Hybrid Lagrangian/Eulerian View of the Global Atmospheric Mass Circulation

In this study, we have diagnosed diabatic heating, meridional adiabatic mass and angular momentum transport, and downward transfer of westerly angular momentum by the pressure torque in isentropic coordinates using daily NCEP-NCAR reanalysis II dataset for 32 years from January 1, 1979 to December 31, 2010. Mass and its associated angular momentum fluxes of instantaneous flow depict a snapshot of mass and angular momentum circulations at that time in the Lagrangian perspective, although they are calculated on the Eulerian latitude-longitude coordinates. The hybrid Lagrangian/Eulerain view of instantaneous flow enables us to delineate how the diabatic heating and the pressure torque drive the local change of air mass and its angular momentum via the meridional mass circulation. One broad hemispheric cell of the meridional mass circulation exists in each hemisphere, which links the tropics to the extratropics and the troposphere to the stratosphere. Warm air mass heated by the diabatic heating in the tropics flows poleward to the extratropics where it sinks by the diabatic cooling. Cold air mass moves back to the tropics by adiabatic equatorward returning flow and diabatic heating near the ground. The air in the poleward branch moves in a down-gradient direction of earth angular momentum whereas the equatorward cold air branch is in an up-gradient direction of earth angular momentum. Embedded in the hemispheric mass circulation are three distinct but connected cells: the tropical Hadley cell, the stratospheric cell (part of Brewer-Dobson circulation in the winter hemisphere), and the extratropical Hadley cell. In the warm air branch of the tropical Hadley cell, poleward angular momentum transport associated with poleward mass fluxes is responsible for intensification of the subtropical jet. The excessive westerly angular momentum of the subtropical jet slows down significantly the poleward advancement of warm air mass as air mass mostly moves towards the east instead of the pole. As the air mass circulates around the subtropical latitudes, air experiences radiative cooling, resulting in downward cross-isentropic mass and angular momentum fluxes. Part of downward mass transport in the subtropics then joins the poleward warm air branch of the extratropical Hadley cell, connecting two tropospheric circulation cells. The remaining part continues to downward crossing isentropic surfaces and merges with the returning cold air branch of the extratropical Hadley cell. The merged air mass together moves equatorward diabatically (and with small portion adiabatically) as the returning flow of the tropical Hadley cell. In the cold air branch of the tropical Hadley cell, the surface frictional torque and mountain torque play a major role in adding the westerly angular momentum to the equatorward returning flow. In the extratropics, the meridional mass circulation is carried out mainly by the baroclinically amplifying (or westward tilted) waves. The westward tilted waves in the extratropical stratosphere account for a net poleward transport of air mass and its angular momentum aloft and a net equatorward transport of air mass and its angular momentum below, as well as a net downward transfer of westerly angular momentum by the pressure torque. As the warm air branch of the tropical Hadley cell, poleward transport of angular momentum aloft gives rise to the intensification of the stratospheric polar jet that in turn slows down the poleward mass transport into the polar region. More intense westward tilted wave activities near the polar jet act to remove the excessive westerly angular momentum aloft by the pressure torque. It helps to pave the way for further poleward mass transport in the warm air branch of the stratospheric circulation to overcome the increment of westerly angular momentum during the poleward air mass advancement. The westward tilted waves in the extratropical upper troposphere do the same thing, and are responsible for the extratropical Hadley cell. Strong downward mass transport from the stratosphere to the troposphere in the extratropics results in 1) much weaker equatorward returning flow in the lower stratosphere than poleward warm air flow in the upper stratosphere and 2) stronger equatorward returning flow of the extratropical Hadley cell than poleward warm air branch of the extratropical Hadley cell. In the cold air branch of the extratropical Hadley cell, the gain of angular momentum transferred from the layers above by the pressure torque helps to pave the way for further equatorward mass transport to overcome the lack of westerly angular momentum on the way to the lower latitudes. The equatorward returning flow of the extratropical Hadley cell is connected to that of the tropical Hadley cell in the subtropics by the diabatic heating near the ground.