The fidelity of a regional coupled model in capturing the relationship between intraseasonal variability and the onset/demise of the Indian summer monsoon

Abstract

Onset and demise of the Indian summer monsoon (ISM) and intraseasonal variability (ISV) embedded within the ISM are dominant climatological phenomena observed over the Indian region. In this study, a quantitative relationship between these two phenomena is assessed and the performance of a regional coupled model in simulating the relationship in the ISM is examined. An objective definition of the local onset and demise of the ISM is used based on more than a century-long India Meteorological Department rain-gauge observation and 10-year-long model simulated precipitation. Two distinct modes of intraseasonal oscillations (ISOs), northwestward propagating 10–20-day periodic high-frequency ISO and northward moving 20–70-day periodic low-frequency ISOs are extracted using Multichannel Singular Spectrum Analysis (MSSA). It is found that nearly 60% of the local onset of the ISM occurs during the positive developing phases of the ISO; whereas, a similar fraction of local demise occurs during positive decaying phases of the ISO across almost entire India. Essentially, the phase-locking of two ISOs creates the most conducive environment for onset or demise of the ISM to occur. In this study we found that the model not only captures the overall structure of the ISO modes, but also simulates the observed distribution of the onset and demise dates of ISM in different ISO phases. The diagnostics described here, with the ability of the model to produce the observed phenomena, provide an essential tool to improve our understanding of the monsoon system and its predictability.