Comparison of Indian summer monsoon rainfall anomalies in response to changes in snow depths and SSTs in a GCM

The slowly varying boundary conditions such as Sea Surface Temperature (SST), snow cover/depth and soil moisture have important roles to play in Indian Summer Monsoon Rainfall (ISMR) variability. Earlier studies have shown strong relationship between inter-annual variations in ISMR and SST anomalies. Some studies also indicate inverse relationship between the winter/spring snow over the Eurasian continent and the following ISMR. The objective of this study is to examine the relative roles of SST and snow depth anomalies in the Indian summer monsoon circulation and associated rainfall. In the present study, a spectral General Circulation Model has been used to conduct the sensitivity studies in the ensemble mode over a period of 20 years. Four types of sensitive experiments have been conducted here by varying the observed and climatological values of both SST and snow depth. Model simulated ISMR agrees well with the corresponding observed value in most of the cases where observed snow depths are provided to the model as initial conditions. Inter-comparison of the results of four experiments reveals that in the absence of initial snow depths, simulated Indian summer monsoon circulation and associated ISMR in the core monsoon region are not close to the respective observed fields even though observed SSTs are prescribed as boundary conditions to the model. The model response in terms of changes in ISMR to the snow depth anomalies in the presence of climatological SST is stronger than that due to SST anomalies in the presence of climatological snow.