Simulation of the Intraseasonal Variations of the Indian Summer Monsoon in a Regional Coupled Ocean-Atmosphere Model

This paper describes a novel simulation of active and break spells of the Indian summer monsoon (ISM) using a relatively high-resolution regional coupled ocean-atmosphere climate model (RCM) run at 10-km grid spacing. Similar to what is seen in observations, the RCM-simulated active (break) spells are characterized by stronger (weaker) rainfall over central India and anomalous low-level atmospheric flow that enhances (weakens) the climatological flow pattern. Highlights of this study include the improved spatiotemporal structure, propagation characteristics, and amplitude of the intraseasonal variations of the ISM rainfall in the RCM simulation as compared with some of the more recent simulations conducted with global models at coarser spatial resolutions. This study's RCM simulation also displays associated variations in the upper ocean, with active (break) spells of the ISM coinciding with colder (warmer) sea surface temperatures (SSTs) in both the Arabian Sea and the Bay of Bengal. These SST anomalies are mainly sustained by corresponding net heat flux anomalies on the ocean surface. The active (break) spells are further associated with shoaling (deepening) of the mixed layer depth, which is critical for the SST response to heat flux. All of these simulated features of intraseasonal variations of the ISM have been seen in earlier observational studies, which further confirms the fidelity of the model simulation and the importance of coupled air-sea interactions and upper-ocean stratification.