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Inter-decadal variations in Japanese Sardine and the Kuroshio Extension

 

Ichiro Yasuda*1 and Masayuki Noto*2

*1 Department of Earth and Planetary Physics, University of Tokyo Also affiliated with Frontier Research System for Global Change

*2 Hokkaido University

 

Japanese sardine is known to greatly vary in inter-decadal time sacles. In 1988, the catch was over 4million tons; it declined abruptly since 1989 and was below 1million tons in 1995. We found the winter-SST in the Kuroshio Extension and its southern recirculation area (KESA: 30-35N, 145-180E) significantly correlates with the mortality coefficient of the Japanese sardine. The warming in the KESA since 1988 possibly causes the collapse of the sardine. The SST variations in the KESA and south of Japan were different from those in the Kuroshio-Oyashio frontal regions in the western/central Pacific (40-45N 145-180E) where a regime-shift around mid-1970s was known: the SSTs in the Kuroshio regions have a longer time scale of over 50 years than in the frontal regions (25 years), and the jump of the Kuroshio SSTs around late-1980s occurred two years earlier in 1988 than in the frontal regions. The shift around mid-1970s in the frontal regions was not observed in the Kuroshio and the KESA where the SST anomalies were already negative in early-1970s. These SST variations well corresponds to sardine catch records. Analyses of heat flux and wind before/after the late-1980s SST jump indicate that the warming since 1988 was caused mainly by the decrease in the net heat flux due to the reduction of wind over the Kuroshio south of Japan (25-35N and 130-150E). A correlation analysis between the SSTs (in the Kuroshio and the KESA) and SLP shows that the low-SSTs lead to low-SLP around the western/central Pacific (centered around 40N and 180) and high-SLP in Siberia (centered around 70N and 150E) which cause a positive wind anomaly south of Japan (increase net heat flux) and positive westerly wind anomaly in 20-35N latitude (possibly spin-up the subtropical gyre). The relation between the SSTs and the wind south of Japan suggests a positive feedback system. Variations in the Kuroshio coulpled with the atmosphere could be one candidate for a source of the inter-decadal North Pacific variability.

 

 

 

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