A pair of Langmuir cells in two laboratory tanks (II) on generation mechanism

Vertical and cross-wind profiles of mean currents were measured systematically in vertical cross-sections of two wind-wave tanks with aspect ratios of order one to study the secondary flow in the tanks. A pair of Langmuir cells turned out to be driven by a close combination of the pressure gradient along the tank and the side-wall effects. That is, part of the adverse pressure gradient produced a parabolic cross-wind profile with the smallest downwind current at the centerline and the largest current along the two sidewalls. As a result, upwelling occurred in the center zone where the return flow was strongest, probably because of the entrainment action of the wind-driven current. In order to compensate for this upwelling, downwelling occurred along the two side-walls from the flow continuity. The resulting vertical circulation formed a pair of Langmuir circulations across the span and served to maintain the parabolic profile formed by the pressure gradient. A positive feedback mechanism is thus found between the primary and secondary circulations through upwelling of the return flow in the center zone. Vertical shears of the span-averaged downwind current measured in two tanks were found to be systematically different from each other. This difference seems to depend on the magnitude of the advective Reynolds stresses in the two tanks.     

太平洋CC区沉积物陆源物质时空分布与大气环流的关系

在玉平洋ＣＣ区沉积物陆源组分定量分离基础之上,研究陆源物质时空分布规律与大气环流的关系。     

湛江湾三维温盐特征季节变化观测分析

利用2017年1−12月的现场观测数据,分析了湛江湾温盐的三维空间结构及季节变化特征。结果表明：(1) 2017年湛江湾各站位年平均温度为23～27℃、盐度为19～27、位势密度为11～17 kg/m³、浮性频率(N²)为7×10⁻⁵～5×10⁻³ s⁻²。浮性频率的垂向结构及水平分布与温度分布类似,而位势密度则与盐度的变化趋势几乎一致;(2)温度季节变化明显,夏季最高,秋季次之,冬季最低,冬夏温差最大达15℃,而盐度季节变化则不大。相较于季节引起的变化,涨落潮对温度以及盐度影响较小。温度跃层夏季最强,10 m处温度最大梯度可达到0.7℃/m,春秋季温跃层抬升至5 m附近,冬季水体上下混合均匀。夏季和秋季存在明显的盐跃层,盐度梯度最大可达到1.1 m⁻¹。跃层上下温盐的季节变化规律一致;(3)水平分布上,从湾顶区、湾颈区、大堤区、浅滩区到湾口区,温度递减,盐度递增,湾顶区和湾口区平均温度差为2.3℃,盐度差为2.7。温盐图分析显示,不同季节水体呈现为不同的温盐条带,湾口区基本为低温、高盐水体,而湾顶区基本为高温、低盐水体,其他区域水体介于上述两者之间。     

大西洋经向翻转环流对岁差响应的气候背景依赖性

大西洋经向翻转环流（Atlantic Meridional Overturning Circulation,AMOC）是气候系统重要的组成部分,其强度变化可直接影响南北半球的热量分配,厘清其变化机理对全球变暖背景下的未来预估至关重要。海洋沉积物记录发现,在晚更新世,AMOC的变化与地球岁差周期有紧密联系,但其物理机理尚不清楚。本文利用海洋−大气耦合气候模型—COSMOS（ECHAM5/JSBACH/MPIOM）模型,通过敏感试验,分析在冰盛期冷期和间冰期暖期气候背景下,AMOC对地球岁差变化的响应机理。结果表明：岁差降低引起的北半球夏季太阳辐射增强,会导致间冰期暖期背景下的AMOC显著减弱,但对冰盛期AMOC的影响并不明显。通过进一步分析发现,在间冰期暖期,夏季太阳辐射增强,造成高低纬大西洋海表的升温,同时促进北大西洋高纬度地区的局地降水,两者导致北大西洋表层海水密度降低,共同削弱大西洋深层水生成。而在冰盛期冷期,大西洋高低纬度地区的响应对AMOC的影响反向—副热带升温触发的海盆尺度低压异常,通过其南侧的西风异常削弱大西洋向太平洋的水汽输送,导致净降水增多,海表盐度下降;同时,高纬度升温造成的海冰减少,促进了海洋热丧失,海表失热变重,有利于大西洋深层水的生成,最终两者的共同作用导致AMOC对岁差变化的响应偏弱。本文系统揭示了不同气候背景下,岁差尺度AMOC变化的控制机理,对理解晚更新世AMOC重建记录中持续存在的岁差周期具有重要启示意义。     

