Formation of the summertime subtropical anticyclones

The monthly mean data set of the NCEP/NCAR reanalysis is employed to investigate the relation between the subtropical anticyclone and diabatic heating. Criteria for defining the location and intensity of the zonal mean subtropical anticyclone are given to study its characteristics. Comparison between the Hadley circulation and the subtropical anticyclone is made. Results show that the maximum convergence of meridional mass flux occurs in the subtropics, resulting in the formation of the subtropical anticyclone. In the free atmosphere, the ridgeline of the subtropical anticyclone deviates completely from the sinking arm of the Hadley cell, with the former being located equatorward of the latter. Due to friction, the subtropical anticyclone in the planetary boundary layer coincides with the sinking arm of the Hadley cell that extends vertically from the planetary boundary layer to tropopause. It is stressed that either in the free atmosphere or in the planetary boundary layer, descent cannot be considered as a mechanism for the formation of the subtropical anticyclone. The theories of thermal adaptation of the atmosphere to external thermal forcing are employed to understand the formation of subtropical anticyclone in the three dimensional domain. Numerical experiments are designed to verify these theories. Results show that strong landsurface sensible heating in the summer subtropics generates not only the strong surface cyclone and upper layer anticyclone over the continent, but also the strong lower layer equatorward flow along the western coast of the continent, breaking the zonal symmetric anticyclone belt along the subtropics and forming the surface anticyclones over the oceans to the west. On the other hand, strong deep convective latent heating generates upper tropospheric equatorward flow that closes off the upper tropospheric subtropical anticyclone to the west of the heating, and lower tropospheric poleward flow that closes off the lower tropospheric subtropical anticyclone to its east. Radiative cooling along the subtropics in the two hemispheres coincides well with the tropospheric sinking and the location of subtropical anticyclone in the planetary boundary layer. Over the eastern oceans in the subtropics, such radiative cooling is strong with its maximum occurring at the top of the planetary boundary layer. It generates strong surface equatorward flow and shifts the centers of the oceanic subtropical anticyclones, which is forced by the land-surface sensible heating, towards the eastern parts of the oceans. In the boreal summer, the formation of the strong South Asian High (SAH) and the North American anticyclone in the upper troposphere and the subtropical anticyclone over the western Pacific (SAWP) in the middle and lower troposphere is therefore considered partly as a result of the convective latent heating associated with the Asian monsoon, but affected by orography and the surface sensible heating over the continent. On the other hand, the formation of the subtropical anticyclones at the surface over the eastern North Pacific and North America is mainly due to the strong surface sensible heating over continents, but affected by radiative cooling over the eastern North Pacific. It is shown that the different kinds of diabatic heating over each continent and the adjacent oceans along the summer subtropics are organized in a quadruplet pattern LOSECOD that forces a specific zonal asymmetric circulation pattern. The global summer subtropical heating and circulation can then be viewed as "mosaics" of such a quadruplet heating and circulation patterns, respectively. The fundamental importance of the land-sea distribution in forming the summertime subtropical circulations is then proved.