September 22, 2020 — In a paper published September 17 in the Journal of Climate, WHOI oceanographers and collaborators at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany use a new model to understand how ocean processes affect marine heatwaves at depth off the west coast of Australia. Known as “Ningaloo Niño,” these extreme warming events have caused mass die-offs of marine organisms, coral bleaching, and potentially permanent ecosystem shifts, all of which impact fisheries and the economies that depend on them.
“This area is a hotspot for increasing temperature and extreme events, with drastic impacts on regional marine species,” said lead author Svenja Ryan. “It’s important to understand where in the water column temperature and salinity changes are happening so you can determine how the ecosystem will be impacted.”
For the first time in the Southern Indian Ocean, Ryan and her co-authors, WHOI physical oceanographers Caroline Ummenhofer and Glen Gawarkiewicz, showed that the effects of marine heatwaves extend to 300 meters or more below the surface along the entire west coast of Australia. They found that during La Niña years, the southward-flowing Leeuwin Current becomes stronger and is associated with warm temperature anomalies at greater depths. These conditions were observed during the 2011 marine heatwave that led to the first-recorded coral bleaching at Ningaloo Reef, a World Heritage site, and extensive loss of a nearby kelp forest. During El Niño periods, the temperature and salinity anomalies associated with marine heatwaves are limited to the ocean surface, showing that complex ocean processes play an important role in the depth-extent of extreme events.
Ryan and her colleagues are using a similar modeling approach to study marine heatwaves in the Northwest Atlantic. “The challenge, wherever you go, is that marine heatwaves have so many drivers,” Ryan said. “Understanding different types of events and their associated depth structure is crucial for regional impact assessment and adaptation strategies, as well as for predicting potential changes in a future climate.”