ATMOSPHERIC PATTERNS RELATED TO EXTREME RAINFALL EVENTS ON THE SOUTHERN REGION OF BRAZIL

Abstract

Agricultural activities at the northwest portion of the State of Rio Grande do Sul - RS, located at the southern region in Brazil, is an important economical issue which constitute great part of the internal income and part of the state and national GDP. The development of different crops in this region particularly depends on atmospheric conditions, mainly during rainy season, which lasts from October to March of the next year. This period is characterized by the development and harvest of important crops in the studied region. In this work we determined the atmospheric patterns for wet and dry periods, between 1978 and 2005, associated to the precipitation observed at RS northwest region. The atmospheric patterns of anomaly fields clearly show opposite signals in dry and wet periods over south and central Brazil, pointing out a clear dipole structure between these two regions. Wet and dry periods are respectively characterized by low and high values of OLR in relation to climatology. OLR anomalies over north and northeast regions of Brazil are constantly in opposite signals in relation to those observed at south region, configuring the dipole structure. Dry and wet periods respectively show negative and positive air divergence anomalies at high levels of atmosphere over the region studied, while atmospheric low levels are characterized by positive and negative divergent anomalies, respectively. During wet and dry periods air circulation evidences the Low Level Jet (LLJ) strengthening and weakening; LLJ is commonly considered as responsible by humidity transportation from north to southeast region of South America and, then, it is related to rain variability.  The results also show that wet periods are related to Subtropical Jet strengthening that contributes to the increasing of atmospheric dynamic instability and, then, to the development and intensification of rainy systems over the region. Variability of sea surface temperature at Subtropical Atlantic and Equatorial Pacific oceans, from 1978 to 2005, sounds to influence considerable fraction of precipitation variability at South of Brazil: positive anomalies of sea surface temperature are observed at Equatorial Pacific portion concomitantly to rain positive anomalies at the South region, for all month analyzed. On the other hand, sea surface temperature variability over Atlantic Subtropical Ocean, that portion adjacent to the south region of Brazil, does not seem to be directly related to the RS rain variability. Therefore, we conclude that the atmosphere over the South Region of Brazil presents distinct patterns during wet and dry periods, coupled to dipole structures at northern and southern South America. The influence of Equatorial Pacific Ocean is much clearer to annual precipitation variability at RS state than Atlantic Subtropical Ocean.