The context of megadrought, drought more severe than any we have experienced over the past 100 years, is assessed in this dissertation. A set of new climate reconstructions including drought, dustiness, and temperature from the south San Juan Mountains in southern Colorado is presented here and provides unforeseen insights into these unusual events. The global context of megadroughts is also analyzed using a network of reconstructions. The new drought record is from bristlecone tree-rings, spans the last 2000 years, and shows two periods with anomalous aridity and drought in the south San Juan Mountains. The later period corresponds with well-characterized medieval climate anomaly (MCA; 900-1400 AD) aridity in southwestern North America (henceforth the Southwest). The earlier interval coincides with the Roman Period (1-400 AD). A severe drought with, almost 50 consecutive years of below average tree-growth, occurs in the middle of the Roman Period during the 2nd century AD. Assessment of Roman and MCA droughts in the context of global climate reconstructions reveals that similar hemisphere scale circulation patterns during both intervals might have contributed to severe aridity in the Southwest. Next relationships between droughts and pluvials in western North America (henceforth the West) and global sea surface temperature (SST) patterns over the last 1100 years are examined. Several methods are used including teleconnection patterns imbedded in tree-ring reconstructed drought maps, and a global network of SST reconstructions. Teleconnection patterns during droughts and pluvials suggest that megadroughts and pluvials were likely forced in part by sequences of anomalous years in the Pacific and Atlantic Ocean, but the analyses also reveals contradictory results that may require new ways of understanding the relationship between SSTs and drought on long timescales. Next, returning to the south San Juan Mountains, we developed a new dust reconstruction from a lake sediment core. The reconstruction illustrates that dustiness has been an important component of Southwestern climate over the past 2941 years. The record shows high dust deposition in the past especially around 900 BC and during the MCA. High dust deposition before recent land use changes suggests that megadroughts or associated periods of aridity were widespread and severe enough to mobilize dust, perhaps resulting in further reductions to mountain snowpack and stream flow. Finally, a new biomarker based temperature reconstruction is presented. The reconstruction spans the last 2000 years and shows that the warmest temperatures during that interval occurred during the Roman Period and the MCA. The record suggests these periods were warmer than today, indicating the San Juan Mountains are a sensitive region to temperature change. Both past warm periods coincide with anomalous drought and dustiness, suggesting that temperature and dust may have acted as megadrought enhancing feedbacks. In summary, this dissertation helps characterize the timing and causes of southwest North American Megadroughts over the past 2000 years; separately addressing changes in moisture balance, dustiness, temperature, hemispheric circulation, and sea surface temperature forcing patterns during these unusual events.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/320004 |
| Date | January 2014 |
| Creators | Routson, Cody Craig |
| Contributors | Overpeck, Jonathan T., Woodhouse, Connie A., Overpeck, Jonathan T., Woodhouse, Connie A., Betancourt, Julio L., Cohen, Andrew S., Meko, David M. |
| Publisher | The University of Arizona. |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | text, Electronic Dissertation |
| Rights | Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. |
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