Distinguishing Spread Among Ensemble Members Between Drought and Flood Indian Summer Monsoon Years in the Past 58 Years (1958-2015) Reforecasts

This study examines the different characteristics of ensemble spread (ESP) between drought and flood years of Indian summer monsoon (ISM) during June to September mean using the Climate Forecast System version 2 (CFSv2). We have analyzed a set of 20-member ensemble seasonal reforecasts for 1958-2015 using CFSv2 initialized in April. The ESP of ISM Rainfall (ISMR) is negatively (positively) correlated with ISMR (NINO3.4) index, indicating that ESP in drought ISMR years seems to be larger than that in flood years. The mean value of ESP for drought ISMR years is larger than flood years. The spatial structure of ISMR composite anomalies during drought years shows better agreement with observed rainfall anomalies in comparison to flood years. As a result, smaller ESP in flood years may suggest that ensemble prediction of ISMR in flood years tends to be overconfident and less reliable due to underestimate of forecast uncertainty, compared to drought years.