Signal-to-noise errors in early winter Euro-Atlantic predictions linked to weak ENSO teleconnections and pervasive North Atlantic jet biases

Long-range winter predictions over the Euro-Atlantic sector have demonstrated significant skill but suffer from systematic signal-to-noise errors. Here, we examine sources of early winter seasonal predictability in across state-of-the-art seasonal forecasting systems. As in previous studies, these systems demonstrate skill in the hindcasts of the large-scale atmospheric circulation in early winter, associated with the East Atlantic pattern. The predictability is strongly tied to the ENSO teleconnection to the North Atlantic, though the systems' response to ENSO is systematically too weak. The hindcasts of the East Atlantic index exhibit a substantial signal-to-noise errors, with the systems' predicted signal generally being smaller than would be expected for the observed level of skill, though there is substantial spread across systems. The signal-to-noise errors are found to be strongly linked to the strength of the ENSO teleconnection in the systems, those with a weaker teleconnection exhibit a larger signal-to-noise problem. The dependency on modelled ENSO teleconnection strength closely follows a simple scaling relationship derived from a toy model. Further analysis reveals that the strength of the ENSO teleconnection in the systems is linked to climatological biases in the behaviour of the North Atlantic jet.