A Change in Climate State During a Pre-Industrial Simulation of the CMIP6 Model HadGEM3 Driven by Deep Ocean Drift

The Pre-Industrial climate model simulation is intended as an equilibrium control experiment that uses constant external forcing, yet in the Coupled Model Intercomparison Project Phase 6 (CMIP6) model HadGEM3-GC31-LL a distinct change in climate state occurs around year 500 of the 2000 year simulation. The global mean near surface air temperature increases by almost 0.5 K associated with a reduction in southern hemisphere sea ice area of almost 20%. Here we show this step change in the state of the climate to be a consequence of the onset of deep convection in the Weddell and Ross Sea gyres. The delayed onset of convection in the gyres is a consequence of a positive, downward, top-of-atmosphere radiative balance, and continual ocean heat up-take during the model spin-up and the initial pre-industrial control simulation. Consequently, model spin-up strategy should be revised to initialize pre-industrial simulations that are in energy balance.

Automated summary

This study finds a distinct change in climate state in the Pre-Industrial climate model simulation of the CMIP6 model HadGEM3-GC31-LL around year 500. The onset of deep convection in the Weddell and Ross Sea gyres is identified as the cause of this step change, which is delayed due to a positive radiative balance and continuous ocean heat uptake during the model spin-up and initial pre-industrial control simulation.