Subtidal Currents Research Articles

Modes of Weather System-Induced Flows through an Arctic Lagoon

With the increasing warming of the Arctic, the summertime ice-free period in the coastal Arctic becomes longer and the water exchange between arctic lagoons and coastal Beaufort Sea becomes more important for land–ocean interaction. This study examined the dynamics of water exchange between the arctic lagoons and the Arctic Ocean under the influence of weather systems (the transient arctic cyclones and hovering Beaufort High pressure system). We implemented rare observations, numerical modeling with the Finite Volume Community Ocean Model (FVCOM), and a forcing-response Empirical Orthogonal Function (fr-EOF) analysis to determine the weather-driven flow patterns and characteristics in the micro-tidal arctic lagoon (Elson Lagoon) with little freshwater discharge. The results were validated for both tidal and subtidal currents with in situ data. The inlets of the lagoon were significantly impacted by wind associated with the weather systems and the flows through the inlets were highly correlated with each other. The fr-EOF analysis for the 1.5-month FVCOM output indicated three significant modes of wind-driven flow. In the deepest (~16 m) northwestern-most inlet, a counter-wind flow occurred more than 96% of the time due to setup and set down of water level inside the lagoon and the vorticity balance related to the wind stress and water depth. For about 60–80% of the time, the exchange flow was out of the lagoon through the northwestern-most and deepest inlet due to the strong easterly winds dictated by the Beaufort High; this dominant flow is mainly caused by the persistent easterly wind as a limb of the Beaufort High pressure system, modified by the transient arctic cyclones with a westerly wind and inward flows at the westernmost inlet of Elson Lagoon. This study shows that the alternating influence from the cyclone-anticyclone weather systems produces a meteorological tide in the subtidal spectrum which dominates the water exchange in the region through the multiple inlets. With the observed increase in cyclone strength and frequency under the warming trend, this may imply a greater contribution from the westerly wind because of the increased cyclonic activities. If this is the case, the inward flow might increase and have an effect on sediment, larval, and nutrient transports through this system.

Modelling assessment of how small-scale vertical movements of infectious sea lice larvae can affect their large-scale distribution in fjordic systems

Sea lice are ectoparasites that can be found in high numbers in and around salmon farms, where they are a threat to fish health and can induce high aquacultural costs. Large numbers of suitable hosts facilitate the infection and subsequent release of their planktonic larvae in the surrounding environment where they can potentially infect wild salmonids, including migrating juvenile fish (smolts). Investigating sea lice spatial distribution is generally done using coupled hydrodynamic and particle tracking models. The quality of these numerical tools is critical to identify areas of higher infection risk to valuable wild salmon populations and thus support sustainable salmon aquaculture. While the transport of salmon lice is mainly affected by physical processes, biological behaviours such as vertical swimming can also play an important role. However, a review of previous sea lice studies shows no clear consensus on their swimming abilities and the parameters implemented in sea lice dispersion modelling. Here, we focus on the Diel Vertical Migration (DVM) behaviour, a vertical migration that sea lice perform within a daily cycle. Our sensitivity study of the infectious copepodid phase of sea lice highlights how their retention potential and spatial distribution within a water body are affected by their vertical swimming velocity and maximum swimming depth. In a fjordic system (Loch Linnhe on the West Coast of Scotland), the vertical position of sea lice can affect their horizontal trajectory due to the two-layer exchange flow of the estuarine circulation. At the surface, transport is mainly due to the wind driven circulation, and the residual seaward current. Lower in the water column, the saltier shelf water is drawn into the sea loch. This can potentially increase the retention of sea lice if they dive deep enough to reach those opposing subtidal currents. Therefore, lack of confidence in sea lice DVM parameterisation may introduce inaccuracy in modelled sea lice distribution. Effective sea lice management in aquaculture would benefit from more observational data like farm sea lice count, sea lice swimming laboratory observations, field observation of sea lice vertical distribution and accurate sea lice quantification in the field. This would help to reduce uncertainties derived from sea lice vertical swimming behaviour parameterisation in lice dispersal models.

