Coupling at semi-annual timescale of ocean-atmosphere processes in the Tropical Western Pacific Warm Pool during the Holocene

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. It is a key region in the global climate system. Variations in sea surface temperature and salinity, precipitation, and oceanic currents across the WPWP exhibit a pronounced semi-annual cycle driven by insolation, latitudinal migration of the Intertropical Convergence Zone (ITCZ), the monsoon system, and the coupled ocean–atmosphere interactions. Recent studies show that this semi-annual variability is modulated by climate oscillations, such as El Nino Southern Oscillation or Indian Ocean Dipole, and may vary over longer periods of time. Here, we provide evidence for modern to mid-Holocene variations in the semi-annual cycle from both proxy data and transient model experiments. Geochemical and sclerochronological records were derived from modern and fossil of giant clam shells (Tridacna spp.) collected from Belitung Island, in the middle of the Karimata Strait in Indonesia. Monthly to daily resolved proxy data is compared to 5 coupled transient simulation models for the Holocene: EC-Earth, AWI-ESM2, MPI-ESM, ISPL-CM5 and IPSL-CM6. Stable isotopes (δ¹⁸O, δ¹³C) from the shells provide reconstructions of sea surface conditions of past environmental conditions. The modern results from the shells are consistent with the measured modern data, supporting the use of giant clam as a proxy for past reconstructions. Model simulations suggest that the structure of the seasonal cycle in the WPWP varied during the Holocene compared to today, driven by orbital forcing of the insolation and internal climate feedbacks. Comparisons across models reveal that they do not converge on the exact magnitude and timing of cold/warm phases but all show warmer and wetter conditions in the second half of the Holocene. Holocene proxy data from the Karimata Strait also show changes in both the amplitude and the structure of this semi-annual variability. Results show an increase in the amplitude of the semi-annual compared to the seasonal cycle during the Holocene. The comparison with transient models highlights an overestimation of variations by the models, which we propose may be related to salinity. Furthermore, the IPSL and EC-Earth models agree with proxy data concerning the amplification of the semi-annual cycle relative to the seasonal cycle.  This multi-proxy model data comparison approach based on Tridacna shells offers new insights into the evolution of seasonal and semi-annual variability in the WPWP from today to the mid-Holocene and a better understanding of the forcing mechanisms.