Controls on propagation of the Indian monsoon onset in an idealised model

The Indian monsoon is a seasonal large-scale circulation system with complex dynamical and thermodynamic interactions. The physical processes are not fully understood. In particular, the mechanisms that control the propagation of the monsoon onset across the Indian continent, against the mid-level wind field, are debated. The Indian monsoon is poorly represented in weather and climate models, with persistent systematic errors making it difficult to accurately forecast the Indian monsoon on subseasonal timescales. A two-layer model based on moisture conservation with a parameterised flux representing convection is developed and used to investigate the competition between dry advection in the upper levels, the rate of moisture replenishment at low levels and the rate of convection from the lower to the upper layers. In a fixed Eulerian frame, the system is initialised at an equilibrium representing pre-onset (May) conditions. Then, changes in the rates of moist inflow and upper level advection are introduced, triggering a transition to a new equilibrium, which reflects the full monsoon state (July-September). The two-layer model reproduces Indian monsoon onset and its progression to the northwest, against an imposed 5 m/s wind in the upper layer. Increasing the parameter representing moist inflow induces a monsoon onset, defined as a threshold of total column moisture, with clear progression from southeast toward northwest India. A lesser wind speed in the upper layer, signifying a weakening mid-tropospheric dry intrusion, allows a more rapid progression of the monsoon onset. A greater upper level wind speed, associated with a strengthening dry intrusion, causes the monsoon onset to retreat. We can quantify the nature of the monsoon onset by deriving an onset speed and the time taken for the system to adjust to a new equilibrium, from analytical theory