NOC Virtual Seminars: Global and regional aspects of ocean heat uptake in CMIP6 simulations of the historical climate
Ocean heat content (OHC) is one of the most relevant metrics tracking the current global heating. Because of its enormous heat capacity the global ocean stores about 90 percent of the excess heat in the Earth System. Therefore simulated OHC time series are a cornerstone for assessing the scientific performance of Earth System models and global climate models. Here we present a detailed analysis of OHC change in simulations of the historical climate (1850-2014) performed with four CMIP6 (Coupled Model Intercomparison Project phase 6) models: UKESM1, HadGEM3-GC3.1-LL, CNRM-ESM2-1 and CNRM-CM6-1. The small number of models enables us to analyse OHC change globally and for individual ocean basins. For the global ocean, the two CNRM models reproduce the observed OHC change since the 1960s closely. The two UK models (UKESM1 and HadGEM3-GC3.1-LL) do not simulate the observed global ocean warming in the 1970s and 1980s, and they warm faster than the observations after 1991. The lack of warming before 1991 is a global phenomenon while the excess warming after 1991 occurs mainly in the Atlantic. Even though all four models share the same ocean component (NEMO ORCA1), the regional ocean heat uptake characteristics differ. The UK models are characterised by a dominance of South Pacific OHC increase before 1991 and strong uptake of heat in the Atlantic after 1991, while in the CNRM models there is no such regime change and Southern Ocean OHC increase dominates throughout the historical simulation.
Figure 1. Ocean heat content anomaly in the global ocean, full depth, in the UKESM1 (red) and HadGEM3-GC3.1-LL (green) historical ensembles compared with five different observational data sets (black, claret, purple). All curves anomalised to the 10-year average 2005-2014. Ensemble means in bold lines. Shading around two of the observational data sets indicates the provided standard error. Ensemble member r17 plotted in teal.