The ocean's engine falters
The coast of Chile is dominated by the Humboldt Current, a cold, north-flowing ocean current that sustains one of the planet's most productive marine ecosystems. This system accounts for up to 20% of the world's total marine fish catch. The cause of this productivity is a process called upwelling, where winds push surface water offshore, allowing deep, cold, nutrient-rich water to rise to the surface. This deep water fertilizes vast blooms of phytoplankton, which in turn support enormous populations of small pelagic fish, most notably the Peruvian anchoveta (Engraulis ringens).
Every two to seven years, this system is radically disrupted by the El Niño-Southern Oscillation (ENSO). During an El Niño event, trade winds weaken, allowing a massive volume of warm water from the western Pacific to move eastward across the ocean in vast, deep pulses called Kelvin waves. When these waves reach the South American coast, they effectively place a cap of warm, nutrient-poor water on the ocean. Sea surface temperatures off the coast can rise by 4°C to 8°C above normal. This stops the important upwelling process, starving the phytoplankton.
The ecological effects are catastrophic. Without their primary food source, anchoveta populations crash. During the major El Niño of 1972-73, the Peruvian anchoveta fishery, which had been yielding over 12 million metric tons per year, collapsed. The powerful 1997-98 El Niño caused a similar collapse, with fisheries in northern Chile and Peru seeing their catch reduced to almost zero. The economic impact on coastal communities is severe, leading to massive layoffs in the fishing industry. It was precisely these recurring, devastating declines in fish catches, first noticed by fishermen in the 1800s, that provided the earliest clues to this massive climate pattern.
A desert drowned in water
The effects of El Niño are not confined to the ocean. The phenomenon dramatically alters atmospheric circulation, bringing extreme weather to otherwise stable climates. Northern Chile hosts the Atacama Desert, the driest non-polar desert on Earth. Some weather stations there have never recorded rain, and the regional average is less than four millimeters per year. The desert's extreme aridity is partly due to the cooling effect of the cold Humboldt Current, which prevents the formation of rain clouds.
During a strong El Niño, the warm ocean water offshore injects enormous amounts of heat and moisture into the atmosphere. This can lead to unusually intense rainfall in the hyper-arid Atacama. In March 2015, one such event dumped the equivalent of seven years' worth of rain on the region in a single night. The effects are twofold. The sudden deluges cause destructive flash floods and landslides, known locally as aluviones, which have devastated towns and infrastructure.
Paradoxically, the rains also trigger a spectacular event known as the desierto florido, or flowering desert. The desert floor, which appears barren, holds dormant seeds of annual plants. The rare, heavy rains trigger mass germination, and within weeks, vast stretches of the normally beige desert are covered in a carpet of colorful flowers. This biological response reveals the hidden life waiting for the rare arrival of water, a direct and beautiful consequence of the same atmospheric shifts that cause such devastation elsewhere.