Statistical analysis of decadal warming and cooling trends in equilibrium water temperature in Japan

In this paper, the seasonal trend of decadal change in equilibrium water temperature was examined in Japan for the recent 50 years from 1963 to 2012. For the time-series data in equilibrium water temperature, the statistical analysis used the Mann-Kendall test to detect a warming or cooling trend, and the Sen's slope to evaluate its quantitative values. The results showed that there were both warming and cooling trends in the decadal changes in the equilibrium water temperature. As for their regional distribution characteristics, the warming trend was evident in the coastal part of the Pacific Ocean in Japan. Both the global warming and urban heat island phenomenon could have had effects on this coastal part. On the other hand, the cooling trend was detected in the northern part, i.e., the Tohoku region and the southern part, north Kyushu region, as well as the coastal part of the Sea of Japan. These parts indicated a strong tendency on either a decrease in short wave radiation or an increase in latent heat transfers in the spring and summer seasons.