Big waves come from big storms. If you like big waves, one of the most exciting things to see is a big, powerful low, spinning up over the ocean, with the prospect of a giant swell heading for the coast in a few days’ time. But what are the optimum conditions for a low to produce the biggest surf? And where and when are these conditions likely to occur?
For a mid-latitude depression to produce the biggest waves, there are a few things that need to be optimal. For example:
* The storm must be spawned in a part of the ocean where the current atmospheric conditions tend to amplify storms; otherwise the low will fizzle out before it gets a chance to really develop;
* Strong winds must blow – preferably hurricane force or more – for at least 12 hours over the same stretch of ocean (the fetch);
* The storm must have plenty of area around it to maximize the fetch, and not encroach upon any land or ice during its swell-producing lifetime. The fetch can be effectively increased if the storm travels along synchronously with the swell it is generating (see my article on dynamic fetch HERE);
* The water must be deep enough so that the waves are not affected by the sea floor while they are being generated.
So, where do these conditions most often coincide the most? In other words, whereabouts in the world’s oceans are you most likely to consistently get the biggest waves?
We can help to answer that question by finding the places where big waves occur most often throughout the year.
A good way of doing this is to choose a particular wave height – say five metres – then find out how many days a year the waves at a particular spot exceed that value. If this is done for many different points on the ocean, contour maps can be drawn, and the places with the most consistent big waves can be seen at a glance.
The data typically comes from reanalysis studies – where huge sets of past meteorological data are plugged into a modern wave-forecasting model, to obtain wave heights going back many years. The data for the maps below were compiled at the Royal Netherlands Meteorological Institute.
In the upper panel (5-m exceedance) you can see that in the Indian Ocean the waves exceed five metres more times throughout the year than anywhere else. In the lower panel (9-m exceedance) you can see that the waves in the North Atlantic exceed nine metres more often than they do anywhere else. On the whole, the maps show that medium-to-large waves are most consistent year- round at spots that receive swells from the Indian Ocean, but really massive swells are more likely to occur in the North Atlantic.
Of course, the size of the waves we can ride when the swell reaches the coast doesn’t just depend on the size of the waves generated in the storm centre; it also depends on local factors. At some spots, the waves are magnified big-time by some bathymetric feature (think Nazaré, Peahi or Puerto Escondido), even though the offshore wave height might not be that big. Also, big waves don’t necessarily mean surfable waves. You might get huge waves close to the storm centre, but these are less likely to be clean or unhampered by local winds.
Cover shot: Matahi Drollet at Teahupoo.