Isentropic diagnostics of mean circulation and water vapour budget for the Asian monsoon.
Lee, Shao Yi.
Date of Issue2012
School of Physical and Mathematical Sciences
The observed Hadley spends most of the year in an asymmetrical state with the win- ter cell dominant. The asymmetrical Hadley reflects the seasonal evolution of the monsoon circulation, since only the monsoon trough seasonally migrates through a large span of latitudes while the location of the oceanic ITCZ remains comparatively fixed. The observed Hadley is often used for comparisons with model-produced global mean meridional circulations. These comparisons may not reflect the actual dynamical regimes that local “Hadleys” lie in, because the global zonal mean meridional circulation is not representative of the meridional circulation in the monsoon and oceanic regions. Therefore, a sectoral zonal mean (“sec-mean”) covering only the Asian monsoon region was applied to pentad-average Japanese 25-year Reanalysis. The study focused on the surface branch of the monsoonal circulation, which is where most of the water vapour transport takes place and also most directly of human interest. Thus, the sec-mean was carried in isentropic coordinates to avoid surface truncation of the residual circulation as happens in the Transformed Eulerian Mean. Circulation and water vapour budgets were diagnosed from the reanalysis. Although expressions of the circulation and water vapour budget were complicated by the combination of sectoral means and isentropic ground intersections, all boundary terms were found to have good physical meaning. In the tropics, the magnitude of the error in potential temperature that results from neglecting the effect of water vapour on specific heat and the gas constant is compa- rable to the magnitude of horizontal potential temperature variation. Therefore, an adjustment was made to the isentropic primitive equations used in this study, to include the effect of water vapour on potential temperature itself. The primitive equations are otherwise dry, and thus may be considered an intermediate step between completely dry equations and those inclusive of moisture processes.
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