What Is: El Niño, La Niña, and a Climate in Flux (Revised)

Scientific Frontline: Extended "At a Glance" Summary: El Niño-Southern Oscillation

The Core Concept: The El Niño-Southern Oscillation is the planet's most consequential mode of interannual climate variability, functioning as a coupled ocean-atmosphere cycle that alternates between warming (El Niño) and cooling (La Niña) phases. Driven by anthropogenic global warming, this historically natural cycle is fundamentally restructuring planetary atmospheric circulation and establishing unprecedented baselines for global weather extremes.

Key Distinction/Mechanism: The oscillation is governed by the Walker Circulation and the Bjerknes feedback loop, where shifts in equatorial trade winds alter oceanic thermocline depth and sea surface temperatures. The phenomenon manifests in two distinct typologies: Canonical (Eastern Pacific) events, which are driven by vertical thermocline displacement, and Modoki (Central Pacific) events, which are governed by horizontal advective currents and produce distinctly different global teleconnections.

Origin/History: Significant historical benchmarks include the 1997 "El Niño of the century" and the powerful 2015–2016 event. The unprecedented 2023–2024 El Niño, formally declared on July 4, 2023, shattered global ocean heat content records, prompting meteorological institutions, including the National Oceanic and Atmospheric Administration, to formally transition to the Relative Oceanic Niño Index in early 2026 to correct for systemic baseline drift caused by global warming.

Major Frameworks/Components:

• Relative Oceanic Niño Index: A modernized climatological metric that subtracts the global tropical sea surface temperature anomaly from the Niño 3.4 region to isolate relative warming and eliminate anthropogenic baseline drift.
• Walker Circulation: A massive east-west overturning atmospheric circulation cell spanning the tropical Pacific that drives deep atmospheric convection and regulates equatorial surface winds.
• Bjerknes Feedback: A highly sensitive, non-linear positive feedback loop where weakened trade winds cause eastern Pacific warming, which subsequently weakens atmospheric pressure gradients and further collapses the trade winds.
• Subsurface Wave Dynamics: Equatorial Kelvin and Rossby waves that dictate the delayed negative thermodynamic feedback strictly required to progress and terminate ENSO phases.
• Atmospheric Teleconnections: Large-scale atmospheric Rossby waves (planetary waves) that transport tropical thermal energy to the extratropics, heavily governed by the Clausius-Clapeyron relation.

Branch of Science: Climatology, Atmospheric Science, Oceanography, and Marine Biology.

Future Application: Advanced empirical orthogonal function analyses and climate models will utilize the Relative Oceanic Niño Index to generate highly precise predictive models of climate extremes. This physical understanding is critical for forecasting macroeconomic teleconnections, managing global agricultural resilience, and mitigating catastrophic ecological events driven by shifting thermodynamic realities.

Why It Matters: The restructuring of this ocean-atmosphere engine triggers severe socioeconomic and material consequences, inflicting multibillion-dollar agricultural shocks and amplifying deadly weather extremes. As global ocean heat content continuously breaches physiological limits, future anomalous warming phases threaten to serve as extinction-level filters, driving widespread trophic collapse and mass mortality for keystone species such as the Peruvian anchoveta (Engraulis ringens) and foundational coral endosymbionts.

As part of the ongoing "What Is" educational series from Scientific Frontline, this comprehensively revised analysis explores the planet's most consequential and dominant mode of interannual climate variability. In 1997, a climatic event of unprecedented scale began to unfold in the tropical Pacific Ocean. Dubbed the "El Niño of the century," it triggered a cascade of extreme weather that reshaped global patterns for over a year, unleashing devastating floods, sparking massive forest fires, and decimating marine ecosystems. Nearly two decades later, the powerful 2015–2016 El Niño, supercharged by a background of long-term global warming, helped propel 2016 to become the hottest year on record.

