Climate change is causing marine ‘coldwaves’ too, killing wildlife

The effects of ocean warming are profound and well-documented. But sometimes changes in the patterns of winds and ocean currents cause seawater 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. More worrying still was the fact these deaths included vulnerable manta rays and even specimens of notoriously robust migratory bull sharks. In southern Africa, bull sharks, whale sharks and manta rays have previously washed up dead following such sudden cold events, especially over the past 15 years.

As we report in Nature Climate Change, the conditions that can drive these killer coldwaves 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.

What’s going on?

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.

Previous research had shown climate change induced changes in global wind and current patterns. So we investigated the potential consequences at particular locations, by analysing long-term wind and temperature data along the south-eastern coast of South Africa and the Australian east coast.

This revealed an increasing trend in the number of annual upwelling events over the past 40 years. We also found an increase in the intensity of such upwelling events and the extent to which temperatures dropped on the first day of each event – in other words, how severe and sudden these cold snaps were.

Read more: The Southern Ocean upwelling is a mecca for whales and tuna that's worth celebrating and protecting

Mass deaths warrant investigation

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.

To investigate the ramifications for marine fauna, we took a closer look at bull sharks. We tagged sharks with tracking devices that also record depth and temperature.

Bull sharks are a highly migratory, tropical species that only tend to travel to upwelling regions during the warmer months. With the onset of winter, they migrate back to warm, tropical waters.

Being mobile, they should have been able to avoid the local, cold temperatures. So why were bull sharks among the dead in this extreme upwelling event?