Intraseasonal Forecasting of the Asian Summer Monsoon in Four Operational and Research Models

The boreal summer intraseasonal oscillation (BSISO) is a dominant tropical mode with a period of 30-60 days, which offers an opportunity for intraseasonal forecasting of the Asian summer monsoon. The present study provides a preliminary, yet up-to-date, assessment of the prediction skill of the BSISO in four state-of-the-art models: the ECMWF model, the University of Hawaii (UH) model, the NCEP Climate Forecast System, version 2 (CFSv2), and version 1 for the 2008 summer (CFSv1), which is a common year of two international programs: the Year of Tropical Convection (YOTC) and Asian Monsoon Years (AMY). The mean prediction skill over the global tropics and Southeast Asia for first three models reaches about 1-2 (3) weeks for BSISO-related rainfall (850-hPa zonal wind), measured as the lead time when the spatial anomaly correlation coefficient drops to 0.5. The skill of CFSv1 is consistently lower than the other three. The strengths and weaknesses of the CFSv2, UH, and ECMWF models in forecasting the BSISO for this specific year are further revealed. The ECMWF and UH have relatively better performance for northward-propagating BSISO when the initial convection is near the equator, although they suffer from an early false BSISO onset when initial convection is in the off-equatorial monsoon trough. However, CFSv2 does not have a false onset problem when the initial convection is in monsoon trough, but it does have a problem with very slow northward propagation. After combining the forecasts of CFSv2 and UH into an equal-weighted multimodel ensemble, the resultant skill is slightly better than that of individual models. An empirical model shows a comparable skill with the dynamical models. A combined dynamical-empirical ensemble advances the intraseasonal forecast skill of BSISO-related rainfall to three weeks.