A study of the sensitivity of the winter mean meridional circulation to sources of heat and momentum

The time and zonally averaged momentum and thermodynamic equations on a spherical earth are combined to formulate an equation for the mean meridional circulation based on the quasi-static assumption discussed in Eliassen (Astrophys. Nor. 1951, 19–60). The forcing terms and coefficients in the equation are taken from previously published zonally and seasonally averaged data, which include the net radiative heating, condensation heating, boundary layer friction and sensible heating, tropical cumulus friction, the mean zonal wind and large-scale eddy transports of momentum and heat in winter. The computed meridional circulation in the Northern Hemisphere compares well to the observations with a three-cell structure of Hadley, Ferrel and polar circulations. To estimate the relative importance in maintaining the meridional circulation a series of solutions is computed by removing one of the individual sources at a time and the solutions are compared to the standard solution. It is found that the Hadley cell maintains the intensity level at only one-third of the standard solution if the condensation heating is absent. Although there is virtually no source in the tropics due to eddy momentum and heat transports, the mid-latitude eddy transport affects the Hadley circulation significantly, suggesting that the cooling due to eddy heat transport in the subtropics is an important component balancing the adiabatic heating in the downward branch of the Hadley cell. The individual effects of radiation, surface sensible heating and surface friction are all comparable, but the contributions in the balance of meridional circulation is, on the global average, not more than one-third of the condensation and resultant eddy effect, although the effects are different for the individual Hadley, Ferrel and polar circulations.