The effects of ocean warming are profound
But sometimes changes in the patterns of winds and ocean currents cause sea water to suddenly cool, instead.
Surface temperatures can plummet rapidly — by 10ºC or more over a day or two.
When these conditions persist for several days or weeks, the area experiences a “coldwave”, which is the opposite of more familiar marine heatwaves.
When a “killer coldwave” manifested along South Africa’s southeast coast in March 2021, it killed hundreds of animals across at least 81 species
As per reports, the conditions that can drive these killer cold waves have grown increasingly common over the past four decades.
Ironically, strengthening winds and currents as a result of climate change can also make these deadly localised coldwaves more likely in places such as the east coasts of South Africa and Australia, potentially putting even highly mobile species such as sharks in harm’s way
Certain wind and current conditions can cause the sea surface to cool, rather than warm.
This happens when winds and currents force coastal waters to move offshore, which are then replaced from below by cold water from the deep ocean.
This process is known as upwelling.
In some places, such as California on the US west coast, upwelling happens regularly along hundreds of kilometres of coastline.
But localised upwelling can occur seasonally on a smaller scale, too, often at the edges of bays on the east coasts of continents due to interactions of wind, current and coastline.
Research showed an increasing trend in the number of annual upwelling events over the past 40 years.
It also found an increase in the intensity of such upwelling events and the extent to which temperatures dropped
During the extreme upwelling event along the southeast coast of South Africa in March 2021, at least 260 animals from 81 species died.
These included tropical fish, sharks and rays
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