Assessment of Indian summer monsoon variability in a regional climate model coupled to a slab ocean model

A suite of Regional Climate Model (RegCM) experiments are performed over south Asia to examine the skill of RegCM to simulate the seasonal and sub-seasonal mean features of Indian summer monsoon (ISM). Because of coupled nature of ISM, three model experiments with RegCM are conducted to examine the skill of coupled and uncoupled configurations of RegCM by forcing it with observed SST (Exp1), coupling it to a simple slab ocean model (SOM) with constant mixed layer depth (MLD; Exp2) and with climatology of MLD (Exp3). The coupled experiments show an overall improvement in several aspects of ISM variability at seasonal and intraseasonal time-scales, despite bias in simulated SSTs. Between coupled experiments; Exp3 reduces biases in SST distribution over the region to the north of Arabian Peninsula, eastern Arabian Sea (AS), and broadly over north Indian Ocean (NIO). Noteworthy is the improved precipitation over central India (CI), head Bay of Bengal (BoB), as well as the representation of easterly wind shear in coupled experiments. At intraseasonal time scales, Exp3 produces spectral peaks above red noise at 25–50-day and 15–20-day periods closely representing the northward propagating intraseasonal mode and quasi-biweekly oscillating mode as in observed precipitation. The improved representation of spatial distribution of intraseasonal activity over NIO as well as the SST and precipitation relationships over head BoB and eastern AS is attributed to better representation of air-sea interaction in Exp3. In brief, the coupling improves the model skill for the true representation of mean ISM variability during boreal summer, and the thorough evaluation of model for longer periods is required to employ it as a downscaling tool for regional climate change studies.