Graduate Thesis Or Dissertation


Evidence for Strengthening Hadley Cells and a Westward Expansion of the Walker Circulation from Assimilation of Paleoclimate Proxy Data Public Deposited

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  • A synthesis of over 2000 paleoclimate proxy records is performed via a data assimilation framework that expands upon previous efforts by implementing a suite of physically-based proxy system models, and which provides the first example of an observationally independent, multi-seasonal (DJFM, JJAS) paleoclimate reanalysis. This methodology is contrasted against previous methods and it is shown that skillful proxy constraint is distributed across a broader range of climate states and provides a slightly more robust reconstruction of variability in known dynamical climate indices (Niño 3.4: r2 = 0.67, southern oscillation index: r2 = 0.71). The seasonal paleo reanalysis-based reconstructions of DJFM and JJAS climate fields are shown to have skill comparable to other modern observational reanalysis products and capture seasonal variability in atmospheric circulation. Evidence is found supporting: a strengthening of the Pacific Walker cell (WC) of roughly 6.5% to 21.5% century-1 and a westward shift of roughly 3° to 10° century-1, a strengthening and widening of the northern hemisphere Hadley cell (NHHC) of roughly 4.5% to 15% century-1 and 0.4° to 1.3° century-1 respectively, as well as a weak, -0.2% to -0.6% century-1, but significant (p<0.01), contracting of the southern hemisphere Hadley cell (SHHC) width over the period 1850-2000 C.E. The high and low frequency variability in the WC has an ENSO and AMO-like structure respectively, and the reconstructions suggest a coupling of the WC and NHHC through these internal modes of variability, along with a coupling of the WC to the SHHC through a mode which has an ITCZ-like structure in the Pacific. The results here are consistent with theoretical arguments that suggest a coupling between the fractional changes in precipitation and atmospheric moisture content and the strength of overturning circulations based on the Clausius-Claperyon relationship, although the constrained reconstruction shows a much greater precipitation response to warming, which changes the sign of the circulation response. This is due to a much greater precipitation response The reduction in record density prior to the 16th century provides an opportunity to measure the advantages that would emerge from efforts to both refine the PSMs and obtain new records that provide a more temporally consistent coverage of proxy observations. Specifically, continuous tropical corals would greatly benefit the skill of the reanalysis.
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