United States

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 degrees Celsius) and purple is cold (‑2 degrees Celsius). The Gulf Stream can be seen as the orange strip extending from the eastern U.S. toward the Atlantic.

Imagine this: you are standing outside in New York City while waiting for a cab. It is in the winter and you are likely freezing. What if you were doing the same thing, but in Porto, Portugal?

Porto shares the same latitude at the Big Apple, but in Portugal you would be about 10 degrees warmer.

This happens for the northeastern coast of the U.S. and eastern coast of Canada. This is also true in other parts of the world. When the northeastern coast of Asia is colder, the Pacific Northwest is warmer.

Researchers at the California Institute of Technology have found an explanation. The culprit is warmer water off the eastern coasts of these continents.

"These warm ocean waters off the eastern coast actually make it cold in winter-it's counterintuitive," said Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering.

By using computer simulations of the atmosphere, the researchers found the warm water off an eastern coast will heat the air above it. That leads to the formation of atmospheric waves, which then draw cold air from the northern polar region. That cold air will then form up just west of the warm water. In that same situation with the Atlantic Ocean, the cold air ends up right over the northeastern U.S. and eastern Canada.

The explanation used to be that the Gulf Stream delivered warm water from the Gulf of Mexico to northern Europe. In 2002, research showed the currents are not able to deliver that much heat. In actuality, they contribute only about 10 percent of the warming.

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.

This image, taken by NASA's Terra satellite in March 2003, shows a much colder North America than Europe‑‑even at equal latitudes. White represents areas with more than 50 percent snowcover. NASA's Aqua satellite also measured sea-ice temperatures. Water between 0 and ‑15 degrees Celsius is in pink, while sea-ice between ‑15 and ‑28 degrees Celsius is in purple.

"It's not that the warm Gulf Stream waters substantially heat up Europe," said Yohai Kaspi, a postdoctoral fellow at Caltech. "But the existence of the Gulf Stream near the U.S. coast is causing the cooling of the northeastern United States."

The researchers' simulations mimic the warm water in the Gulf Stream. That warmer spot produces what is known as Rossby waves. Rossby waves are large atmospheric waves that form when the path of moving air is deflected due to Earth's rotation, which is known as the Coriolis effect. The Coriolis force is responsible for Rossby waves, just it is similar to the way gravity is the force producing waves on the surface of a pond.

In the Northern Hemisphere, the stationary Rossby waves cause air to circulate in a clockwise direction just west of the warm region. East of the warm region, the air swirls in the counterclockwise direction. These motions pull cold air from the north, balancing the heating over the warm ocean waters.

This process also explains why the cold region is just as big for North America and Asia, even though the continents are so different in topography and size. The Rossby wave-induced cooling depends on heating air over warm ocean water. Since the warm currents along western ocean boundaries in the Pacific and Atlantic are similar, the resulting cold region to their west would be similar as well.

The researchers' next step is to build simulations to reflect what happens on Earth in a more realistic way.