Historically, the scientific community has conceptualized the El Niño-Southern Oscillation as a naturally occurring, irregular cyclical alternation between a warming phase (El Niño) and a cooling phase (La Niña). However, the accelerating realities of anthropogenic climate change demand a critical reappraisal. The unprecedented 2023–2024 El Niño event shattered global ocean heat content records and directly triggered a global mass coral bleaching event. By fundamentally restructuring planetary atmospheric circulation, altering marine and terrestrial ecosystems, and driving extreme weather events, this coupled oscillation dictates the baseline rhythm of our global weather. This report delivers an exhaustive synthesis of the structural mechanisms governing the cycle and how a warming global background state is fundamentally altering the planet's most critical climate driver.

The 2023–2024 Event: A Historical Paradigm Shift

The 2023–2024 El Niño event fundamentally redefined contemporary meteorological expectations and established a new, elevated baseline for climate extremes. Formally declared by the World Meteorological Organization on July 4, 2023, and persisting until its dissipation in April 2024, the event ranks as the fifth-most powerful El Niño-Southern Oscillation event in recorded history. However, its interaction with the rapidly warming background state of the global oceans and atmosphere elevated its impacts to historically unprecedented levels. The sheer volume of thermal energy absorbed and subsequently released by the Pacific Ocean during this period overwhelmed historical statistical bounds.

• Global Temperature Anomalies and Heat Content
• Propelled by the synergistic forces of a strong El Niño and continuous anthropogenic greenhouse gas forcing, global surface temperatures in 2024 reached a staggering 1.55 degrees Celsius above the pre-industrial average.
• This unprecedented temperature spike cemented 2024 as the hottest year on record globally, temporarily pushing the planet beyond the critical 1.5 degrees Celsius warming threshold outlined in international climate agreements.
• The upper 2,000 meters of the global ocean absorbed record-breaking amounts of thermal energy. From 2023 to 2024, the ocean heat content from zero to 2,000 meters increased by an estimated 16 plus or minus 8 ZettaJoules.
• This astronomical rate of oceanic warming is thermodynamically equivalent to approximately 1.5 Watts per square meter of heat applied continuously over the entirety of the global ocean surface.
• Sea Surface Temperature Records
• Global sea surface temperatures maintained an unbroken, record-shattering streak for fifteen consecutive months, stretching from April 2023 through June 2024.
• The annual mean sea surface temperature for 2024 ultimately settled at 0.61 degrees Celsius above the 1981–2010 baseline, surpassing the previous record set during the peak of the 2023 anomalous warming phase.
• Subsurface ocean temperatures in the central and eastern Pacific were recorded at more than 8 degrees Celsius above average during the developmental phase of the event, providing immense thermal support for the sustained atmospheric coupling.
• Socioeconomic and Material Toll
• The meteorological extremes driven by the atmospheric reorganization—including severe, multi-season droughts, torrential flooding, and lethal heatwaves—resulted in catastrophic global economic damages.
• Financial assessments estimate the total global damage inflicted by the 2023–2024 event at 103.3 billion United States dollars, reflecting massive agricultural losses, infrastructure destruction, and disruptions to global supply chains.

Methodological Evolution: The Relative Oceanic Niño Index

For decades, the standard climatological metric for identifying, tracking, and measuring the intensity of El Niño and La Niña events has been the Oceanic Niño Index. The Oceanic Niño Index calculates the three-month running average of sea surface temperature anomalies specifically within the Niño 3.4 region, a designated sector of the central equatorial Pacific Ocean bounded by 5 degrees North to 5 degrees South latitude, and 120 degrees West to 170 degrees West longitude. Historically, when this regional anomaly reached or exceeded positive 0.5 degrees Celsius for five consecutive overlapping seasons, an El Niño was declared; conversely, a sustained anomaly of negative 0.5 degrees Celsius or lower indicated a La Niña.

However, the continuous, anthropogenic warming of the global climate system introduced a profound, systematic bias into the traditional calculation. Because the entire global ocean is warming simultaneously, absolute sea surface temperature anomalies in the Niño 3.4 region are artificially inflated by the rising background temperature. This phenomenon, known as baseline drift, occurs even when the specific ocean-atmosphere dynamical coupling characteristic of a true El Niño is absent or remarkably weak. Consequently, the traditional Oceanic Niño Index systematically overestimated the strength and frequency of El Niño events while disproportionately masking the presence and intensity of La Niña events in the modern era.

