Intraseasonal variations of the tropical convergence zones in a simple monsoon model

Abstract

In this thesis we have simulated the salient features of intraseasonal variation of the Indian summer monsoon using a simple monsoon model and unravelled the mechanisms governing these features.

The monsoon model is a zonally symmetric primitive equation model with two levels in the vertical and a meridional resolution of 4°. The new feature of this model is the space–time variation of surface pressure, hitherto not considered by the other two-level zonally symmetric monsoon models. It approximates the geography of the Indian region with a continental cap poleward of 18°N and an ocean to the south of it. The oceanic temperatures are specified, while the continental surface temperatures are calculated with a multi-layer model for the subsurface. Simple models of interactive hydrology, cumulus convection and radiation have been incorporated.

The simulation of the intraseasonal structure with this model is more realistic (vis-à-vis the observations using satellite data) in comparison with the simulations with the other models of this class. In addition to the poleward propagations (which were simulated by previous modelling studies), the present model simulates the presence of an active TCZ in the surface trough zone (henceforth referred to as the active phase). This phase was absent in the simulations by the other models of this class. Upon the removal of space–time variations of surface pressure, this phase is conspicuously absent.

The lifespan of the active phase is about twenty days and is determined by evaporative and surface reheating time-scales of the region between the surface trough and the coastal margin. The strength of the southern TCZ is also an important factor in the lifespan of the active phase.

Moist processes, viz., surface hydrological effects and cumulus convection, play a pivotal role in the simulation of intraseasonal variation of the TCZ. The simulations cease to be realistic in the absence of an interactive surface hydrology and a cumulus parameterization scheme which explicitly incorporates the effects of large-scale low-level convergence and moist static instability of the atmosphere. Simulations with various models of surface hydrology and cumulus parameterization are studied.