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  • Author or Editor: R. G. Gallimore x
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Robert G. Gallimore
and
Donald R. Johnson

Abstract

In the linear theory of the isentropic zonally averaged circulation (Gallimore and Johnson, 1981), stable meridional circulations within the circumpolar vortex are forced by larger scale diabatic beating and angular momentum torques. The linear theory, however, could not describe important nonlinear interactions between the forcing of meridional circulations and the maintenance of the circumpolar vortex nor could the quasi-steady balance be ascertained.

In this study, an isentropic numerical model is developed to investigate the forcing of the meridional circulation and its role and the role of angular momentum torques in maintaining the circumpolar vortex. The model is based on a set of isentropic zonally averaged equations applied to the Northern Hemisphere. The horizontal resolution is 5° of latitude while nine equally spaced isentropic levels are used for vertical resolution. The basic forcing of the model is diabatic heating and zonal pressure and friction torques. The distribution of diabatic heating is prescribed from observational results while the meridional distribution of the zonal pressure torque is determined from a specified separation of the zonally averaged meridional mass transport into geostrophic and ageostrophic modes. The friction torque is internally time dependent. Eddy transport terms are not included and a fixed surface potential temperature is assumed at the lower boundary. The zonally averaged circulation and its forcing were studied in three experiments in which the meridional distribution of the diabatic heating and the pressure torque were varied. After 30 days of numerical simulation a slowly varying state was attained.

The simulation determined a realistic isentropic Hadley circulation spanning the Northern Hemisphere and a zonal wind structure with an upper tropospheric subtropical jet. The structure of the model's zonally averaged meridional circulation is consistent with observations and with the results of the linear theory, i.e., the vertical branches of the direct, stable circulation are forced through tropical heating and polar cooling while the meridional branches are forced by upper tropospheric sources and lower tropospheric sinks of pressure and friction torques. The zonal vortex is maintained through the balance of angular momentum torques and the convergence of the absolute angular momentum transport within the meridional circulation. The results further indicate that non-steady, freely convecting meridional circulations do not occur for realistic forcing.

A comparison of the results in which the heating is varied reveals that the changes in the intensity of the meridional circulation depend on the variations of the meridional heating distribution. Since changes occur in the meridional momentum transport, structural changes in the zonal momentum and torques are also produced. With the same heating, changes in the zonal vortex occur in conjunction with compensating changes in the meridional distribution of the zonal pressure and time-dependent friction torques while the intensity of the total meridional circulation remains independent of this factor.

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Robert G. Gallimore
and
Donald R. Johnson

Abstract

For the axisymmetric general circulation, Eliassen (1951) showed that stable meridional circulations are controlled by large-scale diabatic heating and friction torque. Because the real atmosphere is longitudinally disturbed, a direct meridional response to large-scale diabatic heating is not readily found by conventional zonal averaging in isobaric coordinates. By introducing, the concept of a zonally averaged general circulation in isentropic coordinates, a direct meridional response to diabatic heating is isolated.

Using an approach analogous to Eliassen's, this study has extended the concept of diabatically forced isentropic meridional circulations to include the role of angular momentum torques within the circumpolar vortex. The absolute angular momentum torques comprise three terms: divergence of eddy relative momentum transport, friction and zonal pressure torques. The heating is composed of hemispheric-scale diabatic processes. For a hydrodynamically and statically stable atmosphere, a configuration of tropical heating and higher latitude cooling coupled with a combined upper tropospheric sink and low tropospheric source by the angular momentum torques support a Hadley-type circulation spanning the entire hemisphere.

In low latitudes, an upper tropospheric sink by friction torque and divergence of the eddy relative momentum transport combine with a lower tropospheric source by friction torque to force a dominant ageostrophic mode for the meridional branch of the Hadley cell. Within large-scale middle latitude waves, the zonal pressure torque is a principal mechanism for the transfer of angular momentum from upper to lower isentropic layers and therefore is the dominant upper tropospheric sink and lower tropospheric source of angular momentum in high latitudes. Due to the dominance of the pressure torque in extratropical latitudes, the poleward branch of the upper layers and equatorward branch of the lower layers are mainly geostrophic and mask indirect ageostrophic motion that is forced by friction torque and the divergence of eddy relative momentum transport.

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