[1] Pattern and origins of Kuroshio branches in the bottom water of southern East China Sea (ECS), were carefully examined by numerical simulations based on the Regional Ocean Modeling System (ROMS) together with observations. Model results show that in the bottom water of ECS, the intrusion pattern of Kuroshio is mainly composed of an Offshore Kuroshio Branch Current (OKBC) which, bifurcated from the Kuroshio northeast of Taiwan, flows nearly along the isobath of $100 m, and a Nearshore Kuroshio Branch Current (NKBC) which, originated from the Kuroshio northeast of Taiwan, upwells northwestward gradually from $250 m to $60 m, then turns to northeast around 27.5°N, 122°E, thereafter flows northeastward along the isobath of $60 m, and finally reaches at 30.5°N where it turns to east. Furthermore, we found that the NKBC mostly originated in the Kuroshio subsurface water (120-250 m) east of Taiwan, whereas the OKBC mainly stemmed from the Kuroshio water (60-120 m) east of Taiwan. This pattern and origins of OKBC and NKBC well addressed the observational phenomena that off the coast of Zhejiang province, China, there were colder, less saline, and more phosphate-rich bottom water near the isobath of $60 m rather than near the isobath of $100 m in August 2009. Finally, it is proposed that on southern ECS continental shelf, Kuroshio exhibits its intrusion branches by an anticyclonical stair structure: bottom stair NKBC, middle stair OKBC, and top stair Kuroshio surface branch (KBC).

[1] Using the Regional Ocean Model System, the ocean circulation on the East China Sea (ECS) shelf was examined by a fine-resolution model which was nested in a coarseresolution Pacific Ocean model. The high-resolution simulation shows an accurate volume transport of 2.70 Sv (Sv ≡ 10 6 m 3 s −1 ) through the Tsushima Strait, which is more consistent with the previous 5.5 year observation value (2.64 Sv) than former model results. For the Taiwan Strait it also shows a close volume transport (1.03 Sv) to a recent estimate (1.20 Sv). At the same time the model results reproduced almost all of the known circulation structure on the ECS shelf. In addition, the hindcast of 2009 shows a Kuroshio Bottom Branch Current to the northeast of Taiwan (KBBCNT). The KBBCNT is confirmed by the observational bottom high-salinity water (from 15 August to 2 September 2009) whose distribution is also reproduced by the model results. Tracer and particle experiments were carried out to elucidate the formation of the high-salinity water and the pathway of the KBBCNT. In light of the field observation and numerical experiments, a new pathway of the KBBCNT is proposed: bifurcated from the subsurface water of Kuroshio northeast of Taiwan, it upwells northwestward gradually from 300 to 60 m, then turns to northeast in the region around 27.5°N, 122°E, and finally reaches 31°N off the mouth of the Changjiang River along ∼60 m isobaths, forming the bottom saline water off the coast of Zhejiang province, China.

Off the coast of Zhejiang province, China, algal blooms are frequently observed where the phosphate seems to be an essential ingredient to dominate the growth of the phytoplankton in summer. Therefore, the observed high phosphate distributions off the coast of Zhejiang are closely examined to find out the dominant phosphate origin as well as the underlying forcing mechanism in summer. The observed phosphate distribution has been faithfully reproduced by our numerical model based on the Regional Ocean model System (ROMS). Then, on the basis of the numerical experiments as well as the observations, we propose that the phosphate off the coast of Zhejiang mainly originates from the deep sea water in a special area (122.1°E-z122.5°E, 130 m-300 m deep) along 24.9°N northeast of Taiwan. Also, the forcing mechanism is clearly illustrated. In the bottom water of southern East China Sea, huge phosphate is continuously transported to the area off the coast of Zhejiang by a nearshore Kuroshio branch current which links the phosphate-rich deep sea water to the bottom water off the coast of Zhejiang. Then, off the coast of Zhejiang the transported phosphate-rich water is further upwelled to the surface water due to an upwelling just off the coast of Zhejiang. Then, the upwelled phosphate-rich water is transported offshore in the surface water by the northeastward flowing Taiwan Warm Current, forming a high phosphate tongue which can be easily utilized by the phytoplankton and then immediately explains the observed high chlorophyll tongue off the coast of Zhejiang.Crown

