Kazuaki Tadokoro

Temporal increases of phosphate and apparent oxygen utilization in the subsurface waters of western subarctic Pacific from 1968 to 1998

30-years time series of AOU and phosphate in the Oyashio sub-surface domain showed an increasing trend superimposed on bidecadal oscillation. AOU and phosphate trend on isopycnals between 26.7 and 27.2 σθ increased by an average of 0.9±0.5 and 0.005±0.003 µmol/kg/y, respectively. Salinity on these isopycnals also showed an average linear increase of 0.0008 psu/y. Salinity and density of winter mixed layer, on the other hand, was found to have decreased during the observation period. Observed bidecadal oscillation (average period 20±1 y) in subsurface AOU negatively correlated with that of North Pacific Index (r=−0.88±0.06). As the cause of the linear increase of subsurface phosphate and AOU, we speculated that vertical water exchange in the upper layers of the subarctic North Pacific might have been diminished during this period. A decreasing trend of salinity and density of winter mixed layer in Oyashio that was observed during the same period supported this speculation.

Impact of El Niño events and climate regime shift on living resources in the western North Pacific

Abstract Features of El Nino events and their biological impacts in the western North Pacific are reviewed, focusing on interactions between ENSO and the East Asian monsoon. Impacts of El Nino on the climate in the Far East become evident as ‘cool summers and warm winters’. Effects of climate regime shift on ENSO activities, western boundary currents and upper-ocean stratification, as well as their biological consequences are summarized. These have been: 1. In the western equatorial Pacific, an eastward extension of the warm pool associated with El Nino events induces an eastward shift of main fishing grounds of skip jack and big eye tunas. 2. The surface salinity front in the North Equatorial Current region retreats southward, associated with El Nino events. This leads to a southward shift of the spawning ground of Japanese eel, which is responsible for a reduction in the transport of the larval eels to the Kuroshio and Japanese coastal region, causing poor recruitment. 3. Intensification of winter cooling and vertical mixing associated with La Nina (El Nino) events in the northern subtropical region of the western (central) North Pacific reduces surface chlorophyll concentration levels and larval feeding condition for both Japanese sardines and the autumn cohort of Neon squid during winter–early spring. The semi-decadal scale calm winter that occurred during the early 1970s triggered the first sharp increase of sardine stock around Japan. 4. A remarkable weakening of southward intrusion of the Oyashio off the east coast of Japan during 1988–91, resulted in a decrease in chlorophyll concentrations and mesozooplankton biomass in late spring–early summer of the Kuroshio-Oyashio transition region. Changes occurred in the dominant species of small pelagic fish, through successive recruitment failures of Japanese sardine.

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

[1] We suggest possible mechanisms of decadal-scale variation in PO 4 concentration 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 PO 4 affected production of N. plumchrus due to changes in primary production.

Geographical variations in prosome length and body weight of Neocalanus copepods in the North Pacific

Geographical variations in prosome length and body weight of Neocalanus copepods (N. cristatus, N. plumchrus and N. flemingeri) were investigated on samples from North-South and East-West transects in the North Pacific during spring to early summer in 1998 and 1999. Southward and eastward increasing patterns were pronounced for water temperature, although no significant pattern was observed for chlorophyll a concentrations. All Neocalanus species showed large geographical variations in prosome length and body weight, being smaller in the southern and eastern waters. Comparing the relationship between prosome length and body weight, large deviations (lower body weight at a given prosome length) were evident for the eastern specimens of N. cristatus and N. plumchrus. In stepwise regression analysis, the geographical variations of prosome length and body weight revealed a significantly negative correlation with temperature variations. These results suggest that temperature is a more important environmental factor than chlorophyll a concentration in its effect on geographical variations in prosome length and body weight of Neocalanus copepods in the North Pacific.