PP034-02
Data-Model Comparisons of Tropical Hydroclimate Changes Over the Common Era
Friday, 11 December 2020: 20:34
Virtual
Alyssa R. Atwood, Florida State University, Earth, Ocean, and Atmospheric Science, Tallahassee, FL, United States, David Battisti, University of Washington, Seattle, WA, United States, Elynn Wu, University of California San Diego, San Diego, CA, United States, Dargan M Frierson, University of Washington, Department of Atmospheric Sciences, Seattle, WA, United States and Julian P Sachs, University of Washington, School of Oceanography, Seattle, WA, United States
Abstract:
We re-consider several disparate theories of large-scale tropical hydroclimate changes over the Common Era by reviewing the evidence from 70 paleo-hydroclimate records and assessing the consistency between the regional patterns of reconstructed rainfall and those simulated by transient model simulations of the last millennium. Several robust regionally-coherent patterns emerge from the proxy records. During the Medieval Climate Anomaly (MCA) from 800-1100 CE, a pronounced drying event is observed in both eastern Pacific and northern Mesoamerica records, suggesting a linkage between the tropical Pacific and Atlantic basins during the MCA. During the relatively cool epoch of the Little Ice Age (LIA), an equatorward rainfall contraction over the Asian monsoon/Maritime Continent is inferred from 1400-1700 CE relative to present, consistent with previous findings. During this same period, tropical South American records demonstrate an antiphased dipole pattern, with wet (and/or isotopically depleted) conditions in the southwest and dry (and/or isotopically enriched) conditions in the northeast, consistent with an enhanced South American monsoon and subsidence over northeast Brazil. However, in contrast with several previous hypotheses of tropical Pacific rainfall patterns during the LIA, records from the equatorial Pacific cold tongue do not provide support for a strengthened Pacific Walker Circulation or a large-scale shift of the Pacific ITCZ.
To shed light on the mechanisms of these hydroclimate changes, we compare the reconstructed rainfall patterns to a suite of CMIP5/PMIP3 simulations of the last millennium. All models demonstrate a small zonal mean southward shift in tropical rainfall during the LIA, accompanied by anomalous northward cross-equatorial atmospheric energy transport that is primarily driven by volcanic forcing, land use changes (e.g. cropland expansion in Eurasia), and the surface albedo feedback. Regionally however, the simulations exhibit weak forced long-term tropical rainfall changes that poorly agree with proxy data in regions where proxy records exist, suggesting that either the observed tropical hydroclimate changes were unforced, or the models are not able to capture the forcings, feedbacks, or slowly-varying hydroclimate responses over the last millennium.
