The aim of this project is to find a set of model versions that can shadow recent surface and subsurface ocean changes. Once this model set is identified the range of uncertainty in the Atlantic Meridional Overturning Circulation (AMOC) for these model versions will be assessed.
This experiment uses HadCM3, a coupled model with fully dynamic atmosphere and ocean components, making it the most comprehensive climate model available on climateprediction.net (it is the higher-resolution counterpart of FAMOUS, which has been used in other CPDN experiments). It is run without flux adjustments which “nudge” the climate towards a realistic state, but have an adverse effect on important ocean processes. HadCM3 has been used extensively for climate research, and was one of the models used in the IPCC fourth assessment report.
For this experiment we will use a very substantial ensemble of over 100,000 HadCM3 model restart files, associated with several thousand model parameter configurations, spanning a very large range of model states. Ordinarily, HadCM3 has to be run on a supercomputer, meaning that it can run fast, but only a few ensemble members are possible (of order tens). With your help, running HadCM3 on climateprediction.net means that we can have a much, much larger ensemble, which is needed to properly explore the model’s capabilities.
Initially we will rank the ensemble by consistency with ocean surface temperature observations. Simulations that are inconsistent with observations are disregarded. Simulations that are consistent with observations will then be evaluated for consistency with subsurface temperature observations. We aim to identify approximately 1,000 HadCM3 model-states that are consistent with December 1992 observations. These model states will be subject to initial condition perturbations (to the atmosphere) to generate a 50,000-member ensemble and integrated forwards for a further year before being re-evaluated. Ensemble members that remain consistent will then be re-perturbed and in this way a set of model versions that are consistent with observations identified. The advantage of identifying this set of model versions is that ocean-atmosphere interactions can be simulated in future experiments.
Once this set of model versions is identified the range of AMOC trends can be assessed and compared to the observations obtained during another part of the RAPID-WATCH programme.
ChAAOS (Change in the Atlantic Atmosphere-Ocean System) is part of NERC’s RAPID-WATCH programme. The project involves scientists from the National Oceanography Centre, Durham University, University of Reading, Met Office, University of Oxford, British Antarctic Survey, Imperial College, and London School of Economics.