The most powerful, deadly storms on the planet are beholden to the tiniest of marine creatures.
According to a new study, plankton have the ability to determine whether clusters of tropical thunderstorms spin up into monster hurricanes, and to steer mature storms across large swaths of the ocean.
Plankton are ubiquitous in the ocean, their legions of tiny photosynthetic bodies tinting the blue ocean green with chlorophyll. Even the deep azure of the North Pacific Gyre, a vast "ocean desert" between Japan and California, has a touch of murk to it (pictured below in a satellite composite image). Green water traps light and heat from the sun in the shallows, which raises surface temperatures and makes for prime hurricane-forming conditions.
In a new study upcoming in the journal Geophysical Research Letters (pdf), a team of scientists wondered what would happen if all of the plankton were removed form the Pacific Ocean, leaving nothing but an empty blue basin. Sunlight would pour deeper into the ocean and surface waters would stay cool, but what would be the effect on hurricanes?
"We found that a blue ocean reduces typhoons in the northwest Pacific by 70 percent," lead author Anand Gnanadesikan of the National Oceanic and Atmospheric Administration said, describing a computer simulation of a lifeless ocean that the team put together.
What few storms did form in the simulation hugged the equator, the only area of water warm enough to sustain them. "You got fewer typhoons making it up toward Japan and south China, and more in Vietnam and Cambodia," Gnanadesikan said, adding, "[a perfectly blue ocean] is of course unrealistic, but even a smaller change in ocean color would have a significant effect."
For example, a just slight tweak in ocean color could send 25 percent more hurricanes barreling into Hawaii, an outlook that would have major implications for how communities prepare for wind damage, storm surges, and other hurricane-related hazards.
Fortunately, the ocean isn't fickle enough that we have to worry about every single plankton bloom that comes along. It likely takes several high- or low-plankton years to change the course and intensity of storms significantly.
"It's not a case of if you went and dropped cyanide in the ocean in the path of a hurricane, kill the plankton, and you could suddenly turn it off," Gnanadesikan said.
That's because it takes time for the sun's heat to propagate through the ocean and have its full effect on the atmosphere. In the "blue ocean" experiment, the team found that surface waters stayed cooler, but some of the warm deep water circulated toward the equator. More evaporation and cloud formation over equatorial waters strengthened atmospheric circulation and wind shear (which discourages hurricanes) in more northerly latitudes.
"It's quite a surprise," Gnanadesikan said. "This is something that most people don't think about - why should people who forecast hurricanes care about what color the ocean is? It points out these connections between ocean color and very important weather phenomena."
The implications for the future are far from certain. A recent study in Nature suggested that ocean productivity has been declining steadily since the late 19th century. But the team noted a lull in hurricane activity in the 1960s matches up well with low chlorophyll levels in the Pacific gyres around the same time. Today, chlorophyll is about twice as abundant as it was back then.
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