The discrepancy between model results and observed cooling, Stanford University scientists have now found, comes down mainly to missing meltwater and underestimated rainfall.
"We found that the Southern Ocean cooling trend is actually a response to global warming, which accelerates ice sheet melting and local precipitation," said Earle Wilson, an assistant professor of Earth system science in the Stanford Doerr School of Sustainability and senior author of the March 27 study in Geophysical Research Letters.
As rising temperatures melt Antarctica's ice sheet and cause more precipitation, the Southern Ocean's upper layer is growing less salty - and thus, less dense. This creates a lid that limits the exchange of cool surface waters with warmer waters below. "The fresher you make that surface layer, the harder it is to mix warm water up," Wilson explained.
But this freshening is not fully represented in state-of-the-art climate models - a flaw that scientists have long recognized as a major source of uncertainty in projections of future sea level rise. "The impact of glacial meltwater on ocean circulation is completely missing from most climate models," Wilson said.
Warming events in the Southern Ocean over roughly the past eight years have somewhat diminished the 40-year-long cooling trend. But if sea surface temperature trends around the globe continue to resemble patterns that have emerged in recent decades, rather than shifting toward the patterns predicted in simulations, it would change scientists' expectations for some near-term impacts from climate change. "Our results may help reconcile these global discrepancies," Wilson said.
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," said lead study author Zachary Kaufman, a postdoctoral scholar in Earth system science.
As a result, the Southern Ocean has an outsized influence on global sea level rise, ocean heat uptake, and carbon sequestration. Its surface temperatures affect El Nino and La Nina weather patterns, which influence rainfall as far away as California.
The researchers were surprised to discover 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 said. "This was a surprising result that we are eager to explore further in future work."
For the new study, the researchers sought to avoid this issue by working with a collection of simulations. Using a new ensemble of coupled climate and ocean models from the recently launched Southern Ocean Freshwater Input from Antarctica (SOFIA) Initiative, as well as an older set of models simulating ocean density and circulation changes, the authors analyzed how much simulated sea surface temperatures changed in response to the actual freshwater inputs between 1990 and 2021.
"There's been some debate over whether that meltwater is enough over the historical period to really matter," said Kaufman. "We show that it does."
With the new method, which incorporates simulations from 17 different climate models, the researchers found missing freshwater explains up to 60% of the mismatch in observed and predicted Southern Ocean surface temperatures between 1990 and 2021.
"We've known for some time that ice sheet melting will impact ocean circulation over the next century and beyond," Wilson said. "Our results provide new evidence that these meltwater trends are already altering ocean dynamics and possibly the global climate."
Research Report:The Impact of Underestimated Southern Ocean Freshening on Simulated Historical Sea Surface Temperature Trends
Related Links
Stanford Doerr School of Sustainability
Beyond the Ice Age
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