Circulation and salinity distribution in coastal inlets

The mathematical model developed by Hansen and Rattray based on Pritchard's equations for a coastal-plain estuary has been analysed to study the circulation and salinity distributions in coastal inlets with constant width and depth. Numerical solutions of the basic equations have been obtained without placing any restriction on Rayleigh numbers. A noteworthy contribution of the present analysis is that solutions of equations have been obtained for higher Rayleigh numbers, which was not possible in the earlier model. It is found that the effect of higher Rayleigh numbers is to increase the vertical advection, making the salinities in the upper and lower layers more uniform with a distinct halocline near the mid-depths. Solutions are discussed for some special cases of practical interest.

Freshwater runoff and salinity distribution in the Loxahatchee River Estuary, southeastern Florida, 1980-82 /

Bibliography: p. 35-36.

Mass Transport In The Equatorial Indian Ocean

In the equatorial oceans, the meridional currents are far less energetic than their zonal counterparts. The response of the Equatorial Indian Ocean to the seasonal reversals in the zonal wind field. is quite interesting and unique. A modest attempt, considering the shortcomings in the hydrographic data availability and distribution, is made to evaluate the variability in the zonal transport of mass. in_ both space and time. The peculiarities in its hydrological regime imposed upon by the seasonally varying winds is best appreciated when compared with the quasi permanent circulation characteristics of the Pacific and Atlanti'c.The major features of the tequatorial mass transport is outlined in the introductory chapter of this thesis for the Pacific and Atlantic Mass transport studies in the Indian Ocean, as can be seen from the earlier studies, gis“ the least known and understood, though could have captured the attention of both the experimentalist and the theoretician alike. owing to its complexity. Since in the Indian Ocean, the studies on the zonal mass transport are limited and are confined to the equator only, an attempt has been made to compute the mass transport extending from 5 N to 20 S.

Monthly charts of surface salinity in the Eastern Tropical Pacific Ocean

ENGLISH: Mean monthly maps of surface salinity of the eastern Pacific Ocean were prepared using all available data. Charts were also made of the distribution by one-degree quadrangles of the number of surface salinity observations for each month. Features of interest in the surface salinity distribution are discussed briefly, especially with reference to temporal changes. SPANISH: Se prepararon mapas mensuales sobre la salinidad media de superficie del Océano Pacifico oriental tropical, usando todos los datos disponibles. También se ejecutaron cartas por cuadrángulos de un grado, según el número de las observaciones mensuales de la distribución de la salinidad superficial. Las caracteristicas de interés de las distribuciones de la salinidad superficial se discuten brevemente, especialmente con referencia a los cambios estacionales. (PDF contains 44 pages.)

Evolution of freshwater plumes and salinity fronts in the northern Bay of Bengal

[1] The evolution of freshwater plumes and the associated salinity fronts in the northern Bay of Bengal ( henceforth the bay) is studied using rotated empirical orthogonal function (REOF) analysis and extended associate pattern analysis (EAPA). The results show that sea surface salinity distribution is featured by eastern-bay and western-bay plumes in the northern bay during different seasons. The western-bay plume begins in early July, peaks in late August, and then turns into a bay-shaped plume with the two plumes in either side of the bay, which peaks in late October. The southward extension of the western-bay plume can be explained by the southwestward geostrophic flow associated with the cyclonic gyre in the northern bay, which counters the northeastward Ekman drift driven by wind stress. The offshore expansion of the western-bay plume is induced by the offshore Ekman drift which also produces a salinity front near the east coast of India. The bay-shaped plume appears when the cyclonic gyre shifts westward and a weak anticyclonic gyre occupies the northeastern bay. As the season advances, the western part of the bay-shaped plume decays while the eastern part persists until the following June, which is believed to be associated with the anticyclonic gyre in the northern bay. The evolution of the plumes except the eastern part of the bay-shaped plume in fall can be partly explained by the seasonal variation of mass transport associated with the Sverdrup balance. The fact that the western-bay (eastern-bay) plume appears when surface freshwater flux in the northeastern bay increases ( decreases) dramatically suggests that the plumes are not produced directly by surface freshwater flux. River discharge seems to be the freshwater source for the plumes and has little to do with the evolution of the plumes.

