Quirks

Large cyclical variations in ocean temperature and air pressure occur all across the planet and stir the weather across half the globe. Some recur twice every decade on average, while others come around more than once an year. The "Warm Pool" of the Indian-Pacific Ocean stretches along the equator south of India, through the waters off Sumatra, Java, Borneo and New Guinea and into the central Pacific Ocean. The waters are warmer than any other open ocean. Its fluctuating size and intensity over a period of roughly two decades affects the El Niño Southern Oscillation.

Researchers based at NASA’s Goddard Space Flight Center, led by atmospheric scientist Vikram Mehta, are hot on the trail according to NASA's PR at Earth Observatory. They have found the effects may be felt as far away as Arkansas and may be powerful enough to broaden the extent of El Niño. Their data is available in NASA's Earth Science Data Centers.

The Pool extends 9,000 miles east to west along the equator and 1,500 miles north to south and covers an area four times the size of the USA with a temperature range of 27 to 30°C.

Using sporadic data from 1908 to 1988 they pieced together from early twentieth-century English merchant ships to remote sensing measurements collected by NASA and NOAA satellites they've seen that the size and the temperature varies at many different time scales. The 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, a slow oscillation over a period of 10 to 20 years.

Typically, when the eastern part of the pool is at its peak, such as in 1926, 1943 and 1960, the temperatures will reach 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 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?”

Insolation on the equator never varies, so water temperatures there should be the same the year around. Slight variation being due to the angle of declination (between the Tropics.) In addition, trade winds normally blow from east to west along the equator and sweep warm eastern Pacific surface waters toward the west.

[Wouldn't that tend to "chop"? And push it under?
Would choppy waters in the equatorial oceans in otherwise calm conditions, be more likely to capture radiation than deflect it?]

The warm pool waxes or wanes over a period of about a decade. Would this explain variations in the Southern Oscillation intensities? The heat flux circles 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.

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. The team believes that growth may stem from fluctuating currents several hundred meters below the surface, travelling from an area east of Japan; 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.

Mehta said that the warm pool oscillation’s influence on the world’s climate 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”.

The average temperature of the oscillation is 29°C, the threshold temperature at which air at the ocean’s surface begins to rise rapidly, causing strong atmospheric convection to form 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.

During 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 coincides with increased in rainfall in Australia, the Pacific Northwest and the Mediterranean.

A number of scientific papers show 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 and how the oscillation affects El Niño in the eastern Pacific.

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.

For El Niños —roughly every three to seven years, equatorial winds cease over the winter months and warm water is allowed to build up along the north-western coast of South America.

[Overall air currents go from west to east and head eventually to the southern oceans. Obviously there is a return. But all cloudscapes show the weather heads thataway. So how does the scenario affect the Med?
Beats me!
(Or not, as the case may be.)]

During a La Niña winter, the opposite occurs; trade winds increase and push even more water westward from the eastern Pacific. Both events are known to affect the weather from Australia to the eastern coast of Africa.

[AKA: Into everybody's life a little rain will fall.
But this article is about the equatorial regions not the trade winds. All clouds crossing the South American land mass seem to go south with very few exceptions. In 2005 ONE storm crossed the equator from the Pacific and gave Belize a squirting. And that same year there was an hurricane in Brazilian waters.
There again 2005 was one for the record books.]

The warm pool and the El Niño/La Niña share some of the same waters and 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.

At peak warmth the oscillation during El Niños covered nearly the entire equatorial Pacific.
At coolest point in its oscillation, with an El Niño, most of the warm water stayed near South America.
To test El Niño's impact, Mehta gathered all the precipitation data from North America during a combined 37 El Niño and La Niña events that

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 north-western United States.
[Or to put it another way:
Put it another way.]

Conversely, when the warm pool is large, El Niño and La Niña appear to have very little influence on these regions of the country.
[This is the interesting bit. No southern oscillation can affect precipitation n the northern hemisphere. Clue by four:
It's called the SOUTHERN oscillation for a reason.
(BTW; Anyone who has had cause to resort to a 2 x 4 has reason to suspect that the term is hyperbole - just thought I'd mention it.)]

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. Typically in atmospheric science, an explanation of why a phenomenon occurs is often not worked out until well after the phenomenon is discovered.
[Makes a nice change then?

I clipped the above from:
Earth Observatory, part of the EOS Project Science Office located at NASA Goddard Space Flight Center
webmaster: Goran Halusa

NASA official: Lorraine Remer
last updated: November 3, 2009