Background Pycnocline Depth Constrains Future Ocean Heat Uptake Efficiency

The Ocean Heat Uptake Efficiency (OHUE) quantifies the ocean's ability to mitigate surface warming through deep heat sequestration. Despite its importance, the main controls on OHUE, and on its two-fold spread across contemporary climate models, remain unclear. We argue that OHUE is primarily controlled by mid-latitude ventilation strength in the background climate, itself related to pycnocline depth and stratification. This hypothesis is supported by a strong correlation between mid-latitude (30-60 degrees) OHUE and the near-global average (60 degrees S-60 degrees N) pycnocline depth in CMIP5 and CMIP6 AOGCMs under RCP85/SSP585, and in a parameter perturbation ensemble of ocean GCM (MITgcm) experiments. This correlation explains about 70% of the CMIP5-6 spread in global OHUE. The relationship provides a pathway toward observationally constraining OHUE, and thus reducing uncertainty in projections of future global climate change and sea level rise. The ocean absorbs most of the excess heat in the climate system. How effectively this process reduces surface warming depends on how deeply this heat is stored in the ocean, which varies widely across contemporary climate models. Our study shows that around 70% of the variation in deep heat storage across models is explained by differences is how water from the mid-latitude surface ocean is transported to deeper ocean layers by ocean currents. This difference can be measured by the stratification of seawater density in the base-state climate, a feature that can be observed in the modern ocean. We show this relationship using a new regional decomposition method. The results of this study can be leveraged with ocean observations to reduce uncertainty in future climate and sea level rise projections. Pycnocline depth correlates strongly with Ocean Heat Uptake Efficiency (OHUE) in CMIP5/CMIP6 and MITgcmA regional OHUE decomposition shows that mid-latitude heat uptake and sequestration drives the correlation between OHUE of pycnocline depthInter-model differences in pycnocline depth explain around 70% of the spread in OHUE across CMIP5 and CMIP6