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Envisat altimeter watches Pacific for cold tongue of La Niña

March 06, 2006

Satellite measurements of a steep difference in sea surface height between the western and eastern tropical Pacific support predictions that a La Niña event is in the offing. El Niño's chillier sister, La Niña is linked to opposing but equally wide-ranging shifts in weather patterns.

Hundreds of years ago fishermen off the west coast of Peru noted how periodically around Christmas time the waters grew unusually warm and fish became scarce: a phenomenon they called 'the Christ Child'-El Niño. It begins when a mass of warmer water from the western Pacific moves east, displacing cooler, nutrient-rich waters in the vicinity. This warmer water adds moisture to the atmosphere, raises rainfall levels and disrupts atmospheric circulation on a global basis.

La Niña is an equivalent cooling event during which the warm waters shift westwards to induce upwelling of cold water, reducing rainfall in the eastern equatorial Pacific but increasing it in the west. Researchers now recognise that these twin extremes of El Niño and La Niña are ocean components of a larger phenomenon that extends to the atmosphere, called the El Niño Southern Oscillation (ENSO).

Back in the 1920s meteorologist Sir Gilbert Walker noticed seasonal fluctuations in the air pressure difference across the equatorial Pacific, which he called the Southern Oscillation, and in the 1960s the realisation came this was linked to El Niño and La Niña events. Inter-annual ENSO variations can influence weather patterns worldwide, and researcher seek to combine all available data for enhanced understanding and forecasting.

So today, as the Pacific warm pool shifts westward and La Niña's 'cold tongue' of cool water extends across the eastern Pacific, it is being monitored via a global ocean observing system that includes an important space element.

Sea surface height (SSH) is not constant but varies across the global ocean, with vertical expansion due to increased water temperature being one of the main reasons why: warm water masses can stand up to a metre higher than the surrounding sea. Satellite radar altimeters measure sea surface height down to a maximum accuracy of two centimetres.

Instruments such as the Radar Altimeter-2 (RA-2) on ESA's Envisat bounce 1800 radar pulses per second off the Earth's surface, measuring their return time to the nanosecond to calculate the precise signal distance travelled. The data returned over the open ocean helps to chart changes in sea surface temperature (SST). Altimetry data from Envisat and its predecessor mission ERS-2 are made available in near-real time, then blended with altimetry results from the French/US Jason-1 mission and the US GFO to ensure the best possible global ocean coverage.

Current altimetry-derived sea level anomaly measurements show differences in SSH of 60 cm between the west and east Pacific. This wide-area view provided by altimetry-derived sea level anomaly data complements other in-situ sources, including the 70 buoys of the Tropical Ocean Atmosphere (TAO) array across the equatorial Pacific, operated by the US National Oceanic and Atmospheric Administration (NOAA), the TRIangle Trans-Ocean buoy Network (TRITON) array operated by Japan's Ministry of Education, Culture, Sports and Technology and also the more than 2000 global Argo profiling floats, which provide temperature and salinity profiles at various depths across the global ocean.

These results are then assimilated into sophisticated numerical models by weather centres worldwide, including the Reading-based European Centre for Medium Range Weather Forecasting (ECMWF) to help provide 'initial state' information to make seasonal forecasts of ocean states. The US NOAA Climate Prediction Center at the start of February announced that the conditions for a weak La Niña are in place-with central Pacific SST departing more than -0.5° C for the last three months-and the event is likely to last into late spring and possibly summer.

"The development of a negative anomaly does seem to be well in hand," agrees altimetry expert Christophe Maes of the Institut de Recherche pour le Développement (IRD) in New Caledonia. "It will still take a few months for the scientific community to better comprehend what is going on because the system has often surprised us in the past.

"If it is necessary to be careful in forecasting El Niño then it is all the more necessary for La Niña - not due to any feminine mystique but simply due to our lack of experience. La Niña events are simply less frequent than their male equivalents. This asymmetry has yet to be explained from a theoretical point of view at the current time.\\\

European Space Agency