Where the Sea Meets the Sky
Ocean Modeling Could Lead to More Accurate Weather Forecasts

About the Photo: Oceanographer Huijie Xue.  Every winter, masses of cold, dry Arctic air slide south across the North American continent before encountering a river of warmth from the sub-tropics, the Gulf Stream. The resulting collision tends to create some of New England's most dramatic weather.

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University of Maine oceanographer Huijie Xue loves a good nor'easter.

For her, the excitement is in seeing how sea and sky come together at the inception of such stormy weather. It's that little-understood interaction that Xue is modeling in the research laboratory.

It is basic science that one day could build the foundation for more accurate weather forecasting.

Working with computer models, Xue and her colleagues have already seen how cold winds can draw heat and moisture out of the water, modify water circulation patterns and push the Gulf Stream farther offshore. The net result is a transfer of heat from the ocean to the atmosphere — a major factor in storm development, she says. In fact, the largest sea-to-air transfer of heat ever measured occurred over the Gulf Stream.

Xue is a member of the ocean modeling group and an associate professor in UMaine's School of Marine Sciences. Ultimately, her work will help scientists to understand how the ocean and atmosphere work together to produce day-to-day weather, as well as long-term climate.

As a specialist in scientific modeling, Xue and her colleagues turn theories into sets of equations that are run on some of the world's fastest supercomputers. Their models can reveal details of the natural world that might otherwise remain hidden. They have shown, for example, that heat drawn out of the sea by cold winds can cool the ocean to a depth of more than 100 meters.

In collaboration with some of New England's top marine scientists, Xue has applied her scientific skills to studying the circulation of Penobscot Bay and the Gulf of Maine.

Xue graduated from one of China's leading oceanography programs before coming to the United States in 1986 to earn a graduate degree at Princeton University. It was there that she began to work with researchers who created the Princeton Ocean Model, one of the world's premier ocean circulation models. Later, as a post-doctoral researcher at the University of North Carolina, she worked with scientists who were applying that model to understanding the Gulf Stream and the weather.

In terms of the Gulf Stream's role in climate, Xue says, there's still a lot we don't know. The Gulf Stream carries heat northward, but how much and how it varies with time is poorly understood.

"During the winter when you have polar air coming from Canada — dry and very cold when it hits the ocean — it generates a lot of heat and moisture flux," she says. "The heat and moisture add fuel to the atmosphere cycling system and create very strong storms. That's why we often see a low-pressure system develop along the mid-Atlantic coast just south of New England.

"That's the region where we find what is called an ‘explosive development cyclone,'" says Xue, who has adapted a mathematical model developed at the University of Oklahoma to study tornadoes.

Direct weather observations over the open ocean are key to validating the accuracy of scientific models. However, data collection is expensive, and only two major projects in the last decade have generated data on air and sea temperatures, humidity, wind speeds, cloud cover and other details that could aid researchers.

When conditions are right, scientists on research cruises see water rising like chimneys into the clouds. It's a stunning example of just how close the sea and sky are related, says Xue.

"What we see are chimneys of water vapor rising into the air. Most of the time, the boundary between the air and water is clear-cut, but during those events, the water is exchanging rapidly. The chimneys go from the ocean surface to the bottom of the cloud, and you see a lot of them. It's a stunning scene," she says.

by Nick Houtman
February-March, 2002

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