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What does the recovery of the hole in the ozone layer have to do with a storm surge over the weekend that saw waves nearly 10 metres high, causing significant damage along the South African coast? Scientists explain.
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The recovery of the hole in the ozone layer has played a part in the storm surge over the weekend that had a damaging impact on coastal towns stretching between the Western and Eastern Cape provinces and KwaZulu-Natal, according to a scientist.
The South African Weather Service (SAWS) this week issued a scientific explanation on the storm surge – that saw waves as high as 9.5 metres along the South Coast and between five and seven metres along the East Coast.
A combination of the high waves, spring tide, and "conducive" wind conditions combined to form a "positive storm surge", it said.
Professor Pedro Monteiro from the School for Climate Studies at Stellenbosch University shared that while storms are usual for this time of year – the storm was unusually large because of a number of factors coming together.
Scientists unpack how each of these factors – including the recovery of the hole in the ozone layer – created the conditions for a "perfect storm":
What is a spring tide?
The SAWS explained that tides are affected by the gravitational forces of the sun and moon and the earth's rotation. "The timing and peak of tides are, however, also influenced by factors like the wind and atmospheric pressure," it said.
A spring tide occurs approximately every two weeks during the new and full moon – causing high tides to be slightly higher than usual.
"Spring tides bring more water much further up onto the coast than normal, possibly resulting in chaotic coastal conditions such as coastal flooding and intensifying nearshore hazards such as rip currents," the SAWS said.
Monteiro also noted that the time of year - namely the spring equinox (set to occur around 23 September 2023) - is a high-risk period for being hit by a storm in the Cape. "You have these super high tides, the highest tides in the year," he said. The tides could add between a metre and a metre-and-a-half to the average sea level.
Another factor at play is the differing air pressure. At the centre of the storm, the air pressure is lower than outside the storm.
The ocean feels the air pressure change – and so under the storm, there is a bulge in sea level, explained Monteiro. "The sea level bulges upwards, the bulge contributed to a higher sea-level on top of the spring tide."
So, the spring equinox and the bulge caused by the storm on the sea level contributed together to the storm surge.
READ | Weathering the storm: Task team looks at how it can prepare Western Cape for climate change
Have we seen this storm before?
Monteiro explained that this kind of storm, in particular, was prevalent around 30 to 40 years ago – we just haven't felt it due to a combination of the weather pattern El Niño and the ozone hole.
"The ozone hole has an effect on the atmospheric dynamics in the southern hemisphere," explained Monteiro.
El Niño - which is associated with drought and higher temperatures – has the impact of pushing back cold fronts. This was seen during Cape Town's three-year drought between 2015 and 2018.
The opposite of El Niño is La Niña – and this phenomenon is associated with higher rainfall and lower temperatures. The occurrence of La Niña also means there is less pressure to push back storms.
While El Niño has emerged again, South Africa is not yet feeling its impacts – which also explains the wetter and long-lasting rainfall this winter in the Cape. Secondly, the recovery of the ozone hole is also having an impact.
"We see a combination of the ozone hole recovering, that tends to push the cold fronts closer, northwards," said Monteiro. "And we still not feeling the effect of the next El Niño. All those factors together help us understand why this cold front, and others this winter, came closer to Cape Town."
Why are the waves so high?
Noting the strong winds, Monteiro shared that the longer the wind blows over the ocean, the bigger the waves get.
If a strong wind blows over the ocean for days, it can generate "gigantic" waves. Monteiro said that the waves started being generated when the storm was created off South America. The storm speed and wave speed moved together- essentially with the storm feeding the waves over days, making them "really big".
Is climate change a factor?
The SAWS pointed out that it is difficult to link climate change to a specific weather event without an attribution study.
Professor Liesl Dyson, at the University of Pretoria's Department of Geography, Geoinformatics and Meteorology, noted the damage caused by the large waves but pointed out that this is not the first time that this has occurred.
"…The damage was a result of a strong low pressure south of the country causing large swell and waves but it was the fact that this occurred in conjunction with springtide that caused the most damage. The fact that the two events occurred simultaneously is a coincidence and cannot to attributed to climate change," said Dyson.
But, research shows that extreme weather events are happening more frequently. "… And what this coastal event has shown us is how vulnerable our society and infrastructure are to dangerous weather events," Dyson added.
Monteiro also pointed out that climate change is unlikely to have played a significant role in the storm.
Asked about whether rising sea levels linked to global warming could have contributed to the higher water volumes, Monteiro said it was largely the previously mentioned factors that made the storm "special".
Professor Guy Midgely, acting director for the School for Climate Studies at Stellenbosch University, noted that the recovery strategy from this event will be important. Science shows that climate change will worsen the severity of extreme weather events. And so, it will be important to determine where to rebuild infrastructure – possibly having to retreat more inland from the coast.
The Western Cape government has said it will establish a formal task team to address climate impacts and extreme weather events like the recent storm surge.
