Focus on marine heatwaves – PhD student Youstina Elzahaby

A PhD student on the Moana Project, Youstina Elzahaby’s love of maths has led her on a journey from assessing economic risk towards deciphering marine heatwaves. Youstina is based at the University of New South Wales in Sydney, part of a team led by Prof Moninya Roughan that brings international collaboration to the Moana Project. Her latest work sheds light on what caused recent marine heatwaves in the Tasman Sea, the ocean between New Zealand and Australia.

“There is still so much we don’t know about marine heatwaves,” explains Youstina. “Often, these heatwaves are purely examined in terms of sea surface temperatures. But the water depths they reach are very important for marine life and could be key to understanding the reasons they persist. Looking at their three-dimensional structure allows us to infer what caused them, and what is sustaining them. This is because the water at depth can retain heat, thereby paving the way for future marine heatwaves.”

A marine heatwave is a period of exceptionally warm ocean water. They are caused by ocean currents moving warm water masses into areas where they wouldn’t normally be, or by warm air and sunlight heating up the surface layer of the sea. Sometimes a mixture of both occurs. Marine heatwaves are getting more frequent and can cause devastating impacts on coastal ecosystems, fisheries and aquaculture.

Youstina has a scientific paper in press comparing different methods to determine the forces causing a marine heatwave in the upper ocean. This follows on from another, recently published, scientific paper in Geophysical Research Letters linking what caused Tasman Sea marine heatwaves to their three-dimensional structure.

In this research, she uses the results from hydrodynamic modelling to examine how marine heatwaves form and persist. “Currently, our models can only predict marine heatwaves a week to 10 days out. My work focuses on linking marine heatwave characteristics to circulation and atmospheric conditions, which hopefully will bring us one step closer to being able to predict them a month or further out.”

During a marine heatwave in 2008, the sea surface temperature was substantially above the long-term average (top figure). The anomalous warming in the black box just west of New Zealand reached depths of hundreds of metres (bottom figure), and the subsurface warming signal persisted even once the sea surface temperatures reverted back to normal temperatures from August 2008 onwards. Colour scale: red indicates warmer water than the long-term average, and blue colder.

Advanced warning would help ocean users like aquaculture and fisheries to mitigate the impacts of warmer temperatures.

Despite her already impressive contributions, Youstina’s career in ocean science is just beginning. “I have always loved maths,” she explains. “But I actually started out with Actuarial Studies, a mixture of economics, maths and statistics, which I studied both in Australia and Canada. After working in that field for a while, I returned to university to study maths again, and by chance took a course on fluid dynamics. I was captivated and haven’t looked back since.”

For the remainder of her PhD, Youstina will continue to focus on advancing our ability to predict marine heatwaves.

Youstina’s PhD is supervised by Dr Amandine Schaeffer, Prof Moninya Roughan (both University of New South Wales, Australia), and Dr Sebastien Delaux (MetOcean Solutions).

Check out the Moana Project marine heatwave forecasts here: https://www.moanaproject.org/marine-heatwave-forecast