el niño: a temperature anomaly
DESCRIPTION
- PowerPoint PPT PresentationTRANSCRIPT
El Niño is a large scale oceanographic /
meteorological phenomenon that develops in the
Pacific Ocean, which is associated with extreme
climatic variability. It is the migration, from time to
time, of warm surface waters from the western
equatorial Pacific Basin to the eastern equatorial
Pacific region, along the coasts of Peru and Ecuador.
The warm current (El Niño) temporarily
displaces nutrient-rich up welling cold water
resulting in the heavy harvest of anchovies. The
abundant catch, however, is short-lived. What
follows is a sharp decline in the fish population,
resulting in a lesser catch. At times, warming is
exceptionally strong and ruins the anchovy harvest.
During an El Niño, the physical relationships
between wind, ocean currents, temperature, and
biosphere break down into destructive patterns.
El Niño: A temperature anomaly
The trade winds tend to lose strength with the onset of
springtime in the northern hemisphere. Less water is pushed
westward and, consequently, waters in the central and eastern
Pacific begin to heat up (usually several degrees Fahrenheit)
and the thermo cline tilt diminishes. But the trade winds are
usually replenished by the Asian summer monsoon, and the
delicate balance of the thermo cline tilt is again maintained.
Moreover, the natural spring warming in the central
Pacific is allowed to continue and also spread eastward
through the summer and fall. Beneath the surface, the thermo
cline along the equator flattens as the warm waters at the
surface effectively act as a 300-foot-deep cap preventing the
colder, deeper waters from upwelling. On average, these
waters warm by 3° to 5°F, but in some places the waters can
peak at more than 10°F higher than normal (up from
temperatures in the low 70s Fahrenheit, to the high 80s).
This animation shows the relationship between the direction and intensity of the Pacific trade winds, and the formation of El Niño. The arrows show surface wind dynamics, while the colors represent sea surface temperature. Notice how the warmer water expands, while cooler water contracts.
This series of images shows how the thermo cline tilt changes during the onset of an El Niño. Notice how in January 1997 (left) there is a steep temperature gradient between the western and eastern Pacific Ocean. In April 1997 (center), much of the deep pool of warm water in the west has migrated eastward, lessening the temperature gradient. By July 1997 (right), there is a full-blown El Niño and the thermocline is flat.
Historical Observations
Sir Gilbert Walker
Sir Gilbert Walker
empirically identified that
some notable climate
anomalies—changes in
atmospheric pressure and
circulation—happen around
the world every few years
Professor Jacob Bjerknes
Jacob Bjerknes proposed
that El Niño was just the oceanic
expression of a large-scale
interaction between the ocean
and the atmosphere, and that the
climate anomalies could be
understood as atmospheric
"teleconnections" spreading from
the warm-water regions along the
equator in the mid-Pacific.