To resolve this critical failure and restore the physical validity of oceanic monitoring, the National Oceanic and Atmospheric Administration, in concert with the broader climate science community, formally transitioned to the Relative Oceanic Niño Index in early 2026.

• The Mechanics of the Relative Index: The Relative Oceanic Niño Index functions by subtracting the average sea surface temperature anomaly of the entire global tropics (defined as the latitude band between 20 degrees North and 20 degrees South) from the temperature anomaly observed strictly within the Niño 3.4 region.
• Physical and Atmospheric Justification: By calculating this difference, the new index isolates the relative warming or cooling of the central Pacific against the broader, warming tropical baseline. Near the equator, the Coriolis effect is negligible, meaning horizontal temperature gradients in the free troposphere are small. The tropical atmosphere, therefore, responds to relative temperature differences across the ocean surface rather than absolute temperatures. Deep atmospheric convection is strictly driven by the temperature gradient between the central Pacific and the surrounding tropical oceans.
• Standardization: A mathematical scaling factor is applied to the resulting subtracted difference to ensure that the Relative Oceanic Niño Index shares the same statistical variance as the historical index. This allows meteorological institutions to maintain the traditional positive and negative 0.5 degrees Celsius thresholds for declaring phases, completely removing the confounding influence of the global warming trend.

Manifestations of Diversity: Canonical versus Modoki Flavors

The El Niño-Southern Oscillation is not a monolithic, uniform phenomenon. Advanced empirical orthogonal function analyses, coupled with decades of continuous remote sensing and subsurface buoy arrays, have delineated two distinct physical "flavors" or typologies of El Niño.

• Eastern Pacific (Canonical) El Niño:
• This flavor represents the traditional, historically dominant manifestation, characterized by maximum anomalous warming concentrated in the eastern equatorial Pacific, frequently extending to the coastline of South America.
• Eastern Pacific events are primarily driven by thermocline feedbacks. The initial relaxation of the trade winds allows the oceanic thermocline to deepen in the east, which strongly amplifies sea surface warming by cutting off the upwelling of cold, deep water.
• Dynamically, these events induce a strong Pacific-North American teleconnection pattern in the atmosphere, producing robust secondary warming in the tropical North Atlantic due to anomalously weak surface winds reducing evaporative cooling.
• Central Pacific (Modoki) El Niño:
• In the Modoki variation, the maximum sea surface temperature warming is strictly confined to the central equatorial Pacific, near the International Date Line. This central warm anomaly is distinctly flanked by cooler-than-average waters in both the extreme eastern and western Pacific basins.
• Central Pacific events are largely governed by zonal advective processes—the horizontal movement of warm water by anomalous surface currents—rather than the vertical displacement of the thermocline.
• These events produce remarkably distinct extratropical teleconnections, heavily correlated with an increased frequency of severe droughts in the United States, and crucially fail to produce the subsequent warming in the tropical North Atlantic.

Since the late twentieth century, observational datasets indicate a marked, statistically significant increase in the frequency of Central Pacific Modoki events relative to traditional Eastern Pacific events. Climate models utilizing advanced Coupled Model Intercomparison Project scenarios suggest that this shift is a direct response to anthropogenic forcing, potentially signaling a permanent, structural shift in the planet's primary climate driver.

Mechanism of Action: The Ocean-Atmosphere Engine

Image Credit: Scientific Frontline / NOAA

The Walker Circulation and Equatorial Dynamics

The fundamental thermodynamic engine driving the oscillation is the Walker Circulation, a massive east-west overturning atmospheric circulation cell that spans the tropical Pacific Ocean.

