The Role of Coupled Versus Uncoupled Reanalyses for Initialization of Decadal Climate Predictions

• Applicant: Dr. Iuliia Polkova
• Subject Area: Oceanography
• Term: from 2019 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 436413914

The need for reliable climate prediction is growing in demand for various socio-economic sectors. Coincidently, studies that investigate predictability of the climate system at interannual to decadal timescales demonstrated that the initialization of Earth System Models (ESMs) with the available oceanic and atmospheric observations can improve the ability of models to predict regional surface temperature up to several years ahead. However, the decadal predictions are shown to suffer from model errors and initialization shocks which limit predictability. The magnitude of this effect remains unclear. To eliminate initialization shocks and improve decadal predictions, we need ESMs that are able to estimate coupled initial conditions in a dynamically consistent way. The current project will be concerned with quantifying the impact of dynamically balanced initial conditions on the decadal prediction skill.Among a variety of coupled data assimilation (CDA) methods, the adjoint method is one of the most promising because its result is dynamically consistent with the ESM’s equations. Thus, a dynamically self-consistent space-time evolution of the climate state produced with the adjoint is suitable for initialization of decadal predictions. To this end, we will use the coupled adjoint model developed for the ESM of intermediate complexity CESAM (Centrum für Erdsystemforschung und Nachhaltigkeit Erdsystem Assimilations-Modell) to produce a coupled ocean-atmosphere reanalysis, which will serve as a source of initial conditions for ensembles of retrospective decadal predictions. These simulations will then be compared against the available observations to estimate the prediction skill of CESAM. A decadal prediction framework, which will be implemented within CESAM, will focus on the multiannual variability of large-scale processes and feedbacks acting within the coupled climate system. The advantage of the proposed research is the availability and the computational efficiency of the adjoint and forward CESAM. As programming adjoint codes for ESMs is not a trivial task, it is currently hindering widespread application of the adjoint-based assimilation for initialization of decadal predictions. Therefore, the merits of the adjoint CDA for decadal predictions are still to be understood and demonstrated; this is the major objective of the proposed research project. Thus, in a model-consistent approach, the project will compare initializations based on the coupled ocean-atmosphere reanalysis and based on the widespread strategy in decadal prediction studies of nudging toward the un-coupled reanalyses. Results of the project will serve to guideline future initialization developments for comprehensive ESMs.

• DFG Programme: Research Grants
• Co-Investigators: Dr. Armin Koehl; Professor Dr. Detlef Stammer