Melting ice, more rain drive Southern Ocean cooling

 Global climate models predict that the ocean around Antarctica should be warming, but in reality, those waters have cooled over most of the past four decades.

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 carlifornia

sources-science daily