Figure: Changes in Southern Ocean temperature (left) and salinity (right) over 1950 to 2015 in observations (a, b) and CanESM2 sub-sampled to the observed coverage (c, d), and CanESM2 with full coverage (e, f).

The Southern Ocean has undergone a rapid subsurface warming and freshening over the past several decades. While stratospheric ozone depletion and greenhouse gas increases are both known to play important roles in Southern Hemisphere climate change, the relative contributions of these two forcing agents to observed change in the Southern Ocean has not previously been quantified. The physical drivers of the observed changes are also unclear, though poleward shifts of the Southern Ocean fronts and changing surface fluxes have both been proposed. Recently, the research from the CLIVAR/Clic NORP panel has discovered the driving mechanism of observed temperature and salinity changes in the Southern Ocean. The result is published in the journal Nature Geoscience (Swart et al., 2018). And an associated comment article on it has been published in the “News & Views” of Nature Geosciences (Bindoff, 2018).

In their study,  a new synthesis of all available observations covering the period 1950 to 2015, which show the previously established patterns of warming and freshening (a, b) was produced. To establish the relative roles of different forcing agents, they compared the observed changes to the patterns, or fingerprints, found in various sets of historical climate model simulations, in which the model was run many times with ozone depletion only, greenhouse gas increases only, anthropogenic aerosol increases only, or natural forcing only (volcanoes and solar). Using a detection and attribution analysis, they found that the observed changes are inconsistent with internal variability or natural forcing alone. Rather, the observed changes are mostly attributable to greenhouse gas increases, with ozone depletion playing a detectable, but substantially smaller role. Physically, the model simulations suggest the observed changes in temperature and salinity are driven by increased surface fluxes of heat and freshwater, rather than frontal shifts. The model simulations also allowed us to address the question of whether the observed patterns of change are robust given the sparse observational sampling. By comparing model data sub-sampled to the observed distribution (c, d), with the fully resolved model output (e, f), they find little sensitivity of the zonal-mean and multi-decadal patterns of change to the sparse observational sampling. These results suggest that coarse resolution models can capture broad-scale thermohaline change in the Southern Ocean, and are suggestive that greenhouse gas increases will dominate over ozone recovery moving into the future.

Given the importance of the Southern Ocean in anthropogenic heat and carbon uptake, and its role in Antarctic Ice Sheet melt, understanding past and future thermohaline change is of great significance to the global climate.

Summary written by Neil C. Swart