A Global Linear Balance Model as an alternative to the Quasi-Geostrophic Theory

Abstract

The Quasi-Geostrophic theory has been a valuable framework for understanding synopticand planetary-scale systems in the midlatitudes. By making arguments about the scale of the systems under study, we are able to filter out fast gravity waves and obtain only the slow, synoptic disturbances. However, this framework is invalid in the tropics, as it requires a strong Coriolis force to balance the pressure-gradient forces within the system. In this thesis, we propose and evaluate an alternative framework for describing dry, slow-moving systems in the tropics, which we call the ”Global Linear Balance” (GLB) model. We formulate this on a sphere by considering observations and scaling arguments that apply to synoptic systems all over the globe. We then evaluate its validity for Indian monsoon low-pressure systems (LPSs) and show that it performs better than quasi-geostrophic estimates. We also simulate both systems using a shallow-water model and show that the GLB framework performs much better than QG theory at capturing the evolution of the slow mode in the tropics. This work hence proposes a significantly better alternative to QG theory. While the GLB model is shown to be a good description of dry dynamics in the tropics, another part of the puzzle in understanding tropical systems is the representation of moisture and moist processes. More work is required to integrate these processes into our proposed framework.