El Niño Glossary
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.
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
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 1,000 km
wide, extending westward from the South American coast along the equator
into the central Pacific. The Galapagos Islands lie within the cold tongue.
convection: A self-starting circulation in a fluid (such as
air or water), where a warm current rises into a cool area, and a cool current descends to take
its place. Convection is driven by gravity—warm fluids are usually lighter than denser cold
fluids, and gravity drags the densest material to the bottom.
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 Niño: Spanish for the "Christ Child." Fishermen in Peru and Ecuador used
to use this term to refer to a warming of coastal waters around Christmas
time 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 15 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
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
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
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 and 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
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 and the Southern Oscillation.
westerlies: West to east winds occurring in midlatitudes in both
zooplankton: Tiny sea animals which feed on phytoplankton.
Thanks to the University of Washington, Department of Atmospheric Science