Automatic Algorithm to Define Episodes of SACZ Based on Outgoing Longwave Radiation Data

In this work, an Outgoing Longwave Radiation (OLR) orbital image classification algorithm was developed to identify episodes of South Atlantic Convergence Zone (SACZ). The algorithm considers in the OLR daily field, the spatial pattern of the cloud band associated to the SACZ, and the permanence of this pattern for at least four consecutive days. The SACZ episodes detected by the algorithm present an extensive and well­configured cloud band towards the Southwest Atlantic, occurring simultaneously with the South American Monsoon System (SMAS) and La Plata Basin region. The mean frequency of occurrence was about 1.4 episodes per month, lasting 5.75 days. 65% of the active SACZ occurred during peak summer in the southern hemisphere (December to February) and 35% in November and March. Regarding the SACZ episodes observed by the CPTEC, the agreement was 56%, and the error was 13%. However, through the visual classification of the summers of 1997/98 and 2006/07, it was concluded that both methodologies present errors inherent to the classification processes. The main differences between the methods are that the algorithm tends to capture episodes of SACZ that was well defined and more boreal in relation to those observed by CPTEC, as a consequence of the use of a deterministic method in restriction to the visual analysis. It was also developed a methodology to identify oceanic SACZ episodes (ZCASOCN), which have intense convective activity on the Southwest Atlantic. The ZCASOCN occurs to the north of the average position, with permanence of 6.6 days. The cooling observed in the Southwest Atlantic Ocean a week before ZCASOCN favored the entry of southeast winds in South America, blocking the SACS to the north. A cooling (heating) was also observed below the SACZ cloudiness range (south of SACZ) one week after the convective system pass, which is possibly associated with the SACZ­ cloud­ SST feedback. During the ZCASOCN there is also a tendency for cyclonic circulation in the surface ocean currents, which follows the pattern of winds at 10 m from the surface. The results suggest that the alteration of ocean surface runoff can affect the coastal upwelling along the Brazilian coast.