Isentropic analysis of the Indian Summer Monsoon circulation and its implications for the active and break periods

Abstract:

The atmospheric flow during the Indian Summer Monsoon here is analyzed in isentropic coordinates
in two different ways. First, the lateral mass transport fo air is separated in terms of both the potential temperature
and equivalent potential temperature. This approach, originally developed to analyze the global meridional
circulation, makes it possible to identify the thermodynamic properties of the inflow and outflow.
It is shown here how the properties of various air masses, such as the inflow of warm moist air in the boundary layer,
upper tropospheric outflow, and midlatitudes dry air intrusion, can be systematically identified. Second, we analyze
the vertical overturning in terms of terms of the equivalent
potential temperature of the ascending and subsiding air parcels over the indian subcontinent,
which allows us to further infer the thermodynamic transformation occurring during the monsoon.

This technique is first used to look at the evolution of the flow through the seasonal cycle. We then
further analyze the circulation patterns associated with monsoon breaks and active periods. In doing so, we
identify midtropospheric in usions of dry air from the midlatitudes as a key precursor of monsoon breaks.
The meteorological conditions associated for such intrusion to reach the subcontinent are then discussed.