Journal of Oceanography, Vol. 63 (No. 4), pp. 661-669, 2007
Hiromi Kasai* and Tsuneo Ono
Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Katsurakoi, Kushiro, Hokkaido 085-0802, Japan
(Received 10 July 2006; in revised form 12 March 2007; accepted 12 March 2007)
Abstract: To examine whether the regime shift in 1998 that has been variously reported to have occurred in the oceanographic conditions of the central and eastern North Pacific also occurred in the Oyashio region, western North Pacific, we compared data over the period 19902003. Oceanographic conditions were compared before 1997 with those after 1998, using the A-line dataset (19902003) obtained by the oceanographic surveys of the Hokkaido National Fisheries Research Institute, Fisheries Research Agency (HNFRI/FRA). Seasonal changes of the monthly-mean SST (as temperature in the surface layer) show a significant increase in spring after 1998. After 1998, the mean concentration of chlorophyll a at the surface was higher in spring than that before 1997. This was more remarkable in the main current of the Oyashio. These changes suggest that the spring phytoplankton bloom in the Oyashio region after 1998 was larger in magnitude and initiated earlier. Consumption of nutrients during the spring bloom and standing stock of netplankton also shows a distinct difference between the time period before 1997 and after 1998. These results support the occurrence of the regime shift around 1998 in the Oyashio region. The changes of hydrographical conditions accompanying with the 1998 regime shift are discussed. The hydrographic mechanism of enhancement of primary productivity during the spring phytoplankton bloom was not fully clarified, though. Results in this study may support the usefulness of the A-line dataset for analysis of long-term variability in the western subarctic Pacific.