El Niño impacts biological production acrossentire Pacific Ocean

By Dr. Daniela Turk, Dalhousie University

The ocean contains 50 times more CO2 than the atmosphere.

The large scale phenomenon known as El Niño has a strong influence on weather, rainfall, and ocean physics on a planetary scale. Now a new study by scientists from Dalhousie University, the US National Oceanographic and Atmospheric Administration (NOAA), and the University of Maryland, demonstrates that complex atmospheric-ocean interactions associated with El Niño/Southern Oscillation (ENSO) also have a large impact on the biological productivity and export of organic carbon from the tropical Pacific region. The work is based on a unique combination of ship, buoy and satellite observations in the tropical Pacific, which were published in Science, on July 19.

“The wave-like nature of ENSO sloshes large volumes of water back and forth across the entire equatorial Pacific, a distance of over 12,000 kilometres,” said Dr. Daniela Turk, Department of Oceanography, Dalhousie University, lead author of the study. “This results in changes in the availability of nutrients that fuel the growth of the phytoplankton, the base of the marine food chain, and consequently large-scale changes in the biological
productivity of this region.”

Biological productivity is directly responsible for marine fisheries, and for the export of organic carbon from the surface to deep waters and sediments. This export has implications for climate as well. For example, only about half of the carbon released in the atmosphere through human activities remains in the atmosphere, while the rest ends up in the ocean, or in plants and soils on land. The ocean contains 50 times more CO₂ than the atmosphere, and the vast tropical Pacific is thought to be responsible for 10 to 55 percent of the export of organic carbon from the entire world’s oceans. In addition, interannual changes in global carbon sequestration associated with El Niño/La Niña cycle contribute to the largest known natural year-to-year perturbation of the global carbon cycle.

During El Niño, much of the Pacific Ocean’s capacity to take up and store carbon by biological processes is decreased in the eastern Pacific, but compensated to some extent by increases in the west. During La Niña period, the opposite phase of the oscillation, biological productivity of the east is dramatically increased, while in the west it is diminished. The net basin-scale effect was to decrease the export of carbon to the deep ocean by 30 percent during 1997 – 98 El Niño and to increase it by 40 percent during
1998 – 99 La Niña.

The conclusions are a direct result of the availability of a combination of high precision observations of the surface of the ocean from satellite, and observations of the ocean interior from buoys and ships. “The measurements of the height of the sea surface from the Topex/Poseidon satellite altimeter provide a large scale view of the dynamics of the interior of the ocean when combined with subsurface measurements of temperature and other variables from buoys,” said Dr. Turk.

The Topex/Poseidon sensor built by NASA and the Centre National d’Etudes Spatiales (CNES, France), is a radar that is capable of measuring the height of the sea surface to an accuracy of 2 cm from a distance of over 700 km. The buoy observations were taken from the Tropical Ocean Atmosphere (TAG) array built and operated under the leadership of NOAA. This array consists of over 70 automated data collection buoys in stalled across the tropical Pacific, which provide ocean and weather information on a routine basis. The combination of data provides an unprecedented basinscale perspective on the net effects of El Nino on biological productivity of the tropical oceans.

Contact Dr. Turk, (902) 494-2167, E-mail: neli@raptor.ocean.dal.ca.