Inconsistencies of signatures of Eurasian heat waves in the large-scale Rossby waves in CMIP models

This study identifies discrepancies in signatures of the Eurasian heat waves (EHWs) in the reanalyses and climate models. The scale-dependent analysis considers the global Rossby wave spectrum for the extended boreal summer using daily values of the Rossby wave mechanical energy. We filter Rossby waves using a multivariate, 3D projection of the horizontal velocity and geopotential fields onto a set of orthogonal normal-mode functions. Our previous study has found that Rossby wave energy follows a χ2-distribution with skewness related to the number of degrees of freedom. During EHWs, the skewness of the normalised energy anomaly distributions increases with a corresponding decrease in the number of active degrees of freedom, implying fewer modes involved. Fewer modes indicate a blocking structure that is identified as an increase in the Rossby wave amplitude in the middle troposphere during EHWs. Here, we compare the spatial structure and energy distributions of the troposphere-barotropic Rossby waves in the subset of CMIP5 models with findings from reanalyses. The increase in planetary Rossby wave amplitudes is first identified for the present-day climate. Significant differences among the models are found regarding the change in the skewness of the Rossby wave energy distribution and hence the number of active degrees of freedom during EHWs. The results highlight inconsistencies in simulating the day-to-day variability of planetary Rossby waves during EHWs in the CMIP models despite the overall agreement in the mean circulation and EHW metrics based on surface data reported in previous studies. The results of this study have potential implications for the interpretation of projected changes in Rossby waves and EHWs in future CMIP climate simulations.