September 24, 2005

The Gulf Coast: A Victim of Global Warming?

Continued from page 1

By Wade Roush

But ironically, Emanuel and his group at MIT weren't looking to generate debate when they began analyzing the northern-hemisphere storm records (which came from a commonly-used database compiled by the Hadley Center for Climate Prediction and Research in the UK). The group was mainly interested in how the upper layers of the ocean conduct heat from Earth's lower latitudes to higher latitudes.

"The ocean transports between a quarter and a third of the total heat transported between lower and higher latitudes," Emanuel says. "The atmosphere transports the rest. But the ocean's mechanism is completely different from the mechanism the atmosphere uses. It's been known for years that it's done by the mixing of the upper oceans. But nobody knows what mixes the upper oceans.

"So I was testing the notion that tropical cyclones are primarily responsible for that. If that's true, then the variability in cyclones should correlate with the variability in ocean surface temperatures."

One of Emanuel's early decisions was to measure the power of tropical cyclones, not their frequency.

"If you look globally there are about 90 storms per year," Emanuel says. "That number, although variable, doesn't show any long term trend. The particular metric I developed..iis sensitive to a storm's intensity, meaning its wind speed and duration."

Aside from the fact that the intensity measure might reveal previously undetected climate trends, Emanuel was attracted to it because it is the best predictor of the amount of damage a hurricane will inflict. (One Category 5 hurricane can wreak more havoc on land than dozens of Category 2 or 3 storms.)

Emanuel's measure, called the power dissipation index, is calculated from cyclone wind speeds as observed by satellites, planes, ships, or land stations, and it turned out to provide a new lens on the Hadley data. At first, Emanuel was looking only for signs of "classical" climate signals in the data, such as the well-known El Nino ocean temperature oscillation.  He found them -- but then he saw the drastic increase in hurricane intensity, closely paralleling the sea surface temperature data.

"This database has existed for quite a long time, it just hasn't been looked at in this particular way," says Emanuel. "That happens a lot in science. The [Antarctic] ozone hole sat there in the data for years, and no one looked at it."

The results excited Emanuel, since they supported his idea that tropical cyclones help to mix ocean water and redistribute heat around the globe.