It seems brutally unfair to those on the U.S. East Coast that their Western European counterparts are at least 10 degrees Fahrenheit (5.5 degrees Celsius) warmer on average throughout the winter.
The average January temperature in New York City is 32 degrees F (0 C). The average January temperature for Porto, Portugal, at the same latitude as New York's, is 48 degrees F (8.8 C).
The same phenomenon happens over the Pacific, where winters on the northeastern coast of Asia are much colder than in the Pacific Northwest.
A group of researchers now think they know why this discrepancy occurs between the western and eastern ends of ocean basins — warm water off the eastern coasts of these continents affects the coastal climates.
"These warm ocean waters off the eastern coasts actually make it cold in winter — it's counterintuitive," said study team member Tapio Schneider, a Caltech geoscientist.
Warm air, cold air
It was thought that Western Europe is kept warmer than it's latitude would suggest because of the Gulf Stream, which shoots warm equatorial waters up along the eastern coast of North America and across the North Atlantic to Europe. But the new study shows how atmospheric circulation helps cool the eastern boundaries of the mid-latitude continents.
The warm water off an eastern coast will heat the air above it and lead to the formation of so-called atmospheric waves, drawing cold air from the northern polar region, according to the study, which used computer simulations of the atmosphere. Here's how it works: 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.
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.
"It's not that the warm Gulf Stream waters substantially heat up Europe," said study team member Yohai Kaspi of Caltech. "But the existence of the Gulf Stream near the U.S. coast is causing the cooling of the northeastern United States."
North America & Asia
The cooling effect could account for 30 to 50 percent of the temperature difference across oceans, the study suggests.
It also explains why the cold region is just as big for both North America and Asia, despite the continents' differences in topography and size.
The cooling depends on heating air over warm ocean water. Since the warm currents along western ocean boundaries in both the Pacific and Atlantic are similar, the resulting cold region to their west would be similar as well.
The new study adds to other recent research that found that ocean currents such as the Gulf Stream aren't capable of transporting that much heat to northern Europe, instead contributing only up to 10 percent of their warming.
The study was detailed in the March 31 edition of the journal Nature.