Observed decadal ocean warming (2005-2014) and its contribution to the top of atmosphere energy budget

Abstract:

Quantifying ocean heat content and its change is essential for understanding the response of the climate system to radiative forcing because the oceans are the dominant reservoir of heat on Earth. Here we investigate the 2005-2014 ocean heat content change inferred by Argo gridded products provided by three different groups. Overall, we demonstrate a good agreement among the different products despite a few discrepancies at global and regional scales. We found a decadal increase of ocean heat content of 8.19 +/- 0.68 J/yr. This corresponds to an ocean heat uptake of 0.74 +/- 0.06 W/m2, in good agreement with the IPCC-AR5 estimate report of 0.71W/m2 for 1993-2010. As the Argo coverage is not perfect (coastal regions, marginal seas and semi-enclosed seas are not well sampled by the floats), we compare the global ocean heat content (0-2000m) with and without the aforementioned regions using the ORAS4 ocean reanalysis and find no significant differences between the two estimates. We then show that the Southern hemisphere explains about 90% of the net global ocean heat content increase during 2005-2014 while the Northern hemisphere shows large interannual variability and explains only 10% of the net increase. We also find that the maximum warming is centered at 40°S with two main structures located in the Indian and Pacific oceans, suggesting a possible heat penetration due to the wind-driven mean ocean circulation. We finally compare the annually averaged ocean heating rates with the net TOA flux inferred by CERES and the atmospheric reanalysis ERA-Interim. We find that ocean heating rates show large interannual variability associated with a large spread around the mean estimate (based on the three gridded T/S products). We confirm that the mean ocean heating rate is comparable to the net TOA flux for the last decade within observational uncertainties. However, we find a discrepancy for 2013 that could be due to unsampled regions and/or the significant contribution to another climate reservoir.