Potential Vorticity and the South Asian Monsoon Intraseasonal Oscillations

Regardless of the usefulness of potential vorticity (PV) thinking to understand atmospheric motions at high latitudes, the implications of PV thinking at tropical locations has not been fully explored. The reason seems to be, on one hand, that the equations of motion and the boundary conditions are complicated by the presence of diabatic heating, so that PV is no longer conserved for individual parcels and isentropic surfaces intersect the lower boundary. On the other hand, the inability of PV to capture Kelvin modes seems to limit the generality of the PV concept, the Kelvin mode being very important in the tropics. Nonetheless, PV is a fundamental property of the atmosphere, even at low latitudes, and we seek here to explore its role in the intraseasonal oscillations of the South Asian Monsoon. We investigate both theoretical and observational aspects of the oscillations. First we explore an axisymmetric model to understand the meridional circulation in terms of potential vorticity thinking, and make special emphasis on the solutions found by Plumb and Hou (1992). Next, observationally, we explore the relationship between NOAA outgoing longwave radiation (OLR) and ERA-interim PV on isentropic levels. We further seek signatures of dynamical structures in the observational and reanalysis datasets. We bring to the study the use of dynamic system theory in understanding the monsoon oscillations.