Dynamical simulation of Indian summer monsoon circulation, rainfall and its interannual variability using a high resolution atmospheric general circulation model

Abstract

This paper discusses the simulations of Indian summer monsoon (ISM) using a high-resolution National Center for Environmental Prediction (NCEP) T170/L42 model for a 20-year period (1985-2004) with observed Sea Surface Temperature (SSTs) as boundary conditions and using five initial conditions in the first week of May. Good agreement is found between the observed and simulated climatologies. Interannual variability (IAV) of the ISM rainfall as simulated in individual ensemble members and as provided by ensemble average shows that the two series are found to agree well; however, the simulation of the actual observed year-to-year variability is poor. The model simulations do not show much skill in the simulation of drought and excess monsoon seasons. One aspect which has emerged from the study is that where dynamical seasonal prediction has specific base for the large areal and temporal averages, the technique is not to be stretched for application on short areal scale such as that of a cluster of a few grid point. Monsoon onset over Kerala (MOK) coast of India and advance from Kerala coast to northwest India is discussed based on ensemble average and individual ensemble member basis. It is suggested that the model is capable of realistically simulating these processes, particularly if ensemble average is used, as the intermember spread in the ensemble members is large. In short, the high-resolution model appears to provide better climatology and its magnitude of IAV, which compares favourably with observations, although year-to-year matching of the observed and simulated seasonal/monthly rainfall totals for India as a whole is not good.