The role of thermohaline structure in upper waters of the western Pacific warm pool during deglaciation

The western Pacific warm pool, being the greatest source of heat and water vapor, has a significant impact on global climate change. Furthermore, it has unique properties that distinguish it from other seas, such as the barrier layer, which can govern ENSO via the thermohaline stratification structure. However, it is unclear what the tropical Pacific Ocean's state was throughout the deglacial period and how it influenced global warming. Here we reconstructed the temperature and salinity evolution history of the upper water in the core area of ​​the barrier layer area by collecting sediment core samples from the KX97322-4 station (0ºS, 159.24ºE, 2362 m water depth) of the Ontong-Java station. Based on the analysis of δ¹⁸O and Mg/Ca of two planktonic foraminifera (Globigerinoides ruber and Trilobatus sacculifer) living in different layers of the mixed layer, we reconstructed the temperature and salinity changes in the upper and lower layers of the mixed layer in this region over the past 140,000 years. The vertical temperature and salinity comparison reveals that the barrier layer has tended to strengthen over the two most recent deglaciations. Combined with the reconstruction findings of additional stations, we discovered that the barrier layer was more broadly established during the second termination period, which may explain why MIS5e is warmer than MIS1. This phenomena is linked to increased rainfall in the Indonesian Sea and decreased rainfall in the open sea area of the warm pool. Based on present ocean measurements, it is assumed that an increase in rainfall in the warm pool during the deglacial will result in a deepening of the vertical salinity stratification of the surface water, hence reinforcing the barrier layer. Our findings indicate that lateral halocline intrusion primarily controls the barrier layer on a long time scale, potentially increasing the impact of ENSO on the La Niña-like condition throughout the deglaciation period. During the Holocene, the weakening of the barrier layer correlates to an El Niño-like condition. Reconstructing seawater δ¹⁸O at 185 sites in the Indo-Pacific area revealed that rainfall variations in the western and eastern regions of the warm pool exhibited opposing tendencies. The westward movement of the rain belt may have contributed to the weakening of the barrier layer over the Holocene. Our findings give fresh evidence and a mechanical explanation for long-scale ENSO-like phenomena, particularly during the fast warming deglacial.