Sub-Seasonal Variability of ENSO Teleconnections in Western North America and Its Prediction Skill

The El Niño–Southern Oscillation (ENSO) significantly affects the wintertime surface climate in the western North America (WNA) via the Pacific/North American-like teleconnection pattern. Here, we show that the ENSO teleconnections over the WNA have significant sub-seasonal variation despite the persistence of ENSO-related tropical sea surface temperature anomalies throughout the entire winter. The ENSO–WNA teleconnections become substantially weakened in mid-winter, considerably weaker than those in early and late winter. This mid-winter weakening of the ENSO–WNA teleconnections is attributable to the convection over the equatorial Indian Ocean. In mid-December of El Niño winters, the anomalously-enhanced convection develops over the equatorial Indian Ocean and excites the teleconnection pattern with a two-week time lag. This teleconnection largely opposes the Pacific/North American-like teleconnection pattern that is induced by anomalous convection over the equatorial central Pacific. Such sub-seasonal variation of ENSO teleconnections is poorly predicted by multi-model hindcasts, affecting the mid-latitude prediction skill. It particularly results in a poorer prediction skill of WNA surface air temperature in mid-winter than in late winter. This result highlights a need to better simulate the ENSO-related tropical convections and the associated teleconnections in the model to improve the long-lead prediction of the WNA surface climate.