The estimation of nutrient salt flux from the outer ocean into the coastal region based on the multi-scale ocean model

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In order to estimate and understand the mechanism of nutrients flux from the outer ocean to the coastal region, two different types of coastal area were investigated. In the first case, field observations were carried out within the Kashima coastal region to understand the effect of the Kuroshio Current on the nutrient salt distribution and primary production along the Kashima open coast. The observations showed there is a surge in the concentration of nutrient salts, the duration of the surge, at one or two months. Comparison with observations made in previous periods showed that there are two kinds of response of the Kashima coastal current system to the approaching Kuroshio. When the approaching Kuroshio is not close to the coastal region, an anticlockwise gyre is induced off the Kashima coast and low-salinity and low-temperature water intrudes from the northern area (Jyoban coast) to the coastal region. When the approaching Kuroshio is close to the coastal region, high-nutrient water wells up from the offshore bottom layer. In the second case, numerical experiment were performed for Tokyo bay to understand the intrusion process of oceanic water into a semi-enclosed estuary. The simulation results showed that the oceanic water induction process involves transition between two kinds of flow structure : the normal estuary circulation type (two layers) and middle layer intrusion type (three layers). While the three layer structure is apparent, oceanic water intrudes into the middle layer of eastern part of Tokyo bay and coastal water at surface and bottom layer outflows through the western Dart of Tokyo bay. This agrees well with field observation. An annual analysis based on the numerical experiment results showed that the intensity and structure of oceanic water intrusion exhibits remarkable seasonal variation, hence intrusion in summer is stronger and the core of oceanic intrusion water is closer to the surface than that in winter.