The prevailing trade winds, which blow towards the equator from an easterly direction, force the ocean currents to move in approximately the same direction. Trade ships crossing the Atlantic in the 16th century learnt to exploit this.
Portuguese sailors soon discovered that squeezed within this general westward drift of water was a long thin band of water moving eastwards. This equatorial counter-current, found at the 5-10º northern latitudes, made it easier for ships that had delivered slaves to America to return to Africa for the next transport.
In the 1820s another equatorial counter-current was found in the tropical parts of the Pacific, covering the whole ocean, from the Philippines to Colombia.
The warm water hill
The explanation for this unexpected phenomenon lies in the relative warmth of the counter-currents. When the easterly trade winds drive the warm surface water westwards a kind of “hill” of warm water is created.
The prevailing trade winds provide the resistance to stop the warm water sliding “downhill”. But there is a narrow gap in the dominant easterly wind, where the trades from both hemispheres meet in the inter-tropical convergence zone. The winds there are weak and changeable and therefore the water can flow eastwards freely “downhill”.
This is not the end of the explanation, because for reasons not quite clear, parts of the warm water hill over the western Pacific “break off” with a frequency of 4-8 years, and glide eastwards.
The Coriolis effect makes the currents just north of the equator bear to the right, towards the equator, and the currents just south of the equator turn to the left, towards the equator. While a giant hill of water slowly moves eastwards it is held together by Coriolis forces in what is know as an equatorial Kelvin wave.
The water hill often reaches the opposite side of the Pacific around Christmas, giving it the name El Niño (“Christ child”).
Consequences of El Niño
In a normal situation with westerly currents at the equator, the Coriolis force causes the surface water at the equator in the eastern Pacific to divide, which is the opposite to the way the Kelvin wave is held together.
Therefore, cold, nutrient-rich water, full of fish, upwells to the surface. The switch to El Niño with warm but nutrient-deficient water has serious consequences. The situation becomes a crisis for people and birds caused by the lack of fish, and the weather changes drastically.
While there is usually a drought along the coast in northern Chile, Peru, and southern Ecuador, El Niño causes significant convection leading to downpours with damaging consequences.