影响坦帕湾水交换的三种因素

本文利用高分辨率数值模型,以2001年秋季为例,详细分析了影响坦帕湾水交换的三种因素:潮汐、河流和风。论文共设置了三组实验,驱动力分别为潮汐,潮汐和河流,潮汐、河流和风。模拟结果显示:只有潮汐作用时,由于坦帕湾潮汐较弱,潮程较短,坦帕湾与其临近海域的水交换主要发生在湾口附近;当潮汐和河流共同作用时,由于河流和湾口海水盐度的不同形成了水平密度梯度,在其产生的水平密度梯度力的作用下,坦帕湾形成了表层流向湾外、底层流向湾内的重力环流,从而加强了坦帕湾跟其临近海域的水交换;由湾内指向湾外方向(2001年秋季平均)的风应力加强了流向湾外的表层流,同时水位梯度力发生了反转,变成了由湾口指向湾顶,这加强了流向湾内的底层流,表层流和底层流的加强最终促进了坦帕湾跟其临近海域的水交换;在航道处,水深较深瑞利数较大,该处的重力环流较强,这使得相对于两侧的浅水区,航道处的水交换能力较强。此外,文章还分析了坦帕湾水交换的空间差异,在Old Tampa Bay的西侧和北侧,滞留时间最长,水交换能力最弱。为减少海洋生态灾害发生,今后应重点加强对该地区的生态环境保护。     

珠江口磨刀门水道盐度变化对水文气象要素的频率响应特征

以2010年11月-2011年3月磨刀门水道挂定角、联石湾、竹排沙、平岗和竹银5个测站的盐度、马口水文站流量、三灶站潮位和竹银站的主流向风速为基本数据,应用偏相干谱估计方法分析了各测站盐度变化对潮汐、流量和风的频率响应。经验诊断分析结果表明:1)马口站入海径流存在准1日、半日、1/4日和1/3日等显著的潮致振荡周期;2)各测站盐度变化与潮汐主要振荡周期并不一致,沿磨刀门水道向上游,潮汐对盐度变化的影响递减,竹银站盐度可作为潮汐直接作用下咸潮上溯的平均上限位置;3)消除潮汐和径流的影响后风对盐度变化的作用明显加强;4)各测站盐度变化对潮汐、流量与风的响应过程存在时间超前或滞后但无特定规律,说明磨刀门水道盐度变化过程十分复杂。     

泉州湾水体结构的潮周期变化

泉州湾6个站点的观测数据显示：内湾涨潮流历时由底层向表层逐渐变短，而落潮流历时则逐渐变长；同时，各个站点实测的盐度水深结构也具有明显的潮周期变化特征，这可能与局地水体的层化和混合机制的交替变化密切相关．进一步定量分析S2站位上影响水体结构变化的四种机制发现：外湾的河口环流作用与潮汐张力、风混合以及潮汐混合作用相比要小一个量级．在一般天气条件下的大潮周期，潮汐混合与潮汐张力相互竞争是导致水体结构交替变化的根本原因；小潮周期由于潮流混合作用减弱，水体层化现象得以持续较长时间，风的搅动在特定时刻可以起到削弱层化结构的作用．     

影响南海混合层盐度季节变化的因素分析

通过对1950-2012年的南海混合层盐度数据进行分析,发现影响南海北部和南部盐度季节变化的最主要因素存在很大的差异.在南海北部,影响混合层盐度季节变化的最主要因素是蒸发降水,其次是水平平流.随着逐步南移,蒸发降水对盐度季节变化的影响递减,水平平流的影响逐渐增大;而在南海南部,水平平流的作用超过蒸发降水成为影响盐度的季节变化的最主要因素.在整个南海区域,冬季海水垂直混合变强,混合层变厚,下层高盐海水进入混合层,使混合层海水盐度变高,从而对冬季海水盐度的上升趋势产生促进作用;夏季南海北部混合层底存在上升流,南海东南部由于Ekman输运导致混合层变厚,都会将混合层以下高盐海水带入混合层,使混合层海水盐度变高,从而对夏季海水盐度下降趋势产生阻碍作用,但垂直混合对盐度季节变化的影响不大,远小于蒸发降水和水平平流.     

