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Super El Niño's Kelvin wave could lift California sea levels by up to 30 centimetres
Researchers warn an internal Kelvin wave from this year's super El Niño could lift California sea levels by up to 30 cm once it reaches the coast. The raised water would add to winter storm surges, though the oceanographer quoted expects it to distort tides for weeks.
The Scientist · Science desk
What happened
- The wave began when westerly wind bursts in the far-western Pacific weakened the trade winds, letting warm water and raised seas move east along the equator.
- Blocked by South America, the wave split into two coastal trapped waves that travel north and south for thousands of miles at about 10 km/h.
- California Coastal Commission director Kate Huckelbridge warned coastal residents in a Sept. 11 memo that higher tides magnify storm impacts and told them to prepare for flooding.
- Governor Gavin Newsom declared a state of emergency so the state could move equipment and resources into place before storms hit, The Guardian reported on Sept. 28.
- Webb doubts this El Niño will produce more than one full wave, although a single event can generate several, with the largest usually recorded in winter.
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Why it matters
- contradiction The report says months while Webb says a few weeks, and the shorter window lowers the odds that a winter storm lands on the raised water.
- exposure Central American coasts meet the wave before California does, and Webb counts every eastern Pacific shoreline as vulnerable, while California's emergency declaration covers only one stretch of that coast.
- constraint Low-lying shoreline has less clearance even in calm weather, because the rise lifts low tides as well as high ones whether or not a storm arrives.
The "internal" in the name is what separates this wave from the textbook kind. A Kelvin wave in general is a swell spanning thousands of miles that balances Earth's rotation, David Webb, a physical oceanographer at the UK's National Oceanography Centre, told Live Science [3]. The El Niño variety also moves the boundary between warm, less dense surface water and the colder, denser water beneath it. That layer rises and falls within about 100 metres while the surface lifts [4]. The coastal branches travel at roughly 10 km/h [7], or about 240 km a day [1].
On effect size, Webb's own figure is "tens of centimeters," about a foot [11]. Storms alone can push coastal sea level up by more than a metre, he noted, and climate change has already added at least 20 cm [15][16]. Against a one-metre surge, the wave's 30 cm ceiling [1] is an extra 30% at most. Against a larger surge the share is smaller [2].
The Coastal Commission's concern is about timing. "A key observation from past El Niño events is that the most damaging storms are often those that coincide with the highest tides," Kate Huckelbridge, the commission's executive director, wrote [13]. "Storms that peak during high tides are far more likely to cause damage than if the same storm peaked during low tide," she wrote [14]. The wave raises high tides along with low ones [10], so while it lasts, every high tide starts from a higher level.
The report and its main source differ on how long that is. Live Science's report says the wave could lift sea levels for months [1]. Webb described a shorter window. He said the most likely outcome is a period, or maybe a few weeks, when the normal pattern of tides and tidal currents is distorted, with both high and low tides higher than normal [10]. The wave may also be irregular. Its generation "will have involved changes in the winds at different locations and at different times, so it may not be a particularly clean single wave," Webb said [6].
The thing this doesn't tell you is whether those weeks will overlap the storms. The report says only that the wave will reach California "soon" [1], and the storms this El Niño is expected to bring are forecast for winter [2]. I'd treat 30 cm as the upper limit for a few weeks of raised water. Any storm that arrives inside that window starts from a higher level. A storm that peaks at high tide during those weeks is the case the commission's memo singles out [14].
What to watch
- Tide-gauge readings on the Central American coast as the wave passes, giving its measured height before it reaches California.
- The arrival date of the wave in California compared with the timing of the first major winter storms, especially any that peak at high tide.
- Whether a second full internal Kelvin wave forms this winter, an outcome Webb currently doubts.