Will climate change stir up the Southern Ocean?

The Southern Ocean is a very important sink for carbon dioxide. But a change in the depth of its mixed layer – the uppermost part of the ocean – as a result of climate change could influence two important processes of carbon capture. Firstly, a decrease in mixed-layer depth would reduce the amount of carbon ventilation, lowering carbon dioxide uptake. On the other hand, the same decrease in mixed-layer depth would enable more abundant biological life, resulting in an increased uptake and sedimentation of carbon. The recent Intergovernmental Panel on Climate Change (IPCC) report (Denman et al. 2007) discussed both effects and concluded that the first is more important. This means that a decrease in mixed-layer depth would diminish the role of the Southern Ocean carbon sink.

The mixed layer undergoes intense mixing by surface waves, wind-driven turbulence and convection due to cooling, evaporation at the surface or the addition of brine by sea-ice formation. Due to this mixing, it is homogenous for variables like temperature, salinity and nutrients. The thickness of the layer varies on a whole range of timescales. In the Southern Ocean the mixed layer is generally quite thick, especially in winter. This seasonal thickening is related to cooling, as well as to the expulsion of salt out of the crystal lattice when sea ice is forming, which makes the surface waters more salty and increases their density so that they sink, initiating more profound mixing.

Until now, only a few studies have looked at changes in the mixed-layer depth due to increases in greenhouse gases. And just two of them – Bitz et al. 2006 and Gnanadesikan et al. 2007 – used a current state-of-the-art model. What’s more, researchers have only made qualitative assessments, despite the fact that it’s very important to quantify the possible future changes in the mixed-layer depth. Our recent study (Lefebvre and Goosse 2008) tries to address these problems.

Using 17 coupled models performed for the Fourth Assessment Report of the IPCC, we studied the mixed-layer depth in both the 20th and 21st centuries. For the 21st century we took a moderate scenario (A1B), corresponding to a continuous increase of carbon dioxide concentration up to a level of 720 ppm in 2100.

The study showed that the median of all the models can reproduce mixed-layer depth well. This is especially true in winter, whereas in summer the patches with a thick mixed layer observed in the real world seem to be absent in the median of all the models. This could be due to the averaging procedure, which decreases the spatial variability of the model results. The observations could also be patchy due to lack of sufficient data in many regions. The median of the model results was chosen, instead of the more commonly used mean to eliminate the influence of outliers with an anomalous thick mixed layer. The following features of the observed mixed-layer depth are well reproduced in this median value.

• The bands of shallow mixed layers close to the Antarctic continent and around 30°S, and the band of high mixed-layer depth in between.
• The larger mixed-layer depth in winter than in summer.
• The magnitude of the mixed-layer depth – the model results are in general well in the range of the observed values.

A majority of the models indicate a decrease in mixed-layer depth during the 21st century, as described before for some models. The same effect is seen even more clearly when looking at changes in stratification – defined here as the potential density difference between the surface and 300 m depth. There is a close link between stratification and mixed-layer depth. When stratification increases, the mixed-layer depth decreases and vice versa. But this relation is also influenced by other factors, for instance the strength of the pycnocline.

Models with a large mixed-layer depth during the 20th century tend to change more during the 21st century than those with a smaller mixed layer. As a result, the spread between the models decreases.

The changes in mixed-layer depth also appear to be much smaller in summer than in winter. In other words, the changes are larger in a season with a high mixed-layer depth, and the annual cycle of the mixed-layer depth decreases in amplitude. In summer the mean changes are smaller than the average model grid vertical resolution. As a result questions can be raised about the viability of the quantitative estimations. Arriving at these estimations involves applying several mathematical methods to the data and comparing their results. If these results are close to each other, the robustness of the results is quite high and a large degree of confidence is attached to the results. The different applied methodologies are listed below.

• Mean of all the models.
• Mean of all the models for the best three or best five models. The best models have been defined as those closest to the observations. For each comparison two observational datasets (Levitus 1998 and de Boyer Montégut et al. 2004) were used, so this method supplies four different estimations.
• The regression of the changes in mixed-layer depth on the mixed-layer depth value in the 20th century. Introducing the observed values (of both Levitus and de Boyer Montégut) in this regression provides a quantification of the changes.
• The median of all the model changes.

Specifically, we looked at three bands.

• The 70-60°S band. Here, close to the Antarctic continent, the spread in model results is large, with estimations ranging from no change to a mixed-layer depth decrease of 50 m in winter and small changes in summer.
• The 60-50°S band, where the annual mean changes in mixed-layer depth amount to 3-7 m, ranging from 0-3 m in summer to the order of 10-20 m in winter.
• The 50-40°S band. This band shows a maximum decrease in mixed-layer depth of 4 m in summer and 10 m in winter.

Finally, we showed that the dominant source of the changes in stratification, which is closely linked to the mixed-layer depth, is different in the northern and southern parts of the Southern Ocean. In the north, temperature changes are the most important factor. Warming of the ocean is seen both at the surface and below, but the change in the subsurface is smaller. In addition, because of the shape of the equation of state, the density change due to an increase in temperature is larger for high temperatures than for lower temperatures. As a result, stratification increases and the mixed-layer depth decreases. Further south, salinity changes are predominant. These may be attributed to changes in the hydrology cycle as there is more precipitation in the south, or to changes in both sea-ice formation and salinity fluxes. These points need further investigation.

In conclusion, despite an increase in stratification the changes in mixed-layer depth are quite small. Changes are mostly of the order of 10 m or lower, compared with a winter mixed layer that goes up to several 100 m, which means that their effect on the climate is probably limited. As a result other effects, like the influence of change in wind on three-dimensional ocean circulation, could be more important for changes in the Southern Ocean carbon sink. However, it is clear that the effect of climate change on sink properties needs further study.