Abstract
Southeastern Brazil is the most populated and economically developed region of this country. Its climate consists of two distinct seasons: the dry season, extending from April to September, the precipitation is significantly reduced in comparison to that of the wet season, which extends from October to March. However, during nine days of the 2016 dry season, successive convective systems were associated with atypical precipitation events, tornadoes and at least one microburst over the southern part of this region. These events led to flooding, damages to buildings, shortages of electricity and water in several places, many injuries, and two documented deaths. The present study investigates the synoptic and dynamical features related to these anomalous events. The convective systems were embedded in an unstable environment with intense low-level jet flow and strong wind shear and were supported by a sequence of extratropical cyclones occurring over the Southwest Atlantic Ocean. These features were intensified by the Madden–Julian oscillation (MJO) in its phase 8 and by intense negative values of the Pacific South America (PSA) 2 mode.
Highlights
Climate and weather components that affect directly the population and economy of Southeastern Brazil [1, 2] have been widely studied in recent years, as well as the large-scale forcings from tropical and extratropical origins
It is well known that the climate of Southeastern Brazil is influenced by the South America monsoon system, where during the summer (Dec-Jan-Feb), there is a predominance of intense convective precipitation due to the availability of plentiful heat and moisture over the tropical region [3]. is intense convective precipitation delineates a cloud corridor known as the South Atlantic Convergence Zone (SACZ), which extends from the southwest Amazon Basin and through Southeast Brazil, reaching the Atlantic Ocean [4]
Advances in Meteorology decreases, initiating the dry period over Southeastern Brazil [5]. It is during this period that the South Atlantic Subtropical Anticyclone (SASA) reaches its most westerly position, extending over Southeastern Brazil, which impedes the passage of frontal systems [6]. erefore, during this period, the precipitation events are normally quick, isolated, and not intense
Summary
Climate and weather components that affect directly the population and economy of Southeastern Brazil [1, 2] have been widely studied in recent years, as well as the large-scale forcings from tropical and extratropical origins. We investigated the influence of the most important atmospheric and oceanic phenomena that can affect the weather over the Southeastern Brazil: Madden Julian Oscillation [17], Pacific South America pattern, first and second modes [18, 19]; Indian Ocean Dipole (IOD; [20,21,22]); Southern Annular Mode [23]; and blocking events [24, 25]. PSA teleconnection patterns consist of two distinct modes: PSA 1, related to the El Niño Southern Oscillation (ENSO; Karoly [31]) and PSA 2, associated with the MJO during the winter [18] Both of them have impacts on the climate of South America, and on the rainfall intensity and distribution over São Paulo state. The SP region is considered to be a transition region where the e ect of ENSO could be either to increase or reduce precipitation [48]
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