The Age of El Nino

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. The data 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. "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.