Adjoint Data Assimilation of the Flow on the Southern Flank of Georges Bank: March–June 1999

An adjoint data assimilation method was incorporated into the ECOM-si coastal ocean circulation model and applied to assimilate the flow field on the southern flank of Georges Bank from March to June 1999. The model was driven by tidal forcing consisting of ten tidal constituents at the open boundary, observed winds, and surface heat fluxes. Numerical experiments were conducted following a strategy to understand the critical issues affecting the efficiency and accuracy of the assimilated flow field. The adjoint data assimilation method significantly improved the computational accuracy of subtidal currents, especially for the along-isobath velocity. The integration window length and iteration number were two parameters affecting the assimilation convergence rate toward the observations. In such a nonlinear dynamical system, using a window length close to the M2 tidal period could make the adjoint model difficult to converge, no matter how many iterations were made. Reducing the time interval for the window length speeded up the convergence rate, but it was paid out with the sacrifice of statistical confidence. This assimilation experiment used a 6-h window length, which led to a faster convergence rate in the first ten iterations. The best-assimilated fields that satisfied the error criteria were obtained as the iteration number increased.

A Circulation Model for Inlets Along the Central West Coast of Vancouver Island

ABSTRACT An ocean circulation model has been developed for the central west coast of Vancouver Island, with particular attention to the numerous inlets where salmon farms are located. The ultimate goal is to provide a tool to assist in informing decisions on the siting and management of fish farms and to further understanding of the regional coastal oceanography. The complicated coastline and bathymetry are approximated with an unstructured triangular grid in the horizontal direction and terrain-following coordinates in the vertical. The period of 1 March to 31 July 2016, is simulated using the Finite Volume Community Ocean Model and results are evaluated through comparisons with a combination of acoustic Doppler current profiler, tide gauge, and temperature and salinity observations. The model was found to be accurate in representing (i) tidal elevations and currents everywhere throughout the model domain and (ii) subtidal currents and temperature and salinity along the shelf. However, it fared poorly in inlets where (i) the tidal currents were relatively weak and not the dominant source of mixing and/or (ii) steep bathymetry necessitated smoothing. Sources of these and other inaccuracies, and the changes needed to overcome them, are discussed. Overall, this work highlights the complex and diverse dynamics at play in a glacially-carved coastal ocean, providing an extensive discussion of modelling challenges and potential solutions.

Interactions between Surface Waves, Tides, and Storm-Induced Currents over Shelf Waters of the Northwest Atlantic

A coupled wave–tide–circulation model is used to investigate wave–current interactions (WCIs) over the shelf waters of the Northwest Atlantic (NWA) during Hurricane Earl (2010). WCIs have substantial impacts on hydrodynamics in the upper ocean. The significant wave heights are modulated by WCIs, particularly over regions with strong current gradients, with a reduction up to ~2.1 m (20%) during the storm. Noticeable decreases in surface elevations and tidal currents occur in regions with strong tides such as the Gulf of Maine, mainly due to the wave-enhanced bottom stress. Over regions with weak tidal currents, wave effects on currents are dominated by two competitive processes between wave-induced forces and wave-enhanced mixing. The former strengthens surface currents (up to ~0.55 m/s) and increases the peak storm surge (up to ~0.48 m). The latter is responsible for the reduction in storm-induced surface currents (up to ~0.94 m/s) and anticyclonic modulation of current directions. Vertically, WCIs extend the strong vertical current shear and shift it downward during the storm, which enhances the local mixing and changes the structures of near-inertial oscillations (NIOs). Moreover, tidal currents also change the magnitudes of the NIOs and subtidal currents and affect the intensity of WCIs.

