2025 Theses Doctoral

Untapping a New Climate Archive: Colpophyllia natans Climate Reconstructions in the Tropical Atlantic

Ong, Maria Rosabelle Kwan

The tropical Atlantic is a climatically and oceanographically significant region, influenced by multiple modes of climate variability, both locally and remotely forced. Due to the complexity of the interactions between the ocean and the atmosphere, there is still much that remains uncertain about the dynamics governing this region. Therefore, it is essential to improve our understanding of the short-term and long-term drivers and mechanisms behind these processes.
However, despite significant advancements in observing and monitoring present-day climate, continuous high-resolution records of the tropical Atlantic are still limited. Massive, reef-building corals can provide reliable climate records extending from decades to centuries.

The goals of this dissertation are threefold. First, in Chapter 2, I investigate and evaluate the potential of a ubiquitous, massive boulder brain coral species, Colpophyllia natans, as a novel paleoclimate archive. I assess its reliability and fidelity in recording environmental signals by validating its geochemical records relative to Siderastrea siderea, a more commonly used coral archive in the Atlantic. Due to the complex skeletal structure of C. natans, I developed an effective sampling strategy to carefully recover and extract material from its skeleton. I then establish a Sr/Ca-SST calibration equation for C. natans as a baseline for future Sr/Ca-SST reconstructions. I further demonstrate that C. natans reliably captures SST variability across multiple timescales. Spatial correlation maps between boreal winter reanalysis SST and coral-derived SST show similarity to the tri-pole pattern of SST anomalies associated with the North Atlantic Oscillation (NAO), which is particularly prominent during this season. This finding further supports the hypothesis that C. natans skeletons can be used to track regional, large-scale climate variability in the tropical Atlantic.

Secondly, in Chapter 3, I investigate the drivers of interannual regional hydroclimate variability in the western tropical North Atlantic over the last 58 years. I analyzed an S. siderea coral core to extract Ba/Ca and δ¹⁸Osw to identify and evaluate the primary drivers influencing the Ba/Ca and δ¹⁸Osw variability and assess their potential in tracking regional hydroclimate parameters. While both coral proxies have been widely used to infer hydroclimate conditions, their interpretations are often highly site-specific and require contextual validation. Tobago, our study site, is uniquely situated in a hydrodynamically complex location. The coral is located within the seasonal migration zone of the Intertropical Convergence Zone (ITCZ), which exerts a strong influence on regional precipitation patterns at both seasonal and longer timescales.

Moreover, the site is also located along the trajectory of major western boundary currents that transport relatively fresh surface ocean plumes originating from both the Amazon and Orinoco Rivers. Due to the complex interplay of these influences, it was necessary to undertake a degree of proxy development to isolate and interpret the environmental signal embedded in the coral record.

By comparing the proxies to multiple climate data sets and performing spatial correlation analysis, I found that coral δ¹⁸Osw from our site tracks rainfall at the southern Caribbean region at annual and boreal summer seasons. Meanwhile, coral Ba/Ca tracks rainfall variability across regions of the northern Amazon Basin. When interpreted together, coral δ¹⁸Osw and Ba/Ca provide a unique opportunity to investigate changes in the seasonal migration of the ITCZ and the factors influencing regional rainfall variability. Coral δ¹⁸Osw mainly represent rainfall during boreal summer seasons when the ITCZ is at its northernmost position, while Ba/Ca variations represent rainfall during boreal winter seasons when the ITCZ is at its southernmost position.

Finally, I evaluate the drivers influencing the trends and pattern of rainfall variability in the tropical Atlantic. I found that at interannual timescales, rainfall variability is both influenced by the El Niño Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM) via SST anomalies. During periods of high ENSO variance, tropical Atlantic rainfall is highly sensitive to strong and prolonged El Niño events and caused drier-than-normal conditions in both the southern Caribbean and northern Amazon Basins. Meanwhile, during periods of diminished ENSO variance, positive AMM events or warmer-than-normal SST anomalies in the tropical North Atlantic result in a wetter southern Caribbean region and a drier Amazon Basin. Taken together, I identified two major distinct patterns of drought linked to changes in ITCZ, ENSO variance and meridional SST anomalies, all of which influence rainfall variations in the tropical North Atlantic at interannual timescales.

Finally, in Chapter 4, I used Sr/Ca, δ¹⁸Osw, and Ba/Ca to produce high-resolution monthly records of SST, southern Caribbean rainfall, and northern Amazon Basin rainfall over the last 185 years, spanning from 1831 to 2015, using a long coral core of Colpophyllia natans from Kelleston Drain, Tobago. I used a multiproxy approach to investigate the influences and drivers of longer-term, interdecadal climate variability. I demonstrate that, on interdecadal timescales, the impact of NAO on long-term SST variability is more pronounced. The 20-year smoothed boreal winter SST reconstruction shows sensitivity to changes in long-term NAO variability. I identified two periods of major decoupling between tropical Atlantic SSTs and the NAO associated with the anomalous eastward shift of the northern atmospheric center of action of the NAO.

These findings underscore the robustness of the coral SST reconstruction and its ability to capture long-term, interdecadal environmental trends. This represents a significant contribution to the currently limited marine-based records of the NAO. Additionally, while ENSO may also exert an interdecadal influence on rainfall variability, I found that the interhemispheric temperature gradient modulates the long-term mean position of the Intertropical Convergence Zone (ITCZ) and dictates the patterns of interdecadal variability in the tropical North Atlantic.

Finally, to the best of my knowledge, this is the first time Colpophyllia natans was used in a long-term reconstruction of SST and precipitation using a suite of proxies, such as Sr/Ca, Ba/Ca, and δ¹⁸Osw, unlocking the potential for its use as a reliable marine archive for future climate reconstructions in the wider North Atlantic Ocean region.