Influences of baroclinic eddies on the Hadley cell

The poleward mass flux in the extratropical upper troposphere is primarily due to eddy fluxes of potential vorticity; the equatorward mass flux in the extratropical lower troposphere is primarily due to eddy fluxes of surface potential temperature and potential vorticity. In the subtropics, these eddy mass fluxes connect to the mass fluxes of the Hadley cell. Since a substantial fraction of the mass flux of the Hadley cell does not recirculate within the Hadley cell but connects to extratropical mass fluxes to form overturning cells that span hemispheres, it is possible that mass fluxes due to extratropical baroclinic eddies exert a significant influence on the strength and structure of the Hadley cell.

The extent to which extratropical eddy fluxes of potential vorticity and surface potential temperature can influence the Hadley cell is investigated in numerical simulations of idealized atmospheric circulations. For a dry atmosphere that is heated in a hemispherically symmetric manner, the axisymmetric Hadley circulation is much weaker than the Hadley circulation of the Earth atmosphere. In contrast, the Hadley circulation in the presence of baroclinic eddies is of a strength consistent with observations, suggesting that, in this idealized dry model, baroclinic eddies play a pivotal role in driving the Hadley circulation. Results will be presented that quantify the extent to which extratropical circulation parameters such as the surface potential temperature gradient [and static stability] influence the tropical Hadley circulation in the idealized dry model.