• The Neutral State: Under normal climatological conditions, strong easterly trade winds consistently blow across the Pacific from the Americas toward Asia, physically pushing and pooling warm surface water in the western Pacific. This immense pool of thermal energy drives deep, explosive atmospheric convection over Indonesia and Australasia. The heated, moisture-laden air rises rapidly into the upper troposphere, travels eastward against the surface flow, and eventually cools and subsides over the cooler waters of the eastern Pacific. This subsidence maintains a dominant high-pressure system off the coast of South America, resulting in generally dry terrestrial conditions and reinforcing the easterly surface winds, completing the circulation loop.
• The El Niño Reorganization: A perturbation in the pressure gradient causes the trade winds to weaken or reverse.
• The massive volume of warm water sloshes eastward across the equatorial Pacific as downwelling Kelvin waves, dragging the primary zone of deep atmospheric convection with it.
• This violently reverses normal regional pressure gradients, causing anomalously low pressure and flooding over the normally dry eastern Pacific, and crippling drought over the normally wet western Pacific.
• The La Niña Amplification (Cold Phase): La Niña is a powerful amplification of the neutral state. The easterly trade winds strengthen to a force greater than usual, pushing even more warm surface water toward the far western Pacific. In the eastern Pacific, this intensified westward flow drives an enhancement of upwelling, pulling greater volumes of cold, deep water to the surface. The thermocline becomes steeply tilted, shoaling dramatically in the east. The Walker Circulation goes into overdrive, bringing intense convection and rainfall over Indonesia and the Philippines, while the eastern Pacific becomes even colder and drier.

The Bjerknes Feedback

• The maturation of an El Niño event is governed by a highly sensitive, non-linear positive feedback loop.
• An initial relaxation of easterly trade winds reduces the upwelling of cold water in the eastern Pacific.
• The eastern Pacific warms, weakening the east-west sea surface temperature gradient.
• This directly results in a weaker atmospheric pressure gradient, which causes the trade winds to weaken even further. This mutually reinforcing loop rapidly amplifies the warming anomaly.

Subsurface Ocean Dynamics: Kelvin and Rossby Waves

The feedback loop is ultimately broken by delayed oceanic negative feedbacks that propagate beneath the surface.

• When the easterly trade winds collapse, massive downwelling equatorial Kelvin waves are generated. Trapped by the Coriolis effect, these subsurface waves travel eastward directly along the equator at speeds of roughly two to three meters per second, physically carrying the deep layer of warm water from the western warm pool toward South America. Upon colliding with the eastern boundary, these waves violently depress the thermocline, shutting off nutrient upwelling.
• Concurrently, upwelling equatorial Rossby waves are generated and propagate much more slowly westward across the basin. These slow-moving waves eventually reflect off the convoluted western boundary of the Pacific Ocean (the Maritime Continent). Following reflection, they travel back eastward as upwelling Kelvin waves.
• This complex wave reflection carries the delayed negative thermodynamic feedback strictly required to eventually terminate the El Niño event. The arrival of these upwelling waves forces the shoaling, or shallowing, of the eastern thermocline, cooling the surface and breaking the Bjerknes feedback loop.

Characteristics of ENSO Phases

• ENSO-Neutral Phase
• Relative Oceanic Niño Index: Between -0.5°C and +0.5°C.
• Eastern Pacific Sea Surface Temperature: Average (cool relative to the west).
• Trade Winds: Normal (strong easterlies).
• South American Upwelling: Normal.
• Thermocline Tilt: Moderate (shallow in the east, deep in the west).
• Walker Circulation: Normal strength.
• Primary Rainfall Zone: Western Pacific.
• Southern Oscillation Index: Near zero.
• El Niño (Warm Phase)
• Relative Oceanic Niño Index: Above +0.5°C.
• Eastern Pacific Sea Surface Temperature: Warmer than average.
• Trade Winds: Weaker than average (or reversed).
• South American Upwelling: Weakened or suppressed.
• Thermocline Tilt: Flattened (deep in the east).
• Walker Circulation: Weakened or reversed.
• Primary Rainfall Zone: Central and Eastern Pacific.
• Southern Oscillation Index: Negative.
• La Niña (Cold Phase)
• Relative Oceanic Niño Index: Below -0.5°C.
• Eastern Pacific Sea Surface Temperature: Colder than average.
• Trade Winds: Stronger than average.
• South American Upwelling: Enhanced.
• Thermocline Tilt: Steep.
• Walker Circulation: Strengthened.
• Primary Rainfall Zone: Far Western Pacific (Indonesia).
• Southern Oscillation Index: Positive.