Turbulent mixing and sediment processes in peri-urban estuariesin South-East Queensland (Australia)

An estuary is formed at the mouth of a river where the tides meet a freshwater flow and it may be classified as a function of the salinity distribution and density stratification. An overview of the broad characteristics of the estuaries of South-East Queensland(Australia) is presented herein, where the small peri-urban estuaries may provide an useful indicator of potential changes which might occur in larger systems with growing urbanisation. Small peri-urban estuaries exhibits many key hydrological features and associated with ecosystem types of larger estuaries, albeit at smaller scales, often with a greater extent of urban development as a proportion of catchment area. We explore the potential for some smaller peri-urban estuaries to be used as natural laboratories to gain some much needed information on the estuarine processes, although any dynamics similarity is presently limited by critical absence of in-depth physical investigation in larger estuarine systems. The absence of the detailed turbulence and sedimentary data hampers the understanding and modelling of the estuarine zones. The interactions between the various stake holders are likely to define the vision for the future of South-East Queensland's peri-urban estuaries. This will require a solid understanding of the bio-physical function and capacity of the peri-urban estuaries. Based upon the knowledge gap, it is recommended that an adaptive trial and error approach be adopted for the future of investigation and management strategies.

Mixing process in estuaries

A theoretical expression for vertical profile-of horizontal velocity in terms of its depth-average is derived based on oscillatory boundary layer theory and estuarine flow characteristics. The derived theoretical profile is then incorporated into a vertical quasi-two-dimensional model, which is proved advantageous in more physical implications and less CPU time demand. To validate the proposed model, the calculated results are compared to the field data in the Yangtze River Estuary, exhibiting good agreement with observations. The proposed quasi-two-dimensional vertical model is used to study mixing process, especially dependence of salinity distribution and salt front strength on runoff and tides in estuaries.

El estuario del Plata: donde el río se encuentra con el mar

The Rio de la Plata estuary is one of the largest estuarine environments of South America. It is characterized by a two-layer vertical salinity distribution and the existence of water masses convergence zones (fronts). Oceanographic scenario greatly influence the biology of the planktonic organisms that live in the water column, the benthic organisms that inhabit the soft sediment bottoms of the estuary, and the fishes, which show a differential degree of penetration into the diluted water of the estuary. Short and frequent events of strong weastward winds ('sudestada') alter completely the oceanographic conditions greatly influencing the especies ecology. Biological production of the estuary is high, and most of their organisms have a marine heritage.

The impact of global freshwater forcing on the thermohaline circulation: adjustment of North Atlantic convection sites in a CGCM

On the time scale of a century, the Atlantic thermohaline circulation (THC) is sensitive to the global surface salinity distribution. The advection of salinity toward the deep convection sites of the North Atlantic is one of the driving mechanisms for the THC. There is both a northward and a southward contributions. The northward salinity advection (Nsa) is related to the evaporation in the subtropics, and contributes to increased salinity in the convection sites. The southward salinity advection (Ssa) is related to the Arctic freshwater forcing and tends on the contrary to diminish salinity in the convection sites. The THC changes results from a delicate balance between these opposing mechanisms. In this study we evaluate these two effects using the IPSL-CM4 ocean-atmosphere-sea-ice coupled model (used for IPCC AR4). Perturbation experiments have been integrated for 100 years under modern insolation and trace gases. River runoff and evaporation minus precipitation are successively set to zero for the ocean during the coupling procedure. This allows the effect of processes Nsa and Ssa to be estimated with their specific time scales. It is shown that the convection sites in the North Atlantic exhibit various sensitivities to these processes. The Labrador Sea exhibits a dominant sensitivity to local forcing and Ssa with a typical time scale of 10 years, whereas the Irminger Sea is mostly sensitive to Nsa with a 15 year time scale. The GIN Seas respond to both effects with a time scale of 10 years for Ssa and 20 years for Nsa. It is concluded that, in the IPSL-CM4, the global freshwater forcing damps the THC on centennial time scales.

