Marine heatwaves – prolonged periods of abnormally high ocean temperatures – have become more frequent and intense in recent decades due to climate change. This change is having a profound impact on marine ecosystems, biodiversity, and human life.
According to new research published in the Journal of Geophysical Research, marine heatwaves in the Arctic can be amplified by complex interactions between sea ice, atmosphere and the ocean.
The research was funded by the Ocean Frontier Institute (OFI) and undertaken by scientists at Dalhousie University and other collaborating institutions, such as Fisheries and Oceans Canada, and Foundation for Research and Technology - Hellas in Greece.
“There are many known consequences of extreme warm temperatures in more tropical areas, like coral bleaching,” says Dr. Benjamin Richaud, lead author of the study.
“Marine heat waves can have different consequences on cold region ecosystems, such as a drastic decline of the Alaskan snow crab fishery and melting of sea ice.”
The Arctic, concerningly, is warming faster than anywhere else on the planet.
A pathway for heat into the deeper ocean
By simulating interactions between sea ice, ocean and heat sources and sinks, Dr. Richaud and his colleagues identified processes that can trigger and dissipate marine heat waves in the Arctic.
“What we found is that marine heatwaves are triggered by the atmosphere, which warms the ocean, and then generates a marine heatwave, but where the heat goes afterwards is quite unique,”
he says.
“Most marine heat waves will dissipate with vertical mixing past the mixed layer of the ocean, which means that they provide a pathway for heat from the atmosphere to be redirected towards the deeper ocean.”
The ocean mixed layer is typically a few to 100 metres from the surface and is a dynamic and homogenized ocean zone that plays an important role in ocean-atmospheric heat exchange.
“The heat becomes trapped in deeper ocean layers, is recirculating and moving somewhere else, or is stored for a few months and then resurfacing,” explains Dr. Richaud. “Resurfaced heat can change or delay the way new sea ice forms.”
The role of sea ice
Once the marine heat wave is formed, it can also travel with ocean circulation patterns and move towards sea ice edges, which can trigger more melting of sea ice. Increased sea ice melt can release more fresh water, and in turn lead to stronger stratification of the ocean.
Ocean stratification is the natural separation of the ocean into horizontal layers. In the Arctic, colder, fresher waters lie on top of warmer, more saline waters and insulate the sea ice from those warmer deeper waters.
Sea ice plays a crucial role in climate processes and influences how much solar energy is absorbed or reflected on earth.
“Sea ice reflects solar radiation and has a high albedo,” says Dr. Richaud. “When it melts, it exposes ocean waters, which absorbs solar radiation and in turn melts more sea ice and makes temperatures warmer. This is a positive feedback loop and understanding where the heat comes from and goes during marine heat waves is crucial within this context.”
“This stratification can trap heat in the upper layers of the ocean and prolong and reinforce marine heatwaves,” says Dr. Richaud.
As marine heatwaves are anticipated to become more frequent and severe in the coming centuries, Dr. Richaud hopes these findings can benefit science, policy, and communities.
“By investigating processes generating marine heat waves in the Arctic, we get better at predicting them, and can also help minimize impacts for communities living in the Arctic,”
he emphasized.
About Dr. Benjamin Richaud
Dr. Richaud is originally from France, and recently completed his OFI-funded Ph.D. in Oceanography at Dalhousie University in Halifax, Nova Scotia under the supervision of Dr. Eric Oliver.
His research project examined ice-ocean interactions in the changing Arctic using numerical models. He is now pursuing his interests in sea ice at Université catholique de Louvain in Belgium.
