Influence of the Oyashio Current and Tsugaru Warm Current on the circulation and water properties of Otsuchi Bay,Japan

Mooring and hydrographic observations were conducted from September 2012 to May 2014 at the mouth of Otsuchi Bay, a ria along the Pacific coast of Japan. Our observations quantitatively demonstrated that the circulation and the water properties of Otsuchi Bay are strongly influenced by the Tsugaru Warm Current (TWC) and Oyashio Current (OY) at seasonal and subseasonal time scales. Two bottom-mounted velocity profilers and temperature and salinity measurements beneath the near-surface halocline showed a counterclockwise lateral circulation pattern related to the TWC, which was enhanced from summer to autumn. From winter to early spring, the lateral circulation patterns related to the TWC weakened and the influence of the OY occasionally increased. When the OY was weak, surface flows became an overturning structure, with outflows in the upper layer and inflows in the lower layer. When the OY was strong and passed close to the Sanriku coast, the circulation became highly variable and intermittent. Intrusions of the markedly low-salinity OY water were observed on two occasions and persisted for periods of several weeks to several months. Salinity was sometimes less than 33.7, the lower limit of the typical TWC from late summer to autumn even when the TWC dominates. We suggested that this is the seasonal fluctuations of the TWC itself, as the upstream current of the Tsushima Warm Current is freshened in summer as a result of the influence of the Changjiang River. The surface water was generally fresher in the south of the bay than in the north, suggesting the Coriolis deflection of the river plume.     

Baroclinic circulation and its high frequency variability in Otsuchi Bay on the Sanriku ria coast,Japan

Hydrographic observations were made in Otsuchi Bay on the Sanriku ria coast, Japan, to provide clear images of the baroclinic circulation extending over the bay together with the associated intrusion of lower-layer water (bottom water) from outside the bay. In summer, a prominent baroclinic circulation with flow speeds \({>} 0.1\ \text{ m }\ \text{ s }^{-1} \) extends over the greater part of the bay. A main pycnocline (thermocline), which separates the upper and lower layers, is located at a depth of 15–40 m in and around the bay. The direction of the lower-layer flow (inflow into and outflow from the bay) is opposite to that of the upper-layer flow, which are baroclinically coupled to each other. Moreover, with regard to the lower-layer flow, the inflow tends to occur mainly through the northwestern part of the bay mouth, whereas the outflow tends to occur mainly through the southeastern part. The inflow and outflow alternate on time scales of several to a few tens of hours, and the flow directions are sometimes related to the tidal ones, although the relationship is not applied persistently. In winter, the baroclinic circulation is considerably weaker than in summer, because the stratification breaks down.     

Seasonal variation of residual current in Tokyo Bay,Japan —diagnostic numerical experiments—

The residual currents in Tokyo Bay during four seasons are calculated diagnostically from the observed water temperature, salinity and wind data collected by Unokiet al. (1980). The calculated residual currents, verified by the observed ones, show an obvious seasonal variable character. During spring, a clear anticlockwise circulation develops in the head region of the bay and a strong southwestward current flows in the upper layer along the eastern coast from the central part to the mouth of the bay. During summer, the anticlockwise circulation in the head region is maintained but the southwestward current along the eastern coast becomes weak. During autumn, the preceding anticlockwise circulation disappears but a clockwise circulation develops in the central part of the bay. During winter, the calculated residual current is similar to that during autumn. As a conclusion, the seasonal variation of residual current in Tokyo Bay can be attributed to the variation of the strength of two eddies. The first one is the anticlockwise circulation in the head region of the bay, which develops in spring and summer and disappears in autumn and winter. The second one is the clockwise circulation in the central part of the bay, which develops in autumn and winter, decreases in spring and nearly disappears in summer.     

A three-dimensional numerical study of river plume mixing processes in Otsuchi Bay,Japan

