Southern migration of westerlies in the Northern Hemisphere PEP II transect during the Last Glacial Maximum

摘要

A southward shift of continuous and discontinuous permafrost zones and vegetation belts of Taiga, broad-leaf deciduous and evergreen forests on the islands forming Japan suggests a southern migration of the Northern Hemisphere (NH) summer westerly subtropical jet by around 3-5° of latitude at 135-140°E during the global Last Glacial Maximum (LGM). Since this jet induces the Asian (NH) summer monsoon, the southern shift caused a cooler and drier summer, and the spread of deciduous forest in Southern Japan. The expansion of mountain glaciers on the islands of Japan also supports the southern migration of the subtropical westerly jet. At present, the ablation of snow occurs by heat and precipitation carried by the NH summer monsoon. By comparison, the position of the NH winter westerly subtropical jet remained unchanged, presumably because its course was constrained by the Himalayan Tibetan Plateau. The northern limit of the Kuroshio Current, corresponding to the boundary of the subtropical and subarctic water in the northwestern Pacific Ocean, also migrated by around 3° during the global LGM. The southern migration of the westerly subtropical jet suggests that the current situation where its location shifts during the NH summer to north of the Tibetan Plateau did not occur during the coldest phases of the global LGM, although it did occur intermittently during Marine Isotope Stage (MIS) 3.