No abstract

Seasonal variations of water masses in the East China Sea (ECS) and adjacent areas are investigated, based on historical data of temperature and salinity ( T-S ). Dynamic and thermodynamic mechanisms that affect seasonal variations of some dominant water masses are discussed, with reference to meteorological data. In the ECS above depth 600 m, there are eight water masses in summer but only fi ve in winter. Among these, Kuroshio Surface Water (KSW), Kuroshio Intermediate Water (KIW), ECS Surface Water (ECSSW), Continental Coastal Water (CCW), and Yellow Sea Surface Water (YSSW) exist throughout the year. Kuroshio Subsurface Water (KSSW), ECS Deep Water (ECSDW), and Yellow Sea Bottom Water (YSBW) are all seasonal water masses, occurring from May through October. The CCW, ECSSW and KSW all have signifi cant seasonal variations, both in their horizontal and vertical extents and their T-S properties.Wind stress, the Kuroshio and its branch currents, and coastal currents are dynamic factors for seasonal variation in spatial extent of the CCW, KSW, and ECSSW, whereas sea surface heat and freshwater fl uxes are thermodynamic factors for seasonal variations of T-S properties and thickness of these water masses. In addition, the CCW is affected by river runoff and ECSSW by the CCW and KSW.

Satellite data show that typhoon Chan‐hom did trigger an algal bloom several days after passing by the East China Sea. To investigate dynamic connections between this enhanced chlorophyll‐a and phosphate‐rich Kuroshio subsurface water (KSSW), we set up a fine‐resolution coupled physical‐biological model, which effectively reproduced the oceanic conditions during this typhoon. Furthermore, we released a passive tracer along a zonal transect northeast of Taiwan. The modeled surface tracer variations along the coast of Zhejiang agreed very well with satellite data and chlorophyll‐a changes in the biological model. The lowest structural similarity index between chlorophyll‐a and KSSW was 0.77. Model results along a coastal section imply that typhoon Chan‐hom induced additional upwellings and enhanced vertical mixing, leading to more KSSW outcropping. Through horizontal dynamic diagnostic and vertical velocity decomposition, we distinguished three types of dynamical mechanisms for the upward motions. At first, the linear wind Ekman effect existed around the coastal areas and led to intense Ekman pumping. Second, a pronounced nonlinear effect led to upwellings, and spectrum analysis revealed that this nonlinearity consisted of high‐frequency near‐inertial waves and low‐frequency coastal shelf waves. At last, an eddy field remained after the typhoon, continuously supplying nutrients upward to the surface layer by eddy‐induced Ekman pumping. This study reveals that these mechanisms may be general on the continental shelf, where typhoons can pump phosphate‐rich bottom water into the upper layer and enhance primary productivity there. The results also prove typhoon‐induced nonlinear wave motions on the continental shelf contribute to the outcrop of nutrients.

In 3 consecutive years from 2016 to 2018, extreme ocean warming events, or marine heatwaves (MHWs), occurred during boreal summers in the East China Sea (ECS) and South Yellow Sea (SYS), which is unprecedented in the past four decades based on the satellite record. In this study, we used a high‐resolution hydrodynamic model based on Finite Volume Community Ocean Model (FVCOM) to simulate the evolution of these warming events. An upper ocean temperature budget (0–20 m) analysis based on the model results shows that the shortwave radiation and the ocean advection anomalies jointly contributed to the anomalous warming in the three successive summers (June–August) in the SYS and the north part of the ECS. In addition, the reduction of surface wind speeds during the 2016 and 2017 summers further weakened the vertical mixing, thereby enhancing the anomalous warming in the north part of the ECS adjacent to the SYS. During the three summers, the increases of shortwave radiation were closely related to the East Asian Summer Monsoon (EASM) variability, which reduced the cloud cover in the ECS and SYS, whereas the advection anomalies were mostly associated with regional wind anomalies. In summer 2018, upper ocean heat was transported into the central trough of the SYS, accumulated in an anticyclonic eddy generated by the anomalous wind stress curls. Therefore, despite the primary driver of the MHWs is the EASM variation, regional processes are critical to driving the spatial pattern of the MHW intensity in the ECS and SYS.

The Kuroshio intrusion plays a vitally important role in carrying nutrients to marginal seas. However, the key mechanism leading to the Kuroshio intrusion remains unclear. In this study we postulate a mechanism: when the Kuroshio runs onto steep topography northeast of Taiwan, the strong inertia gives rise to upwelling over topography, leading to a left‐hand spiral in the stratified ocean. This is called the topographic beta spiral, which is a major player regulating the Kuroshio intrusion; this spiral can be inferred from hydrographic surveys. In the world oceans, the topographic beta spirals can be induced by upwelling generated by strong currents running onto steep topography. This is a vital mechanism regulating onshore intruding flow and the cross‐shelf transport of energy and nutrients from the Kuroshio Current to the East China Sea. This topographic beta spiral reveals a long‐term missing link between the oceanic general circulation theory and shelf dynamic theory.