Dissertation Defense: Ana Sena
Title: Interannual Variability of the South American Monsoon in a climate change context
Abstract: The South American Monsoon System (SAMS) is characterized by a well-defined wet and dry season, which is connected to the reversal of anomalous low-level winds east of the Andes. Driven by local and remote processes, the intensity and duration of the SAMS can vary considerably from year-to-year. Improving the understanding of the variability in the SAMS is important not only for the planning of water resources, but also for the global carbon budget due to the presence of the Amazon rainforest. Here, I investigate the mechanisms associated with interannual variability and long-term trends of the SAMS, which can impact the local circulation and precipitation anomalies over South America.
The broad objective of this dissertation is to understand the physical mechanisms of the intraseasonal and interannual variability of the South American Monsoon and how it may change in a warmer climate. This dissertation explores three different aspects of that problem: (1) how the length and intensity of the South American Monsoon may change in a future climate by examining projections in a large ensemble of climate model simulations; (2) how the Indian Ocean Dipole affects the large-scale circulation over South America through extratropical teleconnections and changes in the local Walker circulation; and (3) the role of the Quasi-Biennial Oscillation, a regular and well forecasted mode of climate variability, in modulating the remote effects of the Madden Julian Oscillation in the Southern Hemisphere.
The results shown in this dissertation find that the response of the SAMS to climate change in most areas of South America may be characterized by a dryer dry season and a wetter wet season. However, the Amazon basin is projected to become dryer in all seasons, with possible impacts to forest productivity. Extreme events of rainfall are also projected to become more frequent and intense by the end of the century.
Next, by comparing available observations to a set perturbation experiments in an atmospheric global climate model forced with the SST anomalies due to both signs of the Indian Ocean Dipole (IOD) and a historical simulation from a fully-coupled large ensemble, I analyze the circulation anomalies associated with each phase of the IOD. The simulations reveal that both phases of the IOD can redirect the low-level winds over South America, which changes the advection of moisture to Southeastern Brazil and Southeastern South America. As a result, the response to the positive phase of the IOD is characterized by a dryer-than-usual South Atlantic Convergence Zone, while wet anomalies are found in the La Plata basin during the negative phase of the IOD. This occurs through an extratropical mechanism, composed of Rossby waves excited by the anomalous convection in the tropics that reach South America and redirect the low-level flux in the area. During the positive phase of the IOD, the displacement of the local Walker circulation results in wetting over the Amazon basin.
Finally, I show that the Quasi-Biennial Oscillation (QBO) can modulate the response of the SAMS to the Madden-Julian Oscillation (MJO). When there are easterly winds in the lower stratosphere, the convection anomalies over the MJO are intensified and there are significant anomalies in structure of the extratropical wave train. Therefore, the anomalies associated with the MJO in South America are intensified and extend to greater areas of the subtropical South Atlantic. These results may highlight a new mechanism that can improve subseasonal to seasonal forecasts in South America.