History

How unusual is 2026–27?

Start with this event, then open the historical log behind the instruments. The same rule applies throughout: a similar past reading is context—not a prediction of the same weather or damage.

CURRENT EVENT · 130 YEARS OF CONTEXT · LIKE-FOR-LIKE RONI ONLY

This event

Where 2026–27 sits

Observed and forecast kept separate

Along the bottom

“Start” is the first three-month period at or above +0.5°C. Each step is one month later.

1991–922015–161997–981982–83

Timeline

How humans learned to watch the Pacific

Observation → instrument → today

Compare with nowAdd today’s official reading and each milestone’s comparison limits

Use the same index, season, and baseline for a like-for-like comparison. RONI, ONI, and weekly Niño 3.4 are not interchangeable. Similar peaks do not promise similar evolution, rainfall, impacts, or damage.

1890sdiscovery
What happened
Peruvian observers document unusual warm years
People along the coast had long known the seasonal warm current. Geographers began separating especially strong, disruptive warm years from that normal cycle.
What humans learned
A recurring ocean change could be recognized by the people whose work depended on it.
Why it matters now
The name El Niño came from lived Pacific experience before it became a global climate term.
SOURCE · NOAA AOML · El Niño FAQ ↗LIVE CONNECTION · Read: What is El Niño? →
ThenA local name enters science
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare descriptions of unusual coastal warmth and disruption.
Do not infer: Do not turn narrative reports into a modern temperature index or event ranking.
1920s–30sdiscovery
What happened
Gilbert Walker identifies a pressure seesaw
Walker found that air pressure and climate conditions shifted together across the tropical Pacific. He called the pattern the Southern Oscillation.
What humans learned
The signal was not confined to coastal water; the atmosphere was moving in a Pacific-wide pattern.
Why it matters now
El Niño is not ocean warmth alone; the atmosphere has to be part of the story.
SOURCE · NOAA AOML · El Niño FAQ ↗LIVE CONNECTION · Open: Southern Oscillation guide →
ThenThe atmosphere leaves a signal
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare the direction of the Pacific pressure pattern.
Do not infer: Do not compare early pressure records with today's SOI without matching stations, periods, and methods.
~1960discovery
What happened
Jacob Bjerknes connects the ocean and sky
Bjerknes linked El Niño warmth with Walker's atmospheric pressure pattern and showed that the tropical Pacific behaves as one coupled system.
What humans learned
The warm ocean and the atmospheric pressure seesaw are connected parts of the same system.
Why it matters now
This explains how unusual warmth in one ocean basin can reorganize winds, storms, and weather patterns far away.
SOURCE · NOAA AOML · El Niño FAQ ↗LIVE CONNECTION · Read: Ocean and atmosphere coupling →
ThenTwo clues become one system
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare whether warmth and atmospheric changes span the tropical Pacific.
Do not infer: Do not treat one coastal reading as the strength of the whole event.
1970sdiscovery
What happened
Ocean and atmosphere research becomes ENSO science
Researchers increasingly studied trade winds, thermocline depth, equatorial waves, warm-water movement, and atmospheric feedback as parts of one phenomenon: ENSO.
What humans learned
Winds, stored ocean heat, waves, and storms can reinforce one another and shape an event's evolution.
Why it matters now
The modern Engine page follows this same idea: separate instruments watching connected parts of one machine.
SOURCE · NOAA AOML · El Niño FAQ ↗LIVE CONNECTION · Open: Coupled Pacific explainer →
ThenThe physical engine comes into focus
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare whether surface warmth, trade winds, and tropical convection move together.
Do not infer: The mechanism does not guarantee the timing, peak strength, or local impacts of an event.
1982–83event
What happened
The observing system misses a powerful El Niño
The event was neither predicted nor recognized until it was close to its peak. Sparse real-time Pacific observations left scientists behind the change.
What humans learned
Knowing the mechanism was not enough; scientists needed continuous measurements across the tropical Pacific.
Why it matters now
The failure made continuous ocean measurements and better seasonal prediction an urgent priority.
SOURCE · NOAA PMEL · Building the TAO array ↗LIVE CONNECTION · Compare: 1982–83 record →
Then1982–83 peak +2.40°C RONI
Now+0.98°C RONI · MJJ 2026
Safe comparison: Use one reconstructed index, baseline, and three-month window for both events.
Do not infer: Similar peak ocean values do not mean the atmosphere, evolution, or impacts will match.
1985–94observing system
What happened
Scientists build the Tropical Atmosphere Ocean array
A basin-wide network of moored buoys begins reporting winds, pressure, surface water, and temperatures below the surface in near real time.
What humans learned
A wired ocean could reveal changes below the surface and in the winds before the full event was obvious.
Why it matters now
Scientists could watch the coupled Pacific develop instead of reconstructing it after the fact.
SOURCE · NOAA PMEL · Pacific Ocean TAO ↗LIVE CONNECTION · Open: Pacific Engine →
ThenThe Pacific gains a live sensor network
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare which ocean depths, winds, and locations were observable in each era.
Do not infer: Better coverage can make later events look more detailed without making them physically stronger.
1986discovery
What happened
A dynamical model shows the ocean has memory
Cane and Zebiak demonstrated that large-scale Pacific ocean dynamics could support seasonal ENSO forecasts instead of recognition only after an event had begun.
What humans learned
The ocean stores heat and therefore carries some memory of where the coupled system may move next.
Why it matters now
Heat stored below the surface can provide an early clue—but 1997 later showed that models could still miss the speed and scale of an event.
SOURCE · NOAA PMEL · Forecasting the 1997–98 El Niño ↗LIVE CONNECTION · Open: Forecast range →
ThenPrediction becomes a scientific project
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare what observations and models knew before each event developed.
Do not infer: A successful method in one period does not guarantee an accurate forecast of a later event.
1992observing system
What happened
TOPEX/Poseidon begins mapping Pacific sea level
Satellite altimetry made it possible to map sea-surface height across the basin. Those changes reveal large-scale waves and shifts in heat stored in the upper ocean.
What humans learned
Sea-level patterns can reveal moving upper-ocean heat across places where no buoy is sitting.
Why it matters now
Scientists were no longer limited to scattered points; they could watch a Pacific-wide event organize and move.
SOURCE · NASA Earth Observatory · Eye on the Ocean ↗LIVE CONNECTION · Open: 12-month Pacific evolution →
ThenThe ocean is measured from space
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare the direction and basin-wide shape of changing upper-ocean heat.
Do not infer: Sea-surface height and temperature indices are related evidence, not interchangeable measurements.
1997–98event
What happened
The full ENSO observing system captures a major El Niño
Buoys and satellites followed weakening winds, eastward-moving warm water, a deepening eastern thermocline, and record eastern-Pacific warmth as the event grew.
What humans learned
A major event could finally be watched as a sequence—but its speed and peak could still surprise forecast models.
Why it matters now
For the first time, a major El Niño could be studied across the ocean surface, below it, and through the atmosphere from start to finish.
SOURCE · NOAA PMEL · The 1997–98 El Niño ↗LIVE CONNECTION · Compare: 1997–98 by calendar →
Then1997–98 peak +2.28°C RONI
Now+0.98°C RONI · MJJ 2026
Safe comparison: Compare the same index, season, and event phase; then compare the shape of the Pacific separately.
Do not infer: A matching peak does not imply matching rainfall, damage, or regional outcomes.
2026standard
What happened
NOAA adopts the Relative Oceanic Niño Index
RONI compares Niño 3.4 warmth with average tropical-ocean warmth, then uses a three-month mean. NOAA adopted it for operational ENSO monitoring and prediction.
What humans learned
The ruler itself must adapt when the wider tropical ocean is warming, while still preserving like-for-like history.
Why it matters now
The relative measure aims to keep the Pacific contrast tied more closely to the atmospheric response as the wider tropics change.
SOURCE · NOAA CPC · RONI announcement ↗LIVE CONNECTION · Open: RONI historical record →
ThenThe official yardstick changes
Now+0.98°C RONI · MJJ 2026
Safe comparison: Use NOAA's RONI series for both past and current three-month seasons.
Do not infer: Do not mix RONI with ONI or weekly Niño 3.4 values, and do not treat provisional recent values as final.

