The influence of tropical volcanic eruptions on the climate of South America during the last millennium

Abstract

Currently, little is known on how volcanic eruptions impact large-scale climate phenomena such as paleo-ITCZ position or South American summer monsoon behavior. In this paper, an analysis of observations and model simulations is employed to assess the influence of large volcanic eruptions on the climate of South America. This problem is considered both for historically recent volcanic episodes, for which more comprehensive global observations exist, as well as reconstructed volcanic events for the period 850 C.E. to present that are incorporated into the NASA GISS ModelE2-R simulation of the Last Millennium. An advantage of this model is its ability to explicitly track water isotopologues throughout the hydrologic cycle and simulating the isotopic imprint following a large eruption. This effectively removes a degree of uncertainty associated with error-prone conversion of isotopic signals into climate variables, and allows for a direct comparison between GISS simulations and paleoclimate proxy archives. Our analysis reveals that both precipitation and oxygen isotope variability respond with a distinct seasonal and spatial structure across South America following an eruption. During austral winter, the heavy oxygen isotope in precipitation is enriched, likely due to reduced moisture convergence in the ITCZ domain and reduced rainfall over northern South America. During austral summer, however, precipitation is depleted in heavy isotopes over Amazonia, despite reductions in rainfall, suggesting that the isotopic response is not a simple function of the "amount effect". During the South American monsoon season, the amplitude of the temperature response to volcanic forcing is larger than the rather weak and spatially less coherent precipitation signal, potentially masking the isotopic response to changes in the hydrologic cycle.