海南岛莺歌海近岸的潮汐不对称与潮致余流研究

潮汐不对称与潮致余流在河口海岸区的物质输运中扮演着重要角色。已往的研究表明,在驻波占主导的河口海湾中,涨落潮的历时不对称与流速不对称有较为良好的对应关系。而潮致余流主要由地形与潮波的非线性作用所致。本研究以海南岛莺歌海附近为代表,结合实际观测与数值模型,研究复杂地形的开阔近岸区的潮汐不对称与潮致余流。结果表明,在莺歌海近岸区,涨落潮历时不对称皆表现为涨潮历时短于落潮历时,而流速不对称则出现复杂的空间变化。对流速不对称的机制分解表明,研究区的流速不对称主要由K₁、O₁与M₂的相互作用,以及潮余流与各潮汐分潮的相互作用所控制。其中前者产生涨潮流速大于落潮流速的涨潮优势,而后者则与余流的方向相对应,出现多个涨潮优势与落潮优势的区域。总体而言,研究区的流速不对称由余流与各潮汐分潮的相互作用所决定。这表明,采用涨落潮历时的不对称来确定潮汐不对称的方法在开阔近海区可能并不适用。对潮致余流的研究表明,研究区的欧拉余流远大于斯托克斯余流。欧拉余流表现为多个顺时针与逆时针的涡流。涡流分布与地形具有较好的对应关系,潮流沙脊区多发育顺时针涡流,而深槽区则以发育逆时针涡流为主。摩擦力在涡流的发育中起着重要作用。     

Variability and propagation of Labrador Sea Water in the southern subpolar North Atlantic

The variability of two modes of Labrador Sea Water (LSW) (upper and deep Labrador Sea Water) and their respective spreading in the interior North Atlantic Ocean are investigated by means of repeated ship surveys carried out along the zonal WOCE line A2/AR19 located at 43–48°N (1993–2007) and along the GOOS line at about 48–51°N (1997–2002). Hydrographic section data are complemented by temperature, salinity, and velocity time series recorded by two moorings. They have been deployed at the western flank of the Mid-Atlantic Ridge (MAR) in the Newfoundland Basin during 1996–2004. The analysis of hydrographic anomalies at various longitudes points to a gradual eastward propagation of LSW-related signals, which happens on time scales of 3–6 years from the formation region towards the MAR. Interactions of the North Atlantic Current (NAC) with the Deep Western Boundary Current (DWBC) close to Flemish Cap point to the NAC being the main distributor of the different types of LSW into the interior of the Newfoundland Basin. Comparisons between the ship data and the mooring records revealed that the mooring sites are located in a region affected by highly variable flow. The mooring time series demonstrate an elevated level of variability with eddy activity and variability associated with the NAC considerably influencing the LSW signals in this region. Hydrographic data taken from Argo profiles from the vicinity of the mooring sites turned out to mimic quite well the temporal evolution captured by the moorings. There is some indication of occasional southward flow in the LSW layer near the MAR. If this can be considered as a hint to an interior LSW-route, it is at least of minor importance in comparison to the DWBC. It acts as an important supplier for the interior North Atlantic, distributing older and recently formed LSW modes southward along the MAR.     

