The Amazon basin, which contains about 60% of tropical rainforests in the world, plays vital roles in regulating climate patterns, sustaining ecosystem services, contributing to global biodiversity, and cycling nutrients. These services, however, have been disrupted by human activities within the region due to infrastructure development and resource extraction. These land-use changes have impacts from local to global scales, particularly on climate and hydrologic cycles, but the extent to which is unclear. Therefore, it is essential to examine precipitation variability and look for drivers of changes at multiple spatio-temporal scales. Analysis of hot spots of land-atmosphere interactions highlights the areas where changes in land surface characteristics influence the atmosphere behavior the most. This dissertation focuses on climate variability and land-atmosphere coupling across the Amazon basin. Research questions are addressed in three self-contained chapters. Chapter 2 examines the changes in precipitation amount and intensity using a high-resolution (0.05o spatial resolution) gridded data set (CHIRPS) from 1982 to 2018. Several precipitation indices are developed to analyze trends using the Mann-Kendall test. Our results show landscape-scale changes in the timing and intensity of rainfall events. Specifically, wet areas of the western basin have become significantly wetter since 1982, with an increase of 182 mm of rainfall per year. In the eastern and southern regions, where deforestation is widespread, a significant drying trend is evident. In chapter 3, we aim to examine the impacts of potential tropical reforestation on surface energy and moisture budgets, including precipitation. We simulated changes in heat and moisture fluxes due to tropical reforestation using WRF.V3.9 (Weather Research and Forecast model) to analyze the sensitivity and magnitude of changes to the surface fluxes due to reforestation in the Amazon Basin. We found that the effects of reforestation on the atmosphere were more evident during the dry season; spatial patterns of the changes in atmospheric behavior due to reforestation were consistent with the pattern of land cover change, and the cooling effect of reforestation was evident at seasonal time scale. In chapter 4, following the results of chapter 3 on the effects of land surface characteristics on atmosphere behavior, we aim to find hot spots of strong land-atmosphere (L-A) coupling across the basin at regional scales. Strong land-atmosphere coupling is critical to understanding precipitation dynamics. Therefore, we applied two commonly used coupling approaches at the regional spatial scale and monthly temporal scale. Ultimately, we recommend a new metric considering more physical relationships, interactions, and lag times between variables. We found that the spatial pattern of hot spots is highly dependent on the temporal and spatial scales of analysis. Also, the interactions among variables within the boundary layer play a more important role in determining the hot spots of strong L-A coupling. Overall, the evidence provided here suggests that (1) precipitation distribution has changed over time (1982-2018) with wet areas getting wetter and dry areas getting drier across the Amazon basin; (2) reforestation of deforested regions across the basin moderates atmospheric patterns and behavior; (3) hot spots of strong L-A coupling are highly dependent on temporal and spatial scales of analysis as well as parameters interactions within the boundary layer.
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