Nov 8, 2025
Are the Mass Strandings of Velella velella Linked to El Niño?
What they may be telling us about our changing climate and oceans In late April, thousands of jellyfish-like sea creatures washed up on the coast close to the Golden Gate Bridge in San Francisco. They are called Velella velella, or "by-the-wind sailors," and they have been seen along the Pacific Coast from Santa Barbara to San Francisco, Stinson Beach and north to Oregon.
The El Niño Connection: What Science Says
In recent years, two different phenomena have warmed the oceans. One is El Niño, a weather pattern that originates in the equatorial Pacific and often brings dry weather to the northern United States and wet weather to the southern United States. The other is a marine heat wave, which is brought on by the formation and persistence of a warm water cap.
University of Washington marine biologist Julia Parrish analyzed 20 years of citizen science data to examine the spatio-temporal patterns of mass strandings of V. velella along the Pacific Northwest coast, from Washington to northern California, USA, and found one consistent trigger: warm winters produce Velella springs. When sea surface temperatures run 2°C above average in winter, populations boom offshore. Weakened trade winds, the hallmark of El Niño, then create onshore winds that sweep them to shore.
Mass strandings were recorded in 14 of the study years, with particularly extensive events occurring during 2003-2006 and 2014-2019. These events predominantly took place in spring, often synchronizing in April, and were closely associated with shifts in prevailing onshore winds. Autumn strandings were relatively rare and showed no consistent seasonal pattern. There is a reason these strandings happen almost exclusively in spring. Every year from March through July, prevailing northerly winds push warm surface water away from the California coast, allowing cold, deep water to rise and replace it. This process is called upwelling. Deep, cold ocean water rises towards the surface, bringing lots of nutrients, which brings the phytoplankton and zooplankton. Velella feed on the zooplankton and fish eggs that upwelling produces. Raphael Kudela, professor of ocean science at the University of California, Santa Cruz, explains the connection directly: spring upwelling brings nutrients to the surface, which supports phytoplankton and zooplankton, feeding the very ecosystem that sustains these creatures and drives their population growth offshore.
The link between Velella and El Niño is not limited to California. During 2017 and 2018, scientists recorded the first-ever appearance of Velella velella in the Galápagos Archipelago in Ecuador during a period of elevated sea surface temperatures, an event researchers linked to El Niño–Southern Oscillation events.
Historical records stretching back more than a century show that the densest Velella strandings tend to cluster around El Niño years, when warmer-than-normal surface waters extend across the Pacific.
With the National Oceanic and Atmospheric Administration (NOAA) placing a 61% probability on El Niño emerging between May and July 2026 and the European Centre for Medium-Range Weather Forecasts (ECMWF) forecasting an 80% chance of a strong event with a 22% chance of a “super” El Niño, conditions are set to intensify the environmental drivers already producing the 2026 strandings. The late March 2026 Oregon strandings and the late April 2026 San Francisco events occurring simultaneously across the coast suggest that the pre-El Niño warming of Pacific sea surface temperatures is already producing the ecological conditions that generate these events.
However, despite more than a century of records, the precise mechanisms driving Velella population booms remain incompletely understood.
The Bottom Line
Evidence suggests a strong association between Velella velella mass strandings and El Niño-related ocean warming, particularly through the combined effects of elevated sea surface temperatures and shifting wind patterns. With a high-confidence El Niño forecast for 2026 and ocean surface temperatures already elevated, the conditions that generate Velella strandings are likely to intensify. However, without more research and concrete proof, scientists can’t determine with certainty what’s driving the surge in Velella strandings.
References
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[7] Carrera, M., Trujillo, J., & Brandt, M. (2019). First record of a by-the-wind-sailor (Velella velella Linnaeus, 1758) in the Galápagos Archipelago - Ecuador. Biodiversity Data Journal, 7, e35303. https://doi.org/10.3897/bdj.7.e35303
[8] Callahan, M. (2024, March 25). Weird, jellyfish-like creatures – Velella velella or ‘by-the-wind sailors’ – litter North Coast beaches. The Press Democrat. https://www.pressdemocrat.com/2024/03/25/weird-jellyfish-like-creatures-velella-velella-or-by-the-wind-sailors-litter-north-coast-beaches/
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