No abstract

No abstract

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.

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.

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.

The Changjiang Estuary and adjacent sea area in the East China Sea suffer from frequent harmful algal blooms. However, the relative importance of riverine nutrient input from the Changjiang River and oceanic nutrient input from the Taiwan Warm Current and Kuroshio Current to the development and distribution of summer phytoplankton blooms in this area remains unclear. To address this problem, we deployed a coupled physical-biological model. The coupled model successfully reproduces the main hydrographic and biogeochemical features in this domain. Both satellite observations and model results show two regions of elevated chlorophyll concentrated in this site. Simulated results show that harmful algal blooms in the region north of the Zhoushan Islands are mainly driven by riverine nutrients from the Changjiang River, while algal blooms in the region south of the Zhoushan Islands are mainly controlled by nutrients from the open ocean. Nutrient input, particularly phosphate, from the Kuroshio subsurface water contributes most to the accumulation of dinoflagellate biomass and chlorophyll in the southern region, while the Taiwan Warm Current has less influence. This has implications for nutrient control and land management practices: Although reducing riverine nutrient loads may significantly reduce phytoplankton growth north of the Zhoushan Islands, it will have little effect in the area to the south. Plain Language Summary Blooms of microscopic algae that can be harmful to marine ecosystems and commercial fisheries are increasingly common during the summer months in the Changjiang Estuary and adjacent sea area in the East China Sea. The Changjiang River is traditionally considered as the dominant source of nutrients driving these harmful algal blooms. In this study, we show that although this appears to be true for algal blooms occurring to the north and northeast of the river estuary, algal blooms that occur along the coast to the south of the river estuary are mainly regulated by nutrient input from the open ocean. Intrusion of the Kuroshio subsurface water to the south of the Changjiang Estuary is shown to be particularly important in supplying the dissolved phosphate necessary for the algae in the southern region to bloom in large quantities.

No abstract

No abstract