Internnual-interdecadal variations in plankton biomass and physical environment in the western North Pacific

Abstract

Effects of climate regime shift, ENSO activities and winter monsoon on western boundary currents and upper ocean stratification, as well as their combined effects on plankton biomass and Japanese sardine population are described through analyses of historical time series data. The results are: 1) bidecadal scale variation in the atmospheric condition, coupled with the intensity of winter monsoon influences upper mixed layer depth and density stratification of the ocean, and the volume transport and the current path of the Kuroshio and Oyashio; 2) in the western subtropical North Pacific, weakening of winter cooling and vertical mixing associated with calm and warm winter during the early 1970s, increased surface chlorophyll concentrations in winter in the coastal and offshore water of the Kuroshio, which caused active spawning of mesozooplankton and better feeding condition for sardine larvae; 3) a remarkable weakening of southward intrusion of the Oyashio off the east of Japan during 1988-91 decreased plankton biomass in the Kuroshi-Oyashio transition region in late spring-early summer, and might have caused successive recruitment failures, inducing collapse of Japanese sardine population.