Suppression and enhancement of the spring bloom in the southwestern East Sea/Japan Sea

Abstract

We investigated the variability in surface chlorophyll a in the southwestern region of the East Sea/Japan Sea using OCTS (1997) and SeaWiFS (1999) data, together with in situ data from bimonthly oceanographic surveys. In 1997, the spring bloom appeared to be suppressed in the Ulleung Basin (UB) region, whereas the bloom in the subpolar front (SF) was intensified and continued for four weeks. In 1999, the spring bloom was observed throughout the southwestern region (both UB and SF), but the bloom in the SF region lasted for only one week. Consequently, the overall productivity was higher in 1997 than in 1999, while the spatial difference was much greater in 1997 than in 1999. In 1997, after about seven weeks, copepods became significantly more abundant in the bloom than in the non-bloom area. We relate all these differences to the variability in the volume transport and direction of the Tsushima Current (TC), which is the major input to the East Sea. The volume transport in April–June 1997 was unusually low as compared with that in 1999. The nutrient-poor surface layer of the TC in the UB was much thicker in 1997 and seemed to suppress the spring bloom. However, the front was stationary for more than four weeks and a high-density bloom continued in the SF region for four weeks in 1997. In 1999, the front moved northward rapidly; as a result, the bloom in the SF lasted for one week. In addition, concurrent differences were found in sea-surface height, yearly mean sea level, and the mean position of the SF. Concurrent changes in a suite of oceanographic properties suggest that the unusual conditions in 1997 were related to global variability. A possible relation with ENSO is discussed.