What is an El Niño/Southern Oscillation (ENSO) event? La Niña? How are they connected to each
other? What will be an ideal response?
ANS:Answer
should include:
? Southern Oscillation is the change in atmospheric pressure and wind direction in the
tropical Pacific. Normal conditions have trade winds blowing from the high pressure
area over the eastern Pacific to the low pressure area over the western Pacific. When
Southern Oscillation occurs, the pressure areas shift: high pressure area shifts to the
western Pacific. This then causes the trade winds to weaken or even reverse direction.
? El Niño refers to the effects the Southern Oscillation has on the ocean currents. When
the trade winds weaken, equatorial currents that usually move water westward stop. The
warm water accumulates in the western Pacific. It begins to flow east along the equator.
A rise in sea level and precipitation occurs during these events.
? During El Niño, the cold, nutrient laden current flowing north towards the South
American continent gets interrupted by the warm equatorial waters. This area of the
ocean is dependent on the upwelling of these nutrient-rich waters to sustain the high
levels of biological productivity. Many seabirds and fish migrate or die when those
nutrients are no longer available.
? La Niña events are colder-than-normal circulation events. They have strong currents,
significant upwelling, and stormy conditions associated with them. The upper curve of
the thermocline gets depressed in the west Pacific during these events due to the
renewed trade winds. The eastern Pacific thermocline then becomes shallow.
? El Niño and La Niña are connected to each other. La Niña events are the response to El
Niño events.
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