Rainwater Isotopic Composition in the Ecuadorian Andes and Amazon Reflects Cross-Equatorial Flow Seasonality

Abstract

The variability of the rainfall stable isotopic values (δ2Hp, δ18Op) in the Ecuadorian Amazon to the Andes presents a marked local “altitude” effect. At the same time, this complex orography creates diverse precipitation regimes (unimodal, bimodal, and three-modal) that make it difficult to establish a relationship with the local amount. Nevertheless, stations along these regions show a similar intra-annual isotopic variability, with lower values during MAM and ON. In contrast, higher values are found during DJF and JAS in a w-shaped pattern, suggesting a common regional controller. A monthly δ2Hp and δ18Op collection campaign was established in Central Ecuador (n = 30) to complement stations biased towards the northern and southern parts. Based on back trajectory analysis, the results demonstrated that moisture arrives from two primary sources: the Tropical North Atlantic (DJFM) and the Amazon Basin (JAS). Nevertheless, their convergence (AMJ and ON) is the crucial factor modulating the lowest isotopic values. Precisely, this convergence is stronger at the V-Index region (5° S–5° N, 65°–75° W), where the wind seasonality and reversal at low levels are enhanced, allowing the inter-hemispheric moisture flux transport (cross-equatorial flow). We propose that the amount of rainfall located at the V-Index region is a more robust approach for explaining the δ2Hp and δ18Op variability rather than the local amount.

Drivers of δ18O Variability Preserved in Ice Cores From Earth's Highest Tropical Mountain

“…The results presented here are also consistent with the work of Vargas et al. (2022) who found that the isotopic composition of rainfall in the Ecuadorian Andes is more related to regional precipitation amount than local rainfall, and that decreasing temperatures become the primary driver of isotopic changes during water vapor ascent within the Andes.…”

“…This would be consistent with the interpretations of δ 18 O as a temperature recorder in polar ice cores (e.g., Andersen et al, 2004;Johnsen et al, 1995;Lorius et al, 1985;Rasmussen et al, 2013;Watanabe et al, 1999). The results presented here are also consistent with the work of Vargas et al (2022) who found that the isotopic composition of rainfall in the Ecuadorian Andes is more related to regional precipitation amount than local rainfall, and that decreasing temperatures become the primary driver of isotopic changes during water vapor ascent within the Andes.…”

Stable water isotope signals and their relation to stratiform and convective precipitation in the tropical Andes

“…For example, Aggarwal et al (2016) determined the stratiform fraction for a box of about 275 km x 275 km, whereas in our study, it is calculated for each grid cell (1.5 km x 1.5 km). In fact, we achieved the highest R 2 using a region-wide stratiform fraction (not shown) and similarly, Vargas et al (2022) found the best correlation with δ 18 O P using region-wide precipitation. However, for a better understandability, we used the grid-cell-based calculation of the stratiform fraction.…”

Stable Water Isotope Signals and Their Relation to Stratiform and Convective Precipitation in the Tropical Andes

“…In fact, we achieved the highest R 2 using a region-wide stratiform fraction (not shown). Similarly, Vargas et al (2022) found the best correlation with δ 18 O P using region- wide precipitation. However, for better understandability, we used the grid-cell-based calculation of the stratiform fraction.…”