黄、渤海盐度准三维数值预报模式

黄、渤海盐度的垂直结构具有典型的自模性,而其水平分布又受平流、水平扩散效应及径流等因素的影响。本文根据黄、渤海实测资料拟合了盐度垂直剖面的自模函数,并结合描述表、底层盐度及上均匀层厚度这三个特征量水平分布的方程,形成给出盐度三维结构的准三维模式。在模式中,综合考虑了海面风和热输入的强迫作用以及流场的平流、侧向混和及底层混和影响因素,同时还考虑了径流、蒸发及降水的作用,较客观地反映了盐度的三维分布及其变化的物理过程。试报结果分析表明,模式的功能较好,结果令人满意。     

2017年辽东湾夏季潮汐锋位置变化的分析

近海潮汐锋的分布和变化，主要受表层风摩擦、底层潮混合、净热通量和浮力平流的影响。基于2017年7、8月份辽东湾东部海域的实测数据，并结合ROMS（Regional Ocean Model System）模拟结果，利用考虑浮力平流效应的Stigebrandt公式对夏季辽东湾潮汐锋的位置变化进行了诊断计算，计算结果与ROMS模拟的潮汐锋位置符合较好，进一步探讨了风、净热通量和浮力平流对锋面位置变化的影响。主要结论如下：（1）位于辽东湾北部和东、西沿岸浅水区的潮汐锋呈"几"字形分布；（2）2017年6-7月潮汐锋位置变动不大，7月份仅在辽东湾东、西两岸潮汐锋位置略微向深水区移动，这主要是净热通量整体略微减小和风场略微增大造成的，浮力平流作用效果不够显著；（3）2017年8月辽东湾潮汐锋位置较7月向深水区大幅移动，最大移动距离约为20km，辽东湾8月份净热通量的大幅减弱起到了重要作用，浮力平流使潮汐锋位置向浅水区偏移，其调节效果比较显著。     

一个小型热带河口羽流的季节性变化与洪水事件影响下的特征与动力分析

小河口与大河口的羽流特征具有显著的不同,其更多地受风、径流的变化所控制。文章以海南博鳌万泉河口为例,研究其河口羽流的季节性变化特征,以及洪水对河口羽流的影响。采用动量平衡方法,分析了万泉河口羽流的动力特征。结果表明,万泉河口羽流具有显著的季节性变化,羽流的扩展路径与范围主要受径流和风的控制,夏季时以向外海与向上游方向(相对于开尔文波的传播方向)扩展为主,而冬季时则向下游扩展。夏季的羽流范围大于冬季。一次近2天的洪水可极大地影响羽流的范围及盐度大小,盐度减小1‰的范围可达300km2。但洪水过后,在风与潮汐的混合作用下,河口近岸的盐度可较为快速地恢复到洪水前水平。在羽流的远场区域,流速切线方向主要为压强梯度力与摩擦力的平衡,而在法线方向则为地转平衡;在近场切线方向主要为平流项、压力梯度项与摩擦力项的平衡,而法线方向则主要为压强梯度力与离心力之间的平衡,科氏力项可忽略不计,这与中纬度大河口的近场动力特征具有明显不同。     

丁字湾水文环境特征

本文依据丁字湾水文观测燃料阐述了该湾温，盐度分布及其变化特征，并对潮汐，潮流和余流进行了分析研究，结果表明：１．丁字湾温，盐度分布的月际差异明显，水温８月最高，盐度９月最低，水温日变化主要受太阳辐射的影响，盐度日变化则明显由潮汐所控制，２．该湾潮汐类型属正规半口潮但潮汐日不等现象明显。３．该湾属于正规半日潮流区域，潮流运行以往复流为主，４．８月湾口中部余流从湾外流向湾内，湾口两端余流流向与中部相反     

黄海、渤海盐度准三维数值预报模式

黄海、渤海盐度的垂直结构具有典型的自模性，而其水平分布又受平流、水平扩散效应及径流等因素的影响。本文根据黄海、渤海实测资料拟合了盐度垂直剖面的自模函数，并结合描述表、底层盐度及上均匀层厚度这３个特征量水平分布的方程，给出盐度三维结构的准三维模式。在模式中，综合考虑了海面风和热输入的强迫作用以及流场的平流、侧向混合及底层混合的影响因素，同时还考虑了径流、蒸发及降水的作用，较客观地反映了盐度的三维分布及其变化的物理过程。试报结果分析表明，模式的功能较好，结果令人满意。     

