PhD defense of Clara Boutreux “Interannual climate variability in the tropical Pacific Ocean and the Maritime Continent for the Holocene period” at 2 p.m.

The Tropical Western Pacific Warm Pool (WPWP) constitutes a core component of the global climate system. Acting as a major reservoir of heat and moisture, it has a significant impact on the redistribution of energy between the ocean and the atmosphere and to their redistribution towards higher latitudes. Across the Maritime Continent, variations in sea surface temperature and salinity, precipitation, and ocean circulation exhibit complex seasonality, characterised in particular by a pronounced semi-annual component in the Karimata Strait. This seasonal structure results from interactions between the migration of the Intertropical Convergence Zone (ITCZ), the Asian-Australian monsoon system, and regional ocean circulation, and can be modulated by interannual climate variability. However, this variability remains poorly constrained beyond the instrumental period, while climate simulations disagree on its past and future evolution. This study aims to characterise WPWP climate variability at high temporal resolution throughout the Holocene using modern and fossil biocarbonate archives: Tridacna spp. shells collected from Belitung Island, Indonesia. Their daily growth increments enable environmental variations to be reconstructed at timescales ranging from days to decades. Sclerochronological analyses, combined with trace element ratios (Mg/Ca, Sr/Ca, Ba/Ca) and stable isotopes (δ¹⁸O, δ¹³C), provide insights into variations in temperature, salinity, precipitation, and surface biogeochemical conditions. Modern shells record the bimodal structure of the seasonal cycle, consistent with instrumental observations, as well as variations associated with interannual climate variability. Holocene geochemical and sclerochronological records reveal changes in the amplitude and structure of the semi-annual cycle under the influence of orbital insolation forcing and internal climate feedbacks. Comparisons with transient climate simulations highlight discrepancies in the amplitude and timing of warm and cold phases, despite a shared tendency towards warmer and wetter conditions during the second half of the Holocene. This multi-proxy approach provides new insights into the evolution of seasonal and bimodal climate variability in the WPWP from the present day to the mid-Holocene, contributing to a better understanding of the mechanisms driving these changes.

Location : Building 4, Faculty of Sciences, Nantes (2 rue de la Houssinière)