Investigation of the South American Monsoon during the Miocene Climate Optimum

The Miocene Climate Optimum (MCO ~ 17-14 Ma) is a potential analog for future climate change, with a peak global mean surface air temperature of 7 - 8oC warmer than today. Proxy reconstructions suggest CO2 levels between 400 - 800 ppm, sea level rise of ~50 m above preindustrial, and a weaker equator-to-pole temperature gradient during the MCO. However, past studies have had difficulty simulating a MCO climate state that agrees with proxy records. This study employs the water isotope tracer enabled Community Earth System Model (iCESM1) to simulate MCO climate and investigate the evolution of the South American Monsoon System (SAMS) during this past warm period relative to preindustrial (PI). Our simulations reveal a global warming of 8.21oC relative to PI. Surface temperature over South America ranges between 26 - 31oC and 19 - 24oC for MCO and PI, respectively. The results further show a weakening of the SAMS during MCO with a reduction in summer precipitation and increase in winter precipitation. The SAMS is strongest just below the equator during MCO and over the Andes in PI. Spatial distribution of the d18O of the precipitation shows generally higher isotopic ratios over SA during MCO. The low isotopic ratio over the Andes is not present in the MCO simulation.