Cape Town - Although some have said the seemingly unusually high swells seen along the South African coastline on Wednesday were not unusual, others have questioned the cause and if the waves were related to recent seismic events near Tonga.
South African Weather Service (SAWS) senior marine scientist Marc de Vos said the swells were the result of a big storm in the Southern Ocean – a mid-latitude cyclone, to be exact.
“Winds associated with the storm blew strongly over a large area of the ocean, generating large waves. These waves continue to radiate outward from their source, reaching our coastline on Wednesday,” said de Vos.
Considering the wave height, De Vos said it was not uncommon to see Wednesday’s wave heights at some point during one out of every five summers but overall, they were rare and occurred less than 1 % of the total time during summer (they appeared in roughly one out of five summers).
He said waves with large heights and short periods, or small heights and long periods, were common throughout the year, but the real question was what the likelihood was of the combination of wave heights and periods which occurred on Wednesday for this time of the year.
“Our records indicate that this was indeed very rare. We noted only one such occurrence in 19 summers (in statistical speak, roughly a one-in-twenty year event),” said de Vos.
UCT physical oceanographer Mathieu Rouault said what was unusual about Wednesday’s waves was their swell direction, which was westerly as the swell was usually south-westerly, so the West Coast was more directly exposed to it this time.
The waves were formed by the big storm, which Rouault said did occur in summer in the Southern Ocean, but the storm appeared to be further north than usual, hence the curious westerly direction.
South African Environmental Observation Network (SAEON) and UCT physical oceanographer Jennifer Veitch said the Tongan tsunami would not have impacted South Africa’s coastline and it was unlikely that a Pacific tsunami event would impact South Africa as it was well protected, being situated between the Atlantic and Indian Oceans.
“The recent volcanic explosion at Tonga caused fluctuating sea levels all across the rim of the Pacific ocean, but to the best of my knowledge did not spread to the Indian ocean,” said SAWS Chief Forecaster Kevin Rae.
kristin.engel@inl.co.za