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.
El Niño’s Effect on Algae
Observing
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 entirePacific 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).
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).
Picking Out a Pattern for El Niño’s End
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. Theta beamed back to scientists are used to create weekly maps of the algae.
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.