On our planet, the majority of the heat
is stored in the oceans. They retain more easily the energy
than continents and more than the atmosphere. So the ocean
plays a role as important as atmosphere on the climate.
Under normal conditions the thermocline
is not flat. Its depth is about
50 meters to the east and 150 to 200 meters in the western
Pacific. The waters
of Chile and Peru Seaboard are at a temperature of 64,4-68°F
In the tropical Pacific, the irregular
changes of sea surface temperature (S.S.T. : sea surface
temperature) which occur, ENSO (El Niño Southern
Oscillation) cause local disturbances and have climatic
effects. The two opposite phases of this phenomenon
are : El Niño and La Niña. When there is no
significant SST anomalies then it is named La NADA..
Scientists use the data from the TAO (Tropical
Atmosphere Ocean) / TRITON studying climatic variations
from year to year related to El Niño / La Niña
and consists of 70 moored buoys spanning the equatorial
Pacific between 8° North and 8° South. They measure
the atmospheric conditions of the surface (wind, humidity,
precipitation, radiative flux) and ocean surface conditions
and subsurface down until to 500 m of depth (temperature,
currents and salinity). All is organized by the United States,
France, Japan, and Taiwan.
Their data confirmed the relationship between
changes in the volume of hotter water than 20°C from
0 to 300 m of depth (WWV) and the phenomenon of the cycle
of El Niño Southern Oscillation (ENSO). Because it
was also found that the magnitude of the SST anomalies of
ENSO is directly related to the magnitude of the anomalies
of the WWV zonal averages.
If the WWV is compared to the anomaly of the equatorial
Pacific SST, it is easy to see
that changes in WWV precede the ENSO events. Data source:
It was in 1920 that a British mathematician
Gilbert Walker discovered that when the barometric pressure
is higher than normal in the South Pacific, it tends to
be lower than normal in the Indian Ocean, and the usual
winds are appearing with abundant rain. This climatic
phenomenon occurs every 2 to 7 years.
During the El Niño phenomenon
the thermocline descends in Eastern
Pacific from 15 to 50 meters deeper and rises up to West.
The trade winds
weaken and the waters of Chile and Peru Seaboard are at
a temperature of
78,8-82,4°F (26 to 28°C). The number of hurricanes
affecting Polynesia increases.
CAUSE OF EL NINO
This phenomenon is relaed to the instability
of the couple atmosphere-ocean. Two types of waves travel
in the Pacific. Kelvin waves propagate eastward and Rossby
waves westward. The slow movement of the ripples and the
variation of atmospheric pressure could be one of the causes
of El Niño and la Niña.
The lower the pressure near Indonesia
and Australia, and the higher the pressure near Easter Island,
then trade winds are
stronger sweeping the tropical Pacific from the eastern
Pacific ocean to the west. Warm sea surface water accumulated
in the western Pacific by these winds push up the sea level
of about 50 cm higher and thermocline level is lower near
Indonesia than Ecuador.
But when the pressure increases much near
Indonesia and Australia and then decreases near Easter Island
(Chile), there is an anomaly of the west wind in the central
Pacific and the trade winds
weaken. Then the warm surface waters are driven towards
the East. The surface layer becomes thinner upstream and
downstream plunges. To the east, this increases in depth
is balanced by Kelvin waves that propagate rapidly (3 m/s).
After three months, when they arrive near the western coast,
the waves prevent the upwelling of cold waters rich in fishes.
The increasing in depth of the
thermocline in the east directly affects sea surface
temperature increasing. Then the air is warmed by the increase
in the temperature of the ocean. In the Western Pacific
ocean, the upwelling of the thermocline is spread in the
form of Rossby waves.
Description of ENSO (El Niño and la Niña)
a Southern Oscillation in the Pacific.
The westerlies and trade
winds converge on the area of warm waters
driving warm surface waters. By convention the air is heated
accentuating the anomaly. Different oceanic waves are created
these anomalies which act differently on the thermocline
Régnier helped me correct mistakes, please you to
Im not english speaker, some improprieties can appear
to english masters.
Could you help me reporting by mail any fault you read.
Thank you for all.