Numerical Study On The Kuroshio Intrusion On The East China Sea Shelf And Its Influence On The Biological Process

Using Regional Ocean Model System (ROMS), the ocean circulation on the East China Sea (ECS) shelf was examined by a fine resolution model(5'×5') which was nested in a coarse resolution Pacific Ocean model(10'×10'). In addition, we have developed a 3D phosphate limited physics-biology-chemistry coupled model on the basis of NEMURO (North Pacific Ecosystem Model for Understanding Regional Oceanography), and used the coupled model to study the biological process in the adjacent area of Changjiang River mouth. We have made five important progress as following:1.The high-resolution simulation shows more accurate results than previous model results. The high-resolution simulation show volume transport 2.70 Sv (Sv≡106 m3s-1) through the Tsushima Strait, which is more consistent with the previous 5.5 yrs. (1997.2.21-2002.8.25 yearly mean value)observation value 2.64 Sv than former model results. For the Taiwan Strait, it also shows a close volume transport (1.03 Sv) to recent estimate (1.20 Sv).2. It was proposed that Kuroshio exhibits its intrusion by a anti-cyclonical stair structure which is composed of a nearshore Kuroshio bottom branch current (N-KBBC), an offshore Kuroshio bottom branch current (O-KBBC) and a surface Kuroshio branch current (KBC); the pathways of N-KBBC and O-KBBC have also been elucidated. The bottom Kuroshio intrusion northeast of Taiwan has been carefully investigated by cruise data covering the whole southern East China Sea and an accurate numerical simulation based on ROMS. On the basis of numerical experiments and observational data, we found that originated from the Kuroshio northeast of Taiwan, N-KBBC upwells northwestward gradually from the Kuroshio east of Taiwan, then turns to northeast in the region around 27.5°N, 122°E, and finally reaches at 31°N off Changjiang river mouth along about 60 m isobaths, forming the bottom saline water off the coast of Zhejiang province, China; O-KBBC initially upwells northwestward northeast of Taiwan, then flows northeastward along the 100-m isobath on the East China Sea shelf, and finally one part of it rejoins the Kuroshio mainstream at 28°N and another part of it flows further northeastward; stemmed from Kuroshio surface water, the KBC, as reported in previous studies, turns to ESE after it immediately separated from Kuroshio mainstream and rejoins the Kuroshio mainstream near 27°N.3. It was proposed that N-KBBC originated from the Kuroshio subsurface water (120-300m) east of Taiwan, while the O-KBBC originated from the Kuroshio water (60-120m) east of Taiwan. N-KBBC originated from the deeper water than O-KBBC did. The vertical distribution of N-KBBC and O-KBBC well explained that why there are observational lower salinity, temperature and higher phosphate value in the bottom water close to 60m isobath off the Zhejiang coast than the bottom water close to 100m isobath off the Zhejiang coast. Because observational lower salinity, temperature and higher phosphate value in the bottom water close to 60m isobath off the Zhejiang coast is caused by N-KBBC, while the bottom water close to 100m isobath off the Zhejiang coast is formed by the O-KBBC.4. It was found by numerical calculation that to the adjacent area off Changjiang river mouth, N-KBBC contributed a large amount phosphate which is one order of magnitude larger than the Changjiang river did. In summer, the volume transport of the N-KBBC is estimated at about 0.3 Sverdrup (Sv) which is one order of magnitude larger than that of Changjiang river (about 0.04 Sv). The N-KBBC can continuously transport the phosphate from the east of Taiwan to the area off Changjiang river mouth. The phosphate input of the N-KBBC (5.0×104 ton/month) is one order of magnitude larger than that of Changjiang River(3.0×103 ton/month).5. It was proposed that the high chlorophyll value distribution off the coast of Zhejiang (to the south of 30°N) and the high chlorophyll value distribution zone between Zhoushan and (122.5°E, 31°N), were attributed to the N-KBBC. Using our developed biological model, numerical experiments have been done and show that the high chlorophyll value distribution off the coast of Zhejiang (to the south of 30°N) cannot be generated by Changjiang River, but N-KBBC can do it. Photosynthetic carbon fixation caused by N-KBBC is estimated at about 4.0×105 ton/day.The water exchange between the open sea water and marginal sea water on the East China Sea Shelf is blocked by the Kuroshio. Therefore the cross-shelf Kuroshio intrusion branches play a very important role in the material exchange between the Pacific Ocean and the marginal sea of China.