ENSO teleconnections with the NAE sector during December in CMIP5/CMIP6 models: impacts of the atmospheric mean state

This study examines how atmospheric mean state biases influence the El Niño-Southern Oscillation (ENSO) teleconnections with the North Atlantic-European (NAE) region, using ERA5 reanalysis and a large ensemble of historical simulations from CMIP5 and CMIP6 models. By isolating the contributions from the Niño 3.4 and Tropical Western-Eastern Indian Ocean (TWEIO) regions, we find that in November, the ENSO teleconnection dominates and projects onto the positive phase of the North Atlantic Oscillation (NAO). Later in December, the TWEIO teleconnection becomes dominant, reinforcing the positive NAO phase through a zonal wavenumber-3 Rossby wave train originating from SouthEast Asia (SEA). In general, a systematic improvement in the simulation of the ENSO teleconnection in CMIP6 models compared to CMIP5 is observed, particularly during December. By implementing a clustering approach, we show that models failing to capture the December ENSO teleconnection with the NAE region consistently exhibit a weak Rossby wave source over SEA and overly strong subtropical jet streams in the Pacific and Atlantic Oceans. Using a Rossby wave ray tracing algorithm, we show that enhanced subtropical waveguides in these models effectively trap Rossby waves at lower latitudes, thereby limiting their influence on the NAE region. These stronger subtropical jets are likely linked, via thermal wind balance, to cold sea surface temperature biases in the North Pacific and North Atlantic sectors, common mean state errors in earlier-generation climate models. On the other hand, the underestimated SEA Rossby wave source might be the consequence of a poorly simulated ENSO inter-basin connection with the Indian Ocean.