When we think of climate change, warming and extreme heat often come to mind. Warmer ocean waters threaten the lives of countless marine species. However, a recent study published in Nature Climate Change suggests that climate change may be linked to underwater cold spells. This may seem contradictory to the current global trend of ocean heatwaves, but it's happening, and such events seem to be becoming more common, potentially posing deadly impacts on marine life.
According to Science, in March 2021, South African beaches witnessed the mysterious deaths of giant bat rays, bull sharks, and numerous pufferfish scattered like deflated balloons along the coastline.
These deaths caught the attention of Nicolas Lubitz, the lead author of the paper and a shark researcher from Australia at the time, who was a doctoral student at James Cook University. He collaborated with a group of ecologists and oceanographers to piece together the truth behind the events and broader oceanic trends.
Lubitz and his team focused their study on the Agulhas Current. This fast-flowing current lies near the southeast coast of Africa, sweeping southward along the coasts of Mozambique and South Africa.
Typically, this current carries warm subtropical waters farther south. However, when it moves up onto the continental shelf, it generates eddies, similar to whirlpools formed downstream of boulders in rivers. As these eddies pull water away from the coast, cooler bottom waters from depths of up to 3 kilometers below the sea surface are drawn towards the coast for replenishment. Additionally, easterly winds can push warmer coastal waters offshore, causing the upwelling of colder bottom water.
The researchers found that starting in late February 2021, the convergence of weather and ocean conditions brought chilling results in South African waters. Temperature data from satellites and buoys revealed a large eddy moving southward through the region, coupled with four days of strong easterly winds, which amplified the upward trend of bottom waters. This upwelling caused the sea surface temperature off parts of the South African coast to plummet over 7°C within 48 hours, reaching 10°C. Near the port of Alfred in southeastern South Africa, a subsea temperature tracker showed a drop of over 9°C within a day. This upwelling covered 230 kilometers of ocean and lasted for a week.
Lubitz explained that this is undoubtedly bad news for fish species that prefer warmer waters, such as bull sharks. The coldness of the seawater has exceeded their tolerance, and the sudden, rapid cooling has left them in distress.
"If the seawater temperature drops by 9°C within 24 hours, it's quite deadly," said Lubitz.
Lubitz and his colleagues delved further, studying sea surface temperature changes over 40 years and wind-related data over 30 years in the area. The results revealed that upwelling events in several locations along the Agulhas Current have become more frequent, intense, and longer-lasting, resulting in even lower seawater temperatures. In some places, the number of upwelling events nearly doubled, from about three per year in the 1980s to five or more per year from 2012 to 2022.
To AJ Smit, a coastal ecologist at the University of the Western Cape in South Africa who was not involved in the study, the research "truly proves the existence of ocean cold spells" for the first time, and their scale and intensity are sufficient to impact the entire ecosystem.
However, one of the study's authors, David Schoeman, an ocean climate change ecologist at Southern Cross University in Australia, pointed out that it is still difficult to say whether this trend is being driven by climate change, partly because these upwelling events are highly localized and not well represented in models that study interactions between oceans and climates. Nevertheless, the consistency of these trends with previous predictions about how climate change would more broadly affect ocean currents and their persistence over a long enough period suggests that natural fluctuations alone may not fully explain them. Furthermore, researchers also found that, although less pronounced, cold upwelling events in the East Australian Current have also increased.
