Saturday, September 3, 2011


Hurricane Forcing: Can Tropical Cyclones Be Stopped?

Aug 25, 2011; 9:36 AM ET

Tropical cyclones, or hurricanes as they are known in the regions bordering the Atlantic Ocean, are among nature's fiercest manifestations, capable of releasing as much energy as 10,000 nuclear bombs. Hurricane Katrina leveled New Orleans and the Mississippi Gulf Coast leaving more than 1,800 people dead; Typhoon Morakot killed more people and did more damage to Taiwan than any other storm there in recorded history; and Cyclone Nargis devastated Myanmar (Burma) and resulted in at least 146,000 fatalities.

Could the formation of these storm systems be tempered or even arrested by technical means?

TROPICAL CYCLONE: Could simple man-made structures in the ocean stop tropical cyclones, such as Typhoon Lupit pictured here?

Image: NASA MODIS Rapid Response Team

In June 2009, a plan to reduce the severity and frequency of hurricanes leaked to the public in the form of a patent application under Bill Gates's name (along with many others), resuscitating speculation about a scheme that has been proposed off and on since the 1960s. The core of the idea remains the same: mixing the warm surface waters that fuel tropical cyclones with cooler waters below to drain storms of their energy. But now Stephen Salter an emeritus professor of engineering design at the University of Edinburgh proposes a new—and possibly more realistic—method of mixing.

Salter has outlined in an engineering paper the design for a floating structure 100 meters in diameter—basically a circular raft of lashed-together used tires (to reduce cost). It would support a thin plastic tube 100 meters in diameter and 200 meters in length. When deployed in the open ocean, the tube would hang vertically, descending through the warm, well-mixed upper reaches of the ocean and terminating in a deeper part of the water column known as the thermocline, where water temperatures drop precipitously. The point of this design is to transfer warm surface water into the deeper, cooler reaches of the ocean, mixing the two together and, hopefully, cooling the sea surface. Salter's design is relatively simple, using a minimum of material in order to make the construction of each of his devices cheap (millions of used tires are thrown away each year, worldwide); his scheme would also require the deployment of hundreds of these devices.

Using horizontal wave action at the ocean surface, passive no-return valves would capture energy by closing after a wave has passed through them, allowing the circular interior of each device to raise the level of the seawater within the device by, on average, 20 centimeters. The weight of the gathered warm water would thereby create downward pressure, pushing it down the tube.

The idea is that hundreds of these floating wave-powered seawater pumps would be deployed year-round in areas, such as the eastern tropical Atlantic and the Gulf of Mexico, where hurricanes typically spawn or grow in intensity. (The devices would not, as widely speculated, be deployed only in the path of a hurricane that already formed.)

Salter says he was inspired to invent his device after seeing the damage wrought by Katrina. "I was called to a meeting at [intellectual property firm] Intellectual Ventures where they wanted to talk abut hurricanes, and they were very enthusiastic about it," he says.

The pumps have been named the Salter Sink by the firm, which patented them. Bill Gates was in the session at which Salter proposed the pumps, according to Intellectual Ventures CEO Nathan Myhrvold, and it is the company's policy to list as authors everyone present at a brainstorming session on the patents that are filed as a result of it.

Biological productivity could be side benefit

By mixing warm sea-surface water with the colder water beneath year-round, Salter thinks these pumps could keep the surface temperature below the 26.5 degrees Celsius threshold, beyond which the frequency and severity of hurricanes increase markedly. Salter and some of his co-authors on the original patent think the pump might even increase the biological productivity of the seas in which it's deployed, because it would mix nutrient-rich, deep water with warm, relatively nutrient-poor surface water. Nutrients from deeper parts of the ocean would be brought to within 100 meters of the surface, the deepest that sunlight can penetrate and power the photosynthetic plankton that are the base of the ocean food chain. This would be a boon to fish populations in the ecologically unproductive "biological deserts" of tropical seas where hurricanes spawn and the devices would be deployed. In these areas, little mixing occurs and populations of plankton—and therefore fish—are limited by available nutrients.

Ricardo Letelier, a microbial oceanographer at Oregon State University, however, points out that the effects of increasing available nutrients in the ocean can be unpredictable. "If you were to keep the pumps running continuously…you may allow phytoplankton to bloom," he says. "If you do it for too long, you get a successional pattern where grazers take over and recycle nutrients. And that's one of the problems we've had with iron fertilization experiments—the response of biological systems are not linear."

And Letelier warns that deep ocean waters contain a great deal more dissolved CO2 than surface waters do, because expiring plankton sink in the water column, almost like the rotting leaves on a forest floor. In addition, the solubility of CO2 in water increases with depth and decreasing temperature. As a result, mixing the two layers of the ocean would inevitably lead to significant transfer of CO2 from the biggest carbon sink on Earth—the ocean—to the atmosphere. The process is similar to what happens when you open a carbonated beverage—the drop in pressure causes dissolved CO2 to come out of solution and enter the air.

By Christopher Mims

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