The three-dimensional numerical model SUNTANS is applied to investigate river plume mixing in Otsuchi Bay, an estuary located along the Sanriku Coast of Iwate, Japan. Results from numerical simulations with different idealized forcing scenarios (barotropic tide, baroclinic tide, and diurnal wind) are compared with field observations to diagnose dominant mixing mechanisms. Under the influence of combined barotropic, baroclinic and wind forcing, the model reproduces observed salinity profiles well and achieves a skill score of 0.94. In addition, the model forced by baroclinic internal tides reproduces observed cold-water intrusions in the bay, and barotropic tidal forcing reproduces observed salt wedge dynamics near the river mouths. Near these river mouths, vertically sheared flows are generated due to the interaction of river discharge and tidal elevations. River plume mixing is quantified using vertical salt flux and reveals that mixing near the vicinity of the river mouth, is primarily generated by the barotropic tidal forcing. A 10 ms^?1 strong diurnal breeze compared to a 5 ms^?1 weak breeze generates higher mixing in the bay. In contrast to the barotropic forcing, internal tidal (baroclinic) effects are the dominant mixing mechanisms away from the river mouths, particularly in the middle of the bay, where a narrow channel strengthens the flow speed. The mixing structure is horizontally asymmetric, with the middle and northern parts exhibiting stronger mixing than the southern part of the bay. This study identifies several mixing hot-spots within the bay and is of great importance for the coastal aquaculture system.     

Phytoplankton community structure in Otsuchi Bay,northeastern Japan,after the 2011 off the Pacific coast of Tohoku Earthquake and tsunami

The tsunami caused by the 2011 off the Pacific coast of Tohoku Earthquake seriously damaged the Pacific coast of northeastern Japan. In addition to its direct disturbance, a tsunami can indirectly affect coastal pelagic ecosystems via topographical and environmental changes. We investigated seasonal changes in the phytoplankton community structure in Otsuchi Bay, northeastern Japan, from May 2011, which was 2 months after the tsunami, to May 2013. The phytoplankton species composition in May 2011 was similar to that observed in May 2012 and 2013. The present results are consistent with the dominant species and water-mass indicator species of phytoplankton in past records. These results suggest that there was no serious effect of the tsunami on the phytoplankton community in Otsuchi Bay. Community analysis revealed that two distinct seasonal communities appeared in each year of the study period. The spring–summer community was characterized by warm-water Chaetoceros species, and dinoflagellates appeared from May to September. The fall–winter community was characterized by cold neritic diatoms, which appeared from November to March. The succession from the spring–summer community to the fall–winter community took place within a particular water mass, and the fall–winter community appeared in both the surface water and the Oyashio water mass, suggesting that water-mass exchange is not the only factor that determines the phytoplankton community structure in Otsuchi Bay.     

Swell-dominant surface waves observed by a moored buoy with a GPS wave sensor in Otsuchi Bay,a ria in Sanriku,Japan

Real-time monitoring of wind and surface waves in Otsuchi Bay, Iwate, Tohoku, Japan, commenced in October 2012, using a mooring buoy with an ultrasonic anemometer and a single-mode GPS wave sensor. Wind and wave data are distributed hourly in real time via the Internet along with a chart of their time series. We analyzed data monitored in the first 3 months in order to assess the variability and occurrence of wind and waves and to elucidate the main reasons for wave variation in Otsuchi Bay. The monitoring data revealed that surface waves in the bay were predominantly affected by swells propagated from the northeastern offshore region and that the wave height was significantly correlated with the component of wind velocity toward Otsuchi Bay in the northeastern offshore region that faces the bay mouth. The offshore wind field was expected to provide information useful for predicting coastal waves in a ria bay in Sanriku such as Otsuchi Bay. However, it should be emphasized that the horizontal distribution of the offshore wind field which has a significant effect on the surface waves in a ria bay depends heavily on the topographic shape of the bay.     

象山港水交换特性研究

在验证良好的三维斜压潮流数学模型的基础上,以溶 解态的保守性物质为示踪剂,建立对流-扩散型的海湾水交换数值模型,计算了象山港水体半交换时间和平均滞留时间,并研究了斜压动力对湾内外水交换的贡献。研究结果表明,象山港水交换速度的区域性变化较大,水体半交换时间和平均滞留时间由象山港口门向湾顶逐渐增加,口门附近半交换时间在5d以内,平均滞留时间为5~10 d;湾顶水交换速度缓慢,水体半交换时间为30~35 d,平均滞留时间为35~40d。斜压动力对狭湾外段水交换影响较弱,对狭湾内段有较大的影响。     

