El Nino and La Nina

Normally, trade winds blow east to west across the Pacific, piling warm surface water up near Australia and Indonesia. That warm pool drives rising air and heavy rain there. Off South America, those same winds push surface water away from the coast, so cold, nutrient-rich water wells up from below to replace it, feeding huge fisheries off Peru. In an El Nino, the trade winds weaken or even reverse. Without them holding the warm pool in place, it sloshes eastward across the Pacific toward South America. Rain-producing rising air follows the warm water, so Peru and Ecuador get flooding while Australia and Indonesia, now sitting under cooler water, dry out. The eastward warm water also deepens the thermocline off South America, burying the cold nutrient-rich water too deep for upwelling to reach, and the fishery collapses.

La Nina is the mirror image. Trade winds strengthen beyond their average, piling even more warm water in the western Pacific and pulling more surface water away from South America. Upwelling intensifies, fisheries boom, and the thermocline off South America stays shallow. Australia and Indonesia get heavier rain, while South America tends toward drought.

This topic ties together the wind patterns, ocean circulation, and nutrient upwelling covered earlier in the unit, showing how a single shift in trade wind strength cascades through atmosphere, ocean, and food webs at once. It also sets up how these cycles affect global climate variability and resource availability, since one Pacific event can ripple into weather and economies worldwide.