cyclones.

The scientists’ two 30-day research cruises happened to capture almost perfectly contrasting sets of data. On the first leg the team had clear skies and placid seas, while the second saw their vessel buffeted by three tropical cyclones named Mangkhut, Trami, and Kong-Rey.

While at sea, the researchers measured how fast the water beneath them was moving with a device called an acoustic Doppler current profiler that sends out pulses of sound and measures changes in the frequencies of the returning echoes to infer the velocity of the water the echoes passed through. A long, thin instrument called the Chameleon developed by researchers at Oregon State University measured ocean turbulence as well as temperature, depth, and salinity. The team raised and lowered the Chameleon via cable and when turbulence disturbed a tiny needle on the nose of the instrument, it sent electrical signals back up to the ship that were used to estimate turbulence. 

By combining all these measurements and conducting statistical analysis, the team could finally see how deep the turbulence and heat transfer caused by tropical cyclones traveled as well as how long it lasted.

The scientists found turbulence from the tropical cyclones as far down as their instruments reached and persisted to some degree for three weeks.

“We saw large heat fluxes all the way to 270 meters (885 feet), and we didn’t see a real end to the heat transfer,” said Gutiérrez Brizuela. “As deep as our measurements went and as long as we measured there was active heat transfer.”

The team also found that the turbulence from tropical cyclones traveled beyond the ocean’s surface layer in the form of what’s called a near-inertial wave, which is a type of wave that rolls through the ocean’s interior rather than across its surface. Gutiérrez Brizuela said these near-inertial waves are the main reason the turbulence from a cyclone can last for weeks, adding that it is as though the ocean is ringing or reverberating after the storm’s surface impacts. 

“Our results show that tropical cyclones are transferring enough heat, deep enough in the ocean to suggest that these storms are important players in shaping the conditions of the Pacific Ocean as we know it,” said Gutiérrez Brizuela. “The temperature structure of the tropical Pacific is a big driver of global climate. Without cyclones, the ocean would look different and so would the global climate.”

There are no immediate follow up studies planned, but Gutiérrez Brizuela said that the results highlight the importance of resolving the uncertainty around whether climate change is likely to increase the number of tropical cyclones. “To reliably predict ocean warming, say 30 or 50 years into the future, it looks like we first need to understand how tropical cyclone activity might change,” he said. “This adds a new dimension of uncertainty that I’m going to spend a lot of time thinking about.”

In addition to Gutiérrez Brizuela, Matthew Alford, Shang-Ping Xie, Janet Sprintall, and Gunnar Voet of Scripps Oceanography, Sally Warner of Brandeis University, as well as Kenneth Hughes and James N. Moum of Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences were co-authors of the study. OSU’s Moum was the chief scientist of the research cruises used to collect field data for this study.