Responses of Coastal Upwelling to Tidally Induced Bottom Friction Dynamics and Plume-Modulated Geostrophy: A Process-Oriented Modeling Study

Abstract We used a high-resolution cross-shelf two-dimensional numerical model to investigate the response of coastal wind-driven upwelling circulation to barotropic tidal forcing and lateral buoyant discharge over a broad continental shelf. We found that the tidally amplified asymmetric friction effect arising from the interaction between tidal and subtidal currents modulated the upwelling structure across the shelf. The interaction weakened the water outcropping (upwelling) in the inner shelf due to tidally amplified mixing, but enhanced cross-shore velocity offshore due to tidally induced asymmetric friction effect and nonlinear advection. The enhanced mixing changed the density in the bottom boundary layer and subsequently in the upwelling front, which eventually weakened the geostrophic alongshore flow. The mass and stratification inputs of the lateral buoyant discharge weakened or even reversed geostrophic dynamics for alongshore and upslope transports. The reversed cross-shore density and elevation gradient induced by the buoyant influx weakened the alongshore current and the associated bottom friction effect. The upslope cross-shore transport was reduced due to weakened alongshore flow and the associated bottom Ekman transport. The mass of buoyant influx compensated for the wind-driven offshore transport in the upper layer. The upwelling could be reversed to downwelling when the transport of lateral influx exceeded the wind-driven offshore transport. The responses of upwelling circulation to tidal and lateral buoyancy forcing highlighted in this process-oriented study are fundamental for interpreting more complex wind-driven shelf circulation.

Numerical study of hydrodynamic effects on Manila clam population distribution and transport in the Southwest Laizhou Bay, China

Pelagic larvae of marine organisms are abundant on many continental shelves. Due to their importance as a biological resource and contribution to marine biodiversity, the dynamics of pelagic larvae have received wide attention. Numerical models are an efficient approach to assessing pelagic larval transport driven by various ocean dynamics. However, few studies have focused on the pelagic larval distribution of Manila clam populations and related dynamic factors in the southwestern Laizhou Bay (SLB) during late spring. A hydrodynamic model including realistic meteorologic and oceanic boundary conditions was coupled with a larval transport model to describe the pelagic larval distribution and transport processes. The hydrodynamic model was validated with observational and satellite-derived data. Effects of hydrodynamics and biological behavior on pelagic dynamics were further simulated with numerical experiments. During the early planktonic period, most clam larvae in the SLB were transported eastward or northeastward. Two potential larval settlement areas (LSAs) were recognized: the east side of the dike outside Guangli port and the Lao River estuary. The dominant controlling factor was southerly wind, and larvae's response to river plume or estuarine circulation played a secondary role. Larval transport depended on bottom subtidal currents when late-stage larvae settled near the bottom layer. The hydrodynamic and larval transport model established in this study can be applied to other bays and coastal waters. Understanding the clam larval distribution and transport dynamics in the SLB can provide a reference for predicting LSAs and biological resource management in coastal aquaculture areas.

The Effect of Regional Winds on Subtidal Along‐Inlet Currents at Oregon Inlet, NC

AbstractWind influence on tidal inlet hydrodynamics is examined using 40 days of wind, water level, and current observations collected in Spring 2019 at Oregon Inlet, NC, a large (1 km wide, 1–13 m deep) meso‐tidal inlet with complex delta systems. Wind velocities through the inlet (ranging 0–18 m/s) are modulated at subtidal timescales and are well correlated (R = 0.87) to a subtidal component of the water level slope through the inlet. The subtidal wind and water level slope are also well correlated to the subtidal current along the principal flow axis in the main inlet channel (R = 0.92 and 0.96, respectively). In combination with findings from previous studies, these findings suggest that regional winds induce the subtidal water level slope through the inlet by causing opposing setup/setdown to either side of the inlet. A force balance at the inlet demonstrates that the wind‐induced pressure gradient forces the subtidal currents, with wave forcing and local wind shear acting as lower‐order influences. The magnitude of the subtidal current is substantial, exceeding that of the tidal currents 45% of the time. Cumulatively, these findings indicate that regional winds exert a first‐order control on the currents at Oregon Inlet and cause irregular hydrodynamic patterns not well described by the traditional inlet classification scheme. Regional geographic characteristics may contribute to the high level of wind influence at Oregon Inlet, but similar processes are likely to be important to net flow dynamics at other inlets with large, shallow inland water bodies.

