Early- and late-winter ENSO teleconnections to the Euro-Atlantic region in state-of-the-art seasonal forecasting systems

Abstract

A number of recent studies have highlighted the differences in the northern extratropical response to the El Niño-Southern Oscillation (ENSO) during the early and late part of the boreal cold season, particularly over the North Atlantic/European (NAE) region. Diagnostic analyses of multi-decadal GCM simulations performed as a part of CMIP5 and CMIP6 projects have shown that early winter tropical teleconnections are usually simulated with lower fidelity than their late-winter equivalents. Although some results from individual seasonal forecasting systems have been published on this topic, it is still unclear to what extent the problems detected in multi-decadal simulations also affect initialised seasonal forecasts from state-of-the art models. In this study, we diagnose ENSO teleconnections from the re-forecast ensembles of nine models contributing (during winter 2021/22) to the multi-model seasonal forecasting system of the Copernicus Climate Change Service (C3S). The re-forecasts cover winters from 1993/94 to 2016/17, and are archived in the C3S Climate Data Store. Regression and composite patterns of 500-hPa height are computed separately for El Niño and La Niña winters, based on 2-month averages in November–December (ND) and January–February (JF). Model results are compared with the corresponding patterns derived from the ERA5 re-analysis. Signal-to-noise ratios are computed from time series of projections of individual winter anomalies onto the ENSO regression patterns. The results of this study indicate that initialised seasonal forecasts exhibit similar deficiencies to those already diagnosed in multi-decadal simulations, with a significant underestimation of the amplitude of early-winter teleconnections between ENSO and the NAE circulation, and of the signal-to-noise ratio in the early-winter response to El Niño. Further diagnostics highlight the impact of mis-representing the constructive interference of teleconnections from the Indian and Pacific Oceans in the early-winter ENSO response over the North Atlantic.