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. Plain Language Summary: The Pre‐industrial control simulation of climate models is a reference for climate change simulations. Consequently it should simulate a stable climate. However, if the model is not in radiative equilibrium, the ocean continually accumulates heat, becoming unstable at high latitudes and generating large climate fluctuations. This results in a control simulation that is not a steady state. Consequently the methodologies used to initialize and formulate climate models should be revised to ensure a steady state is achieved. Key Points: The long timescales associated with deep ocean heat uptake suggests simulations may not be equilibriated before forcing is perturbedSouthern Ocean venting of heat reduces sea ice cover and increases global mean temperature, leading to a warmer climate stateNet top‐of‐atmosphere energy imbalance can have implications for the methodologies used for climate model spin‐up [ABSTRACT FROM AUTHOR]