Can We Detect Changes in Amazon Forest Structure Using Measurements of the Isotopic Composition of Precipitation?

Abstract

Large-scale (>500 km) spatial gradients of precipitation oxygen isotope ratios (δ 18 O p ) hold information about the hydrological cycle. They result from the interplay between rainout and evapotranspiration along air-parcel paths, but these counteracting effects are difficult to disentangle, complicating quantification of the effect of land cover change on δ 18 O p . We show that disentangling can qualitatively be achieved using climate model simulations with a land-derived precipitation tracer for tropical South America. We then either vary land cover as observed since 1870 or replace Amazon forests with bare land to determine the resulting signals. Our results indicate that effects of historically changing land cover on annual mean δ 18 O isotope-ratio gradients are small and unlikely detectable, although there is a noticeable signal during the dry season. Furthermore, the effect of changes in water recycling on Amazon δ 18 O p in paleo-records may have been overestimated and need reinterpretation. Plain Language Summary Deforestation causes reduction in precipitation downwind becausetrees act as pumps of water from soils to the atmosphere. This mechanism is primarily important during the dry season. How strong this effect is currently in the Amazon, given that approximately 20% of the forests have been cut, and how important it may be in the future if more forests are being destroyed is of great interest. One indicator of such changes is the east-west difference in heavy water isotope content of precipitation. While preferential rainout of the heavy isotope along air parcel trajectories enhances this difference, transpiration by forests decreases the difference. This is because forests inject water back into the atmosphere that is more enriched than the overlying water vapor. Records of this difference during the last ice age, in particular, have been interpreted in a previous study as providing information on continental recycling. We apply a land-derived water tagging approach in model simulations to investigate the effect of continental recycling on precipitation isotope content and to estimate this effect for varying land cover. We find that a 20% deforestation has only a small impact on precipitation isotope content. Even for a complete deforestation, in contrast to a previous interpretation, thus, only some of the isotopic signal observed during the ice age can be attributed to changes in continental recycling.

The Zonal Patterns in Late Quaternary Tropical South American Precipitation

“…Our approach avoids the assumption that upwind moisture loss is constant through time and, as we discuss in the next section, is consistent with evidence that δ 18 O is not always strongly coupled with local precipitation amount (Ward et al., 2019; Wortham et al., 2017). Within our framework, the wettest time occurs when the isotope gradient is steepest, not when central δ 18 O is lowest, consistent with our understanding of how Δδ 18 O relates to precipitation today (Ampuero et al., 2020; Pattnayak et al., 2019; Salati et al., 1979).…”

Variability and Controls on δ18O, d‐excess, and ∆′17O in Southern Peruvian Precipitation

“…Subcloud evaporation, which is more likely in the winter when the atmosphere is drier, may also play a role in the higher winter d‐excess p values if precipitation condenses from reevaporated vapor (e.g., Risi et al., 2008). Evapotranspiration from the Amazon Basin may amplify this d‐excess p seasonality if the bare‐ground evaporation flux increases during the dry season (Pattnayak et al., 2019).…”

Pacific and Atlantic Multidecadal Variability Relations with the Choco and Caribbean Low-Level Jets during the 1900–2015 Period

“…Barichivich et al [33] found an intensification of Walker's circulation during the late 1990s, associated with strong warming of the tropical Atlantic and cooling of the tropical Pacific. Furthermore, Pattnayak et al [34,35] analyzed the relationship of precipitation in northern and northwestern South America with SST anomalies in the Atlantic after 2000. Therefore, through these interbasin SST gradients, the PDO and AMO SST mean states modulated the CJ and CLLJ variations during the last three decades [30].…”