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.

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.

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.

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.

Reading the Future in a Bed of Algae
El Niño’s Effect on Algae