What is El Niño?
El Niño Southern Oscillation (ENSO) is a large-scale ocean-atmosphere interaction in the tropical Pacific Ocean. ENSO consists of oscillations between a warm phase (El Niño) and a cool phase (La Niña). Each phase typically lasts from nine months to a year, and occurs every three to seven years. The exact timing of ENSO events is difficult to predict more than a few months in advance, and we still don’t really know what factors might trigger El Niño and La Niña events.
El Niño gets its start in ocean dynamics. The graphic below depicts normal conditions in the Pacific. Deep ocean currents push water toward South America and cause cool water to well up. The temperature difference between the cool upwelling in the east and warm pool in the west drives the surface trade winds that flow from the Americas to Australia. This causes rainy conditions in the tropical western Pacific.
Normal conditions in the Pacific.
During El Niño events, the deep, underlying ocean currents that push water toward South America and cause cool upwelling stop flowing. There are still many unknowns about what causes the currents to flow in the neutral phase but stop during El Niño events. When the upwelling stops, warm water distributes itself throughout the Pacific, and without the temperature differential, the trade winds reverse direction. This oscillation inverts the system and tends to bring rain to the eastern Pacific along the coast of Peru and Ecuador in the Americas and causes drying in Australia and Southeast Asia.
El Niño conditions in the Pacific
When an El Niño event occurs, it changes weather patterns throughout the tropics and has cascading effects throughout the sub-tropics and even into the mid-latitudes. During very strong El Niño events, this can result in drought or reduced monsoon rains in the Indian sub-continent and flooding in South America and the central United States.
How does El Niño affect global temperatures?
El Niño years are usually accompanied by elevated global atmospheric temperatures.
Some recent studies have characterized the ENSO phenomenon in terms of shifting heat between the ocean and the atmosphere. This research suggests that the deep ocean absorbs heat from the atmosphere during neutral and cool La Niña years and releases it during El Niño years.
In absence of global warming, this dynamic would result in variable temperatures without a steady trend upward or downward. In fact, scientists can estimate what this would look like, using something called the Multivariate ENSO Index (MEI). If ENSO were the only factor affecting global temperatures, temperatures would jump during El Niño events, but then return to a stable baseline instead of remaining high. Multiple studies have shown that ENSO has had a negligible long-term effect on global temperatures over the past few decades.
But because the planet is experiencing long-term warming, this dynamic is occurring on top of a baseline of increasing temperatures. In real life this results in more of a “stair-climbing” temperature pattern. On a decadal scale, each of the past three decades has been successively warmer than any on record, and collectively the past 30-year period is the warmest in 1400 years. Due to human influence, modern La Niña events now are warmer than El Niño events in the 1970s and 1980s.
Can global climate models predict El Niño?
The full answer is: not exactly. Global climate models take the effect of ENSO events into account in the aggregate, but they cannot predict the timing of individual events. This can cause short-term discrepancies between climate models and observed temperatures. But the models are designed to work over longer timescales, and on the century scale these discrepancies even one another out.
Most recently, some have questioned the recent moderate slowdown in the rate of global surface warming and pointed out a divergence between observations and climate models. Changes in heat exchange with the ocean due to ENSO variability likely plays a role in the trend, which is predicted to change when recent La Niña-like conditions dissipate.
Is the forcefulness of El Niño itself changing?
El Niño events have not significantly influenced long-term temperature trends, but the events themselves may be in the process of becoming more intense. One study of coral growth rings found that El Niño events of the past few decades are more variable and more intense than the norm established over the past 7,000 years.
While the study did not establish a causal link between climate change and intensifying El Niño events, the trend makes sense under the framework suggesting that El Niño discharges heat absorbed during neutral years. In a warmer world there is more heat being trapped every year, so there is more energy for El Niño events to work with. And climate projections suggest that this may continue. A recent study finds human-induced global warming will double the number of extreme ENSO events, raising concern that disruptions from El Niño will worsen in a warmer world.
Is a strong El Niño on the way?
Recent reporting has cited official NOAA Climate Prediction Center estimates of a greater than 50% chance of a return of El Niño this summer. NOAA bases this prediction on a set of models that show the probability of an “ENSO-Neutral” event dropping below 50% by this summer while the probability of an El Niño event rising past 50% at the same time. The models also show the probability for an El Niño event increasing steadily through the end of the year.
Most scientists are now saying that the main question isn’t whether an El Niño will arrive, but how strong it will be. Even a fairly moderate El Niño could break temperature records around the world.