Past events

Strongest observed RONI events

1950—present · calculated from source data

011982–83+2.40°C

021997–98+2.28°C

032015–16+2.25°C

041991–92+2.12°C

051965–66+1.99°C

061972–73+1.97°C

071957–58+1.89°C

082009–10+1.50°C

091986–87+1.46°C

102023–24+1.42°C

How to read this

These are observed three-month RONI peaks, not damage rankings. An event with a similar peak can have a different shape, season, atmospheric response, and regional outcome.

How RONI works →

Loading the latest Pacific readings…

Where 2026–27 sits

How humans learned to watch the Pacific

Peruvian observers document unusual warm years

Gilbert Walker identifies a pressure seesaw

Jacob Bjerknes connects the ocean and sky

Ocean and atmosphere research becomes ENSO science

The observing system misses a powerful El Niño

Scientists build the Tropical Atmosphere Ocean array

A dynamical model shows the ocean has memory

TOPEX/Poseidon begins mapping Pacific sea level

The full ENSO observing system captures a major El Niño

NOAA adopts the Relative Oceanic Niño Index

Strongest observed RONI events

Where 2026–27 sits

How humans learned to watch the Pacific

Peruvian observers document unusual warm years

Gilbert Walker identifies a pressure seesaw

Jacob Bjerknes connects the ocean and sky

Ocean and atmosphere research becomes ENSO science

The observing system misses a powerful El Niño

Scientists build the Tropical Atmosphere Ocean array

A dynamical model shows the ocean has memory

TOPEX/Poseidon begins mapping Pacific sea level

The full ENSO observing system captures a major El Niño

NOAA adopts the Relative Oceanic Niño Index

Strongest observed RONI events