El Niņo monitoring reveals 
surprising extremes

December 6, 1998

SAN FRANCISCO, CA —A portrait of the most intense El Niņo on record this century has provided unprecedented detail on the biological effects of the recent warming event in the equatorial Pacific Ocean, from its beginnings in late 1996 to its abrupt end in early summer this year. That portrait—derived from oceanic and meteorological data delivered by satellites, instrument-equipped moorings, and shipboard cruises—gave researchers the most intimate look yet at the face of El Niņo. At the same time it showed some astoundingly variable features.

"We were able to chart this event at a level of detail not previously possible," said biological oceanographer Francisco Chavez of the Monterey Bay Aquarium Research Institute (MBARI). "This is the first time we’ve ever had a set of biological measurements from moored instruments during an intense El Niņo, and we’ve never seen such low chlorophyll concentrations." Levels of chlorophyll, one of the pigments used by marine phytoplankton (microscopic plants) to transform sunlight into energy for growth, is a prime indicator of biological productivity in ocean waters, which is directly tied to the upwelling of essential nutrients from deep water.

Collaborators on the research included Gene Feldman, of the NASA Goddard Space Flight Center; Michael McFadden, of the Pacific Marine Environmental Laboratory (PMEL), National Oceanic and Atmospheric Administration (NOAA); David Foley, of the Honolulu Lab, National Marine Fisheries Service; and Peter Strutton, also of MBARI.

El Niņo’s dramatic disruption to upwelling is also of keen interest to biologists and chemists because the equatorial Pacific plays a key role in biogeochemical cycles, in particular, the cycling of nitrogen and carbon in the biosphere. The equatorial Pacific is the largest natural source of carbon dioxide to the atmosphere and accounts for one-fifth of the global supply of nitrate to the ocean surface. As evidenced by the powerful climatic events observed around the world during 1997 and 1998, the magnitude and variability of equatorial upwelling has significant consequences for global climate.

Over the past decade scientists have been able to observe the development and progression of El Niņo warmings—and consequent changes in upwelling—thanks to data continuously collected in the equatorial Pacific by the 70 moored buoys of the Tropical Atmosphere Ocean array. Maintained by PMEL-NOAA and collaborating institutions such as MBARI, instruments on the buoys record relative humidity, surface wind velocities, air and sea-surface temperatures, and water temperatures to a depth of 500 meters (1,650 feet). In late 1996 bio-optical and chemical sensors were installed on two buoys maintained by MBARI and a handful of other buoys, allowing researchers for the first time to directly and continuously gauge the fluctuating levels of biological productivity in the equatorial region. These measurements and shipboard sampling of chlorophyll and other ocean properties performed on periodic cruises constitute two of the tools contributing to the El Niņo portrait.

In 1997, with the launch of a NASA satellite bearing a Sea-viewing Wide Field-of-View Sensor (SeaWiFS), a third ocean-monitoring tool came into play. Capable of detecting subtle changes in ocean color that signify various types and quantities of phytoplankton, SeaWiFS shed light on the consequences of El Niņo from another dimension. It was the mooring measurements and SeaWiFS data that revealed surprisingly low and high levels of chlorophyll, coinciding with El Niņo’s strongest phase and the recovery period and transition to La Niņa cooling, respectively. When the warm-water layer extended to its greatest depths (more than 100 meters, or 330 feet), the upwelling of iron and other nutrients necessary for phytoplankton growth virtually ceased, and chlorophyll values derived from mooring data plummeted to less than 0.1 microgram per liter. Carbon dioxide measurements from the moorings indicated that the ocean was near equilibrium with the atmosphere during this period, and that short-term variability was modest.

"If chlorophyll levels have been this low before, we’ve missed seeing it in the 10 years we’ve been collecting shipboard data," Chavez noted. "At the other extreme, during the recovery period and shift to La Niņa conditions in mid-1998, as the water cooled and upwelling surged, the mooring data showed chlorophyll concentrations up to 10 times higher than any we’d previously observed." Carbon dioxide levels and variability increased as upwelling resumed; however, increased productivity induced by La Niņa conditions—as evidenced by the high chlorophyll concentrations—appears to have had a significant dampening effect on sea-surface carbon dioxide levels.

Chavez credits the powerful new tools—remote sensing by satellites and recently installed bio-optical instruments—for providing "a new set of observations that is changing our perspective on El Niņo. It’s an exciting development in ocean monitoring," he contended. "Here you are, sitting on the West Coast of the U.S., and you can chart these significant events in the remote mid-Pacific Ocean." Over the last decade MBARI has vigorously pursued the development of in situ monitoring instruments for continuous data collection, and satellite data received at MBARI figures strongly in the research of Chavez and his colleagues.

El Niņo-La Niņa events and the consequent changes in upwelling ultimately impact certain fisheries; it was, after all, fishermen in Peru who first noted—and named—the local ocean warmings and coolings that affected their livelihood. A new analysis of data collected from offshore Peru, spanning 1955 to 1998, underscores just how closely anchovy catches are linked to the climate events. Richard Barber of Duke University and MBARI’s Chavez studied the time-series data on anchovy catches and sea level, surface temperatures, and the depth of the ocean’s mixed layer (the biological productivity zone). The data had previously revealed that anchovy catches declined with El Niņos, as documented in the 1982-83 occurrence. Now, with the addition of the latest data, Barber and Chavez have found strong evidence that significant increases in anchovy numbers are only associated with La Niņa conditions: cool sea-surface temperatures, low sea levels, and shallow mixed layers conducive to strong upwelling.

The recurrence of an intense El Niņo so soon after the 1982-83 episode—which previously held "event-of-the-century" status—raises questions regarding whether the warmings are becoming more frequent and/or more intense. Chavez’s opinion: "If you look at the record of sea-surface temperatures in Peruvian waters, charted since 1925, the spacing of El Niņos appears roughly consistent. But since 1982 we’ve seen two events that dwarf all the others this century in terms of intensity. Whether or not this proves to be a trend, we’ll have to wait and see, but we’ll be watching more closely and with increasingly sophisticated instruments in the coming years."

Contact: Debbie Meyer, Communications Coordinator