Assessment of Lead-Lag and Spatial Changes in simulating different epochs of the Indian summer monsoon using RegCM4

The present study examines the efficacy of the regional climate model RegCM4 in depicting the onset, active, and break phases of the Indian summer monsoon (ISM). The model is driven by National Centers for Environmental Prediction (NCEP)/NCAR Reanalysis 1 project (NNRP1) data of 28 years (1981-2008) for each monsoon separately. High-resolution gridded rainfall analysis dataset of India Meteorological Department (IMD) and the monsoon epochal report from National Climate Center (NCC) are used for verification. Besides assessment of model performance using qualitative methods, several standard statistical techniques are used to understand the capability of the model in representing monsoonal epochs. A relationship between the SST changes over the Nino-3.4 region and the ISM over all India and the monsoon core region (MCR) is established where a strong dependency in the rainfall variation over the MCR with the SST variation is found than all India rainfall. The performance of the model is satisfactory in portraying the spatial and temporal intraseasonal variability (ISV) of monsoon over MCR and all India regions. It is found that the model performance is higher in simulating the onset and break phases (dry days) than the active phases. The limited skill of the RegCM4 model in representing the active phase is due to the model simulated low temperature and weak pressure gradient over the MCR which prevents the convection and given rise to small and weak active phases over the region. The mechanism from the pre-onset to onset of monsoon, and the monsoon advance between the onset and the monsoon ISV is also well captured by the RegCM4. Weak tropospheric temperature gradient over the Indian subcontinent during the pre-onset period causes delay onset over the Indian subcontinent and the subsequent high tropospheric temperature gradient during the mid of June causes a faster progression of the active/break phases of monsoon. At the same time, simulation of the monsoon phase by the model is heavily regulated by the changes of SST over the Nino-3.4 region. The signal of ENSO variability, forces to the model through the ICBC, modulates the capability of the regional model in identifying the interannual and ISV of monsoon over the MCR region.