Ocean Heat Content in Warm Climates: Pliocene Simulations and Future Comparison

In the present day, global oceans have absorbed most of the excess anthropogenic heat, abating surface temperature warming. The Mid-Pliocene Warm Period (MPWP; ~3.2 million years ago) is often cited as a potential analogue for future climate change due to atmospheric CO₂ levels similar to today (~400 ppm) and global mean surface temperature ~2-4°C warmer than pre-industrial; the MPWP therefore offers a good opportunity to understand how a globally warmer climate stores oceanic heat. We use the PlioMIP2 model ensemble to quantify global ocean heat content (OHC), defined as that in the 0-700 m fixed-depth layer, to present an overview of the spatial characteristics of Indo-Pacific OHC during the MPWP, and to compare global OHC to future scenarios. Simulated MPWP OHC is globally higher than the pre-industrial except for in the Arabian Sea, where the lower OHC is attributed to weakened Northeast monsoon wind strength. Using the dt/dz definition of the thermocline, we find that the thermocline shoals over much of the northern Indian Ocean, including in the Arabian Sea, and deepens in the South China Sea; the equatorial Pacific thermocline warms by ~2°C from pre-industrial to the MPWP without deepening. Globally, MPWP OHC exceeds that of the highest SSP5-8.5 future scenario for the late 21st century (2081-2100). This suggests that the ocean can absorb substantial amounts of heat, though the dynamics of heat uptake remain important for abating surface temperature warming given potential nonlinearities.