03 March 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.
An El Niño event 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.
sea level anomaly in the Tropical Pacific Ocean (Copyright: ESA)
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).
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 as warm water masses can stand up to a metre higher than the surrounding sea. Satellite radar altimeters, such as the Radar Alimeter-2 (RA-2), measure sea surface height down to a maximum accuracy of two centimetres. The data gathered helps to chart changes in sea surface temperature (SST). Data from Envisat and ERS-2 are made available in near real time and combined with similar data from other missions to ensure the best possible coverage.
Global mean sea surface height from altimetry (copyright: ESA)
Current data 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, such as 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 have been fed into 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 coming of La Niña is marked by shifts in global atmospheric circulation patterns, as colder than normal waters in the eastern tropical Pacific suppress rainfall in the vicinity, at the same time as increasing it to the west.
During the northern hemisphere winter, wetter than normal conditions are found over northern Australia and Indonesia. They also occur over the Philippines during the northern hemisphere summer, and Indian monsoon rainfall also increases, particularly in northwest India. Meanwhile South America’s tropical west coast undergoes drier than normal winters.
In turn these shifts affect the position and weaken the intensity of jet streams and the behaviour of storms occurring beyond the tropics in both hemispheres, including a summer decrease in hurricane activity in the eastern tropical North Pacific and a corresponding increase in the number of hurricanes in the tropical North Atlantic. The last La Niña took place during 2000-2001, a weak event compared to the previous La Niña in 1998-2000 – it, and the 1988-89 event together setting the terms of reference for a ‘standard’ La Niña.
El Nino Website