Subseasonal Prediction Performance for Austral Summer South American Rainfall

Abstract Skillful and reliable predictions of week-to-week rainfall variations in South America, two to three weeks ahead, are essential protect lives, livelihoods, ecosystems. We evaluate forecast performance for weekly extended austral summer (November–March) four contemporary subseasonal systems, including a new Brazilian model, at 1–5-week leads 1999–2010. measure by the correlation coefficient (in time) between predicted observed rainfall; we skill Brier score terciles against climatological reference forecast. assess unconditional (i.e., regardless initial condition) conditional based on phase Madden–Julian oscillation (MJO) El Niño–Southern Oscillation (ENSO). All models display substantial mean biases, dry biases Amazonia wet near Andes, which established week 1 vary little thereafter. Unconditional extends 2 all regions except but 3 only over northern, northeastern, southeastern America. Skill upper- lower-tercile 1. Conditional is not systematically or significantly higher than performance; ENSO MJO events provide limited “windows opportunity” improved S2S that region model dependent. may be degraded errors teleconnections regional rainfall, even short lead times.