Using Shortened Spin-Ups to Speed Up Ocean Biogeochemical Model Optimization

The performance of global ocean biogeochemical models can be quantified as the misfit between modeled tracer distributions and observations, which is sought to be minimized during parameter optimization. These models are computationally expensive due to the long spin-up time required to reach equilibrium, and therefore optimization is often laborious. To reduce the required computational time, we investigate whether optimization of a biogeochemical model with shorter spin-ups provides the same optimized parameters as one with a full-length, equilibrated spin-up over several millennia. We use the global ocean biogeochemical model MOPS with a range of lengths of model spin-up and calibrate the model against synthetic observations derived from previous model runs using a derivative-free optimization algorithm (DFO-LS). When initiating the biogeochemical model with tracer distributions that differ from the synthetic observations used for calibration, a minimum spin-up length of 2,000 years was required for successful optimization due to certain parameters which influence the transport of matter from the surface to the deeper ocean, where timescales are longer. However, preliminary results indicate that successful optimization may occur with an even shorter spin-up by a judicious choice of initial condition, here the synthetic observations used for calibration, suggesting a fruitful avenue for future research.