Melting ice, more rain drive Southern Ocean cooling

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.

Reconciling global discrepancies

The mismatch between observed and simulated sea surface temperatures around Antarctica is part of a larger challenge for scientists and governments seeking to prepare for climate impacts. Global climate models generally do not accurately simulate the cooling observed over the past 40 years in the Southern Ocean and the eastern Pacific around the equator or the intensity of the warming observed in the Indian and western Pacific Oceans. There is also a discrepancy between simulations and the observed frequency of La Niña weather conditions, defined by the eastern Pacific being colder than average.

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 Niño and La Niña weather patterns, which influence rainfall as far away as California.

A surprising discovery

To understand the physical mechanism for Southern Ocean cooling -- and enable more reliable projections of its future impacts on Earth's climate system -- Wilson and Kaufman set out to determine how much sea surface temperatures around Antarctica in simulations have cooled in response to freshening. "We naively figured it wouldn't matter exactly where you put the freshwater," Wilson said.

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."

Quantifying the effect of missing meltwater

Previous studies have sought to quantify how Antarctic meltwater affects the global climate system by adding some amount of freshwater to a single climate model simulation, in what scientists have dubbed "hosing" experiments. "You get very divergent results, because people set up their experiments slightly differently, and the models are a little different, and it's unclear if these are really apples-to-apples comparisons," Wilson explained.

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."

Additional co-authors include Yuchen Li, an undergraduate student in the Physics Department in the Stanford School of Humanities and Sciences, Ariaan Purich of Monash University, and Rebecca Beadling of Temple University.

This research was supported by Stanford University, a grant from the NSF Division of Polar Programs, and the Australian Research Council Special Research Initiative for Securing Antarctica's Environmental Future. Li was supported by the Sustainability, Engineering and Science -- Undergraduate Research (SESUR) program in the Stanford Doerr School of Sustainability.