Developing a holistic understanding of monsoon formation with idealized model simulations and theories

Abstract:

Monsoons emerge over a range of land surface conditions and exhibit varying physical characteristics over the seasonal cycle, from onset to demise. Varying the moisture and albedo parameters over land in an idealized modeling framework allows one to analyze the physics underlying the successive stages of monsoon development. To this end we use an idealized moist general circulation model as described in Clark et al. (2018), with an isolated South American continent with 10% of the slab ocean heat capacity and no topography.

Irrespective of the local moisture availability in the idealized model simulations, the seasonal cycles of precipitation and circulation over the South American monsoon sector are distinctly monsoonal with the default surface albedo (0.26). The dry land case (zero evaporation) is characteristic of a shallow overturning circulation with vigorous ascent, transporting water vapor from the ocean. By contrast, the monsoon dynamics with bucket hydrology or unlimited land moisture features deep convection, with moist convection that penetrates the upper troposphere. A series of land albedo perturbation experiments indicates that the monsoon strengthens with the net column energy flux and the near-surface MSE with all land moisture conditions. The analysis supports that inertial instability alone is sufficient for producing a shallow but vigorous circulation and converging a large amount of moisture from the ocean. This mechanism may be key to monsoon onset. Once the land is sufficiently moist, convective instability takes hold and the shallow circulation deepens. These results have implications for monsoon onset and may elucidate the seasonal variations in how surface warming impacts tropical precipitation over land.