Warm water off continents' eastern coasts to blame
Snow cover over North America and Europe in March, 2003, as imaged by a satellite instrument.
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| The Gulf Stream flows northward along the Eastern U.S. then diverges out into the Atlantic Ocean |
March 30, 2011
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| Sea-surface temperatures off Eastern North America; purple = freezing temperature of ocean water. |
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.
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| Surface temperature deviation averaged over northern hemisphere winter months and across 40 years. |
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 counte rintuitive," 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.
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| The northern hemisphere is perturbed with a constant localized heating at the ocean surface. |
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.
To gain insight into the mechanisms that control the atmospheric dynamics, the researchers sped up Earth's rotation in the simulations.
In those cases, the plume of cold air gets bigger--consistent with it being a stationary Rossby-wave plume. 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 scientists have shown how the mechanism causes cooling that extends west of the heat source.
According to the researchers, the cooling effect could account for 30 to 50 percent of the temperature difference across oceans.
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 Rossby-wave induced 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 next step, Schneider says, is to build simulations that more realistically reflect what happens on Earth. Future simulations would incorporate more complex features like continents and cloud feedbacks.
The research was also funded by a NOAA Climate and Global Change Postdoctoral Fellowship and a David and Lucille Packard Fellowship.
-NSF-
The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering. In fiscal year (FY) 2010, its budget is about $6.9 billion. NSF funds reach all 50 states through grants to nearly 2,000 universities and institutions. Each year, NSF receives over 45,000 competitive requests for funding, and makes over 11,500 new funding awards. NSF also awards over $400 million in professional and service contracts yearly.
