Equatorial Surface Winds: Origin of and Response to El Niņo and La Niņa

Ocean surface wind vector is likely the critical variable to predict onset, maintenance and dissipation of the El Niņo and La Niņa phenomena. Eugene Rasmusson and Klaus Wyrtki recognized the need to observe in real-time the surface winds over the ocean along the Pacific equator to improve prediction of the onset and other characteristics of El Niņo and La Niņa. Soon after the 1982-1983 El Niņo, wind-measuring instruments mounted on surface buoys anchored to the ocean bottom began transmitting real-time observations via satellite from several sites along the Pacific equator. The TOGA TAO-TRITON surface moored buoys greatly expanded the in-situ ocean surface vector wind array, but large distances occurred between moorings. The advent of satellite vector wind measurements at 10-m height, first demonstrated in 1978 and sustained after 1991, generated new knowledge about equatorial winds in the Atlantic, Indian and Pacific Oceans. Analyses of SeaWinds-on-QuikSCAT and ASCAT-on-MetOp/A scatterometer datasets showed that from March 2000 to June 2011, when four El Niņos and five La Niņas occurred, the westward wind speed to the west of 150°W was much weaker (slightly stronger) in El Niņo (La Niņa) compared to normal. East of 150°W in the Pacific, the westward wind speed observed in El Niņo was similar to that measured in La Niņa and normal conditions. The longitudinal distribution of 10-m height vector wind components along the equator in the Indian Ocean displayed different characteristics during El Niņo and La Niņa; no such variation was observed in the Atlantic Ocean where the zonal wind component was the same during El Niņo, La Niņa and normal conditions. Analyses of MISR-on-Terra 700-m height wind vectors revealed similar El Niņo/La Niņa results as those at 10-m height. Wind shears between 10- and 700-m heights were larger in the east Pacific in La Niņa compared to normal and El Niņo conditions; no such structure occurred in the Atlantic and Indian Oceans. Comparison of SCAT-on-ERS/1 10-m height wind vectors recorded during the very intense 1997-1998 El Niņo with the winds during the weak-moderate El Niņos in the 2000s will be discussed.