Fortnightly shifts of intrusion depth of oceanic water into Ise Bay

To clarify the time change in water exchanges between Ise Bay and the adjacent ocean, repeated hydrographic observations were conducted along the longitudinal section in Ise Bay. The results show that the mixing condition at the bay mouth (Irago Strait) changed fortnightly in summer. During the spring tides, the strait water below the pycnocline was well-mixed and nearly homogeneous. By contrast, it was weakly stratified during the neap tide. There is a strong negative correlation between the tidal range and the density difference between the upper and lower layers at the strait. In summer, the intrusion depth of oceanic water into the bay and consequent hydrographic conditions inside the bay changed frequently according to the tidal strength. During the spring tides a prominent bottom front was created at the bay mouth, indicating that the strait water, which is a mixture of oceanic and bay waters, intruded through the middle layer. On the other hand, during the neaps, cold and saline oceanic water intruded through the bottom layer into the bay. The intrusion depth is significantly correlated with the tidal range. It is considered that the wellmixed strait water, which has a density equivalent to the middle layer inside the bay, is lighter than the bottom bay water and thus intrudes through the middle layer during the spring tides, while insufficient mixing makes the bottom water at the strait heavier than the bay water, leading to the bottom intrusion during the neap tides.     

Wind- and density-driven circulation in a bay on the Sanriku ria coast,Japan: study of Shizugawa Bay facing the Pacific Ocean

Flow fields in Shizugawa Bay on the Sanriku ria coast, which faces the Pacific Ocean, were investigated using hydrographic observations for the purpose of understanding oceanographic conditions and the process of water exchanges in the bay after the 2011 earthquake off the Pacific coast of Tohoku. In spring to summer, density-driven surface outflow is part of estuarine circulation and is induced by a pressure gradient force under larger longitudinal gradients in density along with lower salinity water in the innermost part of the bay, regardless of wind forcing. In winter to summer, another density-driven current with a thermal structure is induced by a pressure gradient force under the smaller longitudinal density gradients in calm wind conditions. Particularly in winter, Tsugaru Warm Current water can be transported in the surface layer inside the bay. Wind-driven bay-scale circulation with downwind and upwind currents in the surface and deeper layers, respectively, is induced by strong longitudinal wind forcing under the smaller longitudinal density gradients, irrespective of season. Particularly in fall to spring, this circulation can cause the intrusions of oceanic water associated with Oyashio water and Tsugaru Warm Current water in the deeper layer. These results suggest that wind- and density-driven currents can produce the active exchange of water from inside and outside the bay throughout the year.     

Effect of channel bifurcation on residual estuarine circulation: Winyah Bay, South Carolina

The residual circulation pattern of Winyah Bay, the fourth largest estuary on the eastern coast of the US, is examined using stationary and shipborne current measurements during periods of low freshwater discharge. The estuary has a complex morphology with a single channel and narrow banks at the river entrance and the bay mouth, and a bifurcated channel system (main and western channels, respectively) in the middle part that appears to affect the residual circulation.Overall, the upper (single channel morphology) and middle (dual-channel morphology) parts of the estuary exhibit a baroclinic residual circulation. The presence of bifurcated channels in the middle part of the estuary modifies the typical gravitational circulation. The near-bed landward-directed residual flow is stronger in the deeper main channel than the shallower western channel. This is the result of the fact that the magnitude of residual flow scales with the water depth of the channel and it is also influenced by the opposing patterns of channel alignment in the northern and southern junctions. Analytical modeling confirms that the observed residual currents in the upper and middle estuary are density-induced. In the lower estuary, residual flow is directed seaward throughout the water column of the channel while in the adjacent shoals the residual flow is directed landward, suggesting that in contrast to the upper and middle estuary, the residual flow near the mouth is barotropic, controlled by the tides and the channel-bank morphology.     

潮汐驱动下的辽东湾水动力及入海污染物输移特征

通过建立水动力学模型、物质输运模型和年龄模型,对渤海辽东湾潮汐驱动下的水动力状况和污染物输移扩散过程进行了数值模拟研究。结果表明,辽东湾海域入海径流对辽东湾整体流场和水交换过程影响不大,其主要影响集中在河口附近海域。在潮汐的驱动下,辽东湾内形成了复杂的环流结构,辽东湾南北海域分别存在顺时针、逆时针的环流,而辽东湾湾口又存在逆时针环流,使得水交换能力较弱,对辽东湾向外海的物质输运产生不利影响,湾顶附近海域的物质主要通过扩散过程与外海进行交换。年龄模型的计算结果表明,辽东湾河流入海污染物在河口附近停留时间较长,向远区的输运需要较长时间。入海污染物的影响具有局地性,对局部海域水质尤其是辽东湾湾顶的水质会产生不利影响。     

Simulation of Pollutant Transport in Marmaris Bay

The circulation pattern and the pollutant transport in the Marmaris Bay are simulated by the developed three-dimensional baroclinic model. The Marmaris Bay is located at the Mediterranean Sea coast of Turkey. Since the sp ring tidal range is typically 20- 30 cm, the dominant forcing for the circulation and water exchange is due to the wind action. In the Marmaris Bay, there is sea outfall discharging directly into the bay. and that threats the bay water quality significantly. The current patterns in the vicinity of the outfall have been observed by tracking drogues which are moved by currents at different water depths. In the simulations of pollutant transport, the coliforms-counts is used as the tracer. The model provides realistic predictions for the circulation and pollutant transport in the Marmaris Bay. The transport model component predictions well agree with the results of a laboratory model study.     

