The SACZ (South Atlantic convergence zone) is a significant precipitation zone which stretches from the active rainfall area developed over the Amazon Basin in summer. The SACZ is one of the elementary features of the South American monsoon system (Zhou and Lau 1998). The rainfall over the Amazon, which is a strong atmospheric heat source, largely contributes to maintaining the SACZ (Kodama 1993; Figueroa et al. 1995; Mattews et al. 1996). In this paper, we'd like to remark that the SACZ is not a simple slave of the Amazonian rainfall, but the rainfall along the SACZ also may play as a heat source to maintain the SACZ.

Low-level northerly along the western edge of the subtropical high over the South Atlantic is important for maintaining the SACZ, because the northerly supplies much moisture and forms low-level convergence (Kodama 1993). The northerly is considered as a part of monsoon circulation surrounding the monsoon heat low over the South America (Kodama 1992).

We performed vorticity budget analysis using the NCEP reanalysis data. The results suggest that the strong atmospheric heating along the SACZ also may intensify the low-level northerly. The heating maintained by the rainfall along the SACZ forms an updraft along the SACZ. This updraft intensifies the cyclonic vorticity in the lower troposphere by the stretching process. Actually a low-level weak trough is observed along the southwestern side of the SACZ. SLP gradient, and combined northerly, are then intensified along the northeastern side of the SACZ. Note that the low-level trough is not strong in spite of the strong heating. This is because the vorticity generation by the stretching is canceled out by southward advection of lower absolute vorticity from the tropics by the northerly. This suggests that the northerly is partly maintained by the updraft along the SACZ, as a result of vorticity balance between the stretching and the poleward vorticity advection. A positive feedback may exist in rainfall activity of the SACZ, because the moisture supply by the northerly intensifies the rainfall, and the combined updraft, which may intensify the northerly. We will also show some results of AGCM experiments to reinforce this idea.