Large-Scale Common Features of Subtropical Precipitation Zones (the Baiu Frontal Zone, the SPCZ, and the SACZ) Part I: Characteristics of Subtropical Frontal Zones

Abstract

In East Asia, a quasi-stationary frontal zone called the Baiu frontal zone (BFZ) forms during the early summer and provides nearly as much precipitation as the Intertropical Convergence Zone (ITCZ). Since the BFZ has several characteristics different from both the ITCZ and polar frontal zones, Ninomiya (1984) proposed the BFZ should be classified as a subtropical frontal zone.
Using mainly ten-day mean data, we compare the BFZ around Japan with the subtropical portions of the South Pacific Convergence Zone (SPCZ) and the South Atlantic Convergence Zone (SACZ), which are significant precipitation zones in the summer Southern Hemisphere. These three precipitation zones are shown to have several common features as the following.
The BFZ, the SPCZ, and the SACZ (hereafter referred to as the SPZs, the Subtropical Precipitation Zones) commonly form along the subtropical jet on the eastern side of a quasi-anchored trough, which lies to the northeast (in the Northern Hemisphere) or southeast (in the Southern Hemisphere) of the localized active convection of the tropical monsoon. The rainfall amount in the SPZs attains -400 mm/month when they are active. All of the SPZs are characterized by convergence zones with an interior thick moist layer and baroclinic zones with an upper subtropical jet. They are also characterized as poleward boundaries of the moist tropical or monsoon airmass associated with a low-level large gradient of moisture mixing ratio.
Since the evaporation rate is much smaller than the precipitation rate along the SPZs, high rainfall in the SPZs is maintained by the convergence of two types of moisture currents in the SPZs. One is eastward along the SPZ and the other is along the northwestern (in the N.H.) or the southwestern (in the S.H.) periphery of the subtropical high. The latter transports moisture evaporated under the western part of the subtropical high. Generation of convective instability by the differential advection process is found along the SPZs and maintains active convection along the SPZs.
Since the SPCZ and the SACZ have several unique characteristics different from the ITCZ and polar frontal zones but similar to the BFZ, it is concluded that all of the SPZs can be classified as subtropical frontal zones.