Southern Ocean warming will mean a wetter West Coast, US

These teleconnections between the tropical Pacific and far-flung areas are reported in a Cornell University-led computer-model study published in Nature Geoscience.

While other computer models have projected similar precipitation increases generated by a warming Southern Ocean, major uncertainties and a wide range of predictions exist between models.

The new study serves to reduce those uncertainties, which could improve predictions of global mean temperatures and regional precipitation.

"We needed to find the cause of those uncertainties," said Hanjun Kim, the study's co-corresponding author and a postdoctoral associate working with co-authors Flavio Lehner and Angeline Pendergrass, both assistant professors of atmospheric sciences at Cornell. Sarah Kang, professor in the Max Planck Institute for Meteorology in Hamburg, Germany, is the paper's other corresponding author.

"I found that low-altitude cloud feedbacks over the Southern Hemisphere can be one cause of those uncertainties in remote Northern Hemisphere regional precipitation," Kim said. "If we try to reduce the uncertainty of Southern Hemisphere cloud feedbacks, then we can also improve the prediction of global mean temperatures."

The Southern Ocean has a higher capacity for absorbing heat than other bodies of water due to a strong upwelling of deep cold water, but eventually the water will warm and gradually release heat. When this happens, that heat is distributed, creating teleconnections, which are predicted to increase precipitation in East Asia during summers and in the Western U.S. during winters. Such teleconnections are very similar to how El Niño affects weather patterns.

The model predicted that due to the ocean's slow release of heat, the new precipitation patterns could persist for up to 150 years, regardless of efforts to reduce greenhouse gases.

"We can occasionally see these processes today, which allows us to study them," Lehner said, "but we expect in the future for these processes to switch from being an occasional occurrence to being a more permanent state of the system."

Kim found that low-lying clouds over the Southern Ocean act as a key regulator affecting sea-surface temperatures. Accounting for these cloud feedbacks in climate models help explain the uncertainties and variations from one model to another, according to the study.

There are few observational facilities in Antarctica to provide data on cloud feedbacks in the Southern Ocean, so increasing those would in turn improve predictions, Kim said.