Temperature and Hydroclimate Variability in the Southwestern Indian Ocean Since the Little Ice Age reconstructed from Coral Skeletons

<p dir="ltr">Sea surface temperature (SST) and sea surface salinity (SSS) strongly influence ocean circulation, air-sea interactions, and biogeochemistry. Yet identifying their natural variability free of anthropogenic influences is challenging due to limited instrumental data coverage. This thesis investigates the temperature and hydroclimate variability extending back to the Little Ice Age (LIA) in the ecologically and socially important southwestern Indian Ocean (SWIO), using skeletal chemical compositions, Sr/Ca ratios and stable oxygen isotope (d18O) values, of massive Scleractinia (stony corals). Coral-derived SST reconstructions from northeastern Madagascar reveal temporally stable, monsoon-driven coastal upwellings, indicating potential thermal refugia for coral reefs. The unique study site also allows the reconstruction of Northeast (NE) Monsoon variability over the SWIO from coastal SST seasonality. Strong covariance between reefsite SST and broader SWIO conditions enabled reconstruction of regional SST variability from 1671 to 2007. SSS was reconstructed from paired Sr/Ca–d18O data over two key intervals: during the LIA (1671–1736) and from 1886 to 2007. Interannual to interdecadal variations in SST and SSS are closely linked to large-scale climate phenomena, including the Indian Ocean Dipole, El Niño-Southern Oscillation and Atlantic Multidecadal Oscillation. Centennial-scale analysis highlights significant (> 1 °C) warming in the SWIO since the early 20th century. An increase in SSS after 1945 is attributed to changes in remote oceanic precipitation-evaporation balance and wind-driven circulation. During the LIA, intensified NE monsoon, SST comparable to modern values, and higher SSS collectively suggest a southward displacement of the Intertropical Convergence Zone (ITCZ) over the SWIO. Subsequently, our results suggest a northward return of ITCZ at the start of the 18th century, in phase with the proposed ITCZ movement over the western Pacific and seemingly preceding changes over Africa. This study provides a critical baseline for assessing future climate impacts on SWIO ecosystems, particularly regarding the resilience of coral communities.</p>