Atmospheric Mass and Angular Momentum Circulations in Isentropic Coordinates

This study reports a comprehensive diagnostic analysis of diabatic and adiabatic mass and angular momentum fluxes and source/sink terms associated with the atmospheric angular momentum circulation in boreal winter using 32-year daily NCEP-NCAR reanalysis data (1979-2010). The complete diagnosis of the isentropic atmospheric mass and momentum circulations is performed from the total flow prospective without decomposition of the time mean and transient flows and separation of the zonal mean and wavy flows. Embedded in a broad hemispheric cell of the isentropic meridional mass and angular momentum circulations in each hemisphere are three distinct but inter-connected thermally direct circulation cells: the tropical Hadley cell, the stratospheric cell, and the extratropical Hadley cell. The meridional span of the tropical Hadley cell and the subtropical jet intensity is determined mainly by diabatic heating rate in the tropics and cooling rate in the subtropics, which are coupled with the intensity of the tropical Hadley cell. Both the extratropical Hadley and stratospheric cells are driven collectively by diabatic heating/cooling, adiabatic (meridional) mass and its angular momentum transport, and downward transfer of westerly angular momentum by the pressure torque associated with baroclinically amplifying waves that tilt westward with height. We also discuss the role of the surface frictional torque and the mountain torque in maintaining the atmospheric angular momentum circulation. The focus of our analysis is on the simultaneous coupling among the three cells and among these thermodynamic and dynamic processes responsible for the atmospheric mass and angular momentum circulations.