Background
The authors write that "the Southern Ocean is the dominant anthropogenic carbon sink of the world's oceans and plays a central role in the redistribution of physical and biogeochemical properties around the globe," citing Sarmiento et al. (2004). And they therefore state that "one of the most pressing issues in oceanography is to understand the rate, the structure and the controls of the water mass overturning circulation in the Southern Ocean and to accurately represent these aspects in climate models."

What was done
Focusing on five water masses that are crucial for the Southern Ocean overturning circulation - surface subTropical Water (TW), Mode Water (MW), Intermediate Water (IW), Circumpolar Deep Water (CDW) and Antarctic Bottom Water (AABW) - Sallee et al. studied the ability of 21 of the CMIP5 models to simulate what they describe as the most basic properties of each of these water masses (temperature, salinity, volume, outcrop area).

What was learned
Finding #1: "The water masses of the Southern Ocean in the CMIP5 models are too warm and light over the entire water column," with the largest biases being found in the ventilated layers.

Finding #2: "The mode water layer is poorly represented in the models and both mode and intermediate water have a significant fresh bias," while "in contrast to observations (e.g., Rintoul, 2007), bottom water is simulated to become slightly saltier."

Finding #3: "When compared to observation-based reconstructions," the models "exhibit a slightly larger rate of overturning at shallow to intermediate depths, and a slower rate of overturning deeper in the water column."

What it means
In the concluding words of the seven scientists, "many of the biases and future changes identified in this study are expected to have significant impacts on the marine carbon cycle." And these biases and the changes they spawn are therefore not trivial, and must clearly be corrected before they are used to forecast the future of the overturning circulation of the Southern Ocean and its impact on global climate.

References
Rintoul, S.R. 2007. Rapid freshening of Antarctic Bottom Water formed in the Indian and Pacific oceans. Geophysical Research Letters 34: 10.1029/2006GL028550.

Sarmiento, J.L., Gruber, N., Brzezinski, M.A. and Dunne, J.P. 2004. High-latitude controls of thermocline nutrients and low latitude biological productivity. Nature 427: 56-60.