Dynamic Downscaling the South Asian Summer Monsoon From a Global Reanalysis Using a Regional Coupled Ocean‐Atmosphere Model

In this study, we present the results of a regional model (regional spectral model‐regional ocean model [(RSM‐ROMS]) simulation of the South Asian Summer Monsoon (SASM). The RSM‐ROMS integration is carried out at 20 km grid spacing over a period of 25 years (1986–2010). The simulation is forced by global atmospheric and oceanic reanalysis. The RSM‐ROMS simulation shows a realistic alignment of the simulated rainfall along the orographic features of the domain. Furthermore, the RSM‐ROMS simulates the observed feature of convection over continental SASM region being more vigorous with dominance of mixed warm and cold phase hydrometeors in contrast to the dominance of the warm rain process in the neighboring tropical oceans. Similarly, the upper ocean features of contrasting mixed layer and thermocline depths between the northern and equatorial Indian Ocean are also simulated in the RSM‐ROMS. Intra‐Seasonal Oscillation (ISO) of the SASM at 10–20 and 20–70 days are also simulated in the RSM‐ROMS with many of its features verifying with observations. For example, the 20–70 days ISO are of higher amplitude and its meridional propagation is slower in Bay of Bengal compared to that over Arabian Sea. Additionally, RSM‐ROMS shows 12.3 Monsoon Low Pressure Systems (LPSs) per season that is comparable to 14.6 per season from observations. Furthermore, the observed intraseasonal contrasts of LPS between the wet and dry spells of ISO is also reproduced in the RSM‐ROMS. Plain Language Summary The South Asian Summer Monsoon (SASM) climate represents a complex mix of variations across many spatio‐temporal scales over a region with unique topographic and bathymetric features. As a result, the simulation of the SASM climate offers a stiff challenge to numerical climate models. In this study, we evaluate the added value of a regional coupled ocean‐atmosphere model in downscaling the SASM climate from a 2.5° × 2.5° global atmospheric reanalysis and 0.5° × 0.5° global ocean reanalysis to 20 km grid spacing. The regional model demonstrates significant skill in capturing the observed features of the intra‐seasonal oscillations of the SASM besides displaying reasonable fidelity of simulating and improving the mean climate of the SASM. Furthermore, at 20 km grid spacing, the regional model simulates the seasonal and intraseasonal activity of the monsoon low pressure systems that is comparable to observations. Therefore, the regional model at 20 km grid spacing of this study clearly