Winds of Change

A solution finally came from Junye Chen, a graduate student working at NASA’s Goddard Institute for Space Studies in New York under the advisement of Anthony Del Genio and Barbara Carson. Chen realized that changes in large-scale atmospheric circulation had the potential to alter cloud cover and radiation over the entire tropical region. More specifically, air currents that move vertically up and down over the tropics may have fluctuated gradually over the past 15 years, leading to less water vapor and clouds over the tropics and altering outgoing thermal radiation (Chen et al. 2002).

In the tropics, there are two big circulation patterns that control most of the airflow through the troposphere like giant conveyor belts. The Walker circulation transports air east and west along both sides of the equator. Air currents rise high above the warm pool over the far western Pacific and the Indian oceans and then flow east towards the coasts of the Americas. The air currents then descend and travel west, skimming the surface of the Pacific and creating the trade winds. An El Niño or La Niña occurs when this last leg of the Walker circulation breaks down or speeds up. The other major circulation, known as the Hadley circulation, transports air both north and south away (on average) from the equator. In both hemispheres, air in equatorial region rises due to the warmth emitted from the tropical waters and travels away from these regions towards the cooler mid-latitudes. When the air hits roughly 30 degrees latitude on either side of the equator, it cools, plunges back down toward the ocean, and then flows along the surface back toward the equator, thus completing the loop. Together, these two circulation patterns are known as the Hadley-Walker cell.

A large-scale wind pattern transports air and moisture from one side of the Pacific Ocean to the other. Known as the Hadley-Walker circulation, these winds drive both weather and the transport of energy along the equator. Water-laden air rises over the very warm waters of the Western Pacific Ocean. As it rises, the air cools, and the moisture evaporates out, forming clouds and rain. The cool, dry air then moves to the east, and falls to the north and south of the equator. It is likely that this circulation is strengthening, leading to thicker clouds over the regions of upwelling in the western Pacific, and thinner clouds over areas of downwelling. (Illustration by Robert Simmon)

To prove his hypothesis and tie the fluctuations in the upwelling and downwelling legs of the Hadley-Walker cell to changes in radiation and cloud cover data, Chen still had to see past the disruptive, short-term spatial shifts in clouds and air currents that took place during the El Niños and La Niñas of the last 15 years. “I tried to look at the region some other way than spatially. I finally came to the conclusion that you could separate the tropical region into subsidence and convection (areas where downwelling and upwelling currents exist). And that this was a more comprehensive view of the region,” says Chen. He concluded that analyzing cloud patterns and other atmospheric parameters spatially across the tropics would not be entirely necessary. It would be much simpler to narrow the field of view and track thermal radiation and cloud cover over only those regions where various atmospheric conditions pointed to the presence of upwelling and downwelling air currents in the Hadley-Walker cell (Chen et al. 2002).

“When you look at the tropics in this way, El Niño is not dominant anymore, and the long-term decadal change in radiation is,” says Chen. During an El Niño or La Niña, the upwelling and downwelling currents of the Hadley-Walker cell don’t change in area or intensity much, but simply shift. Long-term variations in these currents become more apparent. In essence, analyzing cloud cover and radiation via Chen’s method would be akin to taking note of the change in the number and color of pieces during a checkers game as opposed to simply recording the changing spatial patterns of the pieces on the board.

The results Chen received from his analysis were as expected--on average the thermal radiation escaping the atmosphere seemed to increase markedly over the past 15 years above downwelling legs of the Hadley-Walker circulation. The increase was much greater than the average 4- watt-per-square-meter increase over the entire tropics, suggesting that the change in radiation was due to changes in circulation. To further verify these results, Chen pored over a patchwork of data on cloud amount, upper tropospheric humidity, and vertical wind velocity over the past 15 years in the tropics. In downwelling regions, he found cloud amount and humidity decreased, while in upwelling regions they increased. Vertical wind velocities intensified over both types of regions (Chen et al. 2002). “So we came to the conclusion that the Hadley-Walker circulation as a whole must have intensified over the past 15 years,” says Chen.

Though the data Chen used weren’t thorough enough to prove the hypothesis beyond a shadow of a doubt, the researchers believe that the Hadley-Walker circulation has slowly sped up over the past 15 years. The acceleration likely led to a small increase in cloud cover over upwelling regions and a relatively greater decrease in cloud cover and humidity over downwelling regions. The net effect was less humidity and clouds across the tropics. With diminished water vapor and fewer high-flying cirrus clouds in the upper troposphere, more thermal radiation from the surface was able to make it into space without being re-absorbed. At the same time, clouds overshadowed a smaller portion of the tropics, so more sunlight in the form of short-wave radiation was able to reach the Earth’s surface without being reflected back into space (Chen et al. 2002).

“We are likely seeing a decadal fluctuation here. Right now we are looking at what might lead to such a fluctuation,” says Chen. He believes that this is a natural climate anomaly much like El Niño, La Niña, or the North Atlantic Oscillation. It is a natural part of the rhythms of the Earth’s climate system. But unlike these other anomalies, the Hadley-Walker cell fluctuates over the course of decades instead of years. Chen feels that this phenomenon has no direct relation to global warming or any other hypothesis related to climate change, including the Iris Hypothesis. The Iris Hypothesis states that as sea surface temperature increases due to climate change, the increase will alter the extent of certain types of overlying clouds so that the excess heat is allowed to vent through the top of the atmosphere. Though the Iris Hypothesis may seem to jibe with what has occurred over the past 15 years in the tropics, Chen says that the thermal radiation leaving the top of the atmosphere has increased much too rapidly. Evidence has also shown that the clouds above the tropics are not changing in the ways predicted by the Iris Hypothesis.

Wielicki agrees with Chen’s assessment. He adds, “What we are seeing is that the climate system has multiple ways it can arrange itself and still accomplish a heat balance. In this case as the clouds change, the Earth absorbs more heat at the surface while it radiates more heat from the atmosphere.” Wielicki explains that even from the start, this phenomenon appeared to be a climate fluctuation unrelated to global warming or greenhouse gases. Human-generated greenhouse gases have thus far led to a 0.5-watts-per-square-meter increase in the solar energy absorbed into the atmosphere, while the tropical radiation changes were almost ten times as large.

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The surface winds of the Hadley-Walker circulation appear as the dense white stripe in the center of the map at left. Winds in the eastern Pacific converge near the equator, and then form the trade winds that flow from east to west. Arrows indicate wind direction, while color represents wind speed. (Image courtesy SeaWinds Science Team and Air-Sea Interaction and Climate Team, NASA JPL)