the color of El Niño
by John Weier - August 19, 1999
 
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With the way things are going, El Niño and its counterpart La Niña may be blamed for every natural disaster on the planet. Hardly a month has gone by in the past four years when there hasn’t been a report tying El Niño or its counterpart La Niña to some devastating event. The phenomena have already been linked to everything from tornadoes in the midwestern United States to fires in Indonesia to hurricanes in Central America. But Earth scientists still have much to learn about how the phenomena affect weather systems around the world. Many questions regarding the root cause and physics behind the two events remain unanswered. Predicting exactly when and with what force El Niño or La Niña will strike continues to be elusive.

To improve our understanding of El Niño, Raghu Murtugudde and a team of researchers at NASA’s Goddard Space Flight Center have been observing algae in the Pacific Ocean. They believe that by watching the algae’s movements during El Niños and La Niñas they can gain insight into the processes that drive these events.

Their initial results show promise. Using the first year of data returned from NASA's new Sea-viewing Wide Field-of-View Sensor (SeaWiFS), the scientists have found a way to detect the end of El Niño and the beginning of La Niña a month earlier than anyone else. In the future, the researchers hope to detect other stages of the phenomenas' development and then create models to predict the events' occurrence and their destructive force years in advance.

next El Niño’s Effect on Algae

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

 

El Niño Waves
This beach in Santa Cruz, CA was hammered by large waves throughout the winter of 1997–1998. The frequent storms along the coast of California that season were linked to El Niño. (Photograph courtesy U.S. Geological Survey)

To learn more about El Niño and La Niña, read the fact sheets located in the Earth Observatory Library.

 

El Niño’s Effect on Algae

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"Observing [the algae] in the oceans is much like putting dye in a tank and stirring it up to see where things are moving," Murtugudde said. Algae (phytoplankton) are by far the most abundant form of plant life in the ocean. They are sensitive to light, temperature, currents and winds, and their green chlorophyll can be detected by satellite instruments. Because phytoplankton changes an ocean's color, they are ideal candidates for tracking currents, detecting pollution, and observing meteorological events.

For years scientists have known that El Niño and La Niña change the levels of phytoplankton across the entire Pacific basin. During a normal year, winds gust at a steady rate from east to west across the Pacific and slowly blow the warm surface waters towards Australia and the Indonesian Archipelago. Over a period of time, these winds build up a "warm pool" of water in the western Pacific and leave the eastern Pacific relatively cool. This layer of warm water smothers any upwelling currents, which bring cool, nutrient-rich waters up from the depths of the sea (Njoku et al. 1993). Since phytoplankton can only survive in these nutrient-filled waters, the plants do not usually do well in the western Pacific and thrive in the eastern and central Pacific (Murtugudde et al. 1999).
 

 

diatom
Micrograph of a asterionella japonica—one of thousands of species of phytoplankton. (Photo courtesy George Rowland and the SUNY Marine Sciences Research Center)

To learn more about phytoplankton, see What are Phytoplankton? in the Earth Observatory Library.

May 9, 1998
May 24, 1998

El Niño and La Niña alter the temperature of the surface waters across the Pacific. During an El Niño year, the trade winds in the Pacific die down or reverse direction. The upwelling currents in the east subside, and the pool of warm water in the western Pacific spreads out over the entire basin (Njoku et al. 1993). The phytoplankton in the central Pacific all but disappear, and the population in the eastern Pacific are lowered significantly. The opposite occurs during La Niña. The easterly trade winds pick up and blow even more hot water into the west. The upwelling increases in the central and eastern regions, causing the phytoplankton concentration to explode (Murtugudde et al. 1999).

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back The Color of El Niño’s Effect on Algae

Sea-viewing Wide Field-of-view (SeaWiFS) images of the Galapagos islands and surrounding waters from May 9, 1998 (top) and May 24, 1998 (bottom). The equatorial current shut down by El Niño reappeared over a period of days—indicated by the high concentrations of phytoplankton chlorophyll streaming to the west in the later image. (Courtesy Gene Feldman, SeaWiFS Project)

seawifs pallette

 

Picking Out a Pattern for El Niño’s End

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"With the SeaWiFS satellite, we are able to monitor these changes in ocean color accurately for the first time," said Murtugudde. Though researchers have understood phytoplankton’s reaction to El Niño and La Niña for a couple of decades, there was no way to efficiently monitor the algae across the entire Pacific basin until the launch of SeaWiFS. The instrument is designed to measure the amount of chlorophyll-a (the chemical that makes the phytoplankton green) bobbing around on the ocean’s surface. The satellite that carries the instrument moves in a near-circular orbit from pole to pole and allows SeaWiFS to scan a majority of Earth’s oceans every five days. The data beamed back to scientists are used to create weekly maps of the algae.

