El Niño is the warm phase of the El Niño-Southern Oscillation (ENSO), a natural climate cycle in the tropical Pacific. In normal years, trade winds push warm surface water westward toward Indonesia and Australia, while cooler water upwells along the South American coast. During an El Niño, these winds weaken or reverse. Warm water spreads eastward, the thermocline deepens in the east, and the Walker circulation shifts dramatically. The result is global weather disruptions: increased rainfall in parts of South America and the southern United States, droughts in Australia, Indonesia, and parts of Africa, altered monsoon patterns, and changes in hurricane activity.
Strength is primarily measured by the Oceanic Niño Index (ONI)—the three-month running mean of sea-surface temperature anomalies in the Niño 3.4 region (5°N–5°S, 170°–120°W). Values of +0.5 °C or higher define El Niño conditions; +1.5 °C or higher is strong; and +2.0 °C or higher is classified as “very strong” or a Super El Niño. Complementary indices (MEI, RONI, Niño-1+2, etc.) help capture differences in the spatial pattern of warming, which influence teleconnections.
Which Was the Strongest?
In the modern instrumental record (roughly since 1950), three events stand out:
- 2015–16, with a peak ONI near 2.6 °C,
- 1997–98, around 2.3–2.4 °C, and
- 1982–83, about 2.1–2.2 °C.
These are the strongest Super El Niños of the satellite and buoy era. The 2023–24 event peaked near 2.0 °C and ranked lower.
Most historical and paleoclimate reconstructions point to the 1877–1878 El Niño as the strongest on record. Reconstructed sea-surface temperature anomalies in the eastern and central Pacific often reach or exceed modern records (estimates commonly range from roughly 2.8–3.5 °C or higher, though uncertainty is larger due to sparse observations). Its global impacts were catastrophic. Linked droughts and crop failures contributed to the Great Famine in India, the Northern Chinese Famine, and the Grande Seca in Brazil, with total excess deaths exceeding 50 million—about 3–4 % of the estimated world population at the time. Many experts regard it as one of the worst environmental disasters in human history.
As of mid-August 2026, a new Super El Niño is rapidly intensifying. Weekly anomalies in key Niño regions already reach +1.8 °C to over +3 °C in places. Models and NOAA outlooks assign a very high probability (often exceeding 90 % in recent guidance) that the peak will clearly surpass previous instrumental records, potentially reaching 3.5 °C or higher. If realized, 2026–27 would become the strongest El Niño of the instrumental era and a strong contender for the strongest of all time.
Typical and Documented Impacts on the United States
Strong to very strong El Niños shift and strengthen the subtropical jet stream southward. This produces a more southern storm track. Classic winter-half-year (October–May) patterns include:
- Southern United States and California: Above-average precipitation. Southern California, Arizona, New Mexico, Texas, and the Gulf Coast through Florida tend to be wetter. Flood, mudslide, and river-flood risk rises substantially.
- Northern United States and Midwest: Milder winters with reduced snowfall and fewer cold outbreaks. The northern storm track is suppressed.
- Pacific Northwest: Often drier than average.
- Atlantic hurricane season: Usually below-average activity due to increased wind shear.
- Temperatures: Warmer across much of the North and interior; the South can be cooler due to increased cloudiness and rainfall.
1877–1878 in the United States
This event produced the famous “Year without a Winter.” In the upper Midwest and parts of Canada, winter temperatures ran 8–14 °C above the long-term average—still among the warmest winters in the instrumental record for Minnesota and surrounding areas. Farmers were plowing and planting as early as February in some places. Parts of the West and Southeast saw unusually heavy rain. Warm, wet conditions in the South have been linked to the severe 1878 yellow-fever epidemic (especially in Memphis), which killed thousands. The mild weather disrupted transportation (mud instead of snow) and traditional agricultural calendars.
1997–98 — the modern benchmark
California experienced one of its wettest winters on record, with severe flooding and damages estimated at $2–3 billion in the state alone. The South and Southeast were also wet, and tornado outbreaks occurred. The North was unusually mild and snow-poor. Economically, the U.S. balance was surprisingly positive: roughly $4 billion in direct damages were outweighed by about $19 billion in benefits, mainly from sharply lower heating costs, reduced snow-removal expenses, fewer winter transportation disruptions, and a quiet Atlantic hurricane season. About 189 deaths were recorded, while an estimated 850 lives were saved by the mild winter weather.
2015–16
This event produced the warmest winter on record for the contiguous United States at the time. Much of the North and Midwest was markedly milder. California, however, received far less rain than expected based on 1997–98 analogs, so the multi-year drought was only partially relieved. The Pacific Northwest was wetter in places, while the Southwest and parts of the Southern Plains were drier than the classic El Niño pattern. The more central-Pacific focus of the warming and other concurrent atmospheric patterns (including a persistent ridge off the West Coast) produced these deviations.
Implications for Extreme Events, Including the Developing 2026 Episode
No two El Niños are identical. The precise location and intensity of the warm-water anomaly, background warming from climate change, the phase of the Madden-Julian Oscillation, and other large-scale patterns all modulate the outcomes. A record-strength event on the scale of 1877–78 or a potentially even stronger 2026–27 event substantially raises the odds of the classic signals: elevated flood risk in California and the South, very mild winters in the North, higher coastal water levels and erosion risk along the West Coast (from thermal expansion plus stronger storms), and agricultural disruptions (delayed planting in the South, damage to winter vegetables, mixed effects farther north).
Economic impacts are mixed—large costs from flooding and infrastructure damage offset in part by energy and snow-removal savings. In a warmer climate, some extremes are amplified (higher baseline sea levels plus El Niño-driven water-level increases; hotter background temperatures). At the same time, modern early-warning systems and infrastructure provide far greater adaptive capacity than existed in 1877–78.
The 1877–1878 event remains the historical benchmark for global destructive power. Modern Super El Niños and the developing 2026 episode demonstrate that extreme events remain possible today, with their clearest U.S. fingerprints appearing in winter-half-year precipitation and temperature patterns. The precise regional expression always depends on the details of ocean–atmosphere coupling.
