Greater knowledge of ocean circulation is enabling us to better understand and predict the climate, especially natural catastrophes such as El Niño. This phenomenon, caused by the arrival of anomalous warm water on the coast of Peru, brings severe weather patterns such as drought, flooding and cyclones. It is now possible to predict El Niño from ocean data.
For centuries, Peruvian fishermen have feared the sea warming known as El Niño, which every few years around Christmas, drastically reduces their fishing catches. These El Niño events are part of a broader disruption to normal weather patterns which causes drought, flooding and hurricanes around the world. The 1997-98 event gave scientists a chance to analyse the complex relationship between the ocean and the atmosphere. The key to maximising the opportunity was the satellite altimetry missions. Altimetry data are vital for the early detection, analysis and close monitoring of large-scale tropical climate anomalies, in order to predict when and how events will develop and, ultimately, to anticipate and mitigate their impacts.
From February to April 1997, altimeter satellite data pinpointed a large eastward-moving swelling of waters in the central Pacific. This positive sea-level anomaly, over 10 cm high and increasing as the months went by, peaked near the coast. In August 1998, its signature was plain and south-east Asia was hit by severe drought. Topex/Poseidon tracked the development of El Niño, revealing a maximum anomaly of more than 20 cm in the northern winter. By June 1998, surface height was returning to normal. In July 1998, altimeters revealed favourable conditions for La Niña, which developed in 1999.
Ocean circulation in the tropics is closely related to changes in the trade winds. There is a strong seasonal cycle in the significant ocean circulation parameters: temperature, density and dynamic topography. In the equatorial Pacific, the trade winds usually blow westward, pushing surface waters and creating a build-up of warm waters at the western end of the basin. At the end of the year, when the trade winds decrease, the trend reverses and warm waters start to move eastward. Some years, the trade winds are so weak that this eastward current becomes very strong and crosses the Pacific basin in a few weeks. Heat transfers between ocean and atmosphere are considerable and lead to devastating precipitations and storms when reaching South America. This is “El Niño” which has global repercussions on the climate.
Because the tropical ocean can be regarded as consisting of two different layers, sea level is a good indicator of the upper layer’s heat content. Hence the suitability of using altimetry measurements for studying the exchange of warm water in the tropical Pacific Ocean.
Data use case:
– Cheney, R., L. Miller, R. Agreen, N. Doyle, and J. Lillibridge, TOPEX/POSEIDON: the 2-cm solution, J. Geophys. Res., 99, 24 555-24 563, 1994
– Ji, M., R. W. Reynolds, and D. W. Behringer, Use of TOPEX/Poseidon sea level data for ocean analyses and ENSO prediction: some early results, J. Climate, 13, 216-231, 2000.
– Picaut, J., E. Hackert, A. J. Busalacchi, R. Murtugudde, and G. S. E. Lagerloef, Mechanisms of the 1997-1998 El Niño-La Niña, as inferred from space-based observations, J. Geophys. Res., 107, 10.1029/2001JC000850, 2002.
– Picaut, J., M. Ioualalen, C. Menkes, T. Delcroix, and M. J. McPhaden, Mechanism of the zonal displacements of the Pacific warm pool: implications for ENSO, Science, 274, 1486-1489, 1996.
– Wang, C., and J. Picaut, Understanding ENSO physics, A review, In: Earth’s Climate: The Ocean-Atmosphere Interaction. C. Wang, S.-P. Xie, and J. A. Carton, Eds., AGU Geophysical Monograph Series, 147, 21-48, 2004.
Picaut, J., and A.J. Busalacchi, Tropical ocean variability, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001