Skilful Forecasts of Summer Rainfall in the Yangtze River Basin from November

Variability in the East Asian summer monsoon (EASM) brings the risk of heavy flooding or drought to the Yangtze River basin, with potentially devastating impacts. Early forecasts of the likelihood of enhanced or reduced monsoon rainfall can enable better management of water and hydropower resources by decision-makers, supporting livelihoods and major economic and population centres across eastern China. This paper demonstrates that the EASM is predictable in a dynamical forecast model from the preceding November, and that this allows skilful forecasts of summer mean rainfall in the Yangtze River basin at a lead time of six months. The skill for May-June-July rainfall is of a similar magnitude to seasonal forecasts initialised in spring, although the skill in June-July-August is much weaker and not consistently significant. However, there is some evidence for enhanced skill following El Nino events. The potential for decadal-scale variability in forecast skill is also examined, although we find no evidence for significant variation.