atmospheric pressure Pressure exerted by the atmosphere because of the force of gravity acting on the overlying column of air.

barometer Instrument for measuring atmospheric pressure.

Bjerknes, Jacob University of California professor who recognized that the warm waters of El Niño and the pressure seesaw of Southern Oscillation are part of the same phenomenon.

chlorophyll Greenish plant substance produced by phytoplankton which forms the nutrient base of the ocean ecosystem.

coastal upwelling Strong, wind driven currents along the coasts of California and Peru are deflected westward, away from the coast by the Coriolis force due to the earth's rotation. The water that comes up from below the surface to replace the surface water moving away from the coast is said to be "upwelled." The upwelled water is often cold and rich in nutrients.

Coastal Zone Color Scanner An optical instrument carried aboard a satellite that measures the greenness of the surface water of the ocean, which provides an indication of its chlorophyll content.

cold tongue A band of conspicuously cold surface waters about 1000 km wide extending westward from the South American coast along the equator into the central Pacific. The Galapagos Islands lie within the cold tongue.

Coriolis force Effect of the earth's rotation tending to turn the direction of motion of any object or fluid toward the right in the Northern Hemisphere and the left in the Southern Hemisphere.

easterlies East to west surface winds, that usually extend nearly all the way across the equatorial Pacific from the Galapagos Islands to Indonesia.

ecosystem Community of organisms and their environment.

El Nño Spanish for the "Christ Child". Fishermen in Peru and Ecuador used to use this term to refer to a warming of coastal waters around Christmastime that occurs most years. It has come to be used as a term for abnormal warming events which occur, on an average of two or three times a decade and typically last for a few seasons.

ENSO Acronym for El Niño and Southern Oscillation to describe the linkage between the two phenomena.

equatorial upwelling A band of rising water along the equator that replaces westward-flowing surface water deflected away from the equator by the Coriolis force due to the earth's rotation. The westward flow is driven by the tradewinds.

food web The dependence for food of organisms upon one another, beginning with plants and ending with the largest carnivores.

high-index phase The phase of the Southern Oscillation characterized by higher than normal pressure on the eastern side of the Pacific and lower than normal pressure on the western side. The stronger than normal pressure difference across the equatorial Pacific during the high index phase drives stronger than normal easterly tradewinds.

infrared satellite imagery Just as mosquitoes zero in on their prey by sensing the invisible infrared radiation emitted by their warm bodies, instruments called radiometers carried by satellites in space monitor the infrared radiation emitted from the warm earth. These measurements are used to form infrared images which show the distribution of sea-surface temperature and cloudiness.

jetstream An extended belt of strong westerly winds five to fifteen miles above sea level that often appears in temperate latitudes, particularly during wintertime. Jetstreams form the dividing lines between warm tropical airmasses and cold polar airmasses, and they steer precipitation-bearing weather systems.

low-index phase The phase of the Southern Oscillation characterized by an abnormal weakening of the sea-level pressure contrast across the equatorial Pacific accompanied by a weakening of the easterly tradewinds.

monsoons Rainy seasons over the tropical continents that normally occur during summer, when the continents tend to be warmer than the nearby oceans.

numerical prediction models Computer programs designed to represent, in terms of equations, processes that occur in nature.

phytoplankton Tiny plant species that float freely in the water.

proxy evidence Data obtained through historical records, such as coral samples, tree rings, etc.

real-time observing system Data gathering instruments that provide accurate input for the models used in weather and climate prediction.

Southern Oscillation Episodic strengthening and weakening of the contrast in sea-level barometric pressure between the eastern and western sides of the Pacific. Normally, the pressure is higher on the eastern side, driving the easterly tradewinds in the equatorial belt. The Southern Oscillation causes the pressure contrast and the strength of the easterlies to fluctuate between high-index (strong tradewind) and low-index (weak tradewind) conditions.

thermocline Layer of ocean water a few tens of meters thick that divides the mixed layer (a layer of relatively warm water just below the surface) from colder, deeper water. If one were to descend from the surface in a submarine, water temperature would remain almost constant through the mixed layer and then drop rapidly as one passed through the thermocline. The depth of the thermocline varies in space in time. Along the equator it is usually only a few tens of meters below the surface in the eastern Pacific, compared to 100 meters or more below the surface in the western Pacific. During the onset of El Niño, the thermocline flattens out as the water in the mixed layer surges eastward in response to the weakening of the easterlies. When the thermocline is shallow it is easier for cold, nutrient rich waters to reach the surface to fertilize the food web.

upwelling Term used to describe the process by which cold, nutrient rich water comes up to the surface to replace water that has moved horizontally to some other region. The world's major fisheries coincide with regions of upwelling.

Walker, Sir Gilbert British scientist who recognized the connection between barometer readings at stations on the eastern and western sides of the tropical and South Pacific the Southern Oscillation.

westerlies West to east winds occurring in midlatitudes in both hemispheres.

zooplankton Tiny sea animals which feed on phytoplankton.