AN EMPIRICAL-MODEL STUDY OF THE TROPOSPHERIC MERIDIONAL CIRCULATION BASED ON SAGE-II OBSERVATIONS

This study investigates the tropospheric mean meridional circulation i mportant to the development of opaque clouds and the measurement oppor tunity of the 1.02-mu m channel of the Stratospheric Aerosol and Gas E xperiment (SAGE) II in the troposphere. A simple empirical model is fo rmulated to derive the mean meridional circulation from the 6-year (19 85-1990) statistics of the SAGE II tropospheric measurement frequency. The vertical circulation of the model is assumed to be related to the departure field of the zonally averaged SAGE II measurement frequency from the corresponding global mean in a linear fashion. The proportio nal constant is calibrated with the observed upwelling circulation sta tistics in the tropics. The obtained model vertical circulation is the n used to determine the distribution of meridional velocity according to the continuity equation. The derived model mean circulation feature s the influence from both the diabatic circulation and the eddy quasi- isentropic transport, with a distinct pattern of material advection in to the upper troposphere from both the lower troposphere and the strat osphere. Most significantly, the model circulation is shown to be high ly consistent with the observed free tropospheric aerosol and ozone di stributions, particularly with their seasonal variations given the aer osol and ozone source regions. This high degree of consistency illustr ates the intimate relationship between the large-scale circulation, cl oudiness, and the SAGE II tropospheric measurement frequency, and the robust nature of the empirical model despite the model's simplicity. T he discussion in relating the model circulation to the conventional Eu lerian circulation and the Lagrangian transport, based on isentropic c onsideration, is also provided.