Tropical SST's of March had a significant year-to-year variation. In 1993, an El Nino event occurred and there were no significant heat sources, but only several fragments. In the subsequent years (from 1994 to 1998), there was a big heat source at the Equator. In 1997 , there remained the significant heat source, say, Warm Water Pool, but the location was shified east by 15 degrees compared to the normal position because the greatest El Nino event in this century occurred just two months later. In 1998, there was still a significant heat source in the South China Sea at the Equator, but not the Warm Water Pool because the year of 1998 was an El Nino year.
Tropical SST's of August showed the north limit of SST zone between 25℃ and 26℃ every year. The SST zone between 25℃ to 26℃ could not enter the Sea of Japan in 1993. It reached only to the Tsushima Strait. But in 1994, the zone went far into the Sea of Japan. It reached the central part of the Japan Sea and surrounded the coast line of the mainland of Japan. Japan had a cold summer in 1993 and an intense heat summer in 1994. In the subsequent years, the SST zone entered the Sea of Japan. There was no exception.
SST in the Japan Sea and off Shikoku
The SST's at other places were published by the Japan Meteorological Agency. One was the SST measured by the meteorological robot buoy anchored at the position of 39.7°N, 134.5°E in the Japan Sea. The other was the SST measured by the same type buoy anchored at the position of 29.0°N, 135.0°E off Shikoku. Time series of the ten-day mean SST from 1993 to 1998 was shown in figure 6. The SST off Shikoku had no significant year to year variation. However, the SST in the Japan Sea had an outstanding feature in August. The maximum SST of August was 21.7℃ in 1993. But in the subsequent years, the maximum SST was higher than 25℃. Especially, the maximum SST was 27.9℃ in 1994. These coincided with the cold summer of 1993 and the heat summer of 1994. Therefore, this August SST in the Sea of Japan was a good index to the state of summer in Japan.
Discussion
The monthly mean sea temperature of March at Oki-no-Tori Sima took the minimum in 1997 rather than in 1993. The March SST in the North West Pacific showed that there was no significant heat source but only several fragments of Warm Water Pool in 1993. However, there was a significant heat source to be shifted eastward by 15 degrees just before the biggest El Nino event occurred in 1997. This eastward displacement of the heat source caused to shift the SST zone to the more southward and to cool the sea temperature at Oki-no-Tori Sima. On the other hand, there was a strong heat source in the South China Sea in 1998 which pushed the SST zone to the north. The existence of a heat source at the Equator from the South China Sea to the region of the Warm Water Pool was important to the location of the SST zone. And the location of the SST zone determined the sea temperature at Oki-no-Tori Sima.
Fig. 5 Time Series of SST's in the Sea of Japan and off Shikoku