丁字湾水文环境特征

本文依据丁字湾水文观测资料阐述了该湾温、盐度分布及变化特征，并对潮汐、潮流和余流进行了分析研究．结果表明：1．丁字湾温、盐度分布的月际差异明显．水温8月最高，盐度9月最低；水温日变化主要受太阳辐射的影响，盐度日变化则明显由潮汐所控制．2．该湾潮汐类型属正规半日潮，但潮汐日不等现象明显．3．该湾属于正规半日潮流区域，潮流运动以往复流为主。4．8月湾口中部余流从湾外流向湾内，湾口两端余流流向与中部相反；余流流速湾口中部大于两端。     

Dynamical structure of vertically two-dimensional estuary in steady state

The dynamical structure of a two-dimensional (depth and axial directions) estuary is studied analytically. A set of governing equations describing the time-averaged velocity and salinity in the estuary is used, where all of the external parameters (depth, width, freshwater discharge and horizontal and vertical coefficients of eddy viscosity and eddy diffusivity) are assumed to be constant along the estuary.Two dynamical relations are taken into consideration in the theory. One of them is the dependence of the longitudinal scaleL _d on the balance of longitudinal salt transport, and the other is the relation between the vertical stratification of salinity and the the Prandtl numberP _r=A_v/K_v, whereA _v andK _v denote the coeffcients of the vertical eddy viscosity and diffusivity, respectively. The two relations result in an extension of the parameter range in which the linear balance of momentum holds.A linear state of motion (LM-state) is defined as the state where the momentum balance is linear. The LM-state comprises the so-called vertically homogeneous and the so-called partially mixed state. Perturbation analysis is introduced and dynamical theory is developed in the LM-state. Since the LM-state covers a fairly wide regime with respect to the balance of salt transport, the state is subdivided into three stages: the diffusive, intermediate and advective stages. In the diffusive stage the upstream salt transport is mainly attributed to the horizontal diffusion, whereas in the advective stage it is attributed to advection caused by gravitational circulation. The salinity balance is also linear in the diffusive and intermediate stages, whereas the balance is nonlinear in the advective stage. The advective stage of the LM-state is regarded as a stage bordering the salt wedge state.The longitudinal distribution of depth-mean salinity is found to take an exponential form in the diffusive stage, a nearly linear form in the advective stage and an intermediate form between them in the intermediate stage.     

Winds and buoyancy-driven circulation in the Tampa Bay

INTROOCCrIOWWiththeimplementationoftimeseriesobservationsspanningseveralsynopticweathersystempassages,theimPOrtanceoftheattnosphericallyforcedpartofestuarinecirculationbecameclearinthe1970s.WeisbetgandSturges(1976)examinedthewindeffectindrivingthenetcirculationintheWestPassageoftheNarragansettBay.Usingvelocitymeasurementsofapproximatelyonemonthduration,theyconcludedthatthelocalwindscoulddominatethegravitationalconvectionindeterminingthenetestuarinecirculationprofileofapartiallymixedestu…     

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.     

The principal factors contributing to the flux of salt in a narrow, partially stratified estuary

