A Gulf Stream Collision: Cold Air, Warm Water

Huijie Xue, an assistant professor in the UMaine School of Marine Sciences, has received a $330,000 National Science Foundation grant to continue her studies of the interaction between the Gulf Stream and the atmosphere. The grant is titled Collaborative Research: Coupled Atmosphere-Ocean Model Study of Wintertime Air-Sea Interaction Off the East Coast of North America.

The project could help to improve weather forecasts, although that is not a direct goal of the research, says Xue.

Xue has published reports of her previous work on this topic in the Journal of Physical Oceanography. She began studying these phenomena during her years as a graduate student at Princeton University where she worked 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 understand the Gulf Stream.

Scientists have long known that frequent and sometimes violent interactions between arctic air and the Gulf Stream produce storms. Indeed, the world’s largest transfer of heat from the ocean to the atmosphere has been measured over the Gulf Stream, Xue says.

"We all know there is a storm track along the northeast coast of the United States. What happens is that 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 explains. "The heat and moisture add fuel to the atmosphere cycling system and create very strong cyclonic development. That’s why we always see a low 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.’"

"We know the structure of the Gulf Stream pretty well and why it meanders from north to south. In terms of its climatic role, there’s still a lot we don’t know. We know it carries heat northward, but how much and how it varies with time is poorly understood," she says.Most other research on interactions between the air and the ocean focus on huge areas such as the Pacific basin where periodic El Nino events are associated with significant shifts in weather patterns around the globe. Xue and her colleagues are the first scientists to apply air-sea modeling efforts to a mesoscale geographic area, a term which refers to areas ranging from a few to hundreds of square miles. For their purposes, they are using a mathematical model developed at the University of Oklahoma to study tornadoes.

Among the major hurdles Xue and her colleague face is a lack of direct weather observations over the open ocean. Two major scientific projects in the last decade have generated data on air and sea temperatures, humidity, wind speeds, cloud cover and other details, but modelers have an insatiable appetite for such information. To calibrate and improve their calculations, Xue and other scientists need a steady stream of reliable weather data over the Gulf Stream.

"When we have funding, we make extensive use of aircraft," says Xue. "There are a lot of sensors on board, and we also use cameras to take pictures of the sea state and the clouds."

During research cruises, scientists occasionally see water rising like chimneys into the clouds. "What you see are chimneys of steam rising up 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. It’s very strange. The chimneys go from the ocean surface to the bottom of the cloud, and you see a lot of them," she explains.

In addition to her research, Xue teaches courses at UMaine on the use of mathematical models to understand the circulation of currents in the ocean. She is also conducting research on the circulation of water in Penobscot Bay and in the South China Sea with Chinese and other UMaine scientists.

Next Fall, Xue is planning an international meeting in Bar Harbor to discuss improvements to the state-of-the-art in ocean circulation modeling. The Sigma Coordinate Ocean Model Users Meeting, will be held at the Regency/Holiday Inn Resort Sept. 20-22.

More information about Xue’s Gulf Stream research is available via the Internet at http://athena.umeoce.maine.edu/GALE/GALE.htm.