Schematic diagramme of normal and El Niño situations in the Pacific Ocean

The variability in South American climate is due in part to the Southern Oscillation (SO) and the El Niño phenomenon (EN), also known as El Niño-Southern Oscillation (ENSO), a continuous but irregular cycle of shifts in ocean and atmospheric conditions that affects the whole planet. It usually starts East of the Pacific Ocean and spreads globally for more than a year, with a time interval varying between three and eight years.
 
A complete definition of El Niño is given by Trenberth (1997), who specifies that this phenomenon refers to the coastal warming of the Eastern Pacific basin, but that it is also associated with a more extensive, anomalous ocean warming to the international date line (180°W), and affects all of the Pacific. He adds that the atmospheric component tied to ocean warming is the so-called Southern Oscillation.

The ENSO has two phases: the warm phase, El Niño, and a colder phase known as La Niña, which is characterised by colder than usual sea surface temperatures in the tropical Pacific.  
 
Climate background of the area
 
The climate found on the coasts of Peru and Ecuador is determined by the air above the cold waters of the Eastern Pacific, which is too dense to rise high enough for water vapour to condense and form clouds and rain. Consequently, a large part of these two countries is composed of desert. This desert effectively begins far from the shore, where the cool, dense air also creates a region of high air pressure.

The high pressure system in the East and the low pressure system over the warmer waters of the West move air westward, generating and reinforcing the steady Equatorial trade winds. These winds harvest moisture from the ocean as they blow towards the Western Pacific. There the warm moist air rises, condenses, and produces heavy monsoon rains that nourish the jungles of New Guinea and Indonesia. The sea surface is about 1 to 2 metres higher in Indonesia than in Ecuador, due to the winds which pile up warm surface water in the Western Pacific.

The sea surface temperature is about 8 degrees C higher in the West, with cool temperatures off South America caused by an upwelling of cold water from deeper levels (Humboldt Current). This cold water, rich in nutrients, supports high levels of primary productivity, diverse marine ecosystems, and major fisheries.
 
 
The effect of El Niño
 
Rainfall is found in rising air over the warm waters, and the Eastern Pacific is relatively dry. In El Niño conditions, when the waters off northern Peru are warmer than normal and surface air pressure is consequently lower, the pressure difference between East and West is reduced. This makes the westward trade winds relax, and as the winds falter, the Eastern Pacific suffers a depression in the thermocline levels and an elevation in the West. Big amounts of warm, moist air rises over the Central Pacific instead of further west, stopping the monsoon rains in the region around Indonesia. The opposite effect takes place on the west coasts of North and South America with the generation of rainstorms.