   

 

February 1998
 

While the initial purpose for SeaWiFS was to estimate the amount of carbon dioxide being consumed by algae in the oceans, Murtugudde and his team decided to use its capabilities to view changes in algae across the upper layers of the Pacific. The Goddard team combed the first year of SeaWiFS data to look for any unusual changes in phytoplankton concentrations that might have occurred during the transition from El Niño to La Niña. After examining the image data from January to February 1998, they found something strange: a band of algae extending across the length of the Pacific just north of the equator (Murtugudde et al. 1999). While the appearance of the algae alone suggested a possible end to the El Niño, the real surprise was in the plants' location. "There was elevated chlorophyll just to the north of the equator. This never happens. Everything usually happens on the equator, because the upwelling of the whole ecosystem in the central Pacific is on the equator," said Murtugudde.

Murtugudde explained that the rotation of the Earth causes currents on opposite sides of the equator to move away from each other. Any moving water just north of the equator is pushed further north and any water just south of the equator goes further south. During a normal year the currents produce equatorial upwelling and give rise to beds of algae. When El Niño hits, warm water prevents this upwelling, as it does in many other parts of the ocean, and the algae die off. At the end of the cycle the algal bloom should re-establish themselves at the equator.

  SST Pallette

SeaWiFS Pallette

As indicated by the red (warm) region off the west coast of Peru (top image), El Niño was still going strong in February 1998. To scientists' surprise, phytoplankton were growing just to the north of the equator (bright blue green region in the image second from top).

By February 1999 La Niña had replaced El Niño, and the equatorial Pacific had strong phytoplankton production (bottom pair of images).

Images by Robert Simmon based on data from the Distributed Active Archive Centers at the Jet Propulsion Lab and Goddard Space Flight Center.

 

Febryary 1999
 

In order to understand what was going on, the scientists looked at measurements of the wind speed and water temperature for March and April. The readings not only verified the scientists’ suspicions that the warm waters were retreating to the western Pacific, but also gave them an explanation for the position of the algae. Apparently, El Niño-related changes were also creating changes in the air above the ocean. The winds were not blowing east or west across the equator, but south, and they were pushing warm surface water into the equator.

"If you blow warm water into the equator, it cannot go further south. The water tends to pile up there," he said. Since these winds had blown away the surface waters north of the equator, the upwelling currents shifted and they ended up emerging 200-300 kilometers away. Within days the phytoplankton started to grow north of the equator.

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Reading the Future in a Bed of Algae

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"For the first time we are seeing the transition from El Niño to La Niña well before other measurements become available," Murtugudde said. The researchers had to look at sea surface temperature, sea surface heights and wind speed to verify the results shown on the SeaWiFS satellite maps in January and February. Though the northerly winds and lower temperatures existed during these earlier months, they had not changed enough for the scientists to get a bead on them using standard monitoring equipment, he explained. "The biology reacts much more to sub-surface conditions of the ocean than these other parameters do," Murtugudde said.

Now that he and other NASA Goddard scientists have a way to read the patterns the phytoplankton make, they should be able to detect the end of the next El Niño a month before other, more conventional detection devices do. In the future the team plans to look at what happens to the algae leading up to an El Niño. "It’s quite likely that some of the biological signatures will appear before the next El Niño. This time we will keep an eye out for them. With ocean color data we should be able to see certain things you cannot see with other measurements," Murtugudde said.

His long-term goal is to gather enough data on events like El Niño and La Niña to improve weather forecasting systems. Today, scientists can predict El Niños up to a year in advance, using complex computer simulations and data from other satellites and buoys. However, estimates of the exact months when El Niños and La Niñas begin and end are often very rough. By observing phytoplankton, scientists can track both the motion of the water on the surface and just beneath the surface. This should allow for more comprehensive models and more accurate predictions.

References

  1. Njoku, E. G. and O. B. Brown, Sea Surface Temperature. In Atlas of Satellite Observations Related to Global Change, R. J. Gurney, J.L. Foster and C. L. Parkinson, (Eds.) Cambridge University Press, London, 237-249.
  2. Murtugudde, R. G., S. R. Signorini, J. R. Christian, A. J. Busalacchi, C. R. McClain and J. Picaut, 1999. Ocean Color Variability of the Tropical Indo-Pacific Basin Observed by SeaWiFS During 1997-1998. In press, J Geophys. Res., 2-27.

back Picking Out a Pattern for El Niño’s End

 

Orbview 2
SeaWiFS is carried aboard the satellite OrbView-2, shown here in an animation orbiting over the great lakes. It provides us important information about the oceans and the life within them. (Animation courtesy Jesse Allen, Goddard Visualization Analysis Lab)