THEME 5: CLIMATE VARIABILITY, EL NIÑO AND
RAINFALL PROBABILITIES
5.1 CLIMATE VARIABILITY
Study pages 9 – 12 and 67 – 83 of Tyson (1986).
5.2 EL NIÑO
5.2.1 ENSO BASICS
5.2.1.1 WHAT ARE EL NIÑO AND LA NIÑA?
The term El Niño was first coined more than 100 years ago to describe the
unusually warm waters that would occasionally form along the coast of Ecuador
and Peru. This phenomenon typically occurred late in the calendar year near
Christmas, hence the name El Niño (Spanish for "the boy child", referring to the
Christ child). Today the term El Niño is used to refer to a much broader scale
phenomenon associated with unusually warm water that occasionally forms
across much of the tropical eastern and central Pacific. The time between
successive El Niño events is irregular but they typically tend to recur every 3 to 7
years.
La Niña is the counterpart to El Niño and is characterized by cooler than normal
SSTs across much of the equatorial eastern and central Pacific. A La Niña event
often, but not always, follows an El Niño and vice versa. Once developed, both El
Niño and La Niña events tend to last for roughly a year although occasionally
they may persist for 18 months or more. El Niño and La Niña are both a normal
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,part of the earth's climate and there is recorded evidence of their having occurred
for hundreds of years.
Figures 5.1 and 5.2 show examples of the typical extent of the warming and
cooling in the equatorial Pacific during developed El Niño and La Niña events.
Figure 5.1 Departure of sea surface temperature from the long-term average for an El
Niño during December 1991. Positive values indicate warmer than average
temperatures. Units are in degrees Celsius and contours are drawn at 0.5ºC
intervals. (IRI, 2005).
Figure 5.2 Departure of sea surface temperature from the long-term average for a La
Niña during December 1988. Negative values indicate colder than average
temperatures. Units are degrees Celsius and contours are drawn at 0.5ºC
intervals. (IRI, 2005).
Although El Niño and La Niña events are characterized by warmer or cooler than
average sea surface temperatures in the tropical Pacific, they are also
associated with changes in wind, pressure, and rainfall patterns. In the tropics
where El Niño and La Niña form, rainfall tends to occur over areas having the
warmest sea surface temperature. Figures 5.3 to 5.5 show schematic views of
the links between sea-surface temperatures and tropical rainfall (The sea-surface
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, temperature is shaded with the warm temperatures located to the north-east of
Australia. The dark arrows indicate the direction of air movement in the
atmosphere: upward arrows are associated with clouds and rainfall and
downward-pointing arrows are associated with a general lack of rainfall.).
Normal conditions (Figure 5.3): The warmest water is found in the western
Pacific, as is the greatest rainfall. Winds near the ocean surface travel from east
to west across the Pacific (these winds are called easterlies).
Figure 5.3 Schematic view of sea surface temperature and tropical rainfall in the
equatorial Pacific Ocean during normal conditions. (NOAA, Climate
Prediction Center, 2005).
El Niño conditions (Figure 5.4): The easterlies weaken, warmer than average sea
surface temperatures cover the central and eastern tropical Pacific, and the
region of heaviest rainfall moves eastward as well.
La Niña conditions (Figure 5.5): Could be thought of as an enhancement of
normal conditions. During these events, the easterlies strengthen, colder than
average ocean water extends westward to the central Pacific, and the warmer
than average sea-surface temperatures in the western Pacific are accompanied
by heavier than usual rainfall.
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