Western Pacific Warm Pool Warming and Salinity Front Expansion Since 1821 Reconstructed From Paired Coral δ¹⁸O, Sr/Ca, and Reconstructed δ¹⁸O_sw

Abstract The Southwest Pacific region is of great importance to global climate variability, but instrumental climate observations before the 1980s lack in numbers and quality. Despite efforts in complementing instrumental records with proxy sea surface temperature (SST) and sea surface salinity (SSS) reconstructions based on coral Sr/Ca and δ 18 O sw records, few of them are longer than a century. This study introduces a northwestern extension to the existing records of South Pacific coral study sites with monthly‐resolved Sr/Ca, δ 18 O, and δ 18 O sw reconstructions from Rotuma dating back to 1821. Additionally, we present new monthly‐resolved Sr/Ca and reconstructed δ 18 O sw from a coral from Tonga dating back to 1848. Results reveal 1.5°C warming in the Western Pacific Warm Pool, while the adjacent coral from Tonga shows 1°C warming over the twentieth century. The Rotuma Sr/Ca record reveals thermal stress events impacting the Sr/Ca‐SST relationship in the following months. Coral δ 18 O sw results reveal significant freshening of 0.45 S p (practical salinity unit) in Tonga since the early twentieth century, suggesting the southeastward expansion of the South Pacific Convergence Zone salinity front. The δ 18 O sw inferred SSS provides a valuable extension into the past considering the short and inconsistent instrumental records available. This study demonstrates the utility of coral‐based reconstructions in capturing long‐term and regional climate variations in the Southwest Pacific and the necessity of expanding replicated studies to other underrepresented areas to enhance our understanding of regional climate dynamics.