2006 Volume 84 Issue 2 Pages 365-373
Yamase is the name for local northeasterly winds that blow from the Pacific Ocean to the east coastsof Hokkaido, and the Tohoku District from May to August. They are cool and moist winds accompanied by low-level clouds. In the present study, we investigate the Yamase winds over the oceans around Japan, using ocean surface vector winds observed by a satellite-scatterometer. In order to define the Yamase Phenomena, we employed an Empirical Orthogonal Function (EOF) analysis of the AMeDAS surface air temperatures over the Tohoku District from May to August. Daily mean temperature anomaly (ΔT), which is a deviation from the climatological daily mean temperature, is calculated for the 167 AMeDAS stations. The spatial coeflicient function of the EOF first mode (EOF1), shows synchronous temperature changes in the whole Tohoku District, while that of the EOF second mode (EOF2), shows a seesaw pattern of temperature variation, between the Pacific and Japan Sea sides. The EOF-decomposed temperature variation, and ΔT at the Hachinohe station (a Yamase index), has high correlations; for EOF1, the correlation is 0.78, and for EOF1+2, it becomes 0.89. The Yamase Phenomena represented by the Hachinohe negative ΔT, is thus associated with two different patterns of the temperature variations in the Tohoku District.
The composite wind map made through the sampling of days, with the Hachinohe ΔT < −2°C, shows northeasterly ocean winds blowing toward the Sanriku coast, an anti-cyclonic circulation center in the eastern Okhotsk Sea, and northeasterly surface winds along the Kuril Islands. When the winds approach the Tohoku District in the Pacific Ocean, they separate two flows around 40°N off Miyako. The surface wind fields associated with the cold phenomena of EOF1 and EOF2 corresponding to the Hachinohe negative ΔT are different. The former indicates the northeasterly wind blowing toward the Sanriku coast, and the latter that the northeasterly winds from the sea south of the Kuril Islands, change their direction around 145°E and the easterly winds blow toward the Sanriku coast. The Kitakami Mountains influence significantly on the Yamase winds, over the coastal Pacific Ocean.