The Amazon hydrometeorology: Climatology, variability and links to changes in weather patterns

My thesis focuses on improving the quantification of the hydrological cycle and understanding the atmospheric processes that link weather to climate in the Amazon River basin. By using ERA40 and independent observations, I assess how well we can estimate the surface water budget in the Amazon River basin. I find that ERA40 basin wide annual precipitation (P) overall agrees with observations showing a slight underestimation of 10% in average, whereas runoff (R) is underestimated by a larger margin (∼25%). Observed residual of precipitation and runoff (denoted as P-R) is better estimated by ERA40 P-R than actual ET which includes soil moisture nudging.;The causes for said discrepancies were found to partly relate to soil moisture nudging that needs to be applied during the dry season to produce realistic ET and compensate for the low soil moisture recharge during the previous wet season. Insufficient recharge may in part be caused by underestimation of rainfall amount and intensity; moreover the shallow root layer in the model does not represent the deep soil water reservoir characteristic of the Amazonian forest.;Whether the hydrological cycle and weather patterns in the Amazon have changed during the past few decades is a highly debatable but central question for detecting climate change in the region. The second part of my thesis focus on the physical links between rainfall changes detected in observations, and changes of synoptic scale systems as represented by ERA40. My results suggest that an observed delayed wet season onset is consistent with a decreasing number of cold air incursion (CAI) days in southern Amazon for the period 1979--2001.;The variability of CAI into southern Amazon is related to the variability of SST upstream of South America in the tropical Pacific and Indian Oceans. A Singular Value Decomposition Analysis (SVD) between CAI days and global SST reveal three main modes of co-variability. The first mode describes the effect of the El Nino-Southern Oscillation. During El Nino (La Nina) a strong (weak) subtropical jet stream over South America tends to prevent transient systems from moving to southern Amazon, resulting in decreased (increased) CAI days during SON. The second mode of co-variability shows an anomalously warm western Indian Ocean also related to strong subtropical jet stream, except the jet is positioned farther north in South America, which along with the absence of a well defined subpolar jet stream, favors the northward displacement of transient waves into central South America, but show little response in southern Amazon. The CAI days reconstructed from the first and second modes do not present any significant trend in southern Amazon. CAI days reconstructed from the third mode of co-variability on the other hand, reproduces the SON observed trend in almost its entirety. The third mode of co-variability describes negative (positive) anomalies in CAI days associated with cold (warm) SST anomalies in the eastern tropical Pacific, anomalous wavetrain in the Southern Hemisphere and Walker Cell displacement that are unfavorable (favorable) to the incursion of CAI into southern Amazon. The temporal evolution of this mode correlates negatively with the Pacific Decadal Oscillation, suggesting that the recent gradual shift in PDO polarity reflected on the interannual response of Southern Pacific atmospheric patterns, hence on the behavior of transients propagation. The negative PDO index and its related atmospheric patterns are in agreement with the reduced observed CAI days, which also related to a delayed wet season onset in the southern Amazon.