Improved atmospheric circulation over Europe by the new generation of CMIP6 earth system models

ABSTRACT: Global Climate Models (GCMs) generally exhibit significant biases in the representation of large-scale atmospheric circulation. Even after a sensible bias adjustment these errors remain and are inherited to some extent by the derived downscaling products, impairing the credibility of future regional projections. In this study we perform a process-based evaluation of state-of-the-art GCMs from CMIP5 and CMIP6, with a focus on the simulation of the synoptic climatological patterns having a most prominent effect on the European climate. To this aim, we use the Lamb Weather Type Classification (LWT, Lamb British isles weather types and a register of the daily sequence 736 of circulation patterns 1861-1971. METEOROL OFF, GEOPHYS MEM; 737 GB; DA 1972; NO 116; PP 1-85; BIBL 2P1/2, 1972), a subjective classification of circulation weather types constructed upon historical simulations of daily mean sea level pressure. Observational uncertainty has been taken into account by considering four different reanalysis products of varying characteristics. Our evaluation unveils an overall improvement of salient atmospheric circulation features consistent across observational references, although this is uneven across models and large frequency biases still remain for the main LWTs. Some CMIP6 models attain similar or even worse results than their CMIP5 counterparts, although in most cases consistent improvements have been found, demonstrating the ability of the new models to better capture key synoptic conditions. In light of the large differences found across models, we advocate for a careful selection of driving GCMs in downscaling experiments with a special focus on large-scale atmospheric circulation aspects.We acknowledge the World Climate Research Pro-gram’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Table 1) 3538 J. A. Fernandez-Granja et al.1 3for producing and making available their model output. J.A.F., A.C and J.B. acknowledge funding from the Project INDECIS, part of Euro-pean Research Area for Climate Services Consortium (ERA4CS) with co-funding by the European Union Grant 690462. J.F. acknowledges support from the Spanish R&D Program through project INSIGNIA (CGL2016-79210-R), co-funded by the European Regional Develop-ment Fund (ERDF/ FEDER). We also thank the Santander Climate Data Service (http://scds.es) and our colleagues Antonio Cofiño and Ezequiel Cimadevilla for their support. Sixto Herrera and José M. Gutiérrez provided useful comments on earlier stages of this study. Finally, we thank two anonymous referees for their insightful comments that helped to improve the original manuscrip