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Modeling Southern Ocean Bottom Water Characteristics
Reference
Heuze, C., Heywood, K.J., Stevens, D.P. and Ridley, J.K. 2013. Southern Ocean bottom water characteristics in CMIP5 models. Geophysical Research Letters 40: 1409-1414.

Background
The authors write that "the ability of a model to adequately depict bottom water formation is crucial for accurate prediction of changes in the thermohaline circulation," citing Hay (1993), while also noting that "Southern Ocean deep water properties and formation processes in climate models are indicative of their capability to simulate future climate, heat and carbon uptake, and sea level rise."

What was done
To determine how mankind's best climate modelers are doing in this regard, Heuze et al. "assess how dense water is formed in climate models, and how this impacts the representation of ocean properties at the sea bed," because "the models' ability to depict the present observed state of the climate system [is] a prerequisite for reliable future prediction." So this they thus do for the Southern Ocean in terms of potential temperature, salinity, density and sea ice concentration in 15 CMIP5 historical simulations, where "20 year mean model fields are compared with historical hydrographic data on a grid spacing of 0.5° x 0.5° (Gouretski and Koltermann, 2004), and with the Hadley Centre sea ice climatologies (Rayner et al., 2003)."

What was learned
The four researchers report that "no model" - that's right, no model - "reproduces the process of Antarctic bottom water formation accurately." Rather, "instead of forming dense water on the continental shelf and allowing it to spill off," they indicate that "models present extensive areas of deep convection, thus leading to an unrealistic, unstratified open ocean."

What it means
So what is the status of the climate modeling enterprise? Has it gotten us where we need to go? The answer: Not by a long shot.

References
Gouretski, V.V. and Koltermann, K.P. 2004. WOCE Global Hydrographic Climatology, A Technical Report. Technical Report 35, BSH, Hamburg, Deutschland.

Hay, W.W. 1993. The role of polar deep water formation in global climate change. Annual Review of Earth and Planetary Sciences 21: 227-254.

Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander, L.V., Rowell, D.P., Kent, E.C. and Kaplan, A. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108: 10.1029/2002JD002670.

Reviewed 28 August 2013