El Niño

El Niño is the warm oceanic and coupled atmospheric phase of the El Niño-Southern Oscillation (ENSO), a recurring interannual climate variability pattern characterized by sustained positive sea surface temperature (SST) anomalies across the central and eastern equatorial Pacific Ocean. Originally named El Niño de Navidad by nineteenth-century Peruvian mariners who observed a seasonal warm coastal current appearing around December, the term was expanded by Jacob Bjerknes in the late 1960s to denote the basin-scale ocean-atmosphere phenomenon. Within Earth System Science, El Niño represents the primary natural mechanism of interannual global heat redistribution: by releasing accumulated subsurface thermal energy from the western Pacific into the tropical atmosphere, it reorganizes global weather systems, perturbs planetary biogeochemical cycles, and alters productivity across the marine and terrestrial biosphere.

Because the tropical Pacific basin spans more than 15,000 kilometers along the equator, shifts in its surface thermal structure alter tropical deep convection and excite planetary-scale Rossby wave trains that propagate into the extratropics of both hemispheres. Events recur irregularly every two to seven years and typically persist for nine to twelve months, peaking during boreal winter (November–January) in phase-locked synchronization with the seasonal cycle.

To understand the onset and evolution of El Niño, climatologists contrast the warm phase against the neutral background state of the equatorial Pacific, maintained by the zonal overturning cell known as the Walker circulation.

Neutral State vs. Westerly Wind Bursts

Under neutral conditions, persistent easterly trade winds blow from the eastern Pacific toward the Indo-Pacific Warm Pool (110°E–160°E), piling warm surface water into the western Pacific and raising sea surface height in Indonesia 40 to 60 centimeters higher than along Ecuador and Peru. In the eastern Pacific, this westward-displaced surface layer is replaced from below via equatorial and coastal upwelling. Consequently, the thermocline resides at 150 to 200 meters depth in the western Pacific but shoals to 30 to 50 meters in the eastern Pacific, maintaining the equatorial "cold tongue" and confining deep convection to the Maritime Continent.

Bjerknes Positive Feedback and Equatorial Kelvin Waves

An El Niño episode initiates when the equatorial easterly trade winds weaken across the western and central Pacific, often triggered by Madden-Julian Oscillation (MJO) pulses and synoptic westerly wind bursts (WWBs). When westward wind stress relaxes, the warm water accumulated in the western Pacific surges eastward along the equatorial waveguide as downwelling equatorial Kelvin waves (propagating at 2.0 to 3.0 m s-1 and crossing the basin in 60 to 90 days):

1. Thermocline Depression: The eastern Pacific thermocline deepens by 50 to 100 meters, flattening the basin-wide slope and warming eastern and central Pacific SSTs by +1.0°C to +3.5°C above normal.
2. Convective Migration: As the zonal SST gradient flattens, the primary locus of tropical evaporation and latent heat release migrates eastward toward the International Date Line and the eastern equatorial Pacific.
3. Bjerknes Feedback: Eastward displacement of deep convection lowers sea-level pressure over the central and eastern Pacific while raising pressure over Indonesia and Australia. This reduced pressure gradient further weakens the easterly trade winds, exciting additional downwelling Kelvin waves and locking the Earth System into a self-reinforcing warm phase.

Eastern Pacific vs. Central Pacific Flavors

Observational analyses distinguish between two spatial patterns of El Niño warming: Eastern Pacific (EP) Canonical El Niño, where maximum SST anomalies (+2.0°C to +4.0°C) concentrate in the Niño 3 and coastal Niño 1+2 regions off South America with strong basin-wide thermocline flattening (as in 1982–1983 and 1997–1998); and Central Pacific (CP) or "El Niño Modoki," where maximum warming concentrates near the International Date Line in the Niño 4 and western Niño 3.4 regions, flanked by cooler anomalies in the far west and east (as in 1994–1995 and 2009–2010).

As deep cumulonimbus towers migrate into the central and eastern equatorial Pacific during El Niño, anomalous latent heat release warms the tropical upper troposphere (200–300 hPa), strengthening the subtropical jet streams and exciting stationary Rossby wave trains such as the positive Pacific-North American (PNA) pattern.

Regional Precipitation and Drought Regimes

• Maritime Continent, Australia, and South Asia: Subsidence along the descending branch of the shifted Walker circulation suppresses convection across Indonesia, Papua New Guinea, Malaysia, and Australia, causing meteorological drought, crop failures, and elevated peatland and forest wildfire activity, while often weakening the Indian summer southwest monsoon.
• South and Central America: Along the coastal deserts of Peru and Ecuador, warming coastal waters (Niño 1+2) destabilize the boundary layer, generating convective downpours and flash floods, while southeastern South America experiences enhanced rainfall. Conversely, Colombia, Venezuela, and the northern and central Amazon basin experience subsidence and rainfall deficits.
• North America and Africa: During boreal winter, an extended North Pacific subtropical jet stream directs extratropical storm tracks across California, the U.S. Southwest, and the Gulf Coast, while leaving the Pacific Northwest and northern Rockies warmer and drier. In Africa, El Niño induces drought across southern Africa while increasing short-rain precipitation (October–December) in equatorial East Africa.

Tropical Cyclone Modulation and Global Temperature Spikes

El Niño alters seasonal tropical cyclone activity by modifying vertical wind shear (850–200 hPa). Enhanced upper-tropospheric westerly winds outflowing from central and eastern Pacific convection propagate across the Caribbean Sea and tropical Atlantic (10°N–20°N), increasing vertical wind shear and suppressing Atlantic hurricane formation. Simultaneously, reduced wind shear and deeper upper-ocean thermal content across the eastern and central North Pacific increase hurricane and typhoon activity there. Because the ocean vents stored heat into the atmosphere via upward latent and sensible heat fluxes, global mean surface temperature typically peaks 3 to 5 months after an El Niño maximum, adding +0.10°C to +0.25°C onto the anthropogenic warming trend.

