West Coast ocean heat risk is duration as El Niño strengthens
The risk to the U.S. West Coast's marine life this winter may be how long the ocean stays warm. In an assessment updated September 25, NOAA scientists say El Niño-related warmth is likely to reach coastal waters as an existing marine heatwave fades. That sequence could prolong the period of stress for the California Current ecosystem even if the peak temperature does not climb much higher.
The starting conditions matter. NOAA has tracked a marine heatwave called NEP25A along parts of the central and southern California coast since May 2025. A separate heatwave, NEP26A, is still far offshore and has not reached the coast. NOAA explicitly says neither current heatwave was directly caused by El Niño. The added concern is that El Niño-driven coastal warming could follow the heat already present.
The broader climate forecast is unusually strong, but it is still a forecast. The National Weather Service's September 10 ENSO discussion gives a greater than 90% chance of a very strong El Niño during Northern Hemisphere fall and winter 2026–27. It separately gives a 75% chance that the October–December three-month measure will exceed previous events in the record dating to 1950. The official probability table describes ocean-index strength; it does not assign a 90% chance to any particular fishery closure, storm, or local water temperature.
Warmth can outlast a single heatwave
NOAA's Physical Sciences Laboratory calls its marine heatwave outlook experimental guidance for research. Its September forecast puts the likelihood of marine heatwaves off California and Baja California at 80–100% through the end of 2026. The California Current assessment warns that continued warmth can compress habitat and disrupt the cold, nutrient-rich upwelling on which much of the region's marine food web depends. Researchers name the duration of elevated temperature as the leading risk to living marine resources; they do not say the worst historic ecological effects are already happening.
The distinction between a signal and an outcome is visible in NOAA's observations. Its September assessment says current nutrient-supply indices are around average to slightly below average for the time of year, and that recent fisheries surveys are too early for quantitative conclusions. The agency lists possible effects seen in earlier warm periods—including changes in krill, squid, kelp, and harmful algal blooms—as scenarios to watch, not confirmed results of this winter's event.
What comes next is observable. NOAA says it is monitoring coastal temperatures, upwelling, surveys, and reports from fishing crews. The next monthly ENSO diagnostic discussion is scheduled for October 8. Those observations will show whether El Niño-driven water actually extends the existing heat stress, and which species and fisheries feel it. A high-confidence climate forecast is a reason to watch closely, not a substitute for those measurements.