This ocean phenomenon raises the sea level, and are common during El Niño events. This year’s super El Niño could make them even more intense
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This month, sea levels along the USA’s West Coast – particularly California – are expected to rise by up to 0.3 metres (one foot). This is due to an ocean phenomenon: Kelvin waves. But what exactly are they, and what impacts could they have?
Explaining Kelvin waves
Kelvin waves are extremely long ocean waves that propagate eastward toward the coast of South America. They cause the upper ocean layer of warm water to thicken, raising sea level.
They aren’t curling, crashing waves like those seen at a beach. Instead, they are a ‘disturbance inside the ocean’ – similar to waves in a bathtub, making slow, sloshing movements beneath the surface. While they don’t break, they still have peaks and valleys that change the water depth.
They have been happening for thousands of years, but because sea levels have been rising in recent years, the effects of Kelvin waves can be more pronounced.
Over the next week, the latest Kelvin wave surge will move along the California coast at a speed of around 241 kilometres (150 miles) per day, with experts predicting the waves will continue to pass through from October until February.
These waves, which can be up to hundreds of kilometres long, move at around five miles per hour. Importantly, they are not similar to tsunamis, which are instead caused by sudden movements like underwater earthquakes or landslides, and travel at hundreds of miles per hour.
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The increase in water levels caused by Kelvin waves is only temporary – lasting for several months at most – but can still cause impacts: added water levels can worsen coastal flooding and erosion if it overlaps with high tides, large surf or a coastal storm.
‘If you start with sea levels that are six inches higher than normal, when you have really high tides, big storms or storm surges, the water levels are now six inches higher,’ said research oceanographer with National Oceanic and Atmospheric Administration Fisheries’ Ecosystem Science Division Michael Jacox. ‘The effects can be amplified.’
El Niño and Kelvin waves
‘Kelvin waves are a symptom of El Niño,’ said Dillon Amaya, a professor at North Carolina State University. In a single El Niño year, up to six Kelvin waves can occur. This year, the first Kelvin wave moved up the coast in May and June.
Normally, trade winds push warm surface water westward across the equatorial Pacific. In an El Niño, these winds weaken or reverse, allowing warm water to move eastward and causing Kelvin waves to form deep in the Pacific Ocean.
This year’s El Niño is expected to be the largest in at least 1,000 years, meaning the Kelvin waves that accompany them could be more intense.
Higher tides also magnify the impact of storms, which are more likely in regions of the US during an El Niño. ‘Storms that peak during high tides are far more likely to cause damage than if the same storm peaked during low tide,’ said Kate Huckelbridge, executive director of the California Coastal Commission.
To prepare, California’s governor declared a state of emergency, placing road-closure and snow-removal equipment along highways. Emergency sand is also being placed on beaches as researchers worry that places such as Santa Cruz or Pacifica – with old sea walls – could be affected.
The California Coastal Commission has also encouraged homeowners and residents to prepare for any impacts by adding drainage and ensuring any damage to their properties is addressed in the event of severe weather.
