Super El Niño's Kelvin wave could lift California sea levels by up to 30 centimetres

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 ^[16].

On effect size, Webb's own figure is "tens of centimeters," about a foot ^[9]. 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 ^[14][15]. 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 ^[18].

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 ^[11]. "Storms that peak during high tides are far more likely to cause damage than if the same storm peaked during low tide," she wrote ^[12]. The wave raises high tides along with low ones ^[19], 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 ^[19]. 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 ^[12].