The fluctuating position of the boundary between the Kuroshio (warm) and Oyashio (cold) currents in the mid-latitude western North Pacific affects both heat transport and air–ocean interactions and has significant consequences for the East Asian climate. We reconstruct the paleoceanography of Marine Isotope Stages (MIS) 20–18, MIS 19 being one of the closest astronomical analogues to the present interglacial, through multiple proxies including microfossil assemblage data, planktonic foraminiferal isotopes (δ¹⁸O and δ¹³C), and foraminiferal Mg/Ca-based temperature records, from the Chiba composite section (CbCS) exposed on the Boso Peninsula, east-central Japan. Principal component analysis (PCA) is used to capture dominant patterns of the temporal variation in these marine records, and shows that the relative abundances of calcareous nannofossil and radiolarian taxa are consistent with the water mass types inferred from geochemical proxies. The leading mode (36.3% of total variance) mirrors variation in the terrestrial East Asian winter monsoon (EAWM), reflecting seasonal trends dominated by the winter monsoon system. In the CbCS, this mode is interpreted as reflecting the interplay between the warm Kuroshio and cold Oyashio waters, which is likely related to the latitudinal shift of the subtropical–subarctic gyre boundary in the North Pacific. The second mode (15.4% of total variance) is closely related to subsurface conditions. The leading mode indicates that MIS 19b and 19a are represented by millennial-scale stadial/interstadial oscillations. Northerly positions for the gyre boundary during late MIS 19c, the interstadials of MIS 19a, and early MIS 18 are inferred from the leading mode, which is consistent with a weak EAWM and consequent mild winter climate in East Asia. Nonetheless, the northerly positions for the gyre boundary during late MIS 19c and early MIS 19a were not associated with subsurface warming presumably due to the suppressed gyre circulation itself caused by the weak Aleutian Low. Intermittent southerly positions for the gyre boundary are inferred for the stadials of MIS 19b and 19a. Regional sea surface temperature (SST) comparisons in the western North Pacific reveal that the moderate SSTs during MIS 19a through early MIS 18 were restricted to the mid- to high latitudes, influenced by the weak EAWM. Comparison between MIS 20–18 and MIS 2–1 suggests that glacial MIS 20 and 18 had significantly milder winters than MIS 2, likely related to the relatively weak EAWM.

北太平洋西部中纬度黑潮（暖）流和亲潮（冷）流边界位置的波动影响着热传输和海气相互作用，并对东亚气候产生重大影响。^{我们通过包括微化石组合数据、浮游有孔虫同位素（δ 18} O 和 δ ¹³ C）在内的多种代理数据，重建了海洋同位素阶段 (MIS) 20-18 的古海洋学，MIS 19 是与当前间冰期最接近的天文类似物之一。以及基于镁/钙的有孔虫温度记录，来自日本中东部房总半岛暴露的千叶复合剖面 (CbCS)。主成分分析（PCA）用于捕获这些海洋记录中时间变化的主要模式，并表明钙质超微化石和放射虫类群的相对丰度与从地球化学代理推断的水团类型一致。主模态（总方差的 36.3%）反映了陆地东亚冬季风 (EAWM) 的变化，反映了冬季季风系统主导的季节趋势。在CbCS中，这种模式被解释为反映了温暖的黑潮和寒冷的亲潮水域之间的相互作用，这可能与北太平洋副热带-亚北极环流边界的纬度移动有关。第二种模式（总方差的 15.4%）与地下条件密切相关。主导模态表明 MIS 19b 和 19a 由千年尺度的场域/场间振荡代表。MIS 19c 晚期、MIS 19a 间质层和 MIS 18 早期的环流边界位置偏北，是根据主导模态推断的，这与东亚冬季风较弱以及由此导致的东亚温和冬季气候一致。尽管如此，MIS 19c 晚期和 MIS 19a 早期期间环流边界的偏北位置与地下变暖无关，这可能是由于弱阿留申低压造成的环流环流本身受到抑制。对于 MIS 19b 和 19a 的视场，推断出环流边界的间歇性偏南位置。北太平洋西部区域海表温度（SST）比较表明，MIS 19a至MIS 18早期期间的温和海表温度仅限于中高纬度地区，受东亚冬季风较弱的影响。MIS 20-18 和 MIS 2-1 之间的比较表明，冰川 MIS 20 和 18 的冬季明显比 MIS 2 温和，这可能与相对较弱的 EAWM 有关。