Projected 2026–27 super El Niño could raise east Pacific extreme sea-level risk about tenfold

Why it matters

The authors argue that surface warming alone does not tell you the coastal flood hazard from El Niño. The state of the equatorial ocean and the phase of long-period tide cycles also matter. Adding these to seasonal ENSO forecasts could give earlier warning of coastal extremes. The authors say the 2026–27 background shift alone equals decades of projected sea-level rise.

What they did

The authors combined a tide-gauge sea-level reconstruction, a global tide and surge reanalysis, and ERA5 waves at 3,197 coastal sites. They treated each high water as a trial and compared the chance of exceeding the neutral-month 99th-percentile level in El Niño months with neutral months. They split the change into background sea level and tide–surge shares, and measured how often waves coincide with extremes. They then projected 2026–27 from the link between background sea level and the RONI index, adding astronomical tide cycles and a range of equatorial ocean states.

Key findings

• A typical El Niño raises risk along the eastern Pacific by lifting background sea level about 5 cm; the regional median risk ratio is 2.33 along Mexico–Ecuador, and risk falls around the Maritime Continent (median 0.53).
• Background sea level drives most of the change in the tropical east Pacific (share 0.99 at Mexico–Ecuador). Tides and surges matter more at higher latitudes, such as Alaska–California (background share 0.71).
• Along Mexico–Ecuador the background response steepens for strong events, from 4.2 cm per °C below +1.5 °C to 11.2 cm per °C above it.
• The central 2026–27 projection for Mexico–Ecuador is a risk ratio of 12.9, which moves the neutral 99th-percentile high water from once every 52 days to every 4.0 days. Peru reaches a central estimate of 21.2.
• With a purely linear response, the central Mexico–Ecuador risk ratio falls from 12.9 to 7.9. The 2026–27 background shift equals 40 years of baseline sea-level rise under SSP1-2.6 at Mexico–Ecuador.

Limitations

• This is a preprint that has not been peer reviewed. The central and high 2026–27 scenarios go beyond the largest satellite-era event, so those values are extrapolations, and the steepening rests on few strong events and its form is not firmly fixed.
• Surges and waves are left out of the projection, so risk is likely underestimated where tides and surges carry more of the change. The tide–surge reanalysis also underestimates tropical cyclone extremes.
• The sea-level-rise equivalence leaves out vertical land motion, so relative sea-level rise at a given coast could differ substantially.

Glossary

• RONI: Relative Oceanic Niño Index, a sea surface temperature index for ENSO strength measured against the tropical mean.
• Risk ratio: The chance that a high water exceeds the neutral-month 99th-percentile level during El Niño, divided by the same chance in neutral conditions.
• SSH tilt: The difference in sea-surface height between the eastern and western equatorial Pacific, linked to thermocline depth.
• Shapley value: A method that splits a total change fairly between contributors, here background sea level and tide–surge.