Satellite measurements of a steep difference in sea surface height between the western and eastern Pacific Ocean support predictions that a La Nina event that would impact global weather could be forming, the European Space Agency (ESA) announced.
During a La Nina event, warm water shifts westward in the Pacific Ocean to induce upwelling of cold water, reducing rainfall in the eastern equatorial Pacific but increasing it in the west. The phenomenon is linked to an opposing shift in dubbed El Nino in which a mass of warmer water from the Western Pacific moves east, displacing cooler waters in the vicinity. This warmer water adds moisture to the atmosphere, raises rainfall levels and disrupts atmospheric circulation on a global basis.
Researchers now recognize that these twin extremes are ocean components of a larger phenomenon that extends to the atmosphere, called the El Nino Southern Oscillation. The variation in air pressure can influence weather patterns worldwide and researchers seek to combine all available data for enhanced understanding and forecasting.
Today, as the Pacific warm pool shifts westward and La Nina's cool water extends across the Eastern Pacific, the movement is being monitored via a global ocean observing system that includes space- and surface-based elements. Sea surface height is not constant but varies across the global ocean; vertical expansion due to increased water temperature being one of the main reasons why warm water masses can stand up to a meter higher than the surrounding sea. Satellite radar altimeters measure sea surface height down to a maximum accuracy of 2 centimeters.
Instruments such as the Radar Altimeter-2 on ESA's Envisat bounce 1,800 radar pulses per second off the Earth's surface, measuring their return time to the nanosecond to calculate the precise signal distance traveled. The data returned over the open ocean helps to chart changes in sea surface temperature. Altimetry data from Envisat and predecessor ERS-2 are blended with altimetry results from the French/U.S. Jason-1 mission and other data to ensure the best possible global ocean coverage.
Current altimetry-derived sea level anomaly measurements show differences in sea surface height of 60 centimeters between the West and East Pacific. This data complements information collected by different networks of surface instruments and these results are assimilated into numerical models by weather centers worldwide.
At the beginning of February, the U.S. National Oceanic and Atmospheric Administration's Climate Prediction Center announced that the conditions for a weak La Nina are in place -- with central Pacific sea surface temperatures departing more than 0.5 degrees Celsius for the past 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," altimetry expert Christophe Maes of the Institut de Recherche pour le Developement in New Caledonia, said in a statement. "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.