The role of convection-circulation coupling in expediting South Asian monsoon onset: Insights from SP-CAM

This study emphasizes the role of shallow circulation in transporting lower-level moist static energy northward, thereby intensifying the onset of summer cross-equatorial circulation in the South Asia monsoon region. Previous research has suggested that the monsoon onset can be considered as a transition between an eddy-driven regime and an angular-conserving regime, and momentum budget analyses from these studies support the theory of regime transition (Bordoni and Schneider 2008; Plumb 2005; Geen et al. 2018; Shaw 2014). Additionally, studies have highlighted the significant role of boundary layer entropy during the summer monsoon period (Emanuel 1995; Plumb 2005; Nie et al. 2014), when the circulation operates within the angular-momentum-conserving regime. Adopting this boundary-layer-entropy-centric perspective, many studies emphasizing the role of topography in blocking low entropy inflow from the north and intensifying the South Asian Monsoon (Boos and Kuang 2010; Privé and Plumb 2007; Geen et al. 2014). Meanwhile, the role of synoptic systems and the early onset in the Bay of Bengal (Parker et al. 2016), as highlighted in observational data, in establishing the strong cross-equatorial summer cell remains unclear.

To bridge the gaps between observational studies and theoretical frameworks, this study investigates the mechanisms shaping the evolution of the boundary layer entropy throughout the regime transition. With the goal of understanding the interactions between convective processes and large-scale circulation, we utilize the Superparameterized Community Atmosphere Model (SPCAM), which demonstrates higher convection variability and increased precipitation near the South Asian coastal region compared to traditional global climate models, aligning well with observational data. Compared to simulations without SPCAM coupling, the SPCAM simulations show a more abrupt monsoon onset in South Asia. The sector zonal mean analysis demonstrates that the higher convection variability in SPCAM runs results in more shallow convections before the monsoon onset. Also, the shallow circulation accompanied with these shallow convections can transport higher lower-level entropy northward, causing energy convergence near the coastal region and intensifying the abruptness of the monsoon onset. In contrast, simulations without SPCAM coupling exhibit an unrealistic jump of boundary layer entropy maximum from the equator to the mountainous terrain. Our energy budget analysis highlights that the shallow overturning cell associated with the deep and shallow convections in the coastal regions holds the key for the northward migration of boundary layer entropy maximum. Such a relaxed quasi-equilibrium perspective provides an interpretation for how convection-circulation coupling contributes to the theoretical framework of regime transition.