At some surf spots, the water temperature has more to do with the wind than the air temperature or anything else. Here’s why.
On the southwest tip of Africa, where I’m writing this, the water temperature seems to have a mind of its own. In winter it typically hovers around 15°C and can often drift up to 18°C. But then, sometimes, it can plummet to around 9°C in less than 24 hours. The most logical thing would be if it coincided with a sudden massive drop in air temperature. But it doesn’t. It coincides with a strong southeast wind, famously known as the Cape Doctor or simply the southeaster. As soon as the southeaster starts to blow, the water temperature plunges. Bizarrely, because the southeaster is most common in the summer, the average water temperature is colder in the summer than in the winter.
Elsewhere in the world you can see a similar thing happening. In Galicia and Portugal, for example, the nortada that blows down the coast during summer keeps the water temperature colder than normal. In Peru and Chile, the water is uncharacteristically cold for the latitude, except when the southerly winds die off, in which case the water temperature increases dramatically.
So what is the connection between strong cross-offshore winds and colder-than-normal coastal water temperatures? You might have heard of it: coastal upwelling. Coastal upwelling is the continual rising up of cold water from underneath to replace the warm surface water while the surface water is being pushed away from the coast by the wind.
To understand how it works, we first need to assume that, initially, the coastal water is thermally stratified. That means the warmer, lighter water is sitting on top of the colder, heavier water. Now, if the wind starts to push the warm surface water away from the coast, the colder water underneath will rise up to take its place. If the wind keeps pushing the surface water away from the coast, the Sun won’t get a chance to warm it up before more cold water rises up from underneath. Therefore, the coastal water remains cold as long as this cycle continues. As soon as the wind dies or changes direction, the cycle stops and the surface water once again warms up.
Now, you’d think that a straight offshore wind would be most efficient for blowing the surface water away from the coast. However, thanks to the Coriolis force (that mysterious force that makes lows and highs rotate the way they do – see upcoming article) the surface water being driven by the wind does not move in quite the same direction as the wind. It is deflected approximately 45°. Therefore, it is a side-offshore wind that ends up driving the water straight offshore.
The amount of upwelling depends on the strength of the wind. But that’s also not quite so straightforward. The water movement is proportional to the windspeed squared. In other words, for every doubling of the windspeed the water is pushed four times as hard. This makes coastal upwelling a very efficient mechanism. It is part of the reason why, in Cape Town, one day you can be warm in a three-two and the next day you’ll be cold in a five-four with booties, gloves and a hood.
Cover shot of somewhere in Poland (yep) by Krzysztof Jedrzejak