Fifty Year Trends in Global Ocean Heat Content Traced to Surface Heat Fluxes in the Sub-Polar Ocean

The ocean has absorbed approximately 90% of the accumulated heat in the climate system since 1970. As global warming accelerates, understanding ocean heat content changes and tracing these to surface heat input is increasingly important. We introduce a novel framework by organizing the ocean into temperature-percentiles from warmest to coldest, allowing us to trace ocean temperature changes to changes in surface fluxes and mixing. Applying this framework to observations and historical CMIP6 simulations, we find that 50 +/- 6% of surface heat uptake between 1970 and 2014 is confined to isotherms in the coldest 90% of the ocean volume. These isotherms outcrop over only 23% of the ocean's surface area in the sub-polar regions, implying a disproportionately large heat input per unit area. Additionally, a cooling bias in the CMIP6 models is traced to inaccurate sea surface temperatures and surface heat fluxes into the warmest 5%-20% of the ocean volume.