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
withdrawal. Systematically 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 implement an isolated South American continent with reduced heat
capacity, but no topography in an idealized moist general circulation
model. Irrespective of the local moisture availability, the seasonal
cycles of precipitation and circulation over the South American monsoon
sector are distinctly monsoonal with the default surface albedo. The dry
land case (zero evaporation) is characteristic of a shallow overturning
circulation with vigorous lower-tropospheric ascent, transporting water
vapor from the ocean. By contrast, the monsoon dynamics with bucket
hydrology or unlimited land moisture features deep 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 moist static energy with all land
moisture conditions. The analysis supports that when the land-ocean
thermal contrast is strong enough, inertial instability alone is
sufficient for producing a shallow but vigorous circulation and
converging a large amount of moisture from the ocean even in the absence
of land moisture. Once the land is sufficiently moist, convective
instability takes hold and the shallow circulation deepens. These
results have implications for monsoon onset and intensification, and may
elucidate the seasonal variations in how surface warming impacts
tropical precipitation over land.