WANT to know up to a year in advance if El Ni帽o is coming? Then the best thing to do might be to measure the salt in the ocean.
During an El Ni帽o year, warm surface water spreads eastwards across the equatorial Pacific. This is linked to changes in weather patterns that lead to drought and failure of monsoon rains in Australia and southern Asia, and heavy rains and along the South American coast. Fisheries off South America are also affected.
These changes happen once every three to seven years. Scientists understand why El Ni帽o comes and goes, says Tony Busalacchi of the University of Maryland, College Park, but not why the period varies so much. That makes predicting an El Ni帽o a tricky business.
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The driving forces behind El Ni帽o are thought to be the heat content and density of the surface waters. Previous studies have estimated water density from the temperature data, which is readily available from satellites. But they often neglect salt, which has an effect on density too.
Busalacchi and two Maryland colleagues looked at data from the Pacific between 1980 and 1995 to see how salinity affects the onset of El Ni帽o. They found that low salinity levels in the western Pacific were followed by El Ni帽o conditions six months later. And excess salinity north and south of the equator correlated with warmer waters about 12 months later (Journal of Geophysical Research, vol 107, p 8007).
Those observations suggest that the first thing to trigger an El Ni帽o event is the sinking of cold, salty surface waters on either side of the equator. Then warm, less-salty surface waters spread eastward and trigger the well-known climate changes.
So following salinity data should give researchers a better heads-up on El Ni帽o. NASA has already approved the launch of a satellite called Aquarius in 2006 or 2007, which will remotely measure sea-surface salinity and provide the first global maps of sea salt.