The Global Heat Engine

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When viewing the 216 monthly false-color images consecutively in a time-series animation, distinct large-scale patterns of change become quickly obvious to the eye. The relationship between sea surface temperature and plant productivity becomes apparent too. Reds representing unusually warm waters wax and wane across patches of ocean while the greens of vigorous plant growth, or the browns of drought, roll across landscapes in response.


El Nino and La Nina


Collatz points to the recurring cycles of the El Niño-Southern Oscillation in the equatorial Pacific and Southern Atlantic during the 1980s. Then he notes the subsequent patterns of drought and then vigorous growth that sweep back and forth across South America, as if the continent were the ball in an ongoing ping-pong match between the two mighty oceans.

"What it shows is what you might expect," he observes. "Sea surface temperatures have an impact on the climate (temperature and precipitation) over land and this affects growth of vegetation."


A very strong El Niño brought drought to northern South America in 1983, while a large La Niña brought excess rain in 1989. The vegetation responded by growing poorly in 1983 and vigorously in 1989.

Image by Marit Jentoft-Nilsen, NASA GSFC Visualization and Analysis Lab, based on data from Sietse Los, University of Wales.


QuikSCAT wind data

Dubbed the "global heat engine," Earth scientists have long since recognized that as the ocean releases warmth and moisture into the overlying atmosphere it dramatically influences weather patterns. Anomalously high sea surface temperature, as seen in the equatorial Pacific during El Niño, can drive weather patterns to extremes–producing torrential rains and flooding in some parts of the world and severe drought in others.

But, says Collatz, you cannot expect El Niño to always have the same effects on plant growth across a given region. The impacts of some El Niños are more intense than others.

"Climate oscillations can sometimes interact with one another," explains Collatz. "For instance, the effects of El Niño are sometimes magnified and at other times almost completely cancelled out by the North Atlantic Oscillation (NAO)." (The NAO is an ongoing, long-distance relationship between a high-pressure system over the Azores Islands and a low-pressure system over Iceland. For more details, read Searching for Atlantic Rhythms.)


Trade winds blow from east to west along the equator, carrrying moisture over South America. Evaporation is slowed if the sea surface is cooler than normal, leading to decreased rainfall over adjacent land. Conversely, more evaporation leads to excess rainfall when the sea surface temperature is higher than normal. The image at left shows winds over the Atlantic on June 3, 2001. Arrows correspond to direction, color to velocity.

Image courtesy Seaflux, NASA Jet Propulsion Lab.

Graph of Effects of ENSO and NAO on Brazil

Ultimately, say the authors, this new data set strengthens scientists’ ability to forecast the effects of climate change on vegetation on a global scale. But in order to improve their predictions of what impacts El Niño might have, they need to know what other climate oscillations might affect the strength of El Niño.

"Natural resources, food—lots of things depend upon the healthy growth of vegetation," concludes Collatz. "It is important for us to understand and be able to predict how forests and crops will respond to climate cycles like El Niño."

Toward that objective, the team now has almost 20 years of global observations to give scientists a perspective they’ve never had before. With these new data the team can begin to examine in more detail the roles of the terrestrial biosphere in both the carbon and water cycles.

Collatz adds that the team is already looking ahead to the new NASA satellite sensors now in orbit that are much better calibrated than AVHRR, and they are specifically designed to measure the Earth’s vegetation. Even as they improve upon the quality of the measurements, these sensors—such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), flying aboard OrbView-2, and the Moderate-resolution Imaging Spectroradiometer (MODIS), flying aboard Terra—will extend the heritage of the AVHRR data set well into the new millennium.

  • References
  • Los, Sietse O., G. James Collatz, L. Bounoua, P.J. Sellers, and C.J. Tucker, 2001: "Global interannual variations in sea surface temperature and land surface vegetation, air temperature and precipitation." Journal of Climate, Vol. 14, pp. 1535-1549
  • Myneni, Ranga B., C.D. Keeling, C.J. Tucker, G. Asrar, and R.R. Nemani, 1997: "Increased plant growth in the northern high latitudes from 1981-1991." Nature, Vol. 386, pp. 698-702.

next When Plants are Thriving


Climate cycles—such as El Niño/Southern Oscillation (ENSO) and the North Atlantic Oscillations (NAO) can act in concert, amplifying their effects, or against each other, limiting their effects. This graph shows precipitation anomaly in the area surrounding Nordeste, Brazil. In 1985 the NAO and ENSO both contributed extra rainfall to the region. In 1989 La Niña again brought extra rain to Brazil, but the effects of the NAO reduced the total amount of rainfall.

Graph by Robert Simmon, based on data provided by Sietse Los, University of Wales.