El Niño Is Raising Seas Before the Storms Even Arrive

Sea levels in parts of the eastern Pacific are already up to 40 centimetres above normal as El Niño strengthens, increasing coastal flood risks, according to New Scientist. ^[1]

The October 19 assessment, attributed to atmospheric scientist Paul Roundy of the University at Albany, describes a rise already under way. Roundy said strong bursts of westerly winds could push water levels higher over the next few weeks. ^[1]

A higher sea gives high tides, storm surges and large waves a higher starting point. California has already experienced coastal flooding attributed to El Niño, according to the state's September emergency proclamation, and its State Lands Commission warns of risks to beaches, homes, ports and transport routes. ^[3][4]

The US National Oceanic and Atmospheric Administration's October 8 outlook said El Niño continued to strengthen and was likely to remain strong to very strong through January-March 2027. It put the probability of reaching its historical-strength threshold during October-December at 83%. ^[2]

The Daily Inference reported on October 9 that Chinese monitoring data had put the Pacific warming event above the centre's "super" threshold in September, while NOAA independently identified a strengthening El Niño. The rising sea levels now add a coastal hazard to the developing climate event. ^[1]

The ocean is shifting east

El Niño weakens the trade winds that usually push warm surface water west across the tropical Pacific. As those winds weaken, warm water can move east. Bursts of winds blowing in the opposite direction can generate Kelvin waves, broad disturbances that travel across the ocean and then along coasts, raising sea levels. ^[1]

These are not single breaking waves that arrive and disappear. Their effect is a sustained change in the height of the sea. They take about 60 days to cross the Pacific, and the sea-level response can last for months, according to New Scientist's account of the process. ^[1]

That gives wind events far offshore a route to coastal consequences weeks later. A beach or road can face higher water before a local storm arrives, leaving less room for the extra water and waves that accompany bad weather.

The California State Lands Commission says coastal Kelvin waves are already raising baseline sea levels along the state's coast by six to 12 inches, roughly 15 to 30 centimetres. It identifies flooding and erosion as threats to coastal communities and infrastructure. ^[4]

The temporary rise also sits on a higher long-term ocean. Global mean sea level has risen by roughly 20 centimetres since 1900 because of global warming, according to New Scientist. The El Niño-driven shift adds a further, shorter-lived source of risk. ^[1]

Stronger can mean disproportionately higher

Research by Thomas Monahan and colleagues, described by New Scientist, suggests that sea-level changes become amplified during strong El Niño events. The relationship does not follow a simple straight line in which each increase in the event's strength produces an equal increase in water height. ^[1]

Using past events, the researchers estimated a rise of 24 to 40 centimetres in the Galapagos, depending on how strong this El Niño becomes. Their estimates for Peru and Ecuador were 19 to 33 centimetres. For California, they were nine to 13 centimetres. ^[1]

Those regional estimates are projections based on historical events, separate from Roundy's assessment of current conditions. They point to why further strengthening matters: a more powerful event could produce a disproportionately large coastal sea-level response, rather than a modest step up. ^[1]

The measurement sites behind the reported current maximum of 40 centimetres have not been specified, and Monahan's study is available here only through New Scientist's description, which does not identify its sample size, publication venue or peer-review status. NOAA also cautions that a stronger El Niño makes the expected pattern of impacts more likely without guaranteeing it. ^[1][2]

For coastal planning, the regional differences matter. A Pacific-wide maximum cannot tell a particular town how much water to expect at its waterfront. Local tides, storm conditions and the height of the sea when those events arrive will help determine the flooding. ^[1][4]

The dangerous overlap

California faces several possible additions to the El Niño-driven rise. Long-term lunar cycles could add around five centimetres to sea level there, while storms can produce larger surges and waves, according to New Scientist. ^[1]

The highest tides associated with those lunar cycles are expected during Thanksgiving week and Christmas week. Monahan said those periods coincide with heavy travel and more people being present, adding the potential for disruption during already busy weeks. ^[1]

Daniel Swain of the California Institute for Water Resources said frequent coastal inundation could become severe or record-breaking in some places, with disruption and possible damage. ^[1]

El Niño can also deepen the low-pressure area near the Aleutian Islands and increase storm activity, raising the potential for storm surges. During the 2015-16 El Niño, waves exceeding 15 metres were recorded in Oregon, an example of the wave conditions that can accompany a strong event. ^[1]

California Governor Gavin Newsom proclaimed a state of emergency on September 21 to accelerate preparations. The proclamation directs agencies to coordinate flood-risk reduction and stage supplies ahead of potential impacts, including coastal flooding. ^[3]

The next few weeks will test whether further westerly wind bursts lift sea levels again, as Roundy warned they could. After that, the immediate coastal danger turns on timing: whether elevated water persists when the late-year high tides arrive, and whether storms arrive with them. ^[1]