Variabilidades decadais a multidecadais do sistema de Monção da América do Sul: um estudo observacional e estatístico

Abstract

A monsoon system is characterized by large-scale seasonal wind reversal, due to the thermal gradient between continents and oceans, and that has well-defined dry and wet seasons. However, the South American Monsoon System (SAMS), unlike other monsoons, does not exhibit a change in wind direction, but rather a marked seasonal change in circulation and humidity. The SAMS occurs during the austral summer, with the formation of convection over the northwest of the Amazon region, which extends to the southeast of South America, progressively intensifying. One of the main characteristics of SAMS is the occurrence of the South Atlantic Convergence Zone (SACZ), which is a quasi-stationary moisture convergence zone that can cause intense precipitation when active, or indirectly, its absence can generate a rainfall deficit, as ocurred in the austral summer of 2014/2015 in Southeastern Brazil. One of the importance of SAMS comes from the fact that sectors such as the economy, agriculture, water and energy resources and the livelihoods of a large part of the Brazilian population are heavily dependent on it. SAMS presents variability in several temporal scales, such as intraseasonal and interannual, which are already well detailed in the literature. However, its relationship with lower frequency variability is not fully known. Therefore, this work aims to investigate the relationship between decadal to multidecadal variability and SAMS, focusing on the ZCAS region, using atmospheric and oceanic variables. All study regions showed signs of decadal variability through wavelet analysis, however the most energetic and low-frequency signals were those from Amazon, mainly from 1970 onwards. Decadal precipitation anomalies of Amazon and North of Southeast showed high correlations with decadal anomalies of Pacific SST, characterizing a spatial pattern similar to that of Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscilation (IPO) oscillations. Central-West and South of Southeast had correlations with some areas of Pacific, but these signs were not comprehensive. The combined empirical orthogonal function (EOFc) analysis was able to identify important characteristics of summer circulation in South America on a decadal scale. An intensification of northeast trade winds over the Atlantic was observed, together with an increase in air temperature and humidity in central range of Brazil. The series of temporal coefficients associated with EOFc dominant mode characterized the temporal evolution of SAMS in decadal scale and was defined as a monsoon index, the South American Monsoon Multidecadal Index (SAMMI). In general, the SAMMI had three phases: a negative one between 1937 and 1974, a minor and positive one from 1975 to 1995, and a negative one from 1996 to 2015, and presented a positive and significant correlation with PDO (+0.35 ) and IPO (+0.26) indices. SAMMI was also positively and significantly correlated with decadal precipitation anomalies of Central-West (+0.56), North of Southeast (+0.66) and South of Southeast (+0.59). SAMMI was able to identify the spatial variability of South American monsoon through the dominant mode of main atmospheric patterns, and it mainly resembled the temporal variability of both PDO and IPO.