The Spectrum of Asian Monsoon Variability: An Investigation of Low-Frequency Variability in Paleoclimate Proxies and Climate Models
Pacific Decadal Variability
Proxy system model
AdvisorOverpeck, Jonathan T.
Thompson, Diane M.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction, presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
EmbargoRelease after 12/12/2020
AbstractIn this work we investigate the low-frequency (decadal-centennial) variability of the climate system in Monsoon Asia using a combination of instrumental, paleoclimate proxy, and climate model data. Understanding this critical component of the climate system is essential for accurately assessing the risk of low-probability extreme events such as megadroughts that may arise from the interaction of anthropogenic climate forcing and natural internal variability (Ault et al., 2014). In Appendix A, we use a network of hydroclimate proxies from Monsoon Asia as a case study to compare low-frequency variability in paleo-data to climate model simulations. We take a proxy system modeling approach, using climate output variables from isotope-enabled runs of iCESM, isoGSM, and iCAM5, to simulate synthetic proxy records, which creates a common ground for the comparison. We find that the pseudoproxies based on iCESM do not accurately capture the relative strength of multidecadal-century scale variability in the paleoclimate data. We find that the pseudoproxies based on isoGSM and iCAM5, which are constrained by instrumental observations, appear to match the scaling pattern of variability found in the proxy records. Our results indicate that state-of-the-art, fully coupled climate models are not able to generate a sufficient amount of multidecadal-century scale variability in hydroclimate. We find that one major source of multidecadal scale variability found in the proxies but not in the models comes from the combined interactions among the monsoon, El Niño-Southern Oscillation (ENSO), and the Indian Ocean sea-surface temperature variability. In Appendix B, we investigate the low-frequency variability found in multiproxy ENSO reconstructions, with the goal of understanding how this dominant source of hydroclimate variability on interannual scales may impact teleconnected regions on decadal-centennial time scales. Previous multiproxy reconstructions of ENSO have found a large amount of low-frequency variability but the time series of this low-frequency variability rarely matches between studies. We investigate potential biases in reconstructions of ENSO from the number and geographical distribution of sites, age model uncertainty, and sources of noise present in the individual proxy records. We conduct a series of sensitivity experiments using pseudoproxies to demonstrate that these factors have the potential to bias the spectrum of reconstructions causing them to overestimate the relative amount of decadal-century scale variability in the tropical Pacific. This could help explain why multiproxy ENSO reconstructions show greater multidecadal-century scale variability than is found in observations or climate model simulations. In Appendix C, we introduce a new paleo-monsoon proxy covering the last 2600 years based on grain size variability in a sediment core from Pale Daha, a small lake in western Nepal. We demonstrate that variability in grain size distribution over the last century matches with low-frequency precipitation variability found in 20th Century Reanalysis data (Compo et al., 2011). We identify a major pluvial from 950-1300 CE that matches with a similar pluvial in nearby Sahiya Cave (Sinha et al., 2015), and with increased monsoon circulation over the northern Arabian Sea (Anderson et al., 2002; Gupta et al., 2005). We find evidence that the monsoon precipitation at the site over the last 150 years has been particularly strong as compared to relatively weak monsoon rains from 1300-1800 CE. We then compare the trends found in paleo-monsoon proxies over the last 150 years and find that much of the apparently contradictory trends may be explained by the projected circulation and precipitation responses to anthropogenic forcing. This new interpretation is based on a simulated northward shift in circulation that may increase the relative fraction of precipitation at proxy sites received from the Arabian Sea and decrease the fraction from the Bay of Bengal. This shift in circulation could produce the trend in precipitation stable isotopes found in isotope based paleo-monsoon proxies. If true, these findings indicate that the South Asian Monsoon may already be experiencing the effects of anthropogenic climate change to a degree not yet fully appreciated, and that these changes are likely to continue into the future.
Degree ProgramGraduate College