CO₂ radiative forcing and Intertropical Convergence Zone influences on western Pacific warm pool climate over the past 400ka

The western Pacific warm pool (WPWP) is an important heat source for the atmospheric circulation and influences climate conditions worldwide. Estimating WPWP sensitivity to past radiative forcing perturbations is important for understanding the magnitudes and patterns of current and projected tropical climate change. Here we present a new Mg/Ca-based sea surface temperature (SST) reconstruction over the past 400ka from the Bismarck Sea, off Papua New Guinea, along with benthic foraminiferal δ¹⁸O records and a transient intermediate complexity earth system model simulation. The Mg/Ca-SST record exhibits a close similarity with atmospheric CO₂ content for the whole study period. Our model analysis demonstrates that greenhouse gas forcing is the primary driver for glacial/interglacial SST changes in the entire WPWP region. Mg/Ca-SST in the Bismarck Sea also includes a weaker precessional component, which covaries with reconstructed and simulated local precipitation, and simulated surface currents. We propose that orbitally driven latitudinal shifts of the Intertropical Convergence Zone and oceanic heat advection are responsible for this residual SST variability. On glacial timescales the reconstructed WPWP surface temperature changes over the past 400ka are highly correlated with East Antarctic air temperature variations. The strong effect of greenhouse gas forcings on both records and on global mean temperature variability allows us to determine a scaling factor of 1-1.5 between reconstructed WPWP temperature anomalies and estimates of the global mean temperature.