Thermocline variability in the subtropical northwestern Pacific since the last deglaciation

Thermocline conditions in the tropical western Pacific are closely linked to regional- and basin-scale hydrological variation and may have played a critical role in global climate evolution on various timescales. However, long-term changes in thermocline hydrology in the subtropical northwestern Pacific and their connection to the tropical Pacific remain unclear. Based on planktonic foraminiferal Mg/Ca and δ18O, we reconstructed the temperature and salinity of thermocline water since the last deglaciation, using core OKI02 from the Okinawa Trough (OKT) in the subtropical northwestern Pacific. The results indicated a discrepancy in evolution of both temperature and δ18O between the thermocline and surface water over the past 19 ka. Unlike for surface water, changes in thermocline hydrology correlated with changing conditions in the tropical western Pacific, indicating a close relationship between these regions since the last deglaciation. Thermocline temperature and salinity gradients between the OKT and the tropical western Pacific increased from ∼16.5 to 10 ka BP, reached their maximum values at approximately 10 ka BP, and then decreased from the Early Holocene to present. Variations in temperature and salinity gradients correlated with changes in the El Niño–Southern Oscillation (ENSO) over the past 19 ka, which were coupled with equatorial summer insolation variations. We suggest that the ENSO-like process, forced by equatorial summer insolation, regulated thermocline dynamics in the subtropical northwestern Pacific Ocean since the last deglaciation, by influencing the strength of the Kuroshio Current.