PNAS First Look Blog

Science journalists discuss a selection of new papers from PNAS

Ocean heating from tropical cyclones

Hurricanes apparently pump heat into the ocean, according to direct measurements of the oceans detailed this week in the Proceedings of the National Academy of Sciences. Given how hurricanes are expected to grow in intensity as the climate warms, the oceans may get even warmer and help generate more highly intense storms, potentially leading to a vicious cycle.

Hurricanes and other tropical cyclones vigorously mix warm surface waters with colder waters below, leading to colder surface waters and warmer deeper waters in their wakes. Once the storms pass, while the surface cold dissipates quickly due to sunlight and interactions with the air, the deeper warmth is thought to last much longer, suggesting the net long-term effect of hurricanes is to pump heat into the ocean.

To see how strong this heating effect might be, climate scientist Wei Mei, currently at the University of California, San Diego, and colleagues analyzed satellite-based data on sea surface height to estimate the thermal expansion of water in the wake of hurricanes, particularly those of category 3 to 5 intensity, in the Northern Hemisphere from 1993 to 2009.

“Although previously several studies have been devoted to estimating the effect of hurricanes on warming the ocean, our work is the first to quantify such an effect in a straightforward way by directly monitoring the changes in ocean heat content based on the information obtained from satellite altimetry,” Mei said.

They found that if hurricane activity was distributed evenly over the whole year, they would heat oceans at a rate of roughly 320 terawatts, some 20 times greater than current human power use and approximately 23 times the maximum warming experienced by the rest of the seas on average. This suggests hurricanes are not just the passive results of climate, but play an active role in climate, Mei said.

The scientists found the stronger the hurricanes, the greater the warming effect. Since the number of higher-intensity hurricanes is expected to grow with the warming climate, this effect could help accelerate the warming of the oceans. Since the oceans help power cyclones, a feedback loop could occur, with hurricanes and warming pushing each other to grow, Mei said.

Categories: Earth, Atmospheric, and Planetary Sciences
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