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1. Introduction
The Indian summer monsoon features large spatial and temporal variations in precipitation (Chang et al. 2017). Among the transient features that grow in this region are synoptic-scale disturbances that are often referred to as monsoon low pressure systems. These systems are characterized by slow westward and northward propagation and a horizontal radius of ~2000 km (Godbole 1977; Krishnamurti et al. 1975, 1976; Sikka 1977; Lau and Lau 1990). The India Meteorological Department (IMD) categorizes monsoon low pressure systems according to the strength of surface wind speed. The weakest systems are defined as lows, stronger systems with surface winds between 8.5 and 16.5 m s−1 are defined as monsoon depressions, and the strongest systems are referred to as cyclonic storms (Saha et al. 1981; Krishnamurthy and Ajayamohan 2010; Hunt et al. 2016). Anomalous anticyclones are also observed during the Indian monsoon, which have structures similar to the low pressure systems but with reversed polarity (Krishnamurthy and Ajayamohan 2010). This study focuses on the weaker lows, depressions, and the anomalous anticyclones, which we will collectively refer to as synoptic-scale monsoonal disturbances (SMDs; Krishnamurthy and Ajayamohan 2010; Ditchek et al. 2016).
During their life cycle, monsoon low pressure systems often make landfall over the Indian subcontinent, producing up to half of the total monsoon rainfall received by India (Stano et al. 2002; Ding and Sikka 2006; Yoon and Chen 2005; Yoon and Huang 2012). Conversely, anomalous anticyclones are associated with breaks in the monsoon, with little or no rainfall occurring during the passage of these systems. Thus, understanding SMDs is of critical importance to our understanding of the Indian monsoon and its variability.
In spite of the important role that SMDs have in the monsoon’s hydrologic cycle, very few studies have analyzed how these systems modulate rainfall. Many studies have assumed that SMDs are a result of a variant of baroclinic instability called moist baroclinic instability [Salvekar et al. 1986; Krishnakumar et al. 1992; Krishnamurti et al. 2013; see also Cohen and Boos (2016) for a review on the topic], with precipitation being a result of large-scale quasigeostrophic (QG) ascent in these disturbances (Shukla 1978; Mak 1983; Sanders 1984). Other studies have included moist convection in the form of frictional convergence feedbacks (Goswami 1987).
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