This year we were treated to anexcellent example, yet this begs the next question. Why do we see so much actual variation fromyear to year? I do not think enoughactual variation exists in the Gulf Stream itself to matter much.
Certainly continental weather canbe drawn into the Northeast. Yet theeffect practically disappeared fro a couple of years.
Add this to your kitbag of weathertheory and know it will never be enough.
by Staff Writers
This image, taken by NASA's Terra satellite in March 2003, shows a muchcolder North America than Europe --even atequal latitudes. White represents areas with more than 50 percent snow cover.NASA's Aqua satellite also measured water temperatures. Water between 0 and -15degrees Celsius is in pink, while water between -15 and -28 degrees Celsius isin purple. Credit: NASA/Goddard Space Flight Center Scientific VisualizationStudio; George Riggs (NASA/SSAI).
If you're sitting on a bench in New York City 'sCentral Park inwinter, you're probably freezing. After all, the average temperature in Januaryis 32 degrees Fahrenheit. But if you were just across the pond in Porto , Portugal ,which shares New York
Throughout northern Europe, average winter temperatures are at least 10degrees Fahrenheit warmer than similar latitudes on the northeastern coast ofthe United States and theeastern coast of Canada Pacific Northwest .
Researchers at the California Institute of Technology (Caltech) havenow found a mechanism that helps explain these chillier winters-and the culpritis warm water off the eastern coasts of these continents.
"These warm ocean waters off the eastern coast actually make itcold in winter-it's counterintuitive," says Tapio Schneider, the Frank J.Gilloon Professor of Environmental Science and Engineering.
Schneider and Yohai Kaspi, a postdoctoral fellow at Caltech, describetheir work in a paper published in the March 31 issue of the journal Nature.
Using computer simulations of the atmosphere, the researchers foundthat the warm water off an eastern coast will heat the air above it and lead tothe formation of atmospheric waves, drawing cold air from the northern polarregion. 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 upright over the northeastern United States and eastern Canada
For decades, the conventional explanation for the cross-oceanictemperature difference was that the Gulf Stream delivers warm water from theGulf of Mexico to northern Europe . But in2002, research showed that ocean currents aren't capable of transporting thatmuch heat, instead contributing only up to 10 percent of the warming.
Kaspi's and Schneider's work reveals a mechanism that helps create atemperature contrast not by warming Europe, but by cooling the eastern United States Gulf Stream that causesthis cooling.
In the northern hemisphere, the subtropical ocean currents circulate ina clockwise direction, bringing an influx of warm water from low latitudes intothe 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 theGulf Stream near the U.S. coast is causing the cooling of the northeastern United States
The researchers' computer model simulates a simplified, ocean-coveredEarth with a warm region tomimic the coastal reservoir of warm water in the Gulf Stream . The simulations show that such a warm spot producesso-called Rossby waves.
Generally speaking, Rossby waves are large atmospheric waves-withwavelengths that stretch for more than 1,000 miles. They form when the path ofmoving air is deflected due to Earth's rotation, a phenomenon known as theCoriolis effect. In a way similar to how gravity is the force that produceswater waves on the surface of a pond, the Coriolis force is responsible forRossby waves.
In the simulations, the warm water produces stationary Rossby waves, inwhich the peaks and valleys of the waves don't move, but the waves stilltransfer energy. In the northern hemisphere, the stationary Rossby waves causeair 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 counterclockwisedirection. These motions draw in cold air from the north, balancing the heatingover the warm ocean waters.
To gain insight into the mechanisms that control the atmosphericdynamics, the researchers speed up Earth's rotation in the simulations. Inthose cases, the plume of cold air gets bigger-which is consistent with itbeing a stationary Rossby-wave plume. Most other atmospheric features would getsmaller if the planet were to spin faster.
Although it's long been known that a heat source could produce Rossbywaves, which can then form plumes, this is the first time anyone has shown howthe mechanism causes cooling that extends west of the heat source. According tothe researchers, the cooling effect could account for 30 to 50 percent of thetemperature difference across oceans.
This process also explains why the cold region is just as big forboth North America and Asia , despite thecontinents being so different in topography and size.
The Rossby-wave induced cooling depends on heating air over warm oceanwater. Since the warm currents along western ocean boundaries in both thePacific and Atlantic are similar, theresulting cold region to their west would be similar as well.
The next step, Schneider says, is to build simulations that morerealistically reflect what happens on Earth. Future simulations wouldincorporate more complex features like continents and cloud feedbacks.
The research described in the Nature paper, "Winter cold ofeastern continentalboundaries induced by warm ocean waters," was funded by the NOAA Climateand Global Change Postdoctoral Fellowship, administrated by the UniversityCorporation for Atmospheric Research; a David and Lucille Packard Fellowship;and the National Science Foundation.
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