Note on “Predictability of Northeast Brazil Rainfall and Real-Time Forecast Skill, 1987–98”

We comment on speculations in Folland et al. (2001, hereafter FCRD)about atmospheric and oceanic influences on northeast Brazil (NEB)rainfall in the austral winter dry season. FCRD was mainly concerned with the NEB wet season and its predictability. However, the paper also touched upon the dry season, particularly June and July, when there is sometimes appreciable rainfall. Figure 3d of FCRD showed correlations of the worldwide Global Sea Ice and Sea Surface Temperature version 3 (GISST3)dataset with NEB rainfall, showing areas of fairly strong correlation with Atlantic SST, particularly in the tropical South Atlantic on the South American side of that ocean. FCRD speculated that the high correlation of NEB rainfall with SST in the tropical Atlantic Ocean in June-July might be associated with a displacement of the South Atlantic convergence zone (SACZ)northward. We want to correct this hypothesis, and indicate possible links between June-July NEB rainfall and tropical South Atlantic SST. Analyses of the tropical and subtropical precipitation zones in the Southern Hemisphere have been performed by Kodama (1992, 1993), including the SACZ. Other studies that show influences of the SACZ were reported in Kousky (1988), Liebmann et al. (1999), and Nogues- Paegle and Mo (1997). The SACZ is a band of cloudiness and convective activity oriented northwest-southeast (NW-SE)over eastern South America extending into the tropical to subtropical South Atlantic. It is seen from the end of austral spring (November)up to the beginning of austral autumn (March), being more frequent in the summer months. The SACZ can be detected in both monthly mean precipitation and outgoing longwave radiation (OLR)fields. From about April to October the higher values of precipitation and low values of OLR are displaced northwestward. The SACZ ceases to exist in the austral winter half year. At the same time, there is another band of precipitation over southern Brazil, associated with frequent passages of frontal systems. The SACZ can influence NEB rainfall, but only in the austral summer when it can be displaced northward and affects southern NEB (Chaves and Cavalcanti 2001).