定常态副热带高压与垂直运动的关系

RELATION BETWEEN SUBTROPICAL ANTICYCLONE AND VERTICAL MOTION IN STEADY STATE

文中利用NCEP/NCAR月平均再分析资料研究了定常条件下二维和三维空间上副热带高压与垂直运动的关系 ;比较了Hadley环流和副热带高压动力学的差异。结果显示由于地球自转 ,在副热带地区出现了最大的经向质量通量的辐合 ,导致了纬向平均副热带高压的形成。在热成风关系的制约下 ,除了在北半球夏季 ,通常副热带高压脊线随高度增加向赤道倾斜。Hadley环流的下沉支从对流层顶垂直地延伸到行星边界层。因此副热带高压脊线与该下沉支在自由大气中位置分离 ,年际变化反相 ;在行星边界层中摩擦耗散作用使两者重合 ,年际变化同相 ,但垂直运动对副热带高压的形成不起作用。三维空间上 ,沿行星边界层的副热带高压脊线处为下沉运动 ,副热带大洋东部强烈的下沉运动对应着强烈的向赤道气流。自由大气中 ,垂直下沉运动的分布与副热带高压的分布不同。大洋上副热带高压东部大气下沉 ,副热带高压西部大气上升。这一方面与等熵面的北高南低的倾斜分布在动力学上是一致的 ;另一方面还表明副热带高压的形成也与非绝热加热密切相关。总之 ,不论是在自由大气还是在边界层中 ,不能简单地把下沉运动看成是副热带高压形成的原因。

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 equatorward flow. 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 anticyclones.