Reverberations of the Pacific Warm Pool
 

 

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When scientists speak of large-scale climate events such as El Niño and La Niña, they tend to use the word “anomaly.” Over the past several decades, however, such recurrent changes in the Earth’s climate appear to be anything but. In fact, the more researchers study the Earth’s oceans and atmosphere, the more they are finding that such large-scale cyclical and near-cyclical variations in ocean temperature and air pressure appear to occur all across the planet at many time scales. Some can stir up the weather across half the globe, while others may only affect the coasts of a single country. Some recur twice every decade on average, while others come around every year.

 

Climate Oscillations:
Introduction: El Niño’s Extended Family
Search for Atlantic Rhythms
Reverberations of the Pacific Warm Pool

Coming Soon:
Intertropical Convergence Zone

 

 
Map showing West Pacific Warm Pool
 

 

Of these anomalies, one of the most recent to be discovered takes place in the Indo-Pacific warm pool. This body of water, which spans the western waters of the equatorial Pacific to the eastern Indian Ocean, holds the warmest seawaters in the world. Scientists found that, over a period of roughly two decades, the warm pool’s average annual temperatures and dimensions increase and then decrease like a slowly pulsating beacon.

The effects and origins of these oscillating waters, however, remain something of a mystery. For the past three years researchers based at NASA’s Goddard Space Flight Center, led by atmospheric scientist Vikram Mehta, have been trying to unravel some of the questions surrounding the warm pool. They have been poring over atmospheric and sea surface temperature data from the western Pacific to the eastern United States looking for answers as to why the warm pool oscillates and what effects this oscillation may have on the world’s climate. What they found is that the warm pool’s vacillations may be felt as far away as Arkansas and may be powerful enough to broaden the extent of El Niño.

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The data used in this study are available in one or more of NASA's Earth Science Data Centers.

 

Sea Surface Temperature Palette

The Indian Ocean/West Pacific Warm Pool extends almost half way around the globe, stretching along the equator south of India, through the waters off Sumatra, Java, Borneo, and New Guinea, and into the central Pacific Ocean. The waters of the Warm Pool are warmer than any other open ocean on Earth. Because these waters are hot enough to drive heat and moisture high into the atmosphere, the warm pool has a large effect on the climate of surrounding lands. In fact, the slow fluctuations of size and intensity of the warm pool may be linked with the intensity of El Niño.

 

Running Hot and Cold

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“The dimensions of the Indo-Pacific warm pool are huge. If you look on a map, it extends for 9,000 miles east to west along the equator and 1,500 miles north to south,” says Mehta. “So there’s a lot of warm water sloshing around there.” The warm pool is shaped a bit like a tadpole with its head covering the waters of the western equatorial Pacific between New Guinea and the Samoa Islands and its tail extending through the Indonesian Archipelago and well into the Indian Ocean. All in all, the warm pool blankets an area of the ocean four times the size of the continental United States. Its temperatures average anywhere from 81°F (27°C) at the edges to up to 86°F (30°C) in the center of its eastern bulge.

Mehta and his team began their investigation of the warm pool by analyzing sea surface temperature data of the Pacific from 1908 to 1988. They pieced these data together from a number of different sources ranging from measurements taken aboard early twentieth-century English merchant ships to remote sensing measurements collected by NASA and NOAA satellites. “From these data, we’ve seen that the size and the temperature of the warm pool undergo variability at many different time scales,” Mehta says. He explains that on a yearly basis, this warm water will migrate a little south of its average position during the Northern Hemisphere’s winter and a little north of its average position during the Northern Hemisphere’s summer. Over the course of a La Niña or El Niño year, when the trade winds across the Pacific change for the winter, the eastern edge of the warm pool will often advance or pull back over the period of a few months.

What has piqued Mehta’s curiosity, however, aren’t these relatively short-term, periodic changes in the warm pool, but what happens over the long run. “The interesting thing is that you can see this very slow oscillation over a period of 10 to 20 years,” says Mehta. He explains that when you look at just the yearly average temperatures of the warm pool, they can be seen to grow warmer and then cooler on a periodic basis, roughly every two decades, as if connected to a dimmer switch. Typically, when the eastern part of the pool is at its peak, such as in 1926, 1943, and 1960, the temperatures will go as high as 86°F (30°C) on average in the swollen eastern section and the warm pool will expand. The warm pool will then begin to shrink as the yearly average temperatures drop for roughly another decade, down to less than 84°F (29°C) at the warmest spot. The whole cycle then starts anew.

“So the warm pool is expanding and contracting,” says Mehta. “But where does this excess heat come from to make the warm pool large and where does it go when it’s small?” The sun is always shining bright on the equator so water temperatures there are warm to begin with. In addition, trade winds normally blow from east to west along the equator and sweep warm eastern Pacific surface waters toward the west.
 

  Time Series of Warm Pool Movement

The size of the warm pool waxes and wanes over a period of about a decade. During this time, the excess heat circles around the Pacific and Indian Oceans. The above maps show sea surface temperature anomaly (the difference between normal conditions and measured conditions) before, during, and after a peak in the size and strength of the warm pool. Excess heat circles from the eastern Pacific along the coast of North and South America, to the northern and southern Pacific Ocean, to the West Pacific and Indian Oceans, and back. (Images courtesy Vikram Mehta, NASA GSFC)

Graph of Warm Pool Size

But sunshine and trade winds alone shouldn’t give rise to a localized warm pool in the Indian and western Pacific Ocean or cause these waters to cyclically move up or down a few degrees over 20 years in a rhythmic pattern. To solve this riddle, Mehta and his team have been analyzing atmospheric and subsurface ocean temperature data of the Pacific. “I’ve been using these data to try to piece the whole picture together as to how the changes at the ocean’s surface are tied to what happens deep in the ocean and in the atmosphere,” says Mehta.

The team now believes that the warm pool’s growth may stem from a fluctuation in subsurface currents. These currents, which are located several hundred meters below the ocean’s surface, travel from an area east of Japan to the warm pool. What Mehta’s data reveal is that the currents may be periodically gathering warm surface water from the subtropical Pacific and slowly channeling it beneath the ocean’s waves to the warm pool.

“Currently, we are looking into what happens to the heat when the warm pool shrinks and then grows small,” says Mehta. One theory is that every decade or so the currents turn from hot to cold and gradually decrease the warm pool temperatures. Another theory is that the warm pool reaches a peak temperature and then vents its excess heat into the atmosphere before growing again. Right now the researchers are continuing to look at subsurface ocean currents, air pressure above the warm pool, and various other forms of atmospheric data to find an answer.

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The graph to the left shows the percent change in size of the West Pacific/Indian Ocean Warm Pool over time. The data were filtered to show the long-term fluctuations, a period of 9 to 12 years. (Graph courtesy Vikram Mehta, NASA GSFC)

 

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)