A High‐Resolution 3‐D Circulation Model in a Complex Archipelago on the Coastal Scotian Shelf

AbstractA high‐resolution coastal Finite‐Volume Community Ocean Model (FVCOM) has been configured to simulate the water circulation in the Eastern Shore Islands (ESI) archipelago on the coastal Scotian Shelf. Circulation in this area is characterized by complex interactions between the irregular coastline, a dense archipelago, tides, wind, and subtidal currents. Model outputs show close agreement with the tides, subtidal current, and hydrographic observations. Two circulation regimes within the study area were identified “inshore” and “offshore” of the 60‐m isobath. The balance between pressure gradient and Coriolis effect controls the dominant southwestward current (Nova Scotia Current) in the offshore. Rotary spectra and numerical experiments showed that the tidal current was an important component to the circulation in the inshore. Within the study area there are several clockwise and counter‐clockwise gyres related to the Coriolis effect, surface wind stress, pressure gradient, and the interaction between the irregular topography and current. The combined effects of topography, wind, tide, and Nova Scotia Current regulate the inshore circulation. The topographic features complex the cross‐shore transport. The increased bathymetric slope and strengthened relative vorticity contribute to the inshore cross‐shore current. Here, we found that the joint effect of baroclinicity and bottom relief drives the cross‐isobath transport. The ability of our model to resolve complex coastal circulation patterns provides an important basis on which to better understand how physical oceanography influences the unique ecological processes of this coastal archipelago and ultimately will help to evaluate dispersal and connectivity of coastal species in this area.

EOF Analysis for Wind-driven Along and Cross Shelf Exchange around a Sandbank over the Daesan Inner Shelf (Yellow Sea, Korea)

Ryu, H.-R.; Kim, S.-S.; Kim, D.-H.; Jang, W.-S., and Lee, S.-O., 2021. EOF analysis for wind-driven along and cross shelf exchange around a sandbank over the Daesan inner shelf (Yellow Sea, Korea). In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 141–145. Coconut Creek (Florida), ISSN 0749-0208. It is essential to understand the shelf circulation dynamics qualitatively and, if possible quantitively, since the inner shelf is an important zone located between the surf zone and the continental shelf in terms of the heat exchange, the transport of terrigenous sediment, and so on. In this study, from the compound results of current velocity profiles measured by the moored ADCP and wind observations obtained by KMA (Korea Meteorological Administration) over the Daesan inner shelf located in Yellow Sea, Korea, we found that the inner shelf circulation was mainly driven by a combination of wind stress, pressure gradient, Coriolis force and submarine topography. The Empirical Orthogonal Function (EOF) analysis was used to comprehend the shelf circulation dynamics induced by the wind stress and the wavelet coherence was used to analyze the time-frequency distribution of the coherence between currents, which was reconstructed by EOF method and shelf-aligned winds. The thickness of the surface and bottom frictional layers mainly depended on wind forcing and seabed topography. Also, it was found that the subtidal currents were strongly dominated along-shelf due to the irregular seabed topography such as a sandbank. The vertical structure of the along-shelf currents was mostly uniform regardless of water depth change. In mode 2 of EOF analysis, it was shown that the cross-shelf currents were polarized into two layers. Mostly wind in the ocean may cause down-welling or up-welling along the direction, however, in the study area, variation of wind induced current and the Ekman layer are not clearly distinguishable. Additionally, it is resulted that the cross-shelf exchange caused by the wind was not significant because of complicated seabed topography including Jangan sandbank and tidal force even though the Ekman depth was changed by the wind forcing in the Daesan inner shelf. From a result of wavelet coherence analysis, the correlation between wind and currents was also insignificant according to shelf-aligned U, V directions and water depths.

Subtidal to Supertidal Variability of Reynolds Stresses in a Midlatitude Stratified Inner Shelf

