Impact of cage aquaculture on water exchange in Sansha Bay

Abstract(#br)Influence of cage aquaculture on the flow field and water exchange in Sansha Bay is investigated based on in situ current measurements and output from a two-dimensional shallow water hydrodynamic finite element model (SHYFEM). Without cage influence, the flow is relatively uniform in the vertical except a bottom Ekman layer. An asymmetry of tidal current speed is also observed in Sansha Bay with a dominance of the ebb tide. Near-surface current speed squared in cage-free area is typically larger than that within cage area by a factor exceeding three in deep channels, and by a factor of two in tidal flats. Current speed profiles suggest that cage-induced drag on the flow field can reach as deep as 20 m in the relatively deep channels of Sansha Bay. A set of numerical experiments are designed to quantify the relative effect of cages in tidal flats and channels, respectively, on water exchange using SHYFEM. It is shown that cage aquaculture weakens the local flow but seems to strengthen the flow adjacent to cages. Reducing the frictional drag in channels significantly increases the water exchange rate both locally and in the near-field tidal flats. Therefore, certain clearance or rearrangement of cage aquaculture in channels would be more effective in improving the water exchange in the entire Sansha Bay

Three-dimensional structure of a low salinity tongue in the southern Taiwan Strait observed in the summer of 2005

Cruise observations with CTD (conductivity-temperature-depth) profiler were carried out in the southern Taiwan Strait in the summer of 2005. Using the cruise data, two-dimensional maps of salinity and temperature distributions at depths of 5, 10, 15, 20, and 30 m were generated. The maps show a low salinity tongue sandwiched by low temperature and high salinity waters on the shallow water side and high temperature and high salinity waters on the deep water side. The further analysis indicates that the low salinity water has a nature of river-diluted water. A possible source of the diluted water is the Zhujiang (Pearl) Estuary. Meanwhile, the summer monsoon is judged as a possible driving force for this northeastward jet-like Current. The coastal upwelling and the South China Sea Warm Current confine the low salinity water to flow along the central line of the strait. Previous investigations and a numerical model are used to verify that the upstream of the low salinity current is the Zhujiang Estuary. Thus, the low salinity tongue is produced by four major elements: Zhujinag Estuary diluted water, monsoon wind driving, coastal upwelling and South China Sea Warm Current modifications.National Natural Science Foundation of China [40331004, 40576015, 40810069004, 40821063]; MEL Open Project [MEL0506]; ONR [N00014-05-1-0328, N00014-05-1-0606]; NSF [071003-9222

A Strong Kuroshio Intrusion into the South China Sea and Its Accompanying Cold-Core Anticyclonic Eddy in Winter 2020–2021

Multiple remote sensing datasets, combined with in-situ drifter observations, were used to analyze the Kuroshio intrusion through the Luzon Strait (LS). The results showed that a strong Kuroshio Current Loop (KCL) and accompanying anticyclonic eddy (ACE) existed in winter 2020–2021. As quantitatively demonstrated by the Double Index (DI), the Kuroshio Warm Eddy Index (KWI) had low values during a long sustained period compared to those in all other years in the available historical records. Remarkable kinematic properties (i.e., amplitude, diameter, propagated distance, lifespan and propagating speed) of the accompanying ACE were extracted by automatic eddy detection algorithms, showing that the ACE had a maximum diameter of 381 km and a peak amplitude of 50 cm, which significantly exceeded the previous statistics in winter. The orographic negative wind stress curl southwest of Taiwan Island and the westward Ekman transport through the LS during the winter half year of 2020–2021 both had large values beyond their historical maxima. Hence wind forcing is regarded as the primary forcing mechanism during this event. Alternating cyclonic eddies (CEs) and ACEs approaching on the east of the LS were identified, indicating that the interaction between the Kuroshio and the impinging CEs at proper locations made extra contributions to enhancing the KCL. The accompanying ACE had a distinctive feature of a cold-core structure at the surface layer, so as to be categorized as a cold core ACE (CC-ACE), and the temperature difference between the cold core and outer warm ring was maintained for three months. The generation and long duration of the CC-ACE may be due to the sustaining entrainment supported by the warm water from Kuroshio intrusion and the Northwest Luzon Coastal Current (NWLCC) successively

Source water of two-pronged northward flow in the southern Taiwan Strait in summer

NSFC [40331004, 40810069004]; National Science Council of Taiwan [NSC 97-2621-M-110-002, 96-2628-M-110-002-MY3]; Aim for the Top University PlanIt is generally accepted that the flow is northward in the Taiwan Strait during summer and that the strongest current is detected in the Penghu Channel between the Penghu Islands and the Taiwan Island. This current, the eastern prong flow, is made up of waters from the South China Sea (SCS) and the Kuroshio. North of the Penghu Islands, the current veers to the west before turning northward again because of the shallow Chang-Yuen Ridge, and extends westward off the coast of Taiwan. There is a second prong of northward flow existing between the Taiwan Bank and the China mainland coast. Here, we show with observational data as well as results from a numerical model that this water receives little influence from the Kuroshio and is distinctively cooler, fresher, less oxygenated and more acidic, and contains more dissolved inorganic carbon than waters at the same density level of the eastern prong. Evidence is provided to show that the source water of the western prong should be the subsurface water from the strong upslope advection flowing northward from the SCS to the southern Taiwan Strait and upwelling along the coast during the favorable southwesterly wind. Subsequently, the upwelled water flows over the saddle west of the Taiwan Bank and joins the main flow northwest of the Penghu Islands

