Taroshi-taki, a waterfall located in a mountainous region at Hanamaki City, Iwate Prefecture, sometimes freezes in winter. The circumference of Taroshi-taki has been measured in mid-February every year since 1975. In this region, it is said that in a year when Taroshi-taki freezes (does not freeze) with a larger circumference, a good (bad) harvest can be expected; it will be a hot (cool) summer. This study investigated whether a hot (cool) summer follows a cold (warm) winter in relation to the seasonal march of a temperature anomaly. Because the temperature anomaly must change its sign between winter and summer, we clarified when and why this occurs through a composite analysis in the northern hemisphere, mainly using NCEP/NCAR reanalysis data. Data for 1975-2001 were analyzed because data on energy transport and its budget described below were available only for that period.
During 1975-2001, Taroshi-taki froze (did not freeze) in 17 (10) years. We prepared the composite temperature anomaly for these 17 (10) years from the long-term mean (1971-2000) each month, and investigated its seasonal march. In these 17 (10) years, a negative (positive) temperature anomaly covering all of Japan in winter became positive (negative) in April; the temperature anomaly in winter disappeared in April in each case. Analyses of energy transport and its budget in the atmosphere (apparent heat source and moisture sink) revealed that, in April, when Taroshi-taki froze, the tropical Pacific released more energy to the west than the long-term mean. In April, when Taroshi-taki did not freeze, the spatial pattern of energy transport and its budget were almost opposite.
In years when Taroshi-taki froze, the negative temperature anomaly in winter from eastern Asia to the North Pacific expanded the north-south temperature gradient (enhancement of baroclinity) including this area. During the subsequent April, this caused more atmospheric energy to be transported from the tropical Pacific to the middle latitudes, including Japan, compared to the long-term mean. This made the negative temperature anomaly in winter around Japan positive in April, which was maintained throughout the warm period. It is possible that the freezing of Taroshi-taki in winter represents negative feedback of north-south energy transport from the cold season to the warm season, which constitutes an important cause-and-effect sequence in the climate system.