The suppression of nutrient upwelling in the ocean and the onset of drought across tropical landmasses during El Niño generate pronounced perturbations across the global carbon cycle.

Marine Trophic Collapse and Coral Bleaching

During a strong El Niño, depression of the eastern Pacific thermocline below the depth of wind-induced Ekman pumping cuts off vertical inputs of nitrate (NO3-), phosphate (PO43-), silicic acid (Si(OH)4), and dissolved iron (Fe). Chlorophyll-a concentrations drop sharply, reducing primary production by marine phytoplankton and diminishing organic carbon export via the biological pump. This deficit triggers recruitment failure and migration of forage fish such as the Peruvian anchoveta (Engraulis ringens), alongside mortality among seabirds, marine iguanas, and pinnipeds. Concurrently, prolonged thermal stress (Degree Heating Weeks > 4–8 °C-weeks) causes scleractinian corals to expel their endosymbiotic algae (Symbiodiniaceae), driving pan-tropical mass coral bleaching.

Terrestrial Carbon Fluxes vs. Oceanic Outgassing

El Niño produces the largest natural interannual spikes in the atmospheric growth rate of carbon dioxide (CO2), despite opposing oceanic and terrestrial fluxes:

1. Suppression of Equatorial Pacific Outgassing: Because the eastern equatorial Pacific normally upwells deep water supersaturated in dissolved inorganic carbon (pCO2 > 450–500 μatm), capping this upwelling during El Niño reduces oceanic CO2 efflux to the atmosphere by 0.2 to 0.5 Pg C yr-1.
2. Terrestrial Carbon Sink Reversal: However, drought and heat across the Amazon basin, Indonesia, and southern Africa reduce gross primary productivity via stomatal closure, accelerate soil respiration, and fuel biomass burning. This converts tropical terrestrial carbon sinks into net sources of 2.0 to 3.5 Pg C yr-1, driving net atmospheric CO2 growth surges exceeding 3.0 ppm yr-1.

Operational agencies monitor ENSO via the TAO/TRITON moored buoy array (~70 moorings spanning 8°N to 8°S), Argo profiling floats, and satellites. NOAA classifies El Niño events using the Oceanic Niño Index (ONI), the three-month running mean SST anomaly in the Niño 3.4 region (5°N–5°S, 170°W–120°W). An El Niño episode is declared when the ONI reaches or exceeds +0.5°C for five consecutive overlapping three-month seasons, stratified into weak (+0.5°C to +0.9°C), moderate (+1.0°C to +1.4°C), strong (+1.5°C to +1.9°C), and very strong ("Super El Niño," ≥ +2.0°C) events.

In the recharge-discharge oscillator framework (Wyrtki, 1975; Jin, 1997), trade winds during neutral and La Niña phases build up warm-water volume across the western Pacific, recharging the equatorial thermal reservoir. During El Niño, geostrophic divergence exports this warm water poleward into off-equatorial latitudes (> 5°N/S) while latent heat flux vents energy into the atmosphere—discharging equatorial heat content, shoaling the mean thermocline, and terminating the event by boreal spring.

Following neutral and weak La Niña conditions in 2024–2025, Kelvin wave activity and westerly wind bursts warmed the tropical Pacific in early 2026. NOAA declared El Niño conditions on 11 June 2026; by September 2026, weekly Niño 3.4 SST anomalies reached +1.8°C (exceeding +3.0°C near South America), with ensembles projecting a strong winter peak (> +2.0°C):

• Atlantic Hurricanes: Stronger vertical wind shear suppressed the 2026 Atlantic hurricane season; eight named storms formed through September, but none reached hurricane strength.
• Hydroclimatic and Carbon Impacts: Analogous events indicate drought risk in Indonesia, Australia, southern Africa, and the northern Amazon, heavy rain in coastal Ecuador and Peru, and elevated atmospheric CO2 growth from reduced tropical vegetation uptake.
• Global Temperature: Because global surface temperatures lag tropical Pacific warming by 3 to 5 months, the peak warming contribution is projected for 2027, adding to long-term warming from greenhouse gases.

Why do El Niño events typically peak in December and January?

El Niño is phase-locked to the seasonal cycle. During boreal autumn (September–November), the eastern Pacific cold tongue is well developed and the mean thermocline is shallow, maximizing the Bjerknes feedback. By boreal spring (March–May), solar heating warms the eastern Pacific and the ITCZ nears the equator, weakening the zonal SST gradient and creating a "spring predictability barrier" during which events decay or initiate.

Does El Niño increase or decrease ocean CO₂ emissions?

El Niño decreases direct CO2 outgassing from the equatorial Pacific by 0.2 to 0.5 Pg C yr-1 because the deepened eastern thermocline blocks upwelling of carbon-rich deep water. However, total atmospheric CO2 rises faster during El Niño years because tropical land droughts and wildfires release 2.0 to 3.5 Pg C yr-1 from the terrestrial biosphere.

What is the difference between a canonical Eastern Pacific El Niño and El Niño Modoki?

A canonical Eastern Pacific (EP) El Niño features maximum SST anomalies off South America (Niño 1+2 and Niño 3) and basin-wide thermocline flattening. A Central Pacific (CP) El Niño ("El Niño Modoki") concentrates warming near the International Date Line (Niño 4 and western Niño 3.4), flanked by cooler anomalies in the far west and east.

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