Characterizing Spatial Structure in Climate Model Ensembles

This paper presents a methodology that is designed for rapid exploratory analysis of the outputs from en-sembles of climate models, especially when these outputs consist of maps. The approach formalizes and extends the tech-nique of "intermodel empirical orthogonal function" analysis, combining multivariate analysis of variance techniques with singular value decompositions (SVDs) of structured components of the ensemble data matrix. The SVDs yield spatial pat-terns associated with these components, which we call ensemble principal patterns (EPPs). A unique hierarchical partition-ing of variation is obtained for balanced ensembles in which all combinations of factors, such as GCM and RCM pairs in a regional ensemble, appear with equal frequency: suggestions are also proposed to handle unbalanced ensembles without imputing missing values or discarding runs. Applications include the selection of ensemble members to propagate uncer-tainty into subsequent analyses, and the diagnosis of modes of variation associated with specific model variants or para-meter perturbations. The approach is illustrated using outputs from the EuroCORDEX regional ensemble over the United Kingdom.