El Nino

Typically, the level of ocean water around the world is higher in the
western Pacific and lower in the eastern, near the Western coast of South and
North America. This is due primarily to the presence of easterly winds in the
Pacific, which drag the surface water westward and raise the thermocline
relatively all the way up to the surface in the east and dampen it in the west.
During El Nino conditions, however, the easterlies move east, reducing the
continuing interaction between wind and sea, allowing the thermocline to become
nearly flat and to plunge several feet below the surface of the water, allowing
the water to grow warm and expand. With the help of the National Oceanic and
Atmospheric Administration\'s weather satellites, tracking shifting patterns of
sea-surface temperatures can be made easier. Normally, a "pool" of warm water in
the western Pacific waters exists. Under El Nino conditions, this "pool" drifts
southeast towards the coast of South America. This is because, in a normal year,
there is upwelling on the western South American coastline, and cold waters of
the Pacific rise and push westward. However, during an El Nino year, upwelling
is suppressed and as a result, the thermocline is lower than normal. Finally,
thermocline rises in the west, making upwelling easier and water colder. Air
pressures at sea level in the South Pacific seesaw back and forth between two
distinct patterns. In the high index phase, also called "Southern Oscillation",
pressure is higher near and to the east of Tahiti than farther to the west near
Darwin. The east-west pressure difference along the equator causes the surface
air to flow westward. When the atmosphere switches into the low index phase,
barometers rise in the west and fall in the east, signaling a reduction, or even
a reversal the pressure difference between Darwin and Tahiti. The flattening of
the seesaw causes the easterly surface winds to weaken and retreat eastward.
The "low index" phase is usually accompanied by El Nino conditions.
The easterly winds along the equator and the southeasternly winds that
blow along the Peru and Ecuador coasts both tend to drag the surface water along
with them. The Earth\'s rotation then deflects the resulting surface currents
toward the right (northward) in the Northern Hemisphere and to the left
(southward) in the South Hemisphere. The surface waters are therefore deflected
away from the equator in both directions and away from the coastline. Where the
surface water moves away, colder, nutrient-rich water comes up from below to
replace it which is called upwelling. The winds that blow along the equator also
affect the properties of upwelled water. When there is no wind, the dividing
layer between the warm surface water and the deep cold water would be almost
flat; but the winds drag the surface water westward, raising the thermocline
nearly all the way up to the surface in the east and depressing in the west.
The resulting changes in sea-surface temperature will have an effect on the
winds. When the easterlies are blowing at full strength, the upwelling of cold
water along the equatorial Pacific chills the air above it, making it too dense
to rise high enough for water vapor to condense to form clouds and raindrops. As
a result, this part of the ocean stays indubitably free of clouds during normal
years and the rain in the equatorial belt is mostly confined to the extreme
western Pacific. However, when the easterlies weaken and retreat eastward during
the early stages of an El Nino event, the upwelling slows and the ocean warms.
The moist air above also warms. It would produce deep clouds which make heavy
rain along the equator. The change in ocean temperatures thus causes the major
rain zone over the western Pacific to shift eastward. In this way, the dialogue
between wind and sea in the Pacific can become more and more intense.
Normally, each area of the globe follows a fairly predictable pattern
and receives only that amount of rainfall that it is accustomed to receiving.
However, conditions are quite different during El Nino. During normal years,
when the winds blowing east along the equator are blowing at full strength,
this strip of ocean stays free of clouds and the rain in the equatorial belt
largely confined to the extreme Western Pacific, near Indonesia. But when the
easterlies weaken and retreat eastward during El Nino years, the moist air above
the ocean becomes buoyant enough to form clouds, and the clouds produce heavy
rains along the equator. These rains are only some of the many weather changes
that occur all over the globe