Lagrangian Observation of the Kuroshio Current by Surface Drifters in 2019

In this research, the features of the Kuroshio Current in 2019 were studied based on the observations of 29 self-developed surface current experiment drifters deployed in the western Pacific Ocean and the East China Sea. The Kuroshio flow pattern and velocity magnitude observed in 2019 were largely consistent with the climatology based on the historical drifter dataset, but they still exhibited distinctive characteristics. The intrusion of the Kuroshio into the South China Sea in the spring was observed by a group of drifters crossing the Luzon Strait from east to west, which is a notable departure from the non-intrusion pattern noted to occur in the spring in most of the historical records. A strong intrusion of the Kuroshio into the East China Sea was also observed, taking an anti-cyclonic turn in the northeast of Taiwan Island. Both the drifter trajectories and altimeter-derived dynamical topography captured the large meander pattern of the Kuroshio south of Japan in 2019, with the flow path having a maximum offshore distance of 470 km. In addition, Lagrangian statistics (lateral diffusivity, integral time scale, and integral space scale) were estimated for four selected regions with adequate drifter samplings. The lateral diffusivity had large values along the Kuroshio segment in the East China Sea and small values on the continental shelf of the East China Sea. The integral time scales for the four regions ranged from 0.8 to 3.7 days, with a corresponding integral space scale of 19~128 km

Lagrangian Observation of the Kuroshio Current by Surface Drifters in 2019

In this research, the features of the Kuroshio Current in 2019 were studied based on the observations of 29 self-developed surface current experiment drifters deployed in the western Pacific Ocean and the East China Sea. The Kuroshio flow pattern and velocity magnitude observed in 2019 were largely consistent with the climatology based on the historical drifter dataset, but they still exhibited distinctive characteristics. The intrusion of the Kuroshio into the South China Sea in the spring was observed by a group of drifters crossing the Luzon Strait from east to west, which is a notable departure from the non-intrusion pattern noted to occur in the spring in most of the historical records. A strong intrusion of the Kuroshio into the East China Sea was also observed, taking an anti-cyclonic turn in the northeast of Taiwan Island. Both the drifter trajectories and altimeter-derived dynamical topography captured the large meander pattern of the Kuroshio south of Japan in 2019, with the flow path having a maximum offshore distance of 470 km. In addition, Lagrangian statistics (lateral diffusivity, integral time scale, and integral space scale) were estimated for four selected regions with adequate drifter samplings. The lateral diffusivity had large values along the Kuroshio segment in the East China Sea and small values on the continental shelf of the East China Sea. The integral time scales for the four regions ranged from 0.8 to 3.7 days, with a corresponding integral space scale of 19~128 km

Observations of High-Frequency Internal Waves in the Southern Taiwan Strait

National Basic Research Program of China [2009CB21208]; National Natural Science Foundation of China [41006017, 40976013, 41049905]; Shanghai Oriental Scholar ProgramBased on in situ hydrographic measurements at a station at about 60 m depth near the local shelf break in July 2011 and a satellite image of the sea-surface roughness, we present the evidence of existence and characteristics of high-frequency internal waves (HIWs) in the southern Taiwan Strait. Variations of the thermohaline structure at the observational site were revealed by repeating conductivity-temperature-depth (CTD) profiling measurements of every 2 hours, while fluctuations of the isothermals due to the passage of HIWs were recorded by continuous CTD measurements with a probe positioned approximately in the middle of the pycnocline. A fast sampling frequency of 8 Hz allows the structure of the HIWs to be captured in great detail. The waves were depression waves, with a period of about 6.2 mm and an amplitude of about 25 m. The propagation speed of the waves is estimated to be 0.56 m/s by solving the KdV equation with the observed background stratification. The frequent occurrence of HIWs in the Taiwan Strait is evidenced by the analysis of a MODIS true color image of the sea-surface roughness. The possible generation mechanism of the HIWs is discussed based on a MODIS image and the perturbed KdV equation

Summertime sea surface temperature and salinity fronts in the southern Taiwan Strait

Based on hydrographic survey and satellite data, three fronts were observed in the southern Taiwan Strait in summer. They are the Taiwan Bank Front (TBF), the Southwest Coastal Upwelling Front (SCUF), and the Pearl River Plume Extension Front (PRPEF). In addition to the temperature aspect of TBF and SCUF as indicated in previous studies, we find that TBF and SCUF could also be identified according to salinity. The TBF is closely related to the Taiwan Bank upwelling, tidal mixing, and the Pearl River Plume Extension. Different hydrographic conditions on the southern and northern sides of this front lead to a south-north asymmetric structure of the TBF. The relatively small Simpson-Hunter number around the Taiwan Bank indicates that the TBF may be a tidal front. The SCUF separates the wind-driven cold, saline coastal upwelling water from the warm, less saline offshore water. Owing to frontal instability, SCUF exhibits both short temporal (several days) and small spatial (several kilometres) scales, indicative of intense sub-mesoscale processes. Nonetheless, the weak summertime SCUF was revealed in the log-transformed satellite frontal map. Finally, apart from the commonly observed dominant PRPEF in summer, a bifurcation from the PRPEF was identified in most hydrographic sections. Once generated, this bifurcation is carried by the topography-following current away from the PRPEF. ? 2014 Taylor & Francis