Explanation: Warming and cooling of the Pacific Ocean is most important in terms of general atmospheric circulation.

• The warm water of the central Pacific Ocean slowly drifts towards South American coast and replaces the cool Peruvian current.
• Such appearance of warm water off the coast of Peru is known as the El Nino.
• The El Nino event is closely associated with the pressure changes in the Central Pacific and Australia. This change in pressure condition over Pacific is known as the southern oscillation.
• The combined phenomenon of southern oscillation and El Nino is known as ENSO. In the years when the ENSO is strong, large-scale variations in weather occur over the world.

Source: NCERT – India Physical Environment