The El Nino and La Nina are oceanic phenomenon driven by atmospheric circulation. It is caused by the Walker Circulation.
The Walker circulation is a convective cycle/cell that owes its origin to the gradient of sea surface temperatures along the Equator in the Pacific Ocean.
The Walker circulation is driven by major east-west pressure gradients set up by differences between air rising over heated continents and the warmer parts of the oceans, on the one hand, while air subsiding over continental areas where deep high pressure systems have become established, and in association with subtropical high pressure cells, on the other.
The walker cell was discovered by G.T. Walker, a British meteorologist, who studied these phenomena and named it as Southern Oscillation. The average period of this oscillation is 3 years, but it ranges from 2 to 10 years.
When it was discovered that El Nino seemed to be related to the Southern Oscillation, the occurrences began to be referred to as El Nino Southern Oscillation (ENSO) events.
Near the South American coast, the winds blow offshore, blowing the surface water westward away from the adjacent continental margins. To replace the blown away surface water deep cool water replaces the blown away surface water.
This is called as upwelling. The air above this cool water is stabilised, convection, suppressed and thus, it cannot rise and join the normal meridional circulation.
Instead it flows westwards where it flows as South East Trade winds across the South Pacific to warm the western Pacific where it gains moisture and are heated. It then rises and flows eastwards to complete the cell.
Each year in October, however, the Trade winds slacken and the upwelling is reduced. The warm tropical surface water that was blown up against the western margin begins to flow back eastwards across the ocean’s eastern boundary, it further suppresses the normal upwelling along the coast.
The weakening of the trades is related to the movement of the Indo-Australian low-pressure cell to the east, beginning in October and November. Concurrent with the eastward shift of the Indo-Australian low pressure cell is the movement of the Inter Tropical Convergence Zone (ITCZ) southwards.
The ITCZ’s normal seasonal migration is from 10°N latitude in August to 3°N in February, but during El Nino Southern Oscillation (ENSO) events it may move south of the Equator in eastern Pacific. This shift causes the weakening of the Trade winds.
The warm surface waters remain off the South and Central American coast for an entire year or longer. Upwelling continues to be suppressed and reduced by the overriding warm surface water and the failure of the Trade winds to blow it away. The effect is called as El Nino (meaning the “Christ Child” in Spanish).
This appearance causes the air pressure to drop over large areas of the southeast Pacific while pressure rises over western Pacific. This see-saw variation of air pressure is called the Southern Oscillation.
The shift in pressure pattern causes the Equatorial Easterlies to diminish and to eventually reverse direction. Westerly winds then drag warm surface water towards the east.
When the warm water reaches the South American coast, it is deflected southward along the coastlines of, Peru and Ecuador!
This weakens the Walker circulation but an enhanced Hadley circulation which again increases the Trade winds and keeps the warm water from flowing across in the eastern Pacific, thus, ending El Nino.
When the colder water is re-established in the eastern Pacific, the Hadley cell weakens and conditions are set up for the return of the warm water currents.