El Niño & La Niña

The Walker Circulation and the Oscillation It Supports. Under average conditions, the equatorial Pacific is not thermally symmetric. Persistent easterly trade winds drag warm surface water westward, piling it into the Indo-Pacific Warm Pool near Indonesia, where sea level stands roughly half a meter higher and the surface layer is warmer than 29°C. Because that water is removed from the eastern basin, deeper water rises to replace it along the coast of Peru and Ecuador. This upwelled water is cold and saturated with nitrate and phosphate recycled from decomposing organic matter, and it fuels one of the most productive marine ecosystems on Earth. The boundary between the sun-warmed surface layer and the cold deep ocean — the thermocline — is therefore not horizontal. It lies near 150 meters in the west and shoals to within 40 meters of the surface in the east.

The atmosphere responds to that temperature contrast. Intense convection over the warm pool lifts moist air to the tropopause, releasing latent heat and producing heavy rainfall over Indonesia and northern Australia. That air travels eastward aloft, cools, and subsides over the cool eastern Pacific, where descending air suppresses cloud formation and sustains the coastal deserts of Peru and northern Chile. The sinking branch reinforces surface high pressure, which in turn strengthens the easterly trades. This closed east–west loop is the Walker Circulation, and it is a positive feedback: strong winds maintain the temperature gradient, and the temperature gradient maintains strong winds.

A positive feedback of this kind is powerful but unstable. If the trades weaken even modestly — often triggered by westerly wind bursts in the western Pacific — warm water sloshes eastward as a downwelling Kelvin wave, the thermocline in the east deepens, upwelled water arrives warm instead of cold, the temperature gradient shrinks, and the trades weaken further. The system has entered El Niño. The mirror image, in which the trades intensify and the eastern Pacific becomes anomalously cold, is La Niña. Together these swings comprise the El Niño–Southern Oscillation, or ENSO, which cycles irregularly every two to seven years and is quantified by the Oceanic Niño Index — the three-month running mean sea-surface temperature anomaly in the Niño 3.4 region. An ONI at or above +0.5°C defines El Niño conditions; an ONI at or below −0.5°C defines La Niña.

Why a Pacific Ocean Signal Controls Atlantic Storms. A tropical cyclone is a heat engine. It draws energy from sea water warmer than about 26.5°C through a depth of roughly 50 meters, converts that latent heat into the kinetic energy of rotating winds, and exhausts the spent air outward near the tropopause. The engine only functions if the vertical column stays aligned. When winds at 200 hPa blow at a very different speed or direction than winds near the surface, the storm is sheared: the warm core is displaced away from the low-level circulation, the release of latent heat no longer sits above the center of rotation, and intensification stalls. Meteorologists call this quantity vertical wind shear, and across the Atlantic Main Development Region — the belt from West Africa to the Caribbean between 10°N and 20°N — it is the single most reliable seasonal predictor of hurricane activity.

ENSO reaches the Atlantic through this variable. During El Niño, convection shifts thousands of kilometers eastward into the central Pacific. The upper-level outflow from that displaced convection accelerates westerly winds aloft over the Caribbean and tropical Atlantic, while surface winds remain easterly. The difference between the two levels can exceed 20 knots, which is enough to decapitate most developing systems. El Niño seasons therefore tend to be quiet in the Atlantic even when sea-surface temperatures there are warm. During La Niña the opposite occurs: the upper-level westerlies relax, shear over the Main Development Region can fall below 8 knots, and African easterly waves that would otherwise be torn apart are permitted to organize. The 2005, 2010, and 2020 seasons all occurred under neutral-to-La Niña conditions, and 2020 produced thirty named storms.

The Most Valuable Fish You Have Never Eaten. The Peruvian anchoveta, Engraulis ringens, is a small schooling fish that rarely exceeds twenty centimeters, yet it has supported the largest single-species fishery in human history. The reason is upwelling. Cold water rising along the Peruvian margin carries nitrate to the sunlit surface, phytoplankton bloom in extraordinary densities, and anchoveta — efficient filter feeders that convert plankton directly into flesh — occupy a very short food chain. At the fishery’s peak in 1970, Peruvian vessels landed more than twelve million metric tons in a single year, roughly one-fifth of the global marine catch. Almost none of it reached a dinner plate directly. The fish were reduced to fishmeal and fish oil, then shipped to Europe, North America, and East Asia as protein concentrate for poultry, swine, and later farmed salmon and shrimp.

Then the ocean changed. The severe El Niño of 1972–1973 deepened the thermocline off Peru, so the water pumped upward by the coastal winds came from within the warm layer rather than from the cold, nutrient-rich water below. Primary productivity collapsed, the anchoveta schools contracted into a narrow band of remaining cool water near shore, and the fleet — which had expanded to more than 1,400 vessels — concentrated its effort exactly where the fish were most vulnerable. Landings fell from 12.3 million tons to under 2 million within two years. Roughly 100,000 Peruvian workers in fishing, processing, and shipping lost their livelihoods, and export earnings that had accounted for nearly a third of the nation’s foreign exchange evaporated. Because fishmeal is a globally traded commodity, the shock did not stay in Peru: world fishmeal prices rose sharply, feed manufacturers substituted soybean meal, and soybean prices in the United States more than doubled between 1972 and 1973, raising the cost of chicken and pork for consumers who had never heard of the anchoveta.

The collapse is now understood as a compound event, not a purely natural disaster. El Niño removed the nutrient supply, but sustained overfishing had already reduced the spawning stock and eliminated the older age classes that buffer a population against a bad year. Peru responded over the following decades by building one of the most closely monitored fisheries on Earth: acoustic biomass surveys before each season, a total allowable catch reset twice a year, individual vessel quotas, and immediate closures when juvenile fish exceed a threshold in the catch. During the strong El Niño of 2015–2016 the government canceled an entire fishing season rather than allow the fleet to chase a depleted stock. The decision cost the industry an estimated one billion dollars in a single year and imposed severe hardship on coastal communities, yet the biomass recovered within two seasons. The episode illustrates a difficult principle of resource management: when a climate oscillation removes the productivity that a fishery depends on, the least damaging policy is often the one that hurts most immediately.