RELATION BETWEEN SUBTROPICAL ANTICYCLONE AND VERTICAL MOTION IN STEADY STATE

The NCEP/NCAR monthly mean reanalysis averaged from 1980 to 1995 was employed to investigate the relation between subtropical anticyclone and vertical motion in steady state. Criteria for defining the location and intensity of the zonal mean subtropical anticyclone were given to study its characteristics and seasonal variations. Comparison in dynamics between the Hadley circulation and subtropical anticyclone was made. Results show that due to the earth rotation, the maximum convergence of meridional mass flux occurs in the subtropics, resulting in the formation of the zonal mean subtropical anticyclone. Under the constraint of thermal wind balance, the ridgeline of subtropical anticyclone usually tilts equator-ward with increasing height except in the boreal summer. In the planetary boundary layer, the subtropical anticyclone coincides with the sinking arm of the Hadley cell that extends vertically from the tropopause to the planetary boundary layer, and its annual cycle in intensity is in phase with that of the Hadley cell. The descent above the surface anticyclone is shown to be a result of the frictional dissipation in the planetary boundary layer, and does not contribute to the formation of the anticyclone. In the free atmosphere, the ridgeline of subtropical anticyclone deviates completely from the sinking arm of the Hadley cell with the former usually located to the equator side of the latter, and its annual cycle in intensity is out of phase with that of the Hadley cell.The relation between vertical motion and subtropical anticyclone in the three dimensional domain along the subtropics was also studied. In the planetary boundary layer, the tropospheric sinking coincides well with the ridgelines of subtropical anticyclone. Over the eastern ocean in the subtropics, the strong descent is in coordination with the strong surface equatorwardflow. In the free atmosphere, the distribution of vertical motion differs from that of the subtropical anticyclone. While the distribution of vertical motion could be understood by that of diabatic heating/cooling and the sloping of isentropic surfaces, the displacement between the vertical motion and subtropical anticyclone indicates the requirement of external forcing for the formation of the subtropical anticyclones. It is concluded that in steady state the atmospheric descent either in the free atmosphere or in the planetary boundary layer cannot be considered as a cause for the formation of subtropical anticy clones.