Inter-annual variability of indian monsoon rainfall in the JMA’s seasonal ensemble prediction system in relation to ENSO and IOD

Abstract

The seasonal forecast skill of the Indian summer monsoon rainfall during June to September (JJAS) by the Japan Meteorological agency (JMA) Seasonal Ensemble Prediction System is analysed. The hindcast output valid for JJAS rainfall based on March, April and May initial conditions for a period of 32 years (1979-2010) with 5 ensembles each have been used in this study. In conjunction with significant forecast skill of El Nino the skilful forecast of All India Summer Monsoon Rainfall (AISMR) is found in April ensembles (Significant at 99% level) followed by that of March and May ensembles. The JMA model could capture the AISMR forecast reasonably well during major deficient years like 1982, 1987 and 2009 and the excess year of 1988. However, the excess monsoon season of 1983 was not captured in the model. Similarly, the strongest El Nino year of 1997 associated with slight positive AISMR departure was also not captured in the JMA model. The performance of JMA model in simulating the monsoon rainfall during normal year (1997), excess year (1983) and deficient year (2002) was better understood when the variability of El Nino, Indian Ocean Dipole (IOD) and Equatorial Indian Ocean Oscillation (EQUINOO) in the model are considered simultaneously.