Background Pycnocline depth constrains Future Ocean Heat Uptake Efficiency

TL;DR

This study reveals that the depth of the pycnocline in the background climate significantly influences the ocean's capacity to absorb heat, thereby affecting future climate change mitigation efforts.

Contribution

It demonstrates a strong correlation between pycnocline depth and Ocean Heat Uptake Efficiency across models, highlighting a key control on climate model spread.

Findings

OHUE is strongly correlated with pycnocline depth in models.

Mid-latitude ventilation controls global heat uptake.

Model spread in OHUE is largely due to pycnocline variability.

Abstract

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, as well as its nearly two-fold spread across contemporary climate models, remain unclear. We argue that OHUE is primarily controlled by the strength of mid-latitude ventilation in the background climate, itself related to subtropical pycnocline depth and ocean stratification. This hypothesis is supported by a strong correlation between OHUE and pycnocline depth in the CMIP5 and CMIP6 under RCP85/SSP585, as well as in MITgcm. We explain these results through a regional OHUE decomposition, showing that the mid-latitudes largely account for both: (1) global heat uptake after increased radiative forcing and; (2) the correlation between pycnocline depth and OHUE. Coupled with the nearly equivalent inter-model spreads in OHUE/pycnocline depth, these results imply that mid-latitude ventilation also dominates the ensemble spread in OHUE. Our results provide a pathway towards observationally constraining OHUE, and thus future climate.