Temperatures are even more exceptional below the surface of the Pacific – in places more than 8C above average.
Above-normal sea temperatures are also expected in other parts of the world including the Indian Ocean, tropical Atlantic and around parts of the Australian coast.
With its rich marine life, changes in ocean heat are of particular concern to Australia.
It has been plagued by marine heatwaves in recent years – driven by climate change – leading to widespread coral bleaching and algae blooms, causing “unprecedented damage”.
El Niño is a natural phenomenon, but it tends to bring the kind of extreme heat which also drives those disasters, Dr Alistair Hobday, chief researcher at Australia’s science agency, told the BBC.
This year, marine heatwave forecasts indicate the Great Barrier Reef will be spared the worst of the conditions, giving coral there more time to recover.
“[But] Southern Australia is getting slammed. It had an algal bloom last year, it’s got bird flu now, and it looks like marine heatwaves will be stronger there this summer,” said Hobday.
Last time a marine heatwave hit South Australia and Tasmania, there were starving penguins, fish kills, suffocating algae outbreaks, and jellyfish invasions.
Australia has recently introduced early warning systems for marine crises – this gives time for financial relief for fisheries to be established, relocation of crucial species and increased coral monitoring.
But this event is shaping up to be unprecedented – and Hobday admits he is quite worried.
