If you're sitting on a bench in New York City's Central Park in winter, you're probably freezing.
After all, the average temperature in January is 32 degrees Fahrenheit.
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 is a balmy 48 degrees Fahrenheit.
Throughout northern Europe, average winter temperatures are at least 10 degrees Fahrenheit warmer than 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 in the Pacific Northwest.
Researchers at the California Institute of Technology (Caltech) have now found a mechanism that helps explain these chillier winters--and the culprit is warm water off the eastern coasts of these continents.
"These warm ocean waters off the eastern coasts actually make it cold in winter--it's counterintuitive," says Tapio Schneider, a geoscientist at Caltech.
Schneider and Yohai Kaspi, a postdoctoral fellow at Caltech, describe their work in a paper published in the March 31 issue of the journal Nature.
"This research makes a novel contribution to an old problem," says Eric DeWeaver, program director in the National Science Foundation's (NSF) Division of Atmospheric and Geospace Sciences, which funded the research.
"Traditional wisdom has it that western Europe is warm because of the Gulf Stream, but this paper presents evidence that atmospheric circulation plays an important role in maintaining the colder temperatures found on the eastern boundaries of the mid-latitude continents."
Using computer simulations of the atmosphere, the researchers found that the warm water off an eastern coast will heat the air above it and lead to the formation of atmospheric waves, drawing cold air from the northern polar region.
The cold air forms a plume just to the west of the warm water. In the case of the Atlantic Ocean, this means the frigid air ends up right over the northeastern United States and eastern Canada.
For decades, the conventional explanation for the cross-oceanic temperature difference was that the Gulf Stream delivers warm water from the Gulf of Mexico to northern Europe.
But in 2002, research showed that ocean currents aren't capable of transporting that much heat, instead contributing only up to 10 percent of the warming.
Kaspi's and Schneider's work reveals a mechanism that helps create a temperature contrast not by warming Europe, but by cooling the eastern United States.
Surprisingly, it's the Gulf Stream that causes this cooling.
In the northern hemisphere, the subtropical ocean currents circulate in a clockwise direction, bringing an influx of warm water from low latitudes into the western part of the ocean.
These warm waters heat the air above it.
"It's not that the warm Gulf Stream waters substantially heat up Europe," Kaspi says. "But the existence of the Gulf Stream near the U.S. coast is causing the cooling of the northeastern United States."
The researchers' computer model simulates a simplified, ocean-covered Earth with a warm region to mimic the coastal reservoir of warm water in the Gulf Stream.
The simulations show that such a warm spot produces so-called Rossby waves.
Rossby waves are large atmospheric waves--with wavelengths that stretch for more than 1,609 kilometers (1,000 miles).
They form when the path of moving air is deflected due to Earth's rotation, a phenomenon known as the Coriolis effect.
Similar to how gravity produces water waves on the surface of a pond, the Coriolis force is responsible for Rossby waves.
In the simulations, the warm water produces stationary Rossby waves, in which the peaks and valleys of the waves don't move, but the waves still transfer energy.
In the northern hemisphere, the stationary Rossby waves cause air to circulate in a clockwise direction just to the west of the warm region.
To the east of the warm region, the air swirls in the counterclockwise direction. These motions draw in cold air from the north, balancing the heating over the warm ocean waters.