Northward and southward migrations of frontal zones during the past 40 kyr in the Kuroshio‐Oyashio transition area

Abstract

The variation of sea surface temperature (SST) in the Kuroshio‐Oyashio transition area and the Okhotsk Sea during the last 40 kyr was estimated using long‐chain unsaturated alkyl ketones (alkenones). The temperature difference between the present and the last glacial period was about 5°–6°C in the Kuroshio‐Oyashio transition area, slightly larger than the 4°C previously reported. Moreover, alkenones were not detected before 18 kyr B.P. in or near the Okhotsk Sea, perhaps because of nearly year‐round sea‐ice expansion, which would have prevented the growth of alkenone producers, or because alkenones were less well preserved in surface sediments during the glacial period. The northward and southward migrations of frontal zones were evaluated using differences in alkenone temperatures between stations located in the vicinity of the Subarctic Front (SAF), Subarctic Boundary (SAB), and Kuroshio Bifurcation Front (KBF). All three frontal zones migrated southward during the glacial period, compared with their present positions, and their relative positions were different between the Holocene and glacial periods. In particular, the distance between the SAF and the SAB was reduced during the glacial period compared with at present. The northward and southward migration of the frontal zones may relate to both mid to low atmospheric circulation and polar atmospheric circulation.