Teleconnections: Bridging the Tropics and Extratropics

Through an intricate meteorological process known as teleconnection, the massive release of latent heat from shifting equatorial precipitation triggers vast atmospheric Rossby waves that alter weather patterns globally.

Atmospheric Rivers: During strong El Niño events, an accelerated North Pacific Jet Stream provides a highly efficient track for atmospheric rivers to impact the western coast of North America.

Thermodynamic Amplification: The severity of these storms is intrinsically linked to anthropogenic global warming via the Clausius-Clapeyron relation. This principle dictates that atmospheric moisture capacity expands by approximately 7 percent for every 1 degree Celsius of ambient warming, supplying immense, unprecedented volumes of evaporated water vapor to these weather systems.

Planetary Wave Propagation

The primary physical vectors for these long-distance teleconnections are atmospheric Rossby waves, also known as planetary waves.

• Anomalous, intense tropical convection acts as a massive thermal forcing mechanism, generating Rossby waves that propagate poleward and eastward out of the tropics into the higher latitudes.
• As these planetary waves travel through the atmosphere, they are often captured, accelerated, and guided by the high-altitude polar and subtropical jet streams. The jet streams function as atmospheric "waveguides," dictating the ultimate trajectory of the Rossby waves.
• The peaks and troughs of these transient and stationary waves directly alter the geopotential height fields over the mid-latitudes, setting up semi-permanent ridges of high pressure and deep troughs of low pressure thousands of miles away from the initial equatorial heat source.
• Interactions with the Indian Ocean Dipole: The global climatic influence is frequently modified by regional modes of climate variability. During the 2023–2024 event, the strong Pacific El Niño coupled with a strong positive phase of the Indian Ocean Dipole. Because both systems promote severe subsidence and high pressure over Indonesia and Australia, their concurrent manifestation created a powerful synergistic compounding effect, exacerbating severe droughts across Southeast Asia and amplifying the failure of the Indian monsoon.

Key Global Consequences of ENSO Events

The far-reaching atmospheric teleconnections generate profound, cascading impacts across global ecosystems and human economies.

• Marine Ecosystem Collapse and Coral Bleaching
• The extreme thermal stress generated by severe El Niño events drives catastrophic biological consequences. During the historic 2023–2024 event, extreme heat stress impacted a staggering 84.4 percent of the world's coral reef area, triggering the fourth global mass coral bleaching event.
• The physiological breakdown occurs when heat stress damages the photosynthetic machinery of the Symbiodiniaceae dinoflagellates, causing an overproduction of toxic reactive oxygen species.
• To prevent fatal cellular toxicity, the coral host is forced to expel these vital endosymbionts. Deprived of up to 90 percent of their daily energetic requirements, the corals face rapid starvation, elevated susceptibility to disease, and mass mortality.
• Upwelling Disruptions and Fishery Failures
• The oceanic nutrient factory of the Humboldt Current system is fundamentally dismantled during a strong El Niño. As the oceanic thermocline deepens against the South American coast, upwelled water is drawn exclusively from a warm, nutrient-depleted surface layer rather than the nutrient-dense deep ocean.
• This nutrient cessation triggers a devastating trophic collapse. Keystone species like the Peruvian anchoveta (Engraulis ringens) experience severe phenological mismatches; larvae hatch into an ocean completely devoid of phytoplankton, leading to near-total larval mortality and destabilizing regional macroeconomic systems.
• Global Agricultural Shocks and Inflationary Pressures
• Massive atmospheric restructuring induces multi-season droughts and highly destructive torrential rains across the tropics and subtropics, translating directly into global agricultural shortfalls.
• Extreme rainfall anomalies followed by intense heat in West Africa decimated cocoa yields, surging global prices by 250 percent, while Southeast Asian coffee harvests faced similar meteorological disruptions.
• As extreme droughts impact major grain and sugar regions, reactionary protectionist market behaviors compound global inflationary pressures and severely threaten food security in developing nations.
• Amplified Weather Extremes and Tropical Cyclones
• ENSO dictates hurricane frequency across dual basins. During El Niño, increased vertical wind shear over the tropical Atlantic creates an unfavorable environment, suppressing hurricane formation. Simultaneously, reduced wind shear in the central and eastern Pacific leads to a highly active Pacific hurricane season.
• During La Niña, the pattern violently reverses. Reduced vertical wind shear across the Atlantic basin provides a highly conducive environment for tropical cyclone formation, resulting in an enhanced Atlantic hurricane season with a greater likelihood of landfalling major hurricanes.

