An Oscillation Felt Around the World

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In addition to determining why the warm pool oscillates, perhaps a more important question for both researchers and lay people is what affect does this warm pool oscillation have on the world’s climate? If scientists could discern how the cyclical behavior of the warm pool influences the weather, then there is the possibility that they could better predict future weather patterns by observing warm pool temperatures. Mehta explains that although very little research has been done on the warm pool oscillation’s influence on the world’s climate, there is evidence that it is profound. “A few degrees difference in the warm pool may not seem like much. However, even a small change in a body of water as extensive and warm as this can have great effect on the climate,” says Mehta.


Maps of Change  in Global Rainfall

He elaborates that the warm pool oscillation’s climatic sway stems from the fact that the average temperature of the oscillation is 84°F (29°C). This temperature has a special significance in Mother Nature. It is the threshold temperature at which air at the ocean’s surface begins to rise rapidly, causing strong atmospheric convection (vertical air currents). The upward moving air (called “convection” ) can carry evaporated seawater high into the atmosphere where it condenses to form clouds and entire weather systems. Atmospheric convection can also create a low-pressure zone above the ocean and alter the surrounding air currents. “So as you move up or down by a degree at this temperature, you can make a big impact,” Mehta says.

  The Warm Pool affects the climate, particularly rainfall, around the world. During the months of June, July, and August, a large Warm Pool results in up to 25 percent lower than normal rainfall in Australia and South America, while a smaller than normal Warm Pool is correlated with an increase in rainfall in Australia, the Pacific Northwest, and the Mediterranean. The maps to the left show rainfall while the Warm Pool was large (top) and small (bottom), based on data acquired from 1908 to the present. (Images courtesy Vikram Mehta, NASA GSFC)

Diagram of Convection

A number of scientists have already published papers showing that the oscillation and the corresponding atmospheric changes alter the weather in Australia and in the island nations of the South Pacific. Mehta, however, is focused on the Western Hemisphere. He is curious as to how the oscillation may be altering El Niño in the eastern Pacific.

For those who have forgotten what El Niño and La Niña are all about, in a non-El Niño year, trade winds blow continuously in the winter from east to west along the equator and push the warm surface waters off the coast of South America surface waters toward the west. When El Niño hits periodically—roughly every three to seven years—these equatorial winds cease over the winter months and warm water is allowed to build up along the northwestern coast of South America. During a La Niña winter, the opposite occurs—trade winds increase and push even more water westward, away from the eastern Pacific. Needless to say, both events are known to affect the weather from Australia to the eastern coast of Africa.

  The Warm Pool is important because the water within it is warm enough—28.5 °C—to cause convection. Convection is the process in which hot air at the surface rises, bringing with it moisture evaporated from the ocean. As the air rises it cools, and the water within it condenses to form clouds. The same process leads to afternoon thunderstorms in the summer. In general, air is converging towards the warm pool at the surface, and diverging away from the warm pool at high altitudes. (Image by Robert Simmon, NASA GSFC)

El Nino


palette for el nino sea surface temp

Considering that both the warm pool oscillation and the El Niño/La Niña share some of the same waters, the idea that the two have an influence on one another isn’t far fetched. To see if a connection exists, Mehta compared El Niños from 1909 to 1988 to the warm pool oscillation. He gathered sea surface temperature data of the eastern Pacific from nineteen El Niño events and separated them into two categories—those that occurred when the warm pool was cooler than the average temperature of its oscillation and those that occurred when the warm pool was warmer than average. He then ran a few basic comparisons to see if there were any differences between the two categories.

He found that when the warm pool is at the peak in its oscillation, the warm waters associated with El Niño not only blanket the waters off the coast of South America, but spread out far into the central Pacific. During these winters, the warm El Niño and warm pool waters together covered nearly the entire equatorial Pacific. When the warm pool was at a low in its oscillation, the warm waters characterizing El Niño stayed pretty much confined to South America. “Essentially the difference [between the two variations of El Niño events] lies mostly in the central Pacific,” says Mehta.

He explains that such a change in El Niño may have an impact on everything from the jet stream in the Northern Hemisphere to ocean life along the equator to the moisture content in the air above the Pacific. Some of these large-scale changes in the ocean and atmosphere, in turn, could alter the weather in the United States and Canada. Mehta says that in order to test this hypothesis, he first gathered all the precipitation data from North America during a combined 37 El Niño and La Niña events that occurred between 1908 and 1998. He again split these data into two categories—the winter precipitation data from when the warm pool was above average in its oscillation and those from when the warm pool was below average.

“In the end, we discovered that when the warm pool shrinks down and becomes not so warm you get a much stronger correlation between El Niño and La Niña and the rainfall in the central United States and central Canada. A negative correlation also exists in the [Pacific] Northwest,” Mehta says. In basic terms, when the warm pool is cooler than average, both El Niño and La Niña seem to increase precipitation in the mid-western United States and central Canada and decrease precipitation in the northwestern United States. Conversely, when the warm pool is large, El Niño and La Niña appear to have very little influence on these regions of the country. Mehta warns, however, that these correlation tests cannot tell him how much rainfall has increased or decreased. It can only tell him that El Niño and La Niña seem to be causing more or less precipitation depending on what phase the warm pool oscillation is in.

As is typically the case in atmospheric science, an explanation of why a phenomenon occurs is often not worked out until well after the phenomenon is discovered. Mehta says that right now he and his team are wrestling with atmospheric models and climate data to see if they can uncover how the warm pool is able to affect El Niño and La Niña and the weather in North America. The scientists believe, among other possibilities, that the warm pool could be altering the atmospheric circulation above the Pacific, changing the depth of the top warm layer of the Pacific, influencing the jet stream, or affecting all of the above. “But more research has to be done,” says Mehta. “We are right there at the edge. Exciting, highly statistical results can turn out to be bogus sometimes. You have to be careful.”

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The image above shows the changes in water temperature (color), sea surface height (elevation), and winds (arrows) that occured during the strong El Niño of December 1997. During an El Niño, the trade winds that usually blow from east to west flip-flop and blow from west to east, pushing the water of the West Pacific Warm Pool into the eastern Pacific. The larger the Warm Pool, the stronger the El Niño tends to be. (Image courtesy Greg Shirah, NASA GSFC Scientific Visualization Studio)

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