ENSO teleconnections with the NAE sector during boreal early-winter in CMIP5/CMIP6 models: impacts of the atmospheric mean state

The simulation of teleconnections in climate models can be hindered by biases. This study investigates the impact of model systematic errors on the teleconnections between the El Niño-Southern Oscillation (ENSO) and the North Atlantic-European (NAE) region during early winter (December), using historical simulations from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP). Generally, a positive Indian Ocean Dipole (IOD) event concurs with the onset of an El Niño event. In December, a positive IOD can initiate a Rossby wave-train that propagates from the subtropical South Asian JET (SAJET) sector toward the NAE region, where it forces an atmospheric response that resembles the positive phase of the North Atlantic Oscillation (NAO). Models that fail to simulate the early-winter teleconnection projecting onto the NAO positive phase pattern consistently exhibit a weak Rossby wave source in the SAJET region. Additionally, these models simulate an overly strong subtropical Pacific jet stream, which favors meridionally bent Rossby wave-trains. This waveguide bias is likely due to a cold northwestern Pacific, a mean state bias common to many climate models. These findings suggest that an uncertainty factor regarding the ENSO teleconnection with the NAE may stem from both a degraded Indo-Pacific inter-basin coupling and an overly strong Pacific waveguide.