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.
