From time to time you might hear somebody say something like: “The swell’s out there, but it just needs an onshore wind to push it in.”
In other words the swell is out there but it’s not getting into the coast. All it needs is an onshore wind to drive it in. Sounds logical, right? A swell not quite heading in the right direction, but then being steered into the coast by a gentle push from a few hours of onshore winds. Is this true, or is it a myth?
Well, ocean swell is not like plastic bottles or other floating objects brought onto the shore by onshore winds. It can’t be ‘steered’ by a local wind blowing over the top of it. Not directly anyway. Why? Because ocean waves aren’t objects that move across the surface, like plastic bottles. They are more like ‘packets of energy’ that travel through the ocean. The water particles themselves move in closed circles underneath the waves, and only cover a limited distance.
One thing an onshore wind can do is produce more waves on top of the ones that are already there. If the wind is onshore, the newly-produced waves would be travelling straight towards the coast, and the wave height on the coast would increase. This might give the impression that the onshore wind somehow affected the other swell that was already out there.
However, there is one way that winds might be able to steer swells, but in an indirect way, via currents. As far as I know there are no studies published on this yet, so at the moment it is just a hypothesis.
Swells can change direction by refraction – in other words, if the swell starts to propagate over something that changes its speed. Swells change direction all the time when they reach shallow water, where the depth contours cause them to bend this way and that.
But ocean currents can also cause refraction, albeit in a slightly different way from shallow water. If the waves start to propagate through a current, the current could slow down or speed up part of the wave front, causing the swell to bend. The current does this by effectively moving part of the medium through which the waves are propagating. In fact, currents can interact with swells in many different ways depending on the direction and velocity of one relative to the other.
And here is the final link: one way surface currents can be produced is by the wind. In the deep ocean, the interaction of swells and wind-driven currents is well documented, the most famous case being the Agulhas Current of South Africa (HERE). Near the coast, swells are definitely known to be affected by currents, but these are typically rip or tidal currents. But strong enough winds blowing over the surface near the coast could also generate currents. And if these currents are strong enough and appear in the right places, they could also affect the swells.
Whether this can explain an onshore wind pushing a swell into the coast is a bit of a leap of faith. I’ll leave you to think about it.
Cover shot by Andy Gulliford