Long-term changes of temperature extremes and day-to-day variability in Japan

Long-term changes and short-term variations of extreme temperatures in Japan were analyzed using daily maximum and minimum temperature data at 17 stations for 1931-2005, and 850hPa temperature data at 12 stations for 1961-2005. The analysis was made for (1) monthly highest, lowest and average values of departure from the climatic mean, (2) frequencies of departures exceeding 2σ , and (3) day-to-day temperature variability, which was defined the standard deviation of temperature departure. The result for maximum temperature reveals nearly parallel increase of monthly highest, lowest and average values, at a higher rate in the recent few decades (0.08°C/decade for 1931-2005 and 0.4-0.5°C/decade for 1981-2005), without a trend in day-to-day variability. A similar feature is found for 850hPa temperature. Minimum temperature shows more rapid increase than the other quantities, especially for the monthly lowest value (0.27°C/decade for 1931-2005 and 0.51°C/decade for 1981-2005), with decreasing day-to-day variability. On a short time scale of several months to a year, variations in extreme temperatures are closely related to those of monthly average temperature, and also of day-to-day variability. A lag correlation analysis and a principal component analysis indicate that the anomaly of day-to-day variability tends to last for several months to a year, and is correlated to the anomaly of north-south temperature gradient.