Observations of the velocity and salinity structure of the Tees estuary were made at eight stations along the estuary axis between Victoria Bridge and the sea during the summer of 1975. The measurements were made on ten separate tidal periods covering neap and spring tides.The data were collected over a period of relatively low freshwater flows and the residual current was found to have a strong dependence on the Stokes drift. At the upstream stations, the residuals were more than an order of magnitude greater than the currents anticipated from the freshwater discharge. Although the mean stratification decreased as the tidal range increased, the vertical circulation was stronger on spring tides than on neaps. Vertical variations in the amplitude and phase of the tidal current results in a current which strengthens the vertical circulation. However, this effect only made a relatively small contribution to the observed vertical circulation.The relative contribution of the individual salt flux terms to the net upstream transport of salt varies along the estuary. As the estuary narrows, the contribution by the oscillatory terms dominates that from the shear in the steady state flow. Of these oscillatory terms, the correlation of velocity and salinity fluctuations plays a key rôle in the salt transport. The depth mean values make a greater contribution than deviations from the depth mean and the flux due to phase variations over depth is smaller than either of these. Since the Stokes drift is compensated by a down-stream steady state flow, it does not contribute to the tidal mean transport of salt.At the seaward end of the estuary, the salt fluxes due to the steady state vertical shear and the convariance of the tidal fluctuations act in a complementary way to counter the seaward transport of salt by the freshwater flow. With the possible exceptions of the wide or narrow reaches of the Tees, the longitudinal fluxes of salt due to transverse variations in velocity, salinity and depth and turbulent fluctuations are of secondary importance as contributors to the estuary salt budget.On both neap and spring tides, the computed total salt transports at the Newport and Victoria bridges did not match the values required for a salt balance with the corresponding freshwater flows. These fluxes were probably the cause of the observed downstream displacement of the tidal mean salinity distribution between neap and spring tides.     

WEST: A northern California study of the role of wind-driven transport in the productivity of coastal plankton communities

The “Wind Events and Shelf Transport” (WEST) program was an interdisciplinary study of coastal upwelling off northern California in 2000–03. WEST was comprised of modeling and field observations. The primary goal of WEST was to better describe and understand the competing influences of wind forcing on planktonic productivity in coastal waters. While increased upwelling-favorable winds lead to increased nutrient supply, they also result in reduced light exposure due to deeper surface mixed layers and increased advective loss of plankton from coastal waters. The key to understanding high levels of productivity, amidst these competing responses to wind forcing, is the temporal and spatial structure of upwelling. Temporal fluctuations and spatial patterns allow strong upwelling that favors nutrient delivery to be juxtaposed with less energetic conditions that favor stratification and plankton blooms. Observations of winds, ocean circulation, nutrients, phytoplankton and zooplankton off Bodega Bay and Point Reyes (38°N) were combined with model studies of winds, circulation and productivity. This overview of the WEST program provides an introduction to the WEST special issue of Deep-Sea Research, including the motivation for WEST, a summary of study components, an integrative synthesis of major research results to-date, and background on conditions during field studies in May–June 2001 (the upwelling period on which this special issue is focused).     

Mixed Layer Depth and its Variability in the Eastern Equatorial Indian Ocean as Revealed by Observations and Model Simulations

Mixed layer depth (MLD) variability in the Eastern Equatorial Indian Ocean (EEIO) from a hindcast run of an Ocean General Circulation Model (OGCM) forced by daily winds and radiative fluxes from NCEP-NCAR reanalysis from 2004 to 2006 is investigated. Model MLD compares well with the ～20,000 observations from Argo floats and a TRITON buoy (1.5°S and 90°E) in the Indian Ocean. Tests with a one-dimensional upper ocean model were conducted to assess the impact on the MLD simulations that would result from the lack of the diurnal cycle in the forcing applied to the OGCM. The error was of the order of ～12 m. MLD at the TRITON buoy location shows a bimodal pattern with deep MLD during May–June and December–January. MLD pattern during fall 2006 was significantly different from the climatology and was rather shallow during December–January both in the model and observation. An examination of mixed layer heat and salt budget suggested salinity freshening caused by the advective and vertical diffusive mixing to be the cause of shallow MLD.     

Temporal and spatial variability of sea level and volume flux in the Murderkill Estuary

The instantaneous sea level determined at two sites in the Murderkill Estuary, a tributary of Delaware Bay, results from the superposition of temporal variability operating over different time and spatial scales. Over the relatively short tidal time scales, the semidiurnal tides that represent the dominant tidal constituents in lower Delaware Bay show a modest increase in tidal amplitudes from the bay mouth (Lewes, Delaware), up to Bowers Beach (the mouth of the Murderkill Estuary). However, as the tides propagate into the Murderkill Estuary, the semidiurnal constituents undergo heavy attenuation, resulting in a 48% reduction in tidal amplitude from Bowers to Frederica (approximately the extent of saline intrusion). The diurnal tide, on the other hand, experiences only a 25% reduction in amplitude. The limited tidal asymmetry that is observed may be a result of interaction between flows in the tidal channel and the adjacent salt marsh. At longer time scales, the subtidal sea level experiences no attenuation. The Murderkill Estuary thus behaves like a low pass filter to preferentially damp out high frequency sea level forcing from lower Delaware Bay. The subtidal volume flux in the Murderkill is highly coherent with the time rate of change of sea level, indicating that the Murderkill basically co-oscillates with Delaware Bay in a standing wave fashion over the subtidal time scale. This remote coupling controls more than 90% of the variance in subtidal sea level in the estuary. The surface slopes in the lower bay and the Murderkill Estuary are closely correlated with winds along the orientation of the two waterways, consistent with the effect of local wind on subtidal sea level.     

