Salinity Staircases in the Arctic Ocean

Researchers at the University of Toronto have determined the mechanism responsible for the formation of temperature and salinity “staircases” in the Arctic Ocean. 

Understanding how these vertical structures work will further explain the causes and consequences of rapid Arctic Sea ice loss amid climate change.

 

“Our discovery of a new mechanism of hydrodynamic instability provides insights into the formation of staircase-like structures resulting from the mixing of warm salt water and cooler fresh water,” said Yuchen Ma, a Ph.D. candidate in the department of physics in the Faculty of Arts & Science and lead author of a study published in Physical Review Fluids describing the findings in an interview with phys.org.

 

Known as thermohaline staircases, these step-like variations of temperature and salt concentration are common in a wide range of regions of the global ocean.

 

The findings thoroughly verify a prior analysis by the same authors published in the Journal of Fluid Mechanics in 2020 that confirmed the existence of this new hydrodynamic instability. The validation was achieved by designing a series of direct numerical simulations of turbulence in the Arctic Ocean to understand global ocean circulation better.

 

“The ocean is typically thought of as a highly chaotic and turbulent environment, so it is striking to see such strongly defined layers of salt and heat within it,” says Ma.

 

The flow of heat out of the ocean into the overlying sea ice is enhanced by the company of thermohaline staircases. Therefore, understanding the staircase formation procedure in the polar oceans is essential for developing future global warming projections and informing efforts to mitigate sea ice loss.

 

“In this era of global warming, it is a well-recognized fact that the loss of Arctic Ocean sea ice cover is a critical aspect of this global process,” said University Professor W. Richard Peltier, of the department of physics who is a co-author of the studies and Ma’s Ph.D. supervisor.

“While the extent to which staircase formation is contributing to this loss has yet to be quantified, we can certainly say that the ocean component of the climate models employed to make projections of the global warming process are not able to resolve the staircase formation process.”

The research builds on previous work that focused on understanding global ocean circulation under the ice age conditions from 30,000 to 70,000 years ago.

 

In a previous model of glacial climate, the quick changes from cold to warm weather were confirmed to be caused by an extensive “hole” in the sea ice cover of the North Atlantic Ocean resulting from heat flow out of the ocean into the sea ice. The magnitude of this heat flow was determined by assuming a staircase had formed into the sea below.