Scientists say a giant underwater Kelvin wave, tied to this year’s El Niño, is travelling north along the western coast of the Americas. The wave is a broad, shallow rise in the ocean surface that can add roughly 30 centimetres to sea level as it passes.

A smaller Kelvin wave already arrived in British Columbia in June. Bill Merryfield, a research scientist at Environment and Climate Change Canada, says the next, larger wave is expected within one to two weeks. He added, "We actually did already see over 30 centimetres in some locations in June. And so the coming wave looks like it'll be a stronger one."

Daniel Swain, a climate scientist at the University of California, describes the phenomenon as "a very broad, hundreds of miles wide, but very shallow bulge in the height of the ocean surface, so by maybe a foot or so." This means sea level could rise about a foot (around 30 cm) when the wave reaches the coast.

The extra height becomes most dangerous when it lines up with king tides – the highest tides caused by the combined pull of the sun and moon – and with winter storms that generate storm surges. Merryfield warns that adding 30 centimetres to a king tide, then a storm surge, "is when you really start to get into trouble," potentially leading to flooding and coastal erosion.

Local authorities along the B.C. coast are preparing for the wave’s impact. They expect water levels to rise between six and twelve inches, and are already placing emergency sand on vulnerable beaches to reduce erosion.