• planktonic foraminifera;
  • Mg/Ca;
  • oxygen isotope;
  • western North Pacific;
  • subtropic

[1] Variations in sea surface temperature (SST) and vertical thermal structure in the western subtropical North Pacific, which has the largest air-sea heat flux of the world's oceans, provide insights into the mechanisms of climate change related to air-sea interactions. Here, we present planktonic δ18O and Mg/Ca records from the western subtropical gyre of the North Pacific spanning the last 30 kyrs. The results indicate that subtropical SSTs were approximately 3°C lower during the last glacial than in the Holocene interglacial, indicating that glacial cooling occurred uniformly in the low to mid-latitudes of the western North Pacific. A decrease in intermediate depth temperatures at the late glacial suggests that the formation and/or advection of the subtropical mode water was enhanced due to a strong East Asian winter monsoon. The results suggest that the change in the thermal structure of the subtropical gyre was related to changes in East Asian monsoon activity.