Assessing Northeast Pacific and African Easterly Wave Representation in CMIP6 models

The representation of easterly wave (EW) structure over the east Pacific (PEWs) and Atlantic (AEWs) Ocean during boreal summer in global climate models is examined. Coupled Model Intercomparison Project phase 6 (CMIP6) simulations are compared to the Fifth Reanalysis from the European Centre for Medium-Range Weather Forecasts (ERA5) data. The leading modes are attained on the basis of empirical orthogonal function (EOF) analysis and projections of outgoing longwave radiation (OLR). Lag-regression and pattern correlations analysis were used to determine the model skill in reproducing the observed amplitude, spatial structure, and evolution of PEWs and AEWs. Over the east Pacific Ocean (EPAC), a higher variability in model skill was observed with simulations exhibiting good-to-poor EW representation, whereas good-to-moderate skill was observed for AEWs. Further assessment of PEW revealed that good models exhibit a stronger and narrower intertropical convergence zone, with a stronger meridional gradient in lower free troposphere (LFT) specific humidity. Both good and poor simulations struggled to reproduce the observed horizontal EW LFT temperature structure and mean state, especially over the Panama bight region where insitu generation of waves is most prominent. Vertical moist static energy profiles reveal that both good and poor models have a dry bias, with the highest differences observed across the LFT. Results suggest that the misrepresentation of PEWs could be tied to underlying thermodynamic biases in the models, underscoring the relevance of thermodynamic drivers in the associated EW convection.