Collapse of deep overturning under future climate forcing and impacts on ocean heat and carbon uptake in CMIP6 models

Under future climate forcing, in both strong emission (SSP585) and strong mitigation (SSP126) scenarios, the deep overturning circulation of the Southern Ocean collapses by 2050 in almost all CMIP6 simulations.  The SSP scenario applied ultimately has a relatively small impact on this shutdown, which appears to have commenced in the last few decades of the historical scenario runs – the present day.  The ensemble mean drops to 50% (10±4 Sv) strength, from a pre Industrial strength of 20±9 Sv, with individual models decreasing by over 65%.    This ‘collapse’ occurs over 30-50 years, with most of the temporal variation explained by internal variability within models. 

Associated with the change in overturning strength is a reduction in Antarctic Bottom Water (AABW) volume south of 30S by >2x10¹⁶ m³, driven by a reduction in formation rates around Antarctica.  Here surface warming reduces formation by approximately 4 Sv, whilst freshwater effects (due to sea ice reduction for example) are relatively weak.  Walin analysis shows that subsequent entrainment of Circumpolar Deep Water (CDW) by convecting AABW also significantly decreases, further reducing AABW volume and export.

The reduction in AABW formation results in an expansion of CDW volumes.  CDW upwelling reduced due to the lower cell shutdown is largely compensated by a wind driven increase in upper cell overturning of approximately 25%.  The increased upper cell overturning enhances SAMW export by up to 5 Sv, with a significant boost to net heat export due to both enhanced volumes and temperatures of this water mass.

We explore the wider impact of these Southern Ocean shifts in overturning on the net warming and carbon storage of the global ocean, and the wider global climate.  We contextualize climate model representations of bottom water processes against observations and other model studies to suggest that a deep overturning tipping point may have already been reached.