AbstractWe describe the spatiotemporal variability and vertical structure of turbulent Reynolds stresses (RSs) in a stratified inner shelf with an energetic internal wave climate. The RSs are estimated from direct measurements of velocity variance derived from bottom-mounted acoustic Doppler current profilers. We link the RSs to different physical processes, namely, internal bores, midwater shear instabilities within vertical shear events related to wind-driven subtidal along-shelf currents, and nonturbulent stresses related to incoming nonlinear internal wave (NLIW) trains. The typical RS magnitudes are O(0.01) Pa for background conditions, with diurnal pulses of O(0.1–1) Pa, and O(1) Pa for the NLIW stresses. A NLIW train is observed to produce a depth-averaged vertical stress divergence sufficient to accelerate water 20 cm s−1 in 1 h, suggesting NLIWs may also be important contributors to the depth-averaged momentum budget. The subtidal stresses show significant periodic variability and are O(0.1) Pa. Conditionally averaged velocity and RS profiles for northward/southward flow provide evidence for downgradient turbulent momentum fluxes, but also indicate departures from this expected regime. Estimates of the terms in the depth-averaged momentum equation suggest that the vertical divergence of the RSs are important terms in both the cross-shelf and along-shelf directions, with geostrophy also present at leading-order in the cross-shelf momentum balance. Among other conclusions, the results highlight that internal bores and shoaling NLIWs may also be important dynamical players in other inner shelves with energetic internal waves.

Moored Observations of Transport and Variability of Halmahera Sea Currents

AbstractThe ocean currents in the Halmahera Sea are studied using a subsurface mooring deployed in the Jailolo Strait from November 2015 to October 2017. The subtidal currents of the mooring measurements are characterized by a two-layer system, with the current variability below about 200 m in opposite phases to that in the upper layer. The mean along-strait velocity (ASV) is toward the Indonesian seas in the whole water column, producing an estimated mean transport of 2.44 ± 0.42 Sv (1 Sv ≡ 106 m3 s−1). The errors of the transport calculation based on the single mooring measurements are estimated to be less than 15% using simulations of high-resolution ocean models. A weak current is observed to flow northward during 2017 at the bottom of the strait. The ASV variability is found to be dominated by an annual cycle both in the upper and lower layers. The total transport, however, is dominated by semiannual variability because of the cancelation of the annual transports in the upper and lower layers. The variability of the transport is suggested to be driven by the pressure difference between the Pacific Ocean and the Indonesian seas, as evidenced by the agreement between the satellite pressure gradient and the two-layer transports. The transport of the Jailolo Strait during the 2015/16 super El Niño is found to be nearly the same as that during the 2016 La Niña, suggesting that the interannual variability of the transport is much smaller than the seasonal cycle.

Flow and Sediment Flux Asymmetry in a Branching Channel Delta

AbstractRiver deltas form complex branching patterns that distribute sediment to the coastal sea. The routing and storage of this sediment in deltas are poorly understood. We present results of a 1‐month study of the sediment and water transport through two branches of the Wax Lake Delta on the coast of Louisiana. The two channels maintained near‐equal total partitioning of flow and sediment discharge. East Pass was narrower and had higher tidally averaged velocities, lower tidal velocity fluctuations, more sediment flux, and less alluvial bed cover than Main Pass. We connected these differences to small differences in the geometry of the channels and feedbacks between these differences. East Pass was slightly shorter than Main Pass, leading to a steeper mean free surface profile, yielding higher subtidal currents and advective sediment fluxes. Main Pass received the tides from Atchafalaya Bay earlier than East Pass, allowing tides to reach their junction earlier from Main Pass. This led to Main Pass capturing tidal prism from East Pass and higher tidal velocity fluctuations in Main Pass. These fluctuations caused larger flow convergences in Main Pass, pumping sediment out of East Pass on tidal timescales. Since East Pass had higher mean velocities and lower tidal fluctuations, it spent less time in flow regimes that would allow the formation of alluvial bed cover. By explaining these asymmetries with physical arguments, we showed how geometric differences drive asymmetries in branching systems. Barrier island inlets, coastal lagoons, and other systems may display similar behavior.

Observed cross-shelf suspended sediment flux in the southern Yellow Sea in winter

