A Comparative Study Between Regional Atmospheric Model Simulations Coupled and Uncoupled to a Regional Ocean Model of the Indian Summer Monsoon

We evaluate the impact of air-sea coupling in simulating the characteristics of the Indian summer monsoon (ISM) and its intraseasonal oscillations (ISOs) by using a coupled ocean-atmosphere regional climate model (CRSM) and an atmosphere-only regional climate model (URSM). These 20-km resolution regional climate models (RCMs) were driven by the boundary conditions from Community Climate System Model version 4 (CCSM4) global model. The mean ISM rainfall simulated by URSM and CRSM is comparable with observations and shows an improvement over the CCSM4. The systematic error in the spatial distribution of the mean June-September low-level and upper-level winds, sea surface temperature, and latent heat flux shows that CRSM performs better than URSM, suggesting the benefits of air-sea coupling in the RCM simulations of the mean ISM. Both the CRSM and the URSM perform reasonably and comparably in capturing the spatiotemporal evolution of 10-20-day high-frequency and 20-70-day low-frequency ISOs of the ISM rainfall. These RCMs simulated the observed features of the ISO-filtered rainfall anomalies that propagate at a faster rate across the Arabian Sea compared to that over the Bay of Bengal. Furthermore, the difference in the track density of the monsoon low-pressure systems between the wet and the dry phases of ISOs shows insignificant differences between CRSM and URSM but is found to be superior to the CCSM4 simulation. This study suggests a relative insensitivity of air-sea coupling in the RCM for simulating ISO and LPS features of ISM, which is likely abetted by the realistic ISO signals in the parent CCSM4.