The connection between Labrador Sea buoyancy loss, deep western boundary current strength, and Gulf Stream path in an ocean circulation model

The sensitivity of the North Atlantic gyre circulation to high latitude buoyancy forcing is explored in a global, non-eddy resolving ocean general circulation model. Increased buoyancy forcing strengthens the deep western boundary current, the northern recirculation gyre, and the North Atlantic Current, which leads to a more realistic Gulf Stream path. High latitude density fluxes and surface water mass transformation are strongly dependent on the choice of sea ice and salinity restoring boundary conditions. Coupling the ocean model to a prognostic sea ice model results in much greater buoyancy loss in the Labrador Sea compared to simulations in which the ocean is forced by prescribed sea ice boundary conditions. A comparison of bulk flux forced hindcast simulations which differ only in their sea ice and salinity restoring forcings reveals the effects of a mixed thermohaline boundary condition transport feedback whereby small, positive temperature and salinity anomalies in subpolar regions are amplified when the gyre spins up as a result of increased buoyancy loss and convection. The primary buoyancy flux effects of the sea ice which cause the simulations to diverge are ice melt, which is less physical in the diagnostic sea ice model, and insulation of the ocean, which is less physical with the prognostic sea ice model. Increased salinity restoring ensures a more realistic net winter buoyancy loss in the Labrador Sea, but it is found that improvements in the Gulf Stream simulation can only be achieved with the excessive buoyancy loss associated with weak salinity restoring.     

Numerical study on salinity stratification in the Pamlico River Estuary

The variations of current circulation, salt intrusion, and vertical stratification under different river flow and wind conditions in the Pamlico River Estuary (PRE) were investigated in this paper using a three-dimensional numerical model. The model was calibrated and verified against water level variation, temperature, and salinity variations during 2003 and 2001, respectively. Eight sensitivity tests were conducted with different river flow and wind conditions specified in the model. Model results show that salinity intruded further upstream under scenarios with low flow, downriver local wind, and remote-wind-caused water level set-up conditions. In contrast, the responses of salinity stratification to different environmental forcing functions were different in different portions of the estuary. Salinity stratification was enhanced under high flow condition at the lower part of the estuary, under upriver wind near the river mouth, under downriver wind at the upstream to middle portion of the estuary, and under remote-wind-caused water level set-up condition at the majority of the estuary except near the river mouth. Model results also show that across-channel wind tended to reduce salt intrusion and salinity stratification in the PRE through increased vertical mixing.     

Review of recent findings on the water masses and circulation in the East Sea (Sea of Japan)

Recent findings on water masses, biogeochemical tracers, deep currents and basin-scale circulation in the East/Japan Sea, and numerical modeling of its circulation are reviewed. Warming continues up to 2007 despite an episode of bottom water formation in the winter of 2000–2001. Water masses have definitely changed since the 1970s and further changes are expected due to the continuation of warming. Accumulation of current data in deep waters of the East/Japan Sea reveals that the circulation in the East/Japan Sea is primarily cyclonic with sub-basin scale cyclonic and anticyclonic cells in the Ulleung Basin (Tsushima Basin). Our understanding of the circulation of intermediate water masses has been deepened through high-resolution numerical studies, and the implementation of data assimilation has had initial success. However, the East/Japan Sea is unique in terms of the fine vertical structures of physical and biogeochemical properties of cold water mass measured at the highest precision and their rapid change with the global warming, so that full understanding of the structures and their change requires in-depth process studies with continuous monitoring programs.     

A two-dimensional finite volume hydrodynamic model for coastal areas: Model development and validation

A two dimensional implicit finite volume scheme for solving the shallow-water equations is developed. The effects of the Coriolis force, surface wind stress, and waves are included. A non-uniform rectilinear forward staggered grid is used with Cartesian coordinates. The time integration is performed using the Euler implicit technique. The convective flux is treated using the deferred correction method. The viscous terms are discretized using a second order central difference approximation. The SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm is used for coupling the velocity components and the water elevation gradient for the water level correction. The system of equations is solved sequentially using the Strongly Implicit Procedure (SIP). To simulate wave driven current, a phase averaged wave model is used first to simulate wave transformation and calculate radiation stresses. The performance of the developed model is validated for different sources of external forces and different combinations of boundary conditions. The validation cases include tidal circulation in a harbor and wave induced currents behind a breakwater parallel to the coastline. The model is finally applied to simulate the flow pattern in a closed artificial lagoon and along the coastline near Damietta Port located along the Northern coast of Egypt. Results of the developed model agree well with the published results for the considered cases.