Quantifying cross-shelf and vertical nutrient flux in the Coastal Gulf of Alaska with a spatially nested,coupled biophysical model

The Coastal Gulf of Alaska (CGOA) is productive, with large populations of fish, seabirds, and marine mammals; yet it is subject to downwelling-favorable coastal winds. Downwelling regions in other parts of the world are typically much less productive than their upwelling counterparts. Alternate sources of nutrients to feed primary production in the topographically complex CGOA are poorly known and difficult to quantify. Here we diagnose the output from a spatially nested, coupled hydrodynamic and lower trophic level model of the CGOA, to quantify both horizontal and vertical nutrient fluxes into the euphotic zone. Our nested model includes both nitrogen and iron limitation of phytoplankton production, and is driven by a fine-scale atmospheric model that resolves the effects of local orography on the coastal winds. Results indicate significant “rivers” of cross-shelf nitrogen flux due to horizontal advection, as well as “fountains” of vertical transport over shallow banks due to tidal mixing. Using these results, we constructed a provisional budget of nutrient transport among subregions of the CGOA. Contrary to expectations, this budget reveals substantial upwelling of nutrients over major portions of the shelf, driven by local wind-stress curl. These effects are large enough to overwhelm the smaller downwelling flux at the coast throughout the growing season. Vertical mixing by winds and tides, and horizontal flux from the deep basin, are other substantial contributors to nutrients above the 15-m horizon. These findings help to explain the productivity of this coastal ecosystem.     

Elucidating the dynamics and mixing agents of a shallow fjord through age tracer modelling

The mixing agents and their role in the dynamics of a shallow fjord are elucidated through an Eulerian implementation of artificial tracers in a three-dimensional hydrodynamic model. The time scales of vertical mixing in this shallow estuary are short, and the artificial tracers are utilized in order to reveal information not detectable in the temperature or salinity fields. The fjord's response to external forcing is investigated through a series of model experiments in which we quantify vertical mixing, transport time scales of fresh water runoff and estuarine circulation in relation to external forcing.Using age tracers released at surface and bottom, we quantify the time scales of downward mixing of surface water and upward mixing of bottom water. Wind is shown to be the major agent for vertical mixing at nearly all depth levels in the fjord, whereas the tide or external sea level forcing is a minor agent and only occasionally more important just close to the bottom. The time scale of vertical mixing of surface water to the bottom or ventilation time scale of bottom water is estimated to be in the range 0.7 h to 9.0 days, with an average age of 2.7 days for the year 2004.The fjord receives fresh water from two streams entering the innermost part of the fjord, and the distribution and age of this water are studied using both ageing and conservative tracers. The salinity variations outside this fjord are large, and in contrast to the salinity, the artificial tracers provide a straight forward analysis of river water content. The ageing tracer is used to estimate transport time scales of river water (i.e. the time elapsed since the water left the river mouth). In May 2004, the typical age of river water leaving the fjord mouth is 5 days. As the major vertical mixing agent is wind, it controls the estuarine circulation and export of river water. When the wind stress is set to zero, the vertical mixing is reduced and the vertical salinity stratification is increased, and the river water can be effectively exported out of the fjord.We also analyse the river tracer fields and salinity field in relation to along estuary winds in order to detect signs of wind-induced straining of the along estuary density gradient. We find that events of down estuary winds are primarily associated with a reduced along estuary salinity gradient due to increased surface salinity in the innermost part of the fjord, and with an overall decrease in vertical stratification and river water content at the surface. Thus, our results show no apparent signs of wind-induced straining in this shallow fjord but instead they indicate increased levels of vertical mixing or upwelling during down estuary wind events.