Possible mechanisms of decadal-scale variation in PO4 concentration in the western North Pacific

Abstract

We suggest possible mechanisms of decadal-scale variation in PO 4oncentration in the Oyashio and Kuroshio-Oyashio transition waters, western North Pacific. Significant decreasing and increasing trends in PO 4 have been observed in the surface and mid-layers of the ocean in this region, respectively. Synchronous bidecadal-scale oscillations in PO 4 were also found between the two layers. Differences in the relationship of the trends and the oscillation in these layers suggest that they are driven by separate processes. The trend component may be induced by attenuation of water exchange between the two layers. In contrast, the influence of the 18.6-yr period nodal tidal cycle on the formation rate of intermediate water in the Okhotsk Sea may cause the bidecadal-scale oscillation. The PO 4 concentration showed a significant positive correlation with Neocalanus plumchrus biomass, the dominant mesozooplankton species in the Oyashio and Kuroshio-Oyashio Transition waters. These relationships suggest that the variation in PO4 affected production of N. plumchrus due to changes in primary production. Copyright 2009 by the American Geophysical Union.