Weekend Argus (Saturday Edition)
Furious 122km/h swells ...
WHAT a strange summer we have had. Not only has the southeaster been scarce, but we’ve had a lot of cooler weather, with plenty of cloud and even some welcome rain.
When the Cape Doctor has blown, even in the peak of summer, it has blown more southerly, a wind direction normally associated with autumn as the South Atlantic high begins to move north, and the oncoming low pressure systems shift into the breech, causing the prevalent winds to shift from SE to SW, punctuated by increasingly more regular bursts of NW.
But what really brought home weather patterns that in the words of Alice in Wonderland are becoming “curiouser and curiouser” was the groundswell we gratefully received this week after a long and slow flat spell.
An unusually fierce summer storm about 2000 nautical miles from Cape Town peaked last week Friday with core winds across a fetch of about 500 miles blasting to 80 knots, which is more than double gale-force.
From the end of this conveyor belt of wind marched a swell that had forecasters such as me ‘frothing’ with excitement, as younger surfers might say. The reason for the excitement, shared by many surfers around the Cape, was something that even in winter is as rare as fossilized hens’ teeth from the Proterozoic eon.
The unusual attribute was all about interval. After radiating out from the gigantic pressure bomb that went off in the deep, the swells took three days to reach us, and when they did, each occurred every 22 seconds, otherwise known as wave period. Swells from hurricane-force storm winds are the most powerful, certainly in terms of those formed by wind. To look for anything more powerful, you have to look at other types of originating force, such as undersea earthquakes or space debris slamming into the ocean.
A 22-second swell, which is a manifestation of the wind velocity that created it – has a mathematically constant wavelength of 755 metres. On top of all this volume of ocean that is being moved as the swell glides through it, extremely long swells have massively exponentially more energy than shorter period swells, and they’re moving at incredible speeds.
An individual 22-second swell moves through the ocean at 122 km/h. When it reaches the coast, this energy and speed are expressed in the sheer volume and power of the eventual wave that breaks. Even small waves at these periods are inordinately powerful, often moving too fast for our puny arms to catch them. Early Monday, an intrepid group of surfers in the deep south experienced this. At a powerful reef break, they waited for the sets to come for up to 20 minutes, and then got caught up in a mad blast of ocean power they struggled to deal with.
Unusually spaced long period swells pose the most danger to terrestrially based people because of their deception. A fisherman could literally unpack his box of tricks, bait up, and clamber over to a rock, with the mistaken belief that the sea is flat.
These lulls also make long period swell controversial to surfers. The surfers who watched their spot for 15 minutes and left will claim the ocean was flat to friends who will smugly claim otherwise. While this particular swell was smaller than I thought, the energy was insane when the sets came on Monday. By Tuesday, I paddled out at the same reef break, and even though the period had eased, I saw several waves of immense power. With the volume and power behind it, a swell can quadruple in size. Some wave faces were easily four or five times overhead.
Many surfboards were broken in the carnage, and some people paid their dues in the impact zone. It was fascinating to be part of it.
WEATHER TIP
A lovely burst of cooler NW wind and some rain overnight today brings a solid 6-8 foot swell in light to calm NW leftovers from the fierce burst of wintery weather yesterday. The NW goes moderate to fresh today, breaking up the sea but Muizenberg looks super fun 3’ and clean offshore. Tomorrow, a new swell builds to a solid 8 foot by dusk in moderate to fresh NW. Another great surf day for Muizenberg.