While most of the planet has warmed over the past 40 years, scientists have observed an unusual cooling trend in the surface waters of the Southern Ocean, contrary to predictions and modelling.
A recent study published in Geophysical Research Letter has found that underestimated movement of fresh water into the Southern Ocean – called freshwater fluxes – may help explain why climate models aren’t matching up.
“We found that the Southern Ocean cooling trend is actually a response to global warming, which accelerates ice sheet melting and local precipitation,” says Earle Wilson, an assistant professor of Earth system science at the Department of Earth System Science at Stanford University in California.
Using several climate and ocean models from the Southern Ocean Freshwater Input from Antarctica (SOFIA) Initiative (SOFIA), as well as an older set of models simulating ocean density and circulation changes, the research team examined how melting ice has altered sea surface temperatures over the past three decades.
By comparing simulations with observed freshwater inputs from 1990 to 2021, they discovered that missing freshwater may account for up to 60% of the mismatch between predicted and observed Southern Ocean surface temperatures.
“There’s been some debate over whether that meltwater is enough over the historical period to really matter,” says lead study author Zachary Kaufman, a postdoctoral scholar in Earth system science. “We show that it does.”
As Antarctica’s ice sheets melt and more precipitation falls, the Southern Ocean’s surface becomes less salty and less dense. This creates a barrier, or “lid,” that prevents cooler surface waters from mixing with warmer waters below. Without this mixing, the surface stays colder than expected.
This process, called “freshening”, introduces uncertainty into climate projections, particularly regarding future sea level rise. “The impact of glacial meltwater on ocean circulation is completely missing from most climate models,” Wilson explains.
“We’ve known for some time that ice sheet melting will impact ocean circulation over the next century and beyond,” Wilson says. “Our results provide new evidence that these meltwater trends are already altering ocean dynamics and possibly the global climate.”
Oceans globally have absorbed more than a quarter of the carbon dioxide emitted by human activities and more than 90% of the excess heat trapped in our climate system by greenhouse gases. “The Southern Ocean is one of the primary places that happens,” says Kaufman.
As a result, the Southern Ocean has a significant influence on global sea level rise, ocean heat uptake, and carbon sequestration (capturing and storing carbon).
To better understand these processes, researchers used simulations to study how different types of freshwater input affect sea surface temperatures. They discovered that surface temperatures are much more sensitive to freshwater fluxes concentrated along the coast than those splashing more broadly across the ocean as rain.
“Applying freshwater near the Antarctic margin has a bigger influence on sea ice formation and the seasonal cycle of sea ice extent, which then has downstream impacts on sea surface temperature,” Wilson explains. “This was a surprising result that we are eager to explore further in future work.”
Southern Ocean starting to feel the heat
Do you care about the oceans? Are you interested in scientific developments that affect them? Then our email newsletter Ultramarine is for you.