Random Walk
Warm Waters, Cold Winters
If you’re sitting on a bench in New York City’s Central Park in winter, you’re probably freezing. After all, Manhattan’s average January temperature is 0°C. But if you were just across the pond in Porto, Portugal, which shares New York’s latitude, you’d be much warmer—the average temperature there is a balmy 9°C.
Throughout northern Europe, average winter temperatures are around 10°C warmer than at similar latitudes on the northeastern coast of the United States and the eastern coast of Canada. The same phenomenon happens over the Pacific, where winters on the northeastern coast of Asia are colder than winters in the Pacific Northwest.
Caltech researchers have now discovered an explanation for these chillier winters. The culprit? Warm water. “Warm ocean waters off a continent’s eastern coast actually make it colder in the winter—it’s counterintuitive,” says Tapio Schneider, the Gilloon Professor of Environmental Science and Engineering. Schneider and postdoc Yohai Kaspi described their work in a paper published in the March 31 issue of the journal Nature.
In the Northern Hemisphere, subtropical ocean currents circulate in a clockwise direction, bringing an influx of warm water northward from the low latitudes. In the Atlantic, the Gulf Stream originates off the Florida coast and moves north along the Eastern Seaboard before turning eastward near the coast of North Carolina and heading out into the ocean.
For decades, the conventional wisdom has been that the Gulf Stream heats northern Europe by delivering warm water from the Gulf of Mexico. But in 2002, research showed that the Gulf Stream doesn’t transport enough heat to be directly responsible, contributing to perhaps 10 percent of the temperature contrast between the continents.
Now Kaspi and Schneider have found that the temperature difference isn’t because the Gulf Stream warms Europe, but because the Gulf Stream cools the eastern United States. Their computer simulations of the atmosphere show that the warm water heating the air above it leads to the formation of Rossby waves—atmospheric undulations that stretch for more than 1,000 miles. These Rossby waves are stationary—their peaks and valleys don’t move, but they still transfer energy, drawing cold air down from the northern polar region to form a plume just to the west of the warm water. In our case, this dumps the frigid air right over the northeastern United States and eastern Canada.
The researchers then sped up Earth’s rotation to see how this affected the dynamics. When they did, the plume of cold air got bigger—consistent with it being from a stationary Rossby wave. Most other atmospheric features would get smaller if the planet were to spin faster.
Although it’s long been known that a heat source could produce Rossby waves, which can then form plumes, this is the first time anyone has shown that this can affect continental temperatures. The researchers say the cooling effect could account for 30 to 50 percent of the temperature difference across oceans.
This process also explains why the cold region is equally big in North America and Asia, despite the two continents being so different in topography and size. The Rossby wave–induced cooling depends on heating air over warm ocean water. Since the warm currents along both the Pacific and the Atlantic’s western boundaries are similar, the resulting cold regions farther west would be similar as well.
The next step, Schneider says, is to build simulations that more realistically reflect what happens on Earth by incorporating such complexities as continents and clouds. —MW
This map shows sea-surface temperatures averaged over eight days in September 2001, as measured by NASA’s Terra satellite. Dark red represents warm water (32°C) and purple is cold (–2°C). The Gulf Stream can be seen as the orange strip extending from the eastern U.S. toward the Atlantic.
