Underestimated MJO Variability in CMIP6 Models

The Madden‐Julian Oscillation (MJO) is the leading mode of intraseasonal climate variability, having profound impacts on a wide range of weather and climate phenomena. Here, we use a wavelet‐based spectral Principal Component Analysis (wsPCA) to evaluate the skill of 20 state‐of‐the‐art CMIP6 models in capturing the magnitude and dynamics of the MJO. By construction, wsPCA has the ability to focus on desired frequencies and capture each propagative physical mode with one principal component (PC). We show that the MJO contribution to the total intraseasonal climate variability is substantially underestimated in most CMIP6 models. The joint distribution of the modulus and angular frequency of the wavelet PC series associated with MJO is used to rank models relatively to the observations through the Wasserstein distance. Using Hovmöller phase‐longitude diagrams, we also show that precipitation variability associated with MJO is underestimated in most CMIP6 models for the Amazonia, Southwest Africa, and Maritime Continent. Plain Language Summary Dominant modes (i.e., coherent spatio‐temporal patterns of variability) of the climate system, such as the Madden‐Julian Oscillation (MJO), influence a wide range of weather and climate phenomena worldwide. The ability of state‐of‐the‐art climate models to accurately simulate these modes is crucial for advancing our understanding of the climate system and reliably predicting its future trends. The Coupled Model Intercomparison Project Phase 6 (CMIP6) will be the foundation for the upcoming Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6). Here, we use a wavelet‐based spectral principal component analysis (wsPCA) to quantitatively assess how well historical simulations from 20 CMIP6 models capture MJO as compared to the observations. We first show that the MJO magnitude is not reproduced well in most of CMIP6 models. We then reveal that MJO‐related precipitation variability in the Amazonia, Southwest Africa, and Maritime Continent is significantly underestimated in many CMIP6 models. Our results highlight the need to better simulate the coupled ocean‐atmosphere dynamics in order to improve the representation of MJO in climate models and increase confidence in projected states of MJO for assessing future tropical and extratropical impacts. Key Points A wavelet‐based spectral principal component analysis is used to examine CMIP6 models in reproducing the Madden‐Julian Oscillation (MJO) CMIP6