Publications

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2018
Shi, JR, Xie SP, Talley LD.  2018.  Evolving relative importance of the Southern Ocean and North Atlantic in anthropogenic ocean heat uptake. Journal of Climate. 31:7459-7479.   10.1175/jcli-d-18-0170.1   AbstractWebsite

Ocean uptake of anthropogenic heat over the past 15 years has mostly occurred in the Southern Ocean, based on Argo float observations. This agrees with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), where the Southern Ocean (south of 30 degrees S) accounts for 72% +/- 28% of global heat uptake, while the contribution from the North Atlantic north of 30 degrees N is only 6%. Aerosols preferentially cool the Northern Hemisphere, and the effect on surface heat flux over the subpolar North Atlantic opposes the greenhouse gas (GHG) effect in nearly equal magnitude. This heat uptake compensation is associated with weakening (strengthening) of the Atlantic meridional overturning circulation (AMOC) in response to GHG (aerosol) radiative forcing. Aerosols are projected to decline in the near future, reinforcing the greenhouse effect on the North Atlantic heat uptake. As a result, the Southern Ocean, which will continue to take up anthropogenic heat largely through the mean upwelling of water from depth, will be joined by increased relative contribution from the North Atlantic because of substantial AMOC slowdown in the twenty-first century. In the RCP8.5 scenario, the percentage contribution to global uptake is projected to decrease to 48% +/- 8% in the Southern Ocean and increase to 26% +/- 6% in the northern North Atlantic. Despite the large uncertainty in the magnitude of projected aerosol forcing, our results suggest that anthropogenic aerosols, given their geographic distributions and temporal trajectories, strongly influence the high-latitude ocean heat uptake and interhemispheric asymmetry through AMOC change.

Liu, W, Lu J, Xie SP, Fedorov A.  2018.  Southern Ocean heat uptake, redistribution, and storage in a warming climate: The role of meridional overturning circulation. Journal of Climate. 31:4727-4743.   10.1175/jcli-d-17-0761.1   AbstractWebsite

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60 degrees S and is stored around 45 degrees S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60 degrees S coupled to a surface heat flux increase. In contrast, at 45 degrees S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46 degrees S at a rate of 0.07 ZJ yr(-1) (degrees lat)(-1) (1 ZJ = 10(21) J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42 degrees S at a rate of 0.30 ZJ yr(-1) (degrees lat)(-1), accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.