The Forest Effects on the Isotopic Composition of Rainfall in the Northwestern Amazon Basin

Abstract

In the Amazon basin, intense precipitation recycling across the forest significantly modifies the isotopic composition of rainfall (δ18O, δD). In the tropical hydrologic cycle, such an effect can be identified through deuterium excess (dxs), yet it remains unclear what environmental factors control dxs, increasing the uncertainty of dxs‐based paleoclimate reconstructions. Here we present a 4‐year record of the isotopic composition of rainfall, monitored in the northwestern Amazon basin. We analyze the isotopic variations as a function of the air mass history, based on atmospheric back trajectory analyses, satellite observations of precipitation upstream, leaf area index, and simulated moisture recycling along the transport pathway. We show that the precipitation recycling in the forest exerts a significant control on the isotopic composition of precipitation in the northwestern Amazon basin, especially on dxs during the dry season (r = 0.71). Applying these observations to existing speleothem and pollen paleorecords, we conclude that winter precipitation increased after the mid‐Holocene, as the expansion of the forest allowed for more moisture recycling. Therefore, forest effects should be considered when interpreting paleorecords of past precipitation changes.

Contrasting Southern Hemisphere Monsoon Response: MidHolocene Orbital Forcing versus Future Greenhouse Gas–Induced Global Warming

“…Increased precipitation in Northeast Brazil and the eastern Amazon is coherent with the southward ITCZ migration. However, the observed drying in central part of the Amazon basin, is consistent with the drying effect of the compensating subsidence induced by the enhanced precipitation in Northeast Brazil and in the extreme western portion of the Amazon basin where the moisture comes from the northeast (Venezuela and Colombia) from November to February (Ampuero et al 2020). Thus, besides the ITCZ migration mechanism, there are different mechanisms responsible for subregional precipitation changes.…”

The resilience of Amazon tree cover to past and present drying

“…4). We also adopt the δ 18 O correction that accounts for differential cave temperatures between the sites previously used in references [30,107].…”

Changes in Normalized Difference Vegetation Index in the Orinoco and Amazon River Basins: Links to Tropical Atlantic Surface Temperatures

“…For instance, the southern Amazon basin is an important source of water vapor for southern Brazil, northern Argentina, and the Paraná-La Plata River basins (Martinez and Dominguez 2014), which is transported by the South American low-level jet (SALLJ) that develops on the eastern slope of the Andes (Jones 2019;Marengo 2006;Montini et al 2019;Vera et al 2006b). In addition, the southern Amazon provides water vapor to the tropical Andes (Ampuero et al 2020;Espinoza et al 2020;Ruiz-Vásquez et al 2020). Regarding annual precipitation, the northern Amazon is mainly affected by the meridional migration of the intertropical convergence zone (ITCZ) whereas the southern Amazon is influenced by the South American monsoon system (SAMS) and the South Atlantic convergence zone (SACZ) (Vera et al 2006a).…”