Near-term Mediterranean summer temperature climate projections: a comparison of constraining methods

Abstract

There are several methods to constrain multimodel projections of future climate. This study assesses the quality of four constraining methods in representing the near-term summer temperature projections of the Mediterranean region. Three are based on phasing in ocean surface temperature variations based on observations or decadal predictions, and the method is based on the measuring performance and independence of the individual simulations. The comparison has been carried out with a new framework inspired by the forecast quality assessment of decadal predictions. The framework led to quality estimates of the constrained projection approaches obtained by producing 20-yr temperature estimates every year from 1970 to 2000 and computing quality metrics against observational references. The evaluation results show some differences between constraining approaches. The improvement or deterioration against quality measures of the full, unconstrained, phase 6 of the Coupled Model Intercomparison Project (CMIP6) ensemble shows strong spatial heterogeneity. From the analysis of the selection approaches it is found that the constraints based on sea surface temperature (SST) fields are affected not only by the variability but also by the warming trend. The weighting method generally shows small quality differences with respect to the full CMIP6 ensemble. Despite caveats of the different methods, there is potential to improve the near-term climate projections as some significant quality enhancements were found in some approaches according to the evaluation metrics used. This study suggests a good understanding of the constraining methods, and their forecast quality is required before using them to take informed decisions. Our study opens the door to optimizing these methods for the Mediterranean climate and highlights the need for evaluating the constraints through retrospective assessments against observational references.