The cross-shelf circulation and its transport in the Yellow and East China Seas (YECSs) are very important to global ocean material exchanges and regional environment. A field survey including a long-term mooring, 89 sample stations, 25 drifters, etc., was conducted in the YECSs in the winter of 2015 to investigate the cross-shelf transport of the sediment plume over the Changjiang Bank. The observation shows that the northern East China Sea (ECS) current carries low temperature, low salinity, and high turbidity coastal waters southeastward from the radial tidal sand ridge off the Jiangsu coast to the outer shelf south of Cheju Island. The northern ECS current with an average speed of 4.7 cm/s and in the average direction of 100° from due north is not only measured directly by the 21-day mooring, but also recorded in trajectories of 6 satellite-tracked surface drifters. The mean current of the northern ECS current flows roughly along the 30–50 m isobaths with a small cross-isobath component. The mooring data indicate that the sediment flux is dominated by the mean advection of mean sediment concentration, resulting in an average suspended sediment flux of 52.23 g s−1 m−1 in the direction of 105°. According to the suspended sediment flux decomposition, the flux due to tidal currents is calculated to be one order of magnitude smaller than the advection by subtidal currents, implying that the sediment plume is unlikely generated by the tidal pumping, but is likely maintained by the advection. The amount of the suspended sediments carried by the northern ECS current is estimated to reach 40.62 million tons assuming an average width of 100 km of the turbid plume during December through February. The transport by the northern ECS current is suggested to provide sufficient amount of sediments for the Southwestern Cheju Island Mud to grow in time.

Impacts of subtidal motions and the earth rotation on modal characteristics of the semidiurnal internal tide

Subtidal motions, such as large-scale circulations and mesoscale eddies, frequently occupy on the propagation path of internal waves in the ocean. Through solving a modified Taylor–Goldstein equation with subtidal currents, density stratification, and the earth rotation, this study investigates the impacts of subtidal motions and the earth rotation on the modal characteristics of the semidiurnal internal tide (SIT) based on long-term mooring measurements in the northern South China Sea (SCS). It is shown that the modal characteristics of the SIT are significantly influenced by the time-varying subtidal current and density stratification associated with energetic subtidal motions in the northern SCS. The earth rotation plays a minor role in modulating the modal characteristics of the SIT in the northern SCS, but it becomes effective at middle and high latitudes when strong subtidal current shear associated with subtidal motions is present. Moreover, the strong subtidal current associated with subtidal motions may significantly affect the critical latitude of the SIT. The observational results reported here will help to improve our understanding of the modal characteristics of internal tides under the influence of subtidal motions in global oceans.

Phytoplankton bloom dynamics in turbid, well-mixed estuaries: A model study

To gain insight into mechanisms underlying phytoplankton bloom dynamics in turbid, well-mixed estuaries, experiments were conducted with an exploratory model that couples physical and biological processes. The motivation for choosing exploratory models is that they allow investigation of individual processes in isolation, therefore yielding fundamental insight into the mechanisms of the system. The Ems estuary (between the Netherlands and Germany) was selected as the prototype estuary, in which a zone of high turbidity is observed in the middle and upper reach. Results show that the model is capable of capturing the main features of the observed phytoplankton population density (P) patterns, that is, in the lower reach a spring bloom occurs, followed by a secondary bloom in autumn. Results of sensitivity studies reveal that the along-estuary distribution of suspended particulate matter (SPM) is a determining factor for the along-estuary location of blooms and it largely affects the intensity of blooms. The along-estuary advection of nutrients by the subtidal current is important for obtaining blooms with high intensities. In this model, the seasonally varying water temperature has a larger impact on the timing of spring blooms than the seasonally varying incident light intensity. The occurrence of an autumn bloom is due to the fact that during the summer season, the net specific growth rate of phytoplankton decreases. The latter is likely to result from an optimum water temperature (smaller than the maximum water temperature) for phytoplankton growth. Nevertheless, the occurrence and characteristics of autumn blooms are also influenced by seasonal variations in other aspects, for instance, loss of phytoplankton due to grazing.

Tropical Cyclone‐Driven Sediment Dynamics Over the Australian North West Shelf

AbstractOwing to their strong forcing at the air‐sea interface, tropical cyclones are a major driver of hydrodynamics and sediment dynamics of continental shelves, strongly impacting marine habitats and offshore industries. Despite the North West Shelf of Australia being one of the most frequently impacted tropical cyclone regions worldwide, there is limited knowledge of how tropical cyclones influence the sediment dynamics of this shelf region, including the significance of these episodic extreme events to the normal background conditions that occur. Using an extensive 2 year data set of the in situ sediment dynamics and 14 yearlong calibrated satellite ocean‐color data set, we demonstrate that alongshore propagating cyclones are responsible for simultaneously generating both strong wave‐induced sediment resuspension events and significant southwestward subtidal currents. Over the 2 year study period, two particular cyclones (Iggy and Narelle) dominated the sediment fluxes resulting in a residual southwestward sediment transport over the southern part of the shelf. By analyzing results from a long‐term (37 year) wind and wave hindcast, our results suggest that at least 16 tropical cyclones had a strong potential to contribute to that southwestward sediment pathway in a similar way to Iggy and Narelle.

