An energy conservation analysis of ocean drift in the CMIP5 global coupled models

Climate model simulations of changes to the Earth’s energy budget are fundamental to improving our understanding of both historical and future climate change. However, coupled models are prone to ‘drift’ i.e. spurious unforced trends in state variables, due to incomplete spin-up or non-closure of the energy budget. In this work we assess the globally integrated energy budgets of 25 models in the CMIP5 suite. We show that for many of the models there is a significant disagreement between ocean heat content changes and net top-of-atmosphere radiation. The disagreement is largely time-constant and independent of forcing scenario. Furthermore, the most of the non-conservation seems to occur as a result of energy leaks external to the ocean model realm. After drift correction, the time varying-energy budget is consistent at decadal time scales, and model responses to climate forcing are not sensitive to the magnitude of their drift. This demonstrates that although drift terms can be significant, model output can be corrected post-hoc without biasing results.