On the nature of the Quasi-biweekly Oscillations of the South Asian Monsoon Anticyclone: Implications of the impermeability theorem on the macro-scale circulation of the atmosphere

In late May and June, a strong upper level anticyclone, the monsoon gyre, develops in the upper level circulation. The anticyclone sits over Tibet, covers a considerable portion of the globe and, with an unstable nature, oscillates with a quasi-biweekly periodicity. These oscillations are a recurrent feature of the upper level circulation, and are shown to be associated with mid-latitude potential vorticity that is advected around the eastern flank of the anticyclone and into the tropics, from the South China Sea across South Asia and Africa before returning to higher latitudes. Given their pervasiveness nature and spatial scale a number of questions arise: What is the physical nature of these oscillations? Are they associated with instabilities of the monsoon gyre? Is there a dynamical balance they are forced to meet? By using a shallow water model on a sphere we attempt to gain insight into these questions. We find that the conservation of “potential vorticity substance” in the model (i.e. the impermeability theorem; Haynes and McIntyre, 1992) poses important constraints on the model circulation. Constraints that seem important not only for monsoon-like simulations, but for other possible configurations of the model, thus hinting at constraints on the general circulation. For instance, we argue that, independent of any sources and sinks in the model, a poleward flow everywhere in the domain cannot be attained for long-term simulations. A return flow must also occur in order to conserve absolute vorticity. Additionally, we discuss the implication of these constraints on the formation of Westerly Ducts during the boreal winter. We suggest that the return flow, that owes its existence to a zonally symmetric Hadley-like circulation, might lead to its formation. Thus rendering the Westerly Ducts as fundamental characteristic of the global circulation, tied to the existence of the Hadley cell itself. We further explore the implications of this theorem on the real atmosphere by analyzing ERA-interim data.