Regional Winter Impacts of ENSO

During a typical winter, when the phenomenon's impacts are most pronounced, the shifting jet stream orchestrates a consistent set of probabilistic weather anomalies around the globe.

• United States Southeast:
• El Niño Winter: Cooler and wetter.
• La Niña Winter: Warmer and drier.
• United States Pacific Northwest:
• El Niño Winter: Warmer and drier.
• La Niña Winter: Cooler and wetter.
• Peru and Ecuador:
• El Niño Winter: Warmer and much wetter (flooding).
• La Niña Winter: Cooler and drier (drought).
• Australia and Indonesia:
• El Niño Winter: Warmer and drier (drought and wildfires).
• La Niña Winter: Cooler and wetter (flooding).
• Southern Africa:
• El Niño Winter: Warmer and drier.
• La Niña Winter: Cooler and wetter.
• India:
• El Niño Winter: Weaker monsoon (drier).
• La Niña Winter: Stronger monsoon (wetter).

Conclusion

The evolution, dynamics, and impacts of the El Niño-Southern Oscillation can no longer be scientifically evaluated or modeled in a vacuum of natural, unforced cyclical variability. As decisively demonstrated by the unprecedented meteorological and oceanographic metrics of the 2023–2024 event, aggressive anthropogenic climate forcing has fundamentally and irreversibly altered the thermal baseline upon which this oscillation operates. The systemic methodological transition from the traditional Oceanic Niño Index to the Relative Oceanic Niño Index represents a formal, necessary acknowledgment by the global scientific community that absolute temperature measurements in the Pacific basin are now irrevocably tainted by the background reality of global ocean boiling.

Simultaneously, the physical and spatial manifestations of the phenomenon are shifting in alarming ways. The increasing, statistically significant prevalence of Central Pacific Modoki events relative to historical Canonical events indicates deep, structural changes in equatorial thermocline dynamics and zonal atmospheric advection. Furthermore, the inescapable thermodynamics of the Clausius-Clapeyron relation guarantees that the atmospheric teleconnections of any future El Niño phase will continue to scale upward in their destructive potential. Biologically, the consequences are existential. The rapidly narrowing thermal tolerance windows of critical, foundational marine organisms suggest that future El Niño events will increasingly serve as extinction-level evolutionary filters. As ocean heat content continues to rise, El Niño anomalies will consistently breach physiological limits, driving severe phenological mismatches, catastrophic coral bleaching, and wholesale trophic collapse across the global oceans.

Final Thoughts

The explicit mandate of modern climatology, oceanography, and environmental science is to move beyond the mere observation and statistical cataloging of cyclical climate patterns, and toward a granular, physically robust understanding of how those natural patterns are currently being weaponized by a rapidly warming atmosphere. The analysis presented herein makes it unequivocally evident that the El Niño-Southern Oscillation is no longer functioning merely as a driver of natural climate variability; rather, it has become a powerful magnifying glass for the broader, anthropogenic climate crisis. Achieving a deep, predictive understanding of its shifting thermodynamics, its non-linear and devastating biological impacts, and its profound macroeconomic teleconnections is not merely an academic exercise, but a prerequisite for ensuring global societal and ecological resilience in the twenty-first century.

Be well,
Heidi-Ann Fourkiller

Research Links Scientific Frontline: 

• Scientists Calculate What Could Throw El Niño Out of Balance
• El Niño ‘flavors’ help unravel past variability, future response to climate change
• Mekong Delta will continue to be at risk for severe flooding
• Climate and human land use both play roles in Pacific island wildfires past and present
• La Niña winters could keep on coming
• Potentially hazardous La Niña weather more common, lasting longer

Source/Credit: Scientific Frontline

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Reference Number: wi101225_01 (revised 09/11/2026)

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