Monsoons are a major component of the global climate system affecting floods, droughts and other climate extremes. In this study, we investigated the performance of RSM (Regional Spectral Model) for predicting the summer monsoon over the Indochina Peninsula (ICP) region from 1982 to 2010. The NCEP - Climate Forecast System Reforecast (CFS-reforecast) was used to provide initial and boundary forcing for the RSM configured with an approximately 26?km grid over the ICP. The large-scale fields as well as surface temperature and rainfall of RSM have been evaluated for differing El Ni?o-Southern Oscillation (ENSO) years. The factors affecting changes in the rainfall patterns in ENSO years were determined using the empirical orthogonal function method. In addition, the ability of forecasting onset of summer monsoon was assessed based on the changes in outgoing longwave radiation, rainfall and average zonal wind at 850?hPa. The RSM is satisfactory in terms of forecasting large-scale features in different ENSO conditions. It produces well the interaction between Southwest and Southeast airflow, which are the main characteristics of summer monsoon throughout the study area. The RSM reforecasts are compared to CRU (Climatic Research Unit) data for the distribution of temperature and precipitation; however, their changes due to ENSO condition is inconsistent with CRU data. The extremes maximum and minimum temperatures have reverse signals with ENSO conditions. The extremes maximum 1-day rainfall have a significant change over the gulf of Tonkin and western ICP area due to ENSO condition. RSM results have indicated the effects of terrain and the reverse effects of ENSO condition over ICP area. The onset of summer monsoon is later in summer of El Ni?o year when compared with the other conditions.
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