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Featured Science Paper

Assessment of Southern Ocean mixed-layer depth in CMIP5 models: historical bias and forcing response

Severe weather conditions over the Southern Ocean create thick, well-mixed layers at the ocean surface that are a crucial pathway between the atmosphere and the deeper layers of the ocean. Anthropogenic carbon and heat are drawn down through these thick surface layers and exported in the deep seas for decades to centuries through the large-scale ocean circulation. Our ability to understand and ultimately predict climate is therefore dependent on the ability of climate models to correctly represent both the thick surface layers of the Southern Ocean and its large-scale circulation.

This pair of papers systematically evaluates the representation of the surface layer and Southern Ocean circulation in more than 20 climate models participating in the next IPCC assessment, and identifies the physical processes causing differences between the models. The models consistently produce surface layers that are too shallow and too light, caused by insufficient evaporation over the Southern Ocean. The bias in the surface layer translates into a shortage of newly formed water masses, which are key for the sequestration of heat and anthropogenic carbon. Under increased radiative forcing, such as will happen as greenhouse gas concentrations rise in the atmosphere, the models have generally a consistent response, with an increased volume of deep water uplifted near the ocean surface and an increase in production of younger waters. The results of these papers are important for the IPCC groups to assess the robustness of the representation of heat and carbon cycles in a changing climate, and to pinpoint specific processes to improve in future generations of models.

Link to the full paper in the NERC Open Research Archive


Authors

Sallee, J.-B., Shuckburgh, E., Bruneau, N., Meijers, A.J.S., Bracegirdle, T.J., Wang, Z.

Publication

Journal of Geophysical Research, 118, (4), 1845-1862. 2013