Enhanced and southward North Pacific subtropical gyre circulation following the intensification of Northern Hemisphere Glaciation

The North Pacific Subtropical Gyre (NPSG) is critical for ocean heat distribution and climate regulation in the North Pacific and beyond. However, its variations across major climate transitions over long timescales remain poorly constrained. In this study, we reconstruct sea surface temperature (SST) and salinity (SSS) in the mid-latitude Northwest Pacific from 3.8 to 1.8 Ma using planktonic foraminiferal δ¹⁸O and Mg/Ca ratios from Deep Sea Drilling Project Site 296 and integrate paleorecords from the Kuroshio Current Extension (KCE), California Current (CC), and Alaska Current (AC) regions to investigate NPSG variability during the Late Pliocene Northern Hemisphere Glaciation (NHG). Pronounced increases in SST and SSS at Site 296 after the intensification of the NHG (iNHG) at 2.7 Ma reveal a strengthened Kuroshio Current with enhanced transport of warm saline waters. The concurrent SST increases in the CC and AC areas after ∼2.7 Ma indicate the intensification of all three NPSG boundary currents following the iNHG, resulting in a more robust NPSG circulation. Meanwhile, SSTs in the KCE and northern CC regions decreased considerably, implying southward displacement of the northern boundary of the NPSG. The post-iNHG enhancement and southward shift of the NPSG may have resulted from a more La Niña-like state in the equatorial Pacific and atmospheric circulation reorganization associated with high-latitude cooling in the Northern Hemisphere, as suggested by zonal and meridional SST gradients in the North Pacific. These findings highlight a possible weakening and poleward shift of the NPSG circulation in response to future warming.