Freshwater transport in the coupled ocean-atmosphere system: a passive ocean

Conservation of water demands that meridional ocean and atmosphere freshwater transports (FWT) are of equal magnitude but opposite in direction. This suggests that the atmospheric FWT and its associated latent heat (LH) transport could be thought of as a \textquotedblleft coupled ocean/atmosphere mode\textquotedblright. But what is the true nature of this coupling? Is the ocean passive or active? Here we analyze a series of simulations with a coupled ocean-atmosphere-sea ice model employing highly idealized geometries but with markedly different coupled climates and patterns of ocean circulation. Exploiting streamfunctions in specific humidity coordinates for the atmosphere and salt coordinates for the ocean to represent FWT in their respective medium, we find that atmospheric FWT/LH transport is essentially independent of the ocean state. Ocean circulation and salinity distribution adjust to achieve a return freshwater pathway demanded of them by the atmosphere. So, although ocean and atmosphere FWTs are indeed coupled by mass conservation, the ocean is a passive component acting as a reservoir of freshwater.

Variability of the subtropical shelf front off eastern South America: Winter 2003 and summer 2004

Hydrographic data collected during surveys carried out in austral winter 2003 and summer 2004 are used to analyze the distributions of temperature (T) and salinity (S) over the continental shelf and slope of eastern South America between 27 degrees S and 39 degrees S. The water mass structure and the characteristics of the transition between subantarctic and subtropical shelf water (STSW), referred to as the subtropical shelf front (STSF), as revealed by the vertical structure of temperature and salinity are discussed. During both surveys, the front intensifies downward and extends southwestward from the near coastal zone at 33 degrees S to the shelf break at 36 degrees S. In austral winter subantarctic shelf water (SASW), derived from the northern Patagonia shelf, forms a vertically coherent cold wedge of low salinity waters that locally separate the outer shelf STSW from the fresher inner shelf Plata Plume Water (PPW) derived from the Rio de la Plata. Winter T-S diagrams and cross-shelf T and S distributions indicate that mixtures of PPW and tropical water only occur beyond the northernmost extent of pure SASW, and form STSW and an inverted thermocline characteristic of this region. In summer 2004, dilution of Tropical water (TW) occurs at two distinct levels: a warm near surface layer, associated to PPW-TW mixtures, similar to but significantly warmer than winter STSW, and a colder (T similar to 16 degrees C) salinity minimum layer at 40-50 m depth, created by SASW-STSW mixtures across the STSF. In winter, the salinity distribution controls the density structure creating a cross-shore density gradient, which prevents isopycnal mixing across the STSF. Temperature stratification in summer induces a sharp pycnocline providing cross-shelf isopycnal connections across the STSF. Cooling and freshening of the upper layer observed at stations collected along the western edge of the Brazil Current suggest offshore export of shelf waters. Low T and S filaments, evident along the shelf break in the winter data, suggest that submesoscale eddies may enhance the property exchange across the shelf break. These observations suggest that as the subsurface shelf waters converge at the STSF, they flow southward along the front and are expelled offshore, primarily along the front axis. (C) 2008 Elsevier Ltd. All rights reserved.

Phytoplankton and Hydrography of the Kennebec Estuary, Maine, USA

The biomass, abundance and species composition of phytoplankton in the Kennebec estuary, Maine, USA, were investigated in relation to hydrography and Light regime during 7 seasonal survey cruises. The salinity distribution ranged from 32 at the mouth to between 0 and 5 at the head, depending on the magnitude of freshwater discharge at the time of each survey. Maximum Vertical salinity and temperature gradients were observed at the mouth. while local tidal mixing, combined with the freshwater flow, produced a well-mixed water column at the head of the estuary. The middle portion of the estuary was stratified on flooding and ebbing tides, but was vertically well mixed at high and low tides. Phytoplankton biomass was lowest in winter (chlorophyll a approximate to 1 mu g l(-1)) and highest in summer (up to 10 mu g l(-1)) The phytoplankton species assemblages at the seaward and the riverine ends of the estuary were made up of taxa with corresponding salinity preferences. Both cell numbers and biomass (chlorophyll a) exhibited a bimodal distribution along the length of the estuary in the warmer months, with the middle portions of the estuary having depressed phytoplankton standing stocks compared with the seaward and landward ends. This bimodal distribution was related to Light limitation and nutrient regeