Dear Swapnil,
In 1500s fishermen along Peru’scoastline were shocked to see millions of dead seabirds washed up on the beach. The birds had died because of a decline in the fish population which was their main food source. About the same time sailors noted changes in ocean currents and wind patterns. Since these abnormal conditions reached a peak around December the Peruvians called it El Nino meaning ‘little boy’ in honour of the infant Jesus. An El Nino starts every three to seven years when a vast pool of warm water moves eastward in the tropical Pacific Ocean affecting air circulation and rainfall patterns over the world.

Normal year
Under normal conditions there is a high-pressure condition over eastern Pacific Ocean and a low-pressure condition exists over western Pacific. This makes Trade Winds blow westwards from the high-pressure area to the low pressure one. The winds also push seawater west towards Australia and Indonesia. Sea surface is actually two feet higher around Indonesia than it is near the South American coast. The water that blows westward is replaced by cold water from deeper levels and a vibrant ecosystem thrives on this cold, nutrient-laden seawater that rises to the surface along the South American coastline. So the cool sea surface feeds a vibrant fishery in Peru and Ecuador. In contrast the sea surface near Indonesia (western Pacific) is hot and as water evaporates from the ocean, it creates clouds and brings normal rains to surrounding regions.

El Nino year
During an El Nino, the high and low-pressure conditions weaken and Trade Winds weaken too. This allows warm seawater around Indonesia to surge towards South America as the ocean sloshes east-wards, seeking to level itself out. The upward movement of cold water along South America stops and the fish die in huge numbers because they don’t get the supply of nutritious, cold water. This is often the most visible sign of El Nino.

Once the warm waters move eastward, the winds and currents change. This causes weather upheavals around the world bringing rains to the normally dry American coast and droughts to other parts. The El Nino in 1982-83, for instance, caused droughts in southern Africa, India, Indonesia and southern Peru. On the other hand heavy rains flooded Ecuador, Northern Peru and Cuba while hurricanes lashed Tahiti and Hawaii.

Global warming
El Nino years tend to be warmer than usual and may lead to a global heat wave. But the reverse can happen too. Scientists explain that global warming could intensify El Nino if warm temperatures increased evaporation from land. The extra moisture in the air could strengthen storms and floods associated with El Nino.