Paleoclimate simulations of middle Miocene and early Eocene South America: a quantitative analysis of the regional atmospheric water cycle

Abstract:

Investigating the evolution of the South American hydroclimate and its response to tectonic and environmental changes throughout the Cenozoic era can provide mechanistic understandings of hydroclimate changes in the past and future. Here, we examine the distributed impacts of tectonic (paleogeography and orography), and environmental (CO₂ and vegetation) forcings on the South American hydroclimate and water isotope systems through simulations of the early Eocene (~55-50 Ma), and the middle Miocene (~20-14 Ma) using the NCAR isotope-enabled, Community Earth System Model (iCESM1.2). Mechanisms for changes in the regional atmospheric water cycle are quantitatively explored using the water tagging capability of iCESM1.2.

Our simulations suggest that the Cenozoic widening of the Atlantic Basin increases the fetch over the ocean, which enhances the tropical easterly moisture flux into South America. Surface uplift of the Andes further strengthens the South American Low-Level Jet and redistributes the tropical moisture flux toward the subtropical interior of the continent. Lowering CO₂ concentrations decreases the water vapor residence-time and transport distance which allows more localized moisture recycling. The expansion of the Amazon basin enhanced local evapotranspiration but have little impact on the continental moisture flux distribution. These findings have implications for the hydroclimate and water isotope systems over the Andean Mountains and the Amazon forest.