Spin-up of UK Earth System Model 1 (UKESM1) for CMIP6

For simulations intended to study the influence of anthropogenic forcing on climate, temporal stability of the Earth's natural heat, freshwater, and biogeochemical budgets is critical. Achieving such coupled model equilibration is scientifically and computationally challenging. We describe the protocol used to spin-up the UK Earth system model (UKESM1) with respect to preindustrial forcing for use in the sixth Coupled Model Intercomparison Project (CMIP6). Due to the high computational cost of UKESM1's atmospheric model, especially when running with interactive full chemistry and aerosols, spin-up primarily used parallel configurations using only ocean/land components. For the ocean, the resulting spin-up permitted the carbon and heat contents of the ocean's full volume to approach equilibrium over 5,000 years. On land, a spin-up of 1,000 years brought UKESM1's dynamic vegetation and soil carbon reservoirs toward near-equilibrium. The end-states of these parallel ocean- and land-only phases then initialized a multicentennial period of spin-up with the full Earth system model, prior to this simulation continuing as the UKESM1 CMIP6 preindustrial control (piControl). The realism of the fully coupled spin-up was assessed for a range of ocean and land properties, as was the degree of equilibration for key variables. Lessons drawn include the importance of consistent interface physics across ocean- and land-only models and the coupled (parent) model, the extreme simulation duration required to approach equilibration targets, and the occurrence of significant regional land carbon drifts despite global-scale equilibration. Overall, the UKESM1 spin-up underscores the expense involved and argues in favor of future development of more efficient spin-up techniques.