Long-term trend analysis of equilibrium water temperature in Japan over the recent decades

Abstract

The annual mean equilibrium water temperatures (EWTs) of Japan over the past 60 years (1963–2022) and the seasonal mean EWTs over the last 30 years (1993–2022) were analyzed, focusing on the effects of the recent acceleration of global warming. EWT was calculated theoretically using meteorological observation data. Mann-Kendall test and Sen’s slope were used to detect trends and calculate changing rates. The annual mean EWT analysis showed that the upward trend was statistically significant at all 119 target points in Japan during 1963–2022, with an average value of 2.96 (°C/60 years). An abruptly strong upward trend was detected from the 1980s to the early 1990s, and a consistent upward trend has been observed from the 2010s to the present. The statistical analysis of seasonal mean EWTs during 1993–2022 showed that the trend detection rate was highest in spring at 89.1%, with the temperatures showing an upward trend. In winter, the changing trend was not statistically significant across Japan. Furthermore, the rise in EWT in summer was most noticeable in northeastern Japan, whereas no changing trend was detected in southwestern Japan, due to the influence of changes in shortwave radiation.