Análise sinótica e de grande escala de ondas de frio extremas no Sudeste do Brasil no século XX

Abstract

The 20th century was marked by several intense cold waves in Brazil. Due to the impacts of cold extremes on public health and agriculture, this study aimed to investigate 12 episodes that occurred in Southeast Brazil between the years 1961 and 2017. In addition, we performed a case study for one of these events, which occurred in 1985, due to its magnitude and duration. For this purpose, the ERA20-C reanalysis data was used in order to elaborate the synoptic fields, compositions and for the application of ray tracing. Regarding the synoptic patterns of the 12 events, there is a similarity in the structure of the waves in the upper atmosphere, which, in general, present ridges amplified in the East Pacific Ocean and amplified troughs over South America (SA). This wave pattern is important to direct the flow from south to north at low levels, generating cold horizontal advection in the Southeast Region. Compositions of the 250 hPa geopotential height anomaly show a Rossby wave train that departs from the center-east of the South Pacific Ocean towards SA two days before the extreme cold at the study site. The ray tracing analysis confirm the compositions and reveals that the source region of the number 2 and 3 planetary waves is the central-eastern part of the South Pacific Ocean. It is suggested that the ridges that amplify from the East Pacific Ocean towards the Patagonia and, later, to the Southwest Atlantic Ocean, are a response to the events of the Rossby wave break that occur in the AS longitudes. The results obtained for the 1985 cold wave show that the amplification of an existing long wave over the South American continent contributed to a blocking pattern, which explains the consecutive days of cold advection in the Center-South of Brazil and, consequently, in the Southeast Region. This is because the blocking structure in the longitudes of the SA - forced from a disturbance in the wave train of the Southern Hemisphere (SH) generated in the Indian Ocean - established a dipole baric field in the Southwest Atlantic Ocean, characterized by a high block in the Falkland Islands and low pressure on the east coast of the South Region of Brazil. The performance of both systems intensified the pressure gradient that directed the south/southeast winds in the interior of the continent for several days. The persistence of this atmospheric configuration has generated records of minimum temperatures and a wide area covered by frost in several states, including the snow record at Pico das Agulhas Negras, within the Itatiaia National Park.