Temperature trends in Japan: 1900-1996

Long-term trends in annual, seasonal, and monthly mean temperature (abbreviated as AMT, SMT, MMT, respectively) in Japan are investigated. The magnitude of a trend is measured by assuming it to be linear. The statistical significance of a site trend is assessed by the Mann-Kendall (MK) with consideration of serial correlation. The statistical field significance of trends in three major climatic regions: Hokkaido (I), areas adjacent to the Japan sea (II), and to the Pacific Ocean (III), is evaluated by the bootstrapping test which preserves cross-correlation among sites. From 1900 to 1996, AMT increased from 0.51 to 2.77 degreesC averaged across all 46 sites. At the regional scale, AMT increased by 1.38, 1.08, and 1.32 degreesC in regions I, II, and III, respectively. The trends at both sites and regions are statistically significant even at the significance level (alpha) of 0.005. SMT increased from 0.47 to 3.69 degreesC at all the 19 available sites with the highest increases in winter and spring. Except for a few series, the changes in SMT are statistically significant at alpha = 0.01. The upward trends in SMT are statistically significant even at alpha = 0.001 in both regions II and III. MMT at 19 sites increased within a wide range from 0.17 to 4.12 degreesC. The increases are largest in the winter and spring months, and most of the site increases are statistically significant at alpha = 0.05. The trends are statistically significant at alpha = 0.025 and 0.001 in regions II and III, respectively. The trends in both SMT and MMT in region III are larger than those in region II.