Marine heatwaves, how the ‘silent killer’ of underwater life works

the Marine heat waves It is the extreme temperature phenomena of sea ​​water Which can have serious effects on Underwater life. size Environmental damage It depends not only on the surface signature that can be easily observed, but also on the structure of the heat wave at depth. But due to the lack of subsurface field observations, The vertical structure of heat waves at sea is not well understood. It is known that the oceans are also experiencing rising temperatures. When water exceeds the seasonal limit for five or more days, it is a Marine heat wave. They cause their worst damage in the summer, when the oceans are already warmer, but they can happen at any time of the year.

More than 90% of the heat trapped by greenhouse gases ends up in the oceans. So it is not surprising that heat waves are becoming more intense and frequent. This year, scientists noticed out-of-range rates, they report in new research just published in Nature, Earth and Environment Connectionsresponsible for Amandine Schiffer from the School of Mathematics and Statistics at the University of New South Wales in Sydney, Australia. The specialist confirms that “since April of this year, the average temperature of the world’s oceans has reached the highest level ever recorded.”

Since the 1980s, satellites have revolutionized ocean science by allowing daily measurements of sea temperatures. But satellites are watching from above. They can’t see what’s going on beneath the surface. “Our new research explores what happens in deep waters. It shows that marine heat waves don’t just happen at the surface there. In the most destructive cases, heat can even penetrate the sea floor. Surprisingly, some waves only affect the sea floor,” Schaefer stressed.

While marine creatures are usually only seen on the surface of the ocean, there is life everywhere. Fish, algae, sponges, cold-water corals, shellfish, and crustaceans live on the shallow sea floor of continental shelves (the sunken parts of continents).

The depth of these shallow oceans is, on average, less than 100 metres. When the shelf ends, there is usually a steep descent into the bowels of the ocean, with miles of water between the surface and the sea floor.

Marine heat waves destroy life in the seas covering the continental shelf. The creatures here are sensitive to temperature extremes, just like those on the surface. The specialist adds: “But for them, this term is different from what we consider extremist.” If you’re used to 12°C temperatures, a 15°C heatwave can be devastating. When marine heat waves hit, they can kill. “More than a billion marine organisms died during a single heatwave off the coast of the western United States and Canada in 2021. This year, extreme heatwaves hit most of the oceans during the boreal summer.”

Fish and other creatures that can move do this, heading towards the poles or deeper places in search of cooler waters. Those who cannot, must endure it or die. Heat waves can trigger migration. New species arrive seeking refuge and can change the ecosystem.

The seas covering the continental shelf are relatively shallow compared to the miles of deep ocean water. However, it is impossible to see what is happening below using satellites or high-frequency radars.

The sea is a hostile environment. Tools are exposed to high pressure, corrosive salt water, and marine organisms such as shellfish and sponges that settle on them. This is one reason why there is only very limited data on long-term trends in subsurface temperatures. But these records are vital for calculating typical temperatures for this time of year and determining what constitutes an extreme.

“Australia “It’s one of the few places that produces this kind of valuable, long-term data,” Schiffer says. Off the southeast coast are several oceanographic anchors: a floating array of sensors fixed to the bottom. “One has been measuring daily temperatures from the surface to the seafloor, at a depth of 65 metres, since 1993. Our previous research found that marine heat waves at depth can actually be more intense and last longer compared to the surface,” the expert said. .

In their new research, they took a closer look at temperature data. “We have discovered – continues the specialist – that marine heat waves are of different types and have different causes. We also found that some types of marine heatwaves are more likely during certain seasons. For example, in winter they usually extend from the surface to the sea floor. They occur when the strong, deep, warm Australian East Current meanders westward toward the coast. “As the current swings up the continental slope, it carries warm water over the shelf and closer to the coast.”

In summer, Australia experiences two very different types of heatwaves. The first occurs when the weather is good, with few clouds, and more of the sun’s heat reaches the oceans. They can also occur when winds are weaker and the ocean cools less due to evaporation. These heat waves are limited to the surface and a few meters below.

Then there’s a second, “a very strange heat wave system that only appears near the sea floor,” Schiffer reports. They occur when strong winds create currents that push warm, shallow water to the bottom. On the east coast, these currents come from cold southerly winds. So, even while you’re shivering in the cold wind South Ocean, the ocean floor may be suffocated by a heat wave. These can be the most destructive to ecosystems, but they go almost unnoticed.

New research shows that marine heatwaves have different flavours.

“This is important, because it will allow us to better predict whether a heat wave is about to hit our oceans, predict which parts of the water column are about to be affected, and which ecosystems are at risk,” Schaefer says. Of course, slowing ocean warming and preventing marine heatwaves from damaging ecosystems means significantly reducing carbon emissions. But while we’re working on it, This knowledge could buy us time to find strategies to reduce the number of underwater deaths and damage to tourism and fishing. The specialist concluded that it depends on the survival of these ecosystems.