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Mechanisms and Attribution of Changes in Austral Summer Precipitation related to the South Atlantic Convergence Zone

  • Author(s): Zilli, Marcia T.
  • Advisor(s): Carvalho, Leila M. V.
  • et al.
Abstract

Austral summer (DJF) precipitation over tropical South America (SA) is characterized by the South American Monsoon System (SAMS) and the South Atlantic Convergence Zone (SACZ). The increase in atmospheric temperature and water vapor content over the SA during the last decades of the 20th century could affect the duration and amplitude of the SAMS and the intensity of the SACZ. This research examines the spatial variability of precipitation trends over SE Brazil, focusing on the SACZ. More specifically, this study investigates trends in precipitation over Southeastern Brazil (SE Brazil) and examines changes in the position and intensity of the SACZ. SE Brazil is the most densely populated region in the country with a large portion of this population living in urban centers. The SACZ is important for agriculture and water supply for millions of people. One of the main goals of this research is to identify mechanisms associated with the observed changes in the characteristics of the SACZ during the last three decades of the 20th century, and examine the relative contribution of natural and anthropogenic forcing to precipitation trends. The first chapter investigates the pattern of spatial variability of precipitation trends over the coastal region of SE Brazil. This study shows that over the southern portion of the study area, precipitation is increasing due to the increase in the frequency and intensity of extreme events. Over the northern portion of the area, while the intensity of extreme events is increasing, the number of precipitating days is decreasing. This spatial pattern of precipitation trends suggests a poleward shift of the SACZ, which is investigated in the second chapter. Chapter 2 focuses on the underlying mechanisms associated with changes in precipitation intensity related to the position of the SACZ. Decadal variations in the mean state of the atmosphere suggest that the observed changes in precipitation over SE Brazil are associated with a weakening of the poleward winds along the eastern Brazilian coast that reduces the dynamic support necessary for convection along the equatorward margin of the SACZ. Additionally, this analysis also identifies a decrease in low-to-mid troposphere (700hPa) moisture over the tropical Atlantic in the recent decade that further reduces the moisture transported into the convective margin of the SACZ. Both mechanisms contribute to reducing precipitation over eastern tropical Brazil and characterize the poleward shift of the SACZ. The final chapter focuses on evaluating the contribution of natural variability and anthropogenic-related forcings to the poleward shift of the SACZ and drying conditions over eastern Brazil at the end of 20th century. Simulations from different scenarios of the Coupled Model Intercomparison Project phase 5 (CMPI5) models capable of reproducing the SACZ climatology suggest significant contribution of anthropogenic forcing on the SACZ-related precipitation trends. Despite the large discrepancies in the simulated precipitation trends, similarities among the ensemble members provide compelling evidence that the poleward shift of the SACZ in the last three decades of the 20th century is largely enhanced by anthropogenic forcing. Collectively, the three chapters of this dissertation characterize the recent changes in precipitation related to a poleward shift of the SACZ and give novel insights into the influence of anthropogenic-related forcing on these changes. These findings advance the scientific understanding of the consequences of recent climate variability and change over eastern tropical South America, particularly over the SACZ.

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