ORCID

https://orcid.org/0000-0002-4233-2308

Date of Award

Summer 2024

Language

English

Embargo Period

7-30-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Atmospheric and Environmental Sciences

Program

Atmospheric Science

First Advisor

Mathias Vuille

Committee Members

Aubrey Hillman, Brian Rose, Jason Smerdon

Keywords

Paleoclimate, Climate dynamics, Climate variability, Stable oxygen isotopes, South American Summer Monsoon, Hydroclimate

Subject Categories

Climate

Abstract

Terrestrial archives of oxygen isotope records are particularly important for the study of South American hydrology, where hydroclimate observations over the historical period are sparse and often not long enough to characterize multidecadal to centennial-scale changes. Knowledge of such low-frequency changes is critically important for interpreting and constraining projections of future change and detecting the emergence of forced change beyond the envelope of natural variability. This dissertation focuses on the past, present, and future of South American tropical and subtropical climate, with an emphasis on the mature phase of the monsoon season (December, January, and February; DJF) using oxygen isotopes in precipitation to interpret variability and change in the water cycle and climate, or hydroclimate. By utilizing an extensive network of diverse terrestrial archives of oxygen isotopes (such as cave records, lake records, ice core records, and tree ring cellulose records), observational networks, and isotope-enabled global climate models, a multi-data synthesis was achieved, which maximizes the strengths of these diverse data sources. Through this oxygen-isotope lens, regional hydroclimate responses to changes in internal variability of the climate system and external forcing influences, such as anthropogenic change, have been documented.

Chapter 2 characterizes regionally coherent modes of South American Summer Monsoon (SASM) variability over the Last Millennium (850 – 1850 Common Era (CE)) using paleoclimate records and isotope-enabled climate models. Two modes were defined as indices for monsoon strength and South Atlantic Convergence Zone activity, respectively. These indices are useful as temporally complete descriptions of regional climate variability against which discrete paleoclimate records can be compared. Additionally, this work utilized fully coupled, isotope-enabled climate models run as single-forcing experiments to investigate the influence of external forcing to highlight regionally coherent, multi-centennial departures from the mean climatological state during the Last Millennium. This chapter was published in Orrison et al. (2022) in the journal Climate of the Past.

Chapter 3 characterizes the spatiotemporal footprints of the Pacific low- (multidecadal) and high-frequency (interannual) variability within terrestrial archives (proxy records), observations, and model simulations of oxygen isotopes in precipitation and water vapor during the past 120 years (1880 – 2000 CE). Utilizing an atmosphere-only climate model forced with observed sea surface temperatures highlights the Walker circulation response linking Pacific sea surface temperature anomalies with the stable oxygen isotope response of the water cycle over South America. This chapter is in revision for publication in the Journal of Geophysical Research - Atmospheres.

Chapter 4 isolates the novel connection found between oxygen isotope signals identified in the hydrologic cycle over South America and a regionally defined index of the South American Hadley circulation during the mature phase of the SASM (DJF). This work finds that the trends of weakening and expansion that characterize the regional Hadley circulation over the course of the 20th and 21st century are coupled to the enrichment in heavy isotopes in terrestrial archives of precipitation. This work utilizes a fully-coupled isotope-enabled climate model with transient simulations to examine this relationship in the historic period (1920 – 2009) and into the future (2010 – 2098 CE), thereby establishing a long-term historical context for projected future monsoon weakening based on a stable oxygen isotope framework. This chapter is in preparation for submission to the journal Geophysical Research Letters.

License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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