The Western Pacific Warm Pool (WPWP) is the largest reservoir of warm surface waters on Earth with a permanent sea surface temperature (SST) of >28°C and it is considered the steam engine that drives atmospheric circulation on our globe. It represents a major heat source for the global atmosphere and a location of deep atmospheric convection and heavy rainfall. Small variations in the sea surface temperature (SST) in the WPWP influence the location and strength of convection in the rising limb of the Hadley and Walker circulations, perturbing planetary scale atmospheric circulation, atmospheric heating globally, and tropical hydrology.
Rainfall is one of the most challenging atmospheric parameters to reconstruct to this day, thus the general lack of rainfall proxy records in the Western Pacific Warm Pool. To address this, we conducted biomarker-based rainfall estimates in the WPWP. Analyses of plant-wax n-alkanes and their stable carbon (δ¹³C) and hydrogen (δD) isotopic composition were performed on 50 samples from gravity core GeoB17429-2 located off the northern coast of Papua New Guinea (PNG). Our data offers insights into the hydroclimate change and aims to uncover the intricate ocean-atmosphere couplings and their determinants, such as ITCZ, ENSO, and Walker circulation changes that took place over the past 15.000 years. In addition, Community Earth System Model (CESM) climate simulations were carried out to identify the drivers and forcing of the rainfall changes in the WPWP.
The hydrogen (δD) isotopic composition of GeoB17429-2 reveals four distinct periods of decreased rainfall during the last deglaciation, at ~9.5-8.5 ka, ~4-6 ka, and at ~1.5-1 ka.  Those drier conditions as inferred from the δD data correlate with heavier δ¹³C_wax, indicating a higher water use efficiency under drier conditions.
Our findings argue for a significant Middle Holocene ENSO-related precipitation change that is driven by the equatorial September insolation maxima. The equatorial insolation maximum caused a warming in the WPWP thermocline and an intensified zonal thermal gradient across the equatorial Pacific, leading to a stronger Walker circulation, which reduced the interannual ENSO activity and thus decreased rainfall over PNG.