The Spectrum of the Indian Summer Monsoon: a Proxy System Modeling Approach

Abstract:

Paleoclimate data often suggest that there is more hydroclimate variability in the multidecadal-century scales than is projected by climate models. This proxy-climate model mismatch could be caused by 1) a deficiency of climate models in capturing real long-period variability in the climate system or 2) biases in the proxy systems that amplify long-period variance. In this study we use proxy system models for oxygen isotopic systems (speleothem, ice core, tree-ring cellulose) to determine how much of the proxy-climate model mismatch is due to filtering of the climate signal by these proxies. We focus on the region impacted by the Indian summer monsoon and use output data from isotope-enabled climate models to drive our proxy system models. We find that much but not all of the proxy-climate model mismatch can be explained by biases in the proxy systems. Speleothem systems exhibit much stronger biases than tree-ring cellulose and ice cores due to significant reservoir times for water in karst. Even after using proxy system models to create pseudoproxies, the paleodata tend to have greater long-period variance (longer, more severe droughts and pluvials) than do the forward modeled pseudoproxies. However, access to long runs for isotope-enabled models from more modeling agencies is required before generalizations can be made about such model-proxy comparisons.