The influence of volcanic eruptions on the climate of tropical South America during the last millennium in an isotope-enabled general circulation model

Abstract

Abstract. Currently, little is known on how volcanic eruptions impact large-scale climate phenomena such as South American paleo-Intertropical Convergence Zone (ITCZ) position and 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 tropical South America. This problem is first considered for historically recent volcanic episodes for which more observations are available but where fewer events exist and the confounding effects of El Niño-Southern Oscillation (ENSO) lead to inconclusive interpretation of the impact of volcanic eruptions at the continental scale. Therefore, we also examine a greater number of reconstructed volcanic events for the period 850 CE 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 records.Our analysis reveals that both precipitation and oxygen isotope variability respond with a distinct seasonal and spatial structure across tropical 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, more negative values of the precipitation isotopic composition are simulated 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, complicating the isotopic response to changes in the hydrologic cycle.

Forward modeling of δ18O in Andean ice cores

“…Most PMIP3 models are not isotope-enabled (although see Colose et al [2016]), and we expect to solve for western Amazon Basin vapor values by way of offline calculations. Most PMIP3 models are not isotope-enabled (although see Colose et al [2016]), and we expect to solve for western Amazon Basin vapor values by way of offline calculations.…”

“…Looking forward, we intend to drive our model with GCM output for the last millenium, such as from the Paleoclimate Modelling Intercomparison Project III (PMIP3). Most PMIP3 models are not isotope-enabled (although see Colose et al [2016]), and we expect to solve for western Amazon Basin vapor values by way of offline calculations. As more isotope-enabled and intermediate complexity last millenium simulations [Dee et al, 2014] become available this work can be expanded.…”

The South American monsoon variability over the last millennium in climate models

“…Some of these problems still persist in the PMIP3 LM simulations (PAGES 2k-PMIP3 group, 2015). Furthermore, a recent model simulation of the global monsoon during the LM, performed in a non-PMIP3 experiment, indicates that the NH summer monsoon responds more sensitively to GHG forcing than the SH monsoon rainfall, which appears to be more strongly influenced by solar and volcanic forcing (Liu et al, 2012;Colose et al, 2016;Novello et al, 2016). Hence, a stronger sensitivity of SAMS rainfall to LM forcing estimations and the inadequate response of current GCMs to such forcings may also bias the CMIP5/PMIP3 simulations of the summer SAMS rainfall during the past millennium.…”

Comparing proxy and model estimates of hydroclimate variability and change over the Common Era

“…The best way to analyze isotope proxy information is to compare the proxy data to simulations from climate models that incorporate isotope tracers. Recent simulations that do include isotope tracers have greatly simplified the task of undertaking proxy-model comparisons for a large component of the proxy data that are available during the CE (Colose et al, 2016a). Intercomparison projects such as the SWING2 project are also compiling the results from isotope-equipped atmospheric climate models (Sturm et al, 2010;Conroy et al, 2013) to provide the community with a diverse set of simulations to probe the drivers of spatiotemporal water isotope variability.…”

“…Figure 6a characterizes precipitation anomalies after volcanic eruptions, while Fig. 6b and d separate the precipitation response to the volcanic forcing of differing meridional structures in CESM LME (18 ensemble members; see Colose et al (2016a) for details on eruption classifications) during boreal winter in the Amazon and boreal summer in the Sahel, respectively. Although there is considerable spread within a given classification related to different eruption magnitudes and internal variability, there is a strong tendency for an increase in local precipitation if the aerosol loading is preferentially located in the opposite hemisphere.…”