Date of Award

Summer 2026

Language

English

Embargo Period

8-1-2026

Document Type

Master's Thesis

Degree Name

Master of Science (MS)

College/School/Department

Department of Atmospheric and Environmental Sciences

Program

Atmospheric Science

First Advisor

Mathias Vuille

Second Advisor

Oliver Timm

Keywords

ITCZ, Hydroclimate, Speleothem, Paleo data Assimilation, Last millennium, Paleoclimate

Subject Categories

Atmospheric Sciences | Climate

Abstract

Hydroclimate variability over tropical South America is strongly influenced by meridional shifts in the Intertropical Convergence Zone (ITCZ), yet the spatial expression and dynamical controls of these changes during the last millennium remain poorly understood. This work investigates hydroclimate variability and ITCZ migration over the last millennium (850-1850 CE) using a new isotope-enabled data assimilation (DA) reconstruction made from a suite of speleothem δ¹⁸O records from the neotropics on both sides of the ITCZ. The reconstruction targets the March-August (MAMJJA) season in order to capture the rainfall regimes most relevant to central Colombia and northeastern Brazil. The focus on these two regions was motivated by the availability of new high-resolution speleothem δ¹⁸O records that allow isolating ITCZ-related hydroclimate variability over the neotropics.

An offline particle-filter DA approach was employed to reconstruct hydroclimate conditions from 850 to 1850 CE using the isotope-enabled Community Earth System Model (iCESM v1) as a prior and a network of 8 oxygen isotope records compiled for assimilation. Proxy records and the model equivalents were averaged into 5-year intervals prior to assimilation to account for differences in temporal resolution and reduce proxy-specific noise. A meridional δ¹⁸O gradient between the Rainha Cave record in northeastern Brazil (RN) and the central Colombia Caracos record (CAR) was used to characterize large-scale hydroclimate variability and infer meridional ITCZ displacements.

Results reveal pronounced multi-centennial hydroclimate variability with predominantly strong gradient conditions during the Medieval Climate Anomaly (MCA, 950-1150 CE) and predominantly weak gradient conditions during the Little Ice Age (LIA, 1500-1850 CE). Strong gradient years were associated with enhanced precipitation and isotopically depleted conditions over Central America, the Caribbean and northern tropical South America, while rainfall was reduced over eastern and northeastern Brazil. This is consistent with a northward displacement of the ITCZ. Weak gradient years exhibit the opposite hydroclimate structure and were associated with a southward ITCZ displacement. Atmospheric circulation at 500 hPa, 850 hPa and 200 hPa, together with Pacific Walker Circulation (PWC) diagnostics indicate that these hydroclimate states were embedded within a coupled tropical circulation framework linking ITCZ variability and Pacific climate dynamics. Strong gradient years exhibited an El Niño-like configuration, characterized by a weakened and eastward shifted PWC, whereas weak gradient years displayed a more La Niña-like state.

Overall, this study demonstrates that hydroclimate variability over tropical South America during the last millennium was shaped by the combined influence of ITCZ migration and PWC variability. These findings improve our understanding of long-term tropical hydroclimate variability and provide important context for interpreting future climate change across the neotropics.

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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