LAUSR

Abstract Variations in thermoclinal conditions play an important role in the evolution of the Western Pacific Warm Pool (WPWP) and in connecting the tropical and extratropical regions of the Pacific Ocean. However, secular thermoclinal changes in the northwestern part of the WPWP and their forcing mechanisms are not yet well understood, particularly on orbital timescales. In this study, we report thermoclinal variation at Site MD06-3047 B in the western Philippine Sea over the last 700 kyr. Our data show that variations in thermocline temperature and salinity in the study area were correlated with changes in Earth’s obliquity, with warmer and saltier (colder and fresher) thermocline waters corresponding to high (low) Earth-axial obliquity. At high obliquity, increases in thermocline temperature and salinity may have been linked to advection of warm North Pacific tropical water. In the study area, a lower (higher) vertical temperature gradient in the upper water column suggests a deeper (shallower) thermocline during glacials (interglacials). The main factor leading to thermoclinal deepening during glacials was strengthening of the East Asian winter monsoon (EAWM), and more La Nina-like conditions may have contributed to concurrent subsurface warming and thus thermoclinal deepening. In-phase thermoclinal deepening in the northern and peri-equatorial southern WPWP during glacial stages may have been triggered by changes in glacial boundary conditions, such as increased ice sheet thickness, through intensification of the EAWM and strengthening of the tropical Trade Winds.