南海北部东沙岛附近的内潮和余流特征

采用东沙岛附近的一个长达9个月的锚定潜标的观测资料对南海北部的正压潮、内潮和余流情况进行了分析,得到了当地正压潮和内潮的特征。此处正压潮流以全日潮为主,秋、冬季相对较大,春季相对较小;正压余流受海盆尺度环流和地形的限制,在潜标观测期间的秋、冬、春三季基本以偏西向的正压流为主。内潮同正压潮一样,也以全日分潮为主,潮流椭圆随水深发生旋转,在110—120m附近存在内潮非常弱的一层。斜压余流在2009年2—3月比较异常,这是由于在此其间有一个中尺度涡经过。对此潜标数据采用经验正交函数分解的方法进行分析,发现海流的各个主要EOF模态与内波的垂向模态结构有一定的关联。     

象山港内湾潮汐应变对横向流及其余环流垂向空间结构的调控研究

横向流 (垂直于海湾主轴方向的流动) 对横向动量以及物质分布具有重要影响。已往研究表明,潮汐应变对横向流的垂向空间结构具有重要的调控作用。但这种认识仅局限于强层化海区,弱层化条件下潮汐应变对于横向流空间结构的影响仍未可知。为此,本文以象山港为例,基于实测数据阐释了弱层化条件下潮汐应变对横向流及其余环流垂向空间结构的调控作用。结果显示,象山港内湾横向流的垂向空间结构随大小潮呈现出明显的变化规律。大潮时,潮汐应变现象明显,涨潮时较强的垂向混合使得横向流在高潮阶段呈现出一层结构;落潮时垂向混合较弱,横向流在低潮时呈现两层结构。小潮时,潮汐应变受到抑制,垂向混合在涨落潮时均较弱,因此横向流在高低潮阶段均呈现出两层结构。经过潮时均进一步得到的横向余环流呈现出上层向南、下层向北的两层结构。由于潮汐应变的大小潮变化,横向余环流的反转深度反转点自大潮到小潮呈现出上升的趋势。     

Numerical experiments on wind-driven circulations and associated transport processes in Suruga Bay

The dynamics of the wind-driven circulations and surface transport processes in Suruga Bay have been examined by performing numerical experiments. While strong winds exist outside the bay, the winds inside the bays are greatly reduced, which generates a strong wind stress curl in winter and autumn. In particular, in winter, a strong positive curl region is located across the bay mouth, and a strong surface circulation with counterclockwise rotation is generated beneath it. The circulation is nearly geostrophic, but is not affected by the bottom topography in the deep bay. It is suggested that intense surface water exchange through the bay mouth occurs in winter, whereas it is not active in the other seasons when no significant vorticity is supplied on the bay mouth from the atmosphere. Moreover, we propose a hypothesis that the atmospheric wind stress curl will cause the frequent appearance of the counterclockwise circulation in winter in the real ocean.     

Wind-dependent formation of phytoplankton spring bloom in Otsuchi Bay,a ria in Sanriku,Japan

Spring blooms of phytoplankton composed of centric diatoms developed in late February, March, and April in Otsuchi Bay on Sanriku ria coast, Japan. During this period, associated with prolonged seasonal west wind (>1 day), intense exchange of waters occurred between inside and outside the bay: outflow of nearsurface brackish water over inflow of oceanic water at depth. This circulation interrupted formation of the blooms, and transported phytoplankton populations seaward. By such water movements, a significant amount of nutrients in the bay was carried out, otherwise replenished into the bay, depending on water masses located outside the bay. Owing to irregular features of wind events, a bloom lasted from several days to a week. From February to April, supply of nutrients seemed to be replete except for the latter half of the bloom period, and estimates of the critical depth exceeded the depth of the bottom consistently. Thus, net growth of phytoplankton was expected throughout the observation period, and potentially blooms could be formed. However, the blooms were only formed under calm weather. We hypothesize that the exchange of waters dilutes populations in the bay, and that formation of the bloom, that is, accumulation of biomass depends on a balance between the growth of phytoplankton and the dilution of bay water.     

