Validation of 40 year multimodel seasonal precipitation forecasts: The role of ENSO on the global skill

The skill of seasonal precipitation forecasts is assessed worldwide—grid point by grid point—for the 40 year period 1961–2000, considering the ENSEMBLES multimodel hindcast and applying a tercile-based probabilistic approach in terms of the relative operating characteristic skill score (ROCSS). Although predictability varies with region, season, and lead time, results indicate that (1) significant skill is mainly located in the tropics—20 to 40% of the total land areas; (2) overall, September–October (March–May) is the most (least) skillful season; and (3) the skill weakens (with respect to the 1 month lead case) at 4 months lead—especially in June–August—although the ROCSS spatial patterns are broadly preserved—particularly in Northern South America and the Malay Archipelago. The contribution of El Niño–Southern Oscillation (ENSO) events to this 40 year skill is also analyzed, based on the idea that the seasonal predictability may be mainly driven by El Niño and La Niña precipitation teleconnections and, consequently, limited by the ability of the different seasonal forecasting models to accurately reproduce them. Results show that the ROCSS spatial patterns for (1) the full period 1961–2000 and (2) El Niño and La Niña events are highly correlated—over 0.85. Moreover, the observed teleconnection patterns are properly simulated (predicted)—with spatial correlations around 0.8—by most of the models at both 1 and 4 months lead time.