ZMT - Leibniz Zentrum für Marine Tropenforschung

Abstract/Summary:
The Southwest Pacific is a critical region for understanding wider regional climate dynamics, yet it has been historically under-monitored by instrumental and proxy observations.
To address this gap, proxy data from long- lived tropical corals are essential as they compensate for the late start of instrumental observations and provide insights into past oceanographic variability. My research uses coral-based proxies to reconstruct past sea surface temperature (SST) and sea surface salinity (SSS) in this region, focusing on the West Pacific Warm Pool (WPWP) and the South Pacific Convergence Zone (SPCZ).
New, monthly-resolved coral-based time series from Rotuma, Fiji, and Nomuka Iki, Kingdom of Tonga, extending back to pre-industrial baselines to 1821 and 1848, respectively, extend the geographical coverage of the South Pacific coral network.
Monthly-resolved Sr/Ca and reconstructed δ18Oseawater (sw) from paired Sr/Ca and δ18O measurements from previously undocumented island of Rotuma indicate a significant warming trend of up to 1.45 °C in the West Pacific Warm Pool since the beginning of the 20th century. Reconstructed δ18Oseawater (sw) from paired Sr/Ca and δ18O show that the low salinity water (“fresh”) pool of the SPCZ has been extending over the last century as the Tonga coral records a freshening trend. These findings are crucial for understanding how key oceanic features that play an essential role in global weather patterns are changing and under which influences.
The newly generated coral-based time series in this work serve to join other SPCZ coral records in constructing of composite indices (SPCZcoral indices) to describe the regional trends and reconstruct past interannual variability. SPCZcoral indices were developed from coral geochemical time-series across multiple locations (Rotuma and Savusavu islands (Fiji), Nomuka Iki Island (Tonga), and Rarotonga). These indices extend instrumental SSS records by approximately 100 years and improve the resolution of past composite SST and SSS reconstructions of the region. SPCZcoral indices provide a clearer picture of SPCZ interannual variability, particularly in relation to El Niño Southern Oscillation (ENSO) events. The results demonstrate differential responses to Central Pacific and Eastern Pacific El Niño events, enabling us to reconstruct historical SPCZ behavior. An important contribution of the SPCZc SSS is the reconstruction of the SPCZ precipitation axis shifting drastically equatorially from its usual diagonal position, in some cases leading to drought events in the SPCZ region, known as the SPCZ zonal events.
This body of work contributes to the field of paleoclimatology by providing new coral-based records from the SPCZ, and specifically the WPWP area, extending historical climate records of the SPCZ and improving our understanding of the natural variability and anthropogenic impacts on the Earth's climate system.