What is an El Niño/Southern Oscillation (ENSO) event? La Niña? How are they connected to each other?

Answer 1

· 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.