Deep heatwaves missed
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Experts say marine heatwaves in the deep ocean often go undetected.
Marine heatwaves (MHWs) and cold spells are increasingly impacting both the ocean surface and the deep ocean.
While these events at the surface have been extensively monitored over the past decade, a new study has uncovered that 80 per cent of MHWs below 100 metres are independent of those occurring at the surface.
The research, carried out by CSIRO and the Chinese Academy of Sciences, highlights a major gap in understanding deep-sea warming.
MHWs, defined as prolonged temperature increases, can disrupt marine ecosystems by damaging coral reefs and causing species displacement.
Rising global temperatures have driven an increase in these events, with notable impacts recorded along Australia’s East Coast, Tasmania, the northeast Pacific, and the North Atlantic.
Traditionally, satellite technology has been used to track MHWs, focusing primarily on surface temperatures.
However, recent findings published in Nature, reveal that subsurface MHWs are significantly underreported, which adds to a critical gap in understanding ocean warming.
The study used data from more than two million global ocean temperature profiles, marking the first global analysis of its kind.
“Our research reveals that MHWs are often hidden below the surface and occur separately to those on the surface,” said CSIRO researcher Dr Ming Feng.
“These findings deepen our understanding of the frequency and intensity of extreme temperature events under the ocean surface, and possible implications.”
A key factor influencing these deep-sea heatwaves is ocean eddies - large swirling currents that redistribute heat, nutrients, and oxygen within the ocean.
“Eddies play an important role in temperature variability and are often linked to MHWs below the surface,” Dr Feng says.
In the East Australian Current region, data from Australia's Integrated Marine Observing System shows that more than 70 per cent of deep-sea MHWs occur within ocean eddies.
These swirling currents can also affect other oceanic conditions, such as acidity, oxygen levels, and nutrient availability, which further complicates the impact of subsurface heatwaves.
The study's findings suggest that ocean eddies not only contribute to subsurface temperature extremes but have also intensified these events over recent decades.
This is largely due to an increase in energy contained within the eddies, driven by global warming.
The researchers say understanding the role of eddies in subsurface MHWs is critical for better assessing and predicting these events as the climate continues to warm.
The authors stress that monitoring these events more closely is essential, given their potential to affect marine life cycles and distribution, particularly in the deeper ocean where many species reside.