Dynamics of ’Internal’ Interannual Variability of the Indian Summer Monsoon in a GCM

Seasonal mean prediction and predictability experiments with General Circulation Models (GCM’s) indicate that a significant fraction of observed interannual variability (approximately 50% or more) of the Indian summer monsoon may be due to ’internal’ low frequency variability (Goswami, 1998). Potential predictability studies with long observations also indicate similar contribution of ’internal’ low frequency (LF) variability to observed interannual variability (IAV) of the Indian summer monsoon (Ajayamohan and Goswami, 2003). The ’internal’ LF variability act as a background of unpredictable noise mixed with the predictable ’externally’ forced signal. Improvement in seasonal mean prediction would require successful extraction the ’signal’ from the background of noise of comparable magnitude. In order to develop such a technique, a clearer understanding of physical mechanism(s) responsible for the ’internal’ LF variability is required but is not currently available. The objective of the present study has been to attempt to unravel the mechanis m responsible for the observed ’internal’ LF variability. Since clean separation between the ’internal’ and ’external’ component of interannual variability from observations is difficult, we take recourse to a GCM experiment to achieve the goal. The CCM3 model with T42 horizontal resolution and 18 levels in the vertical is integrated for 25 years without any external interannual forcing, with only annual cycle of solar external forcing and annual cycle of SST as boundary forcing. The seasonal long term mean of the simulations are found to be reasonable when compared with observations (Fig.1) .Any interannual variability of the simulated Indian summer monsoon (or any other climate system for that matter) is, therefore, of internal origin. It is shown that mo del simulates interannual variability of the Indian monsoon comparable in amplitude to the observed interannual variability. (Table 1). It is also shown that spatial structure of the simulated IAV of the monsoon is similar to that of the observed IAV of the monsoon. As the model simulates significant and realistic internal IAV of the summer monsoon, an understanding of the model internal IAV may provide insight towards understanding the observed internal IAV.