The way 'seiches' create a rhythmic slosh in enclosed bays

Picture a massive bay acting like a giant, liquid pendulum. That’s a seiche. It’s not the tide rolling in; it’s the whole body of water doing a slow-mo wobble from one shore to the other.

A heavy gust or pressure shift gives the bay a nudge, piling water up. When that force lets go, gravity tries to flatten it out, but the water overshoots, sloshing back and forth until it settles.

It’s nature’s own bathtub effect, keeping a steady groove long after the wind has bailed.

Wait, does the size of the bay change how fast it sloshes?

Spot on, mate. Just like a shortboard snaps through turns faster than a longboard, the shape of the bay dictates the beat. A massive, deep harbor takes ages to swing back, while a tiny inlet might zip back and forth in minutes.

It’s all about the 'natural frequency.' Every body of water has its own favorite tempo. If the wind pulses at just the right time to match that rhythm, the slosh gets bigger and stays alive way longer.

Think of it like a playground swing. If you kick at the perfect moment, you keep the momentum. In a huge bay, that 'kick' just takes a lot more time to travel from one end to the other.

Can that perfect timing actually turn the slosh into something dangerous?

Too right it can, mate! When that 'kick' matches the bay's heartbeat perfectly, it’s called resonance. Like pumping your legs on a swing until you’re nearly looping the bar, the water doesn't just wobble; it stacks up high.

In the Great Lakes, a nasty storm can sync with the slosh and surge the water level by meters in a flash. It looks like a tide on fast-forward, swallowing piers and catching boaties off guard.

It’s not an earthquake tsunami, but a 'meteo tsunami'—a beast born purely from the wind and the water’s own stubborn rhythm.

So how do you spot a 'meteo tsunami' if there's no earthquake warning?

That’s the tricky bit, mate. Since there’s no tectonic rattle to wake you up, you’ve gotta keep your eyes on the sky. These sneaky devils are triggered by a sudden, massive jump in air pressure—like a giant invisible hand pushing down on the ocean.

If a fast-moving storm front or a pressure spike hauls across the bay at the exact same speed the wave wants to travel, they lock together. The storm keeps feeding energy into that wave, making it grow into a monster as it nears the shore.

To someone on the sand, it might just look like a weirdly fast-rising tide or the water suddenly sucking out to sea. By then, the 'kick' has already landed, and you'd better be heading for high ground pronto.

Why does the wave turn into a monster only when it nears the shore?

Think of it like a crowded lineup at the beach. In the deep ocean, the wave has heaps of room to move underneath. It’s just a deep-bellied swell passing under your board—you barely feel the lift.

But as the seafloor rises, the bottom of the wave starts dragging on the sand. It’s like the wave is tripping over its own feet. Since it can't go down, all that energy gets squeezed upward, stacking the water into a vertical wall.

It’s the ultimate crunch time, mate. The energy that was spread out in the deep gets compressed into a tiny space, turning a gentle wobble into a heavy, shore-breaking beast.