A Massive Coccolithophorid Bloom Observed in Mikawa Bay, Japan

In April 1996, a massive algal bloom of the coccolithophorid Gephyrocapsa oceanica developed in both Chita Bay and Atsumi Bay which comprise the bay known as Mikawa Bay of Japan. It was the first record of such a bloom in this area. In Chita Bay, the bloom persisted until the middle of May, however in Atsumi Bay, it remained until early June. From the analysis of salinity, water temperature, and current velocity and direction data, it is considered that the following mechanism accounts for the occurrence and maintenance of the bloom: Before the bloom, the standing crop of phytoplankton was poor, resulting in relatively rich nutrients throughout the bay. Thereafter, with the influx of oceanic water into Mikawa Bay, high salinity occurred firstly in Chita Bay. Under these hydrographic conditions, the bloom occurred first in Chita Bay, and extended throughout the bay with the clockwise circulation of water into Atsumi Bay. In Chita Bay, the bloom was influenced by rainfall and G. oceanica flowed out from this area. Whereas, in Atsumi Bay, the bloom persisted for longer due to the clockwise circulation and another influx of oceanic water.     

Numerical study of baroclinic tides in Luzon Strait

The spatial and temporal variations of baroclinic tides in the Luzon Strait (LS) are investigated using a three-dimensional tide model driven by four principal constituents, O₁, K₁, M₂ and S₂, individually or together with seasonal mean summer or winter stratifications as the initial field. Barotropic tides propagate predominantly westward from the Pacific Ocean, impinge on two prominent north-south running submarine ridges in LS, and generate strong baroclinic tides propagating into both the South China Sea (SCS) and the Pacific Ocean. Strong baroclinic tides, ∼19 GW for diurnal tides and ∼11 GW for semidiurnal tides, are excited on both the east ridge (70%) and the west ridge (30%). The barotropic to baroclinic energy conversion rate reaches 30% for diurnal tides and ∼20% for semidiurnal tides. Diurnal (O₁ and K₁) and semidiurnal (M₂) baroclinic tides have a comparable depth-integrated energy flux 10–20 kW m⁻¹ emanating from the LS into the SCS and the Pacific basin. The spring-neap averaged, meridionally integrated baroclinic tidal energy flux is ∼7 GW into the SCS and ∼6 GW into the Pacific Ocean, representing one of the strongest baroclinic tidal energy flux regimes in the World Ocean. About 18 GW of baroclinic tidal energy, ∼50% of that generated in the LS, is lost locally, which is more than five times that estimated in the vicinity of the Hawaiian ridge. The strong westward-propagating semidiurnal baroclinic tidal energy flux is likely the energy source for the large-amplitude nonlinear internal waves found in the SCS. The baroclinic tidal energy generation, energy fluxes, and energy dissipation rates in the spring tide are about five times those in the neap tide; while there is no significant seasonal variation of energetics, but the propagation speed of baroclinic tide is about 10% faster in summer than in winter. Within the LS, the average turbulence kinetic energy dissipation rate is O(10⁻⁷) W kg^{− 1} and the turbulence diffusivity is O(10⁻³) m²s⁻¹, a factor of 100 greater than those in the typical open ocean. This strong turbulence mixing induced by the baroclinic tidal energy dissipation exists in the main path of the Kuroshio and is important in mixing the Pacific Ocean, Kuroshio, and the SCS waters.     

Tidal truncation and barotropic convergence in a channel network tidally driven from opposing entrances

Residual circulation patterns in a channel network that is tidally driven from entrances on opposite sides are controlled by the temporal phasing and spatial asymmetry of the two forcing tides. The Napa/Sonoma Marsh Complex in San Francisco Bay, CA, is such a system. A sill on the west entrance to the system prevents a complete tidal range at spring tides that results in tidal truncation of water levels. Tidal truncation does not occur on the east side but asymmetries develop due to friction and off-channel wetland storage. The east and west asymmetric tides meet in the middle to produce a barotropic convergence zone that controls the transport of water and sediment. During spring tides, tidally averaged water-surface elevations are higher on the truncated west side. This creates tidally averaged fluxes of water and sediment to the east. During neap tides, the water levels are not truncated and the propagation speed of the tides controls residual circulation, creating a tidally averaged flux in the opposite direction.