The spatio-temporal variation of wintertime subtidal currents in the western Taiwan Strait

A new data set of observations by six cruises of ship-mounted acoustic doppler current profiler (SADCP) and three 40 d long bottom-mounted ADCPs (BADCPs) is employed to reveal the spatiotemporal variability of tidal and subtidal currents in the western Taiwan Strait (TWS) during winter season. The results confirm the existence of intense cotidal lines for M2 tidal current, which is located north of 25°N. In this case, no existence of an amphidromic point can be identified. It is also revealed that the counter-wind current (CWC) can extend through the whole western TWS and even occupy the entire water column during winter monsoon relaxation. However, this CWC is observed to be thoroughly overwhelmed by the downwind China coastal current (CCC) during the two big monsoon bloom events in the winter of 2007, and the CCC consequently extends southward throughout the western TWS instead. Most importantly, the variation of the spatial extent for the CWC and the CCC in the western TWS is found to be well explained by the first two modes of the vector empirical orthogonal function (VEOF) analysis, that is, it is mainly controlled by a wind-driven quasi barotropic current as the first mode and slightly modulated by a relatively weak background current with a first-order baroclinic structure as the second mode.

Lateral variability of subtidal flow at the mid‐reaches of a macrotidal estuary

AbstractTransverse variations of tidal and subtidal flow were investigated in a macrotidal and convergent estuary. This was accomplished by combining data analysis of current velocities and water density with numerical modeling at the mid‐reaches of the Gironde Estuary (France). Nonlinear mechanisms responsible for overtide generation and hence subtidal flows were found to vary across the estuary and from neap to spring tides. Subtidal flows were driven by a combination of internal asymmetry, tidal advective accelerations, nonlinear effects of water level variations, quadratic friction, and river discharge. The quarter‐diurnal overtide band (D4) in the flow was generated by internal asymmetry and tidal advective accelerations during neap tide. The ratio of quarter‐diurnal to squared semidiurnal bands (D4/D22) was largest (>0.3) in sections of the channel showing subtidal outflow. River discharge increased from neap to spring tides causing a subsequent increase of seaward subtidal currents. During spring tide, D4 was generated by tidal advective accelerations and quadratic friction combined with river discharge, rather than by internal asymmetry. The sixth‐diurnal overtide (D6) in the flow was comparable to D4 for both neap and spring tides. Largest D6/D23 ratios were found in the shallowest cross‐channel locations during both neap and spring tides.

Quantifying the Effect of Salinity Stratification on Phytoplankton Density Patterns in Estuaries

To quantify the effect of salinity stratification on phytoplankton density (denoted as P) patterns, experiments were conducted with an idealised model that couples physical and biological processes. Results show that the idealised model is capable of capturing the main features of observed P patterns in the Columbia River estuary during the spring season: during weak stratification, P is almost vertically uniform with values decreasing towards the estuary mouth, whereas during strong stratification, high values of P extend further seawards but are confined to the upper layer. Sensitivity studies reveal that the strong vertical gradients of P can only occur if the intensity of turbulence (measured by depth-averaged values of vertical eddy viscosity and eddy diffusivity) is weak. The advection of P by subtidal currents is important in obtaining a smaller along-estuary gradient of P during weak stratification and in obtaining a smaller horizontal gradient and a larger vertical gradient of P during strong stratification. Accounting for stratification controlled vertical distribution of vertical eddy viscosity and eddy diffusivity is necessary for obtaining realistic P patterns if stratification is strong, but not if stratification is weak. A higher osmotic stress, which leads to faster loss of phytoplankton in salt water, results in a larger along-estuary gradient of P if stratification is weak and in a larger vertical gradient of P if stratification is strong.