The hydrological effects of forest cover loss are difficult to discern in the case of large‐scale basins with gradual changes and difficult to isolate when climate variability is also present. In the present study, we evaluated the effects of climate variability and human activity on the annual streamflow in a basin in the Amazon arc of deforestation. We statistically analyzed the components of the annual water balance and monthly streamflow and used the currently used Tomer‐Schilling, elasticity, and decomposition of Budyko‐type curve methods to separate climate‐induced changes and anthropogenic effects. Annual series of the monthly maximum and minimum streamflow, total streamflow, and total reference evapotranspiration presented statistically significant increasing trends. No significant trend was observed for precipitation. The greatest change in the average annual runoff coefficient was observed between the first (1973–1984) and second (1985–1994) analyzed periods. Even with the continuous reduction in the forested area, the third (1994–2004) and fourth analyzed periods (2003–2016) showed only relatively small changes, most likely due to the intensity of slash‐and‐burn activities and vegetation regrowth. The methods showed that deforestation was the primary cause of the streamflow changes, but with different intensities, and a small recuperation was observed in the last analyzed period. On average, the annual water yield would increase between 26% and 58% after the first time interval without the opposite effect of climate variability, which must be considered in basin management. Future research should focus on analyzing the water storage and the dependence of the precipitation‐runoff relationship from the climate.

Fifteen years (1987–2001) of rain gauge-based data are used to describe the intraseasonal rainfall variability over tropical Brazil and its associated dynamical structure. Wavelet analysis performed on rainfall time series showed significant peaks centered roughly in periods of 30–70 days, particularly in the eastern southeastern Amazon and northern northeast Brazil. A significant enhancement of precipitation with maximum anomalies in a northeastward oriented band over tropical Brazil is evidenced from empirical orthogonal function (EOF) analysis of 30–70-day filtered rainfall anomalies during rainy season (January to May). Lagged/lead composites revealed that, on a global scale, the Madden–Julian oscillation (MJO) is the main atmospheric-mechanism modulator of the pluviometric variations on intraseasonal timescale in the eastern Amazon and northeast Brazil. A coherent northward expansion of rainfall across tropical Brazil is evident during the passage of MJO over South America. Regionally, the establishment of a quasi-stationary deep convection band triggered by the simultaneous manifestation of south Atlantic convergence zone (SACZ) and intertropical convergence zone (ITCZ) explains the intensified rainfall over these regions. Such regional mechanisms are dynamically embedded within the eastward-propagating MJO-related large-scale convective envelope along tropical South America/the Atlantic Ocean. These features occur in association with a significant intraseasonal evolution of the lower-level wind and sea-surface temperature (SST) patterns, particularly in the Atlantic Ocean, including a coherent dynamical connection with atmospheric circulation, deep convective activity over South America and rainfall over tropical Brazil. Copyright © 2006 Royal Meteorological Society

The influence of the large-scale climatic variability dominant modes in the Pacific and in the Atlantic on Amazonian rainfall is investigated. The composite technique of the Amazon precipitation anomalies is used in this work. The basis years for these composites arc those in the period 1960-1998 with occurrences of extremes in the Southern Oscillation (El Niño or La Niña) and the north/south warm (or cold) sea surface temperature (SST) anomalies dipole pattern in the tropical Atlantic. Warm (cold) dipole means positive (negative) anomalies in the tropical North Atlantic and negative (positive) anomalies in the tropical South Atlantic. Austral summer and autumn composites for extremes in the Southern Oscillation (El Niño or La Niña) and independently for north/south dipole pattern (warm or cold) of the SST anomalies in the tropical Atlantic present values (magnitude and sign) consistent with those found in previous works on the relationship between Amazon rainfall variations and the SST anomalies in the tropical Pacific and Atlantic. However, austral summer and autumn composites for the years with simultaneous occurrences of El Niño and warm north/south dipole of the SST anomalies in the tropical Atlantic show negative precipitation anomalies extending eastward over the center-eastern Amazon. This result indicates the important role played by the tropical Atlantic in the Amazon anomalous rainfall distribution.