Evaluation of the Contributions of Year-Specific Climate Anomaly, Nationwide Warming, and Urban Warming to Hot Summers in Japan

Using surface observation data for the past hundred years, the contributions of year-specific climate anomaly, nationwide warming, and urban warming to hot summers in Japan were evaluated. A number of indices in temperature were defined to indicate the severity of summer heat in each year. Then, the year-to-year time series of each index was divided into a year-specific component and a temporally smoothed component, and the latter was divided into a nationwide non-urban component and an urban component. The results show that the non-urban component began to increase after the 1990s, which is approximately attributable to global warming, although there are some temperature variations on the yearly to multidecadal scales related to the Pacific Decadal Oscillation (PDO) and the Southern Oscillation (SO), whereas urban warming became apparent since the 1960s at stations in highly urbanized areas. For the recent record-breaking summer heat, the contributions of the year-specific temperature anomaly, nationwide warming, and urban warming are all evaluated to be of the order of 1 °C.