Roles of the Atmospheric Heat Sources in Maintaining the Subtropical Convergence Zones: An Aqua-Planet GCM Study

Abstract

The subtropical convergence zones (STCZs), that is, the Baiu frontal zone and subtropical portions of the South Pacific and South Atlantic convergence zones, are unique subtropical rainfall systems connected to the tropical monsoons to the west. An aqua-planet GCM without any land was used to study diabatic heating contributions in tropical monsoons and the STCZs that maintain the STCZs. An STCZ was simulated in the GCM without any influence of land when a monsoonlike, namely, strong zonally localized off-equatorial heat source, was put in the Tropics. The monsoonlike heat source has two roles:1) making a strong subtropical jet through a local Hadley circulation in the summer hemisphere and 2) making a low-level poleward moisture flow, which supplies considerable moisture to the STCZ. Upward motion maintained by the diabatic heating in the STCZ induces a low-level trough along the west and poleward side of the STCZ, keeping the vorticity balance between stretching and advection of low absolute vorticity air from the Tropics. This trough intensifies the low-level wind along the STCZ. This wind maintains strong convergence in the STCZ, together with the poleward moisture flow from the Tropics. The upward motion also induces an upper-level trough and ridge with the STCZ between, as a result of vorticity balance between shrinking and eastward vorticity advection by the mean westerlies. The radiation of a stationary Rossby wave from the STCZ was also suggested.