All the surfers were frothing, from Frankie the old ballie to Kalani the seven-year-old grom. A big swell was indicated on all their favourite forecast apps, a swell more akin to conditions they expect during the stormy months of winter. They were not disappointed: on Wednesday 19 January, a huge swell delivered unusually large and unusually westerly waves to the western seaboard of the Cape Peninsula as well as further up the west coast.

According to Marc de Vos at the South African Weather Service, these wave heights are observed in one out of every five summers, but the extreme wave heights of these waves, coupled with the large periods (a maximum wave height of 11m with a period of ~ 18s was measured), made this a one-in-20-year event.

Most of the usual surf spots were maxed-out, jet-skis towed the big-wave surfers into behemoths and the groms caught novelty waves in places where waves don’t really belong. Further marking this event as one for the records, is that it caused the wreck of the Antipolis, an oil tanker that ran aground in 1977 and has been a popular diving site, to be washed ashore.

What caused this unusual summer swell? 

It is tempting to link this event with the Tongan tsunami that occurred only five days earlier. However, tsunami waves are extraordinarily fast waves, moving at an average speed (in deep water) of about 700 km/h, meaning that it would take the Tongan tsunami less than a day to reach South Africa (without taking into account the continent of Australia that would impact its trajectory and provide the African coast with shelter from the wave).

In fact, the event was caused by a big mid-latitude cyclone in the Southern Ocean that Mathieu Rouault, associate professor and researcher at the Marine Research Institute at the University of Cape Town, observed to be further north than usual for this time of the year, which is consistent with the unusually westerly swell direction.

Mid-latitude cyclones are low-pressure weather systems of up to about 2 000 km in diameter that are associated with intense winds that last three to five days, during which time energy is transmitted into the ocean, forming waves of various sizes and wavelengths in a chaotic sea. As these waves move radially outward from their generation area, the smaller ones with shorter wavelengths move away more slowly than the larger ones, with larger wavelengths and periods, thus tidying up into a coherent groundswell that manifests as clean, well-spaced and powerful waves as they approach the coast.

Mid-latitude cyclones are formed and travel eastward in the westerly-wind belt in the Southern Ocean. The westerly-wind belt shifts northward during winter months, hence the propensity for storms and big seas off the South African coast then, rather than during summer months.