Water in the South Pacific tends to circulate counter-clockwise, driven by trade winds and the Coriolis effect. The Humboldt Current, consisting of cold water from the Antarctic, flows north along the coast of South America. This brings nutrients to the surface, making phytoplankton thrive, and so also the fish that feed on them. It also cools down the air, reducing its capacity to hold moisture, and so dries out the coast from northern Chile up to southern Ecuador. Very little rain falls in this area - Lima is the second driest capital city in the world, after Cairo - and many plants survive only because of dew forming at night.
This counter-clockwise flow means that there’s a gradient in air pressure and water temperature across the Pacific, with the east normally cold with high pressure, and the west warm with low pressure. This varies throughout the year, with the east usually warmest around Christmas, which led fishermen to call this El Niño, in another example of the enormous influence of the Catholic Church in this part of the world. The flow of cold water will sometimes diminish or stop for months at a time, bringing hot moist air, and lots of rain, to the coast in what, once it’s lasted for long enough, is called an El Niño Event. The diminished Humboldt Current also means less food for phytoplankton, which can have serious effects on fish stocks.
So, if you happen to live, say on the north coast of Peru, in a house of adobe bricks, and eat fish and whatever you can carefully grow, an El Niño event can be pretty catastrophic. Your fields are waterlogged, your home is being washed away, and there aren’t any fish to catch.
The picture above is part of a (restored) wall at Chan Chan. A smaller adjacent wall has the same pattern with the fish going in the opposite direction, north to south. This, we were told, shows that the Chimú understood that the Humboldt Current usually brings fish from the south, but sometimes El Niño reverses the flow.