El Niño is a global weather disturbance that comes along every 4 to 7 years when trade winds blowing across the Pacific Ocean weaken or even reverse. (Why they do this, no one knows.) Normally these winds blow from the Americas toward Australia, pushing sun-warmed surface waters from east to west. Warm water accumulates near Australia in a region call ‘the warm pool’. What happens when the trade winds falter? That warm water sloshes eastward back across the Pacific Ocean. One can see it in NOAA and NASA satellite maps of sea surface temperature and sea surface height: a band of warmer, higher water stretching along the equator from the mid-Pacific to the coasts of Peru and Ecuador.
This warm strip has multiple influences on global patterns of winds and humidity. For instance, it diverts the course of the jet stream–a “conveyor belt” for storms–that in turn affects weather over much of the globe, especially the North American continent. In terms of Pacific sea surface temperatures, the 2002-03 El Niño has been far milder than the mammoth El Niño of 1997-98. Recent images from the NASA/CNES Jason-1 satellite show that the mid-equatorial Pacific was only 2°C warmer than average in 2002, compared to the huge, long-lasting tongue of water that was as much as 5°C warmer than average and piled up along the coasts of the Americas in 1997. A comparison of El Nino’s warm strip has been donne for Dec. 1997 and Dec. 2002. Sea surface temperature anomalies in these maps were computed from measurements of sea surface temperature collected by the AVHRR sensor on the NOAA polar orbiting satellites.
Meanwhile in Ecuador and Peru, countries where El Niño usually brings torrential rains and flooding, the 2002 El Niño has had little effect. New England has also experienced contrary weather: The El Niño winter of 1997 was unusually warm. This winter it has been bitterly cold.