Exercise 2 solution

During an El Nino: the trade winds weaken (and may even reverse in places); warm water that was piled up in the west spreads eastward, so sea surface temperature in the eastern Pacific rises well above normal; the upwelling of cold, nutrient-rich water off Peru weakens or shuts down, so plankton and anchovy catches collapse, and heavy rain can fall on the normally dry coast.

In a La Nina all of these reverse: the trade winds strengthen, the eastern Pacific becomes cooler than normal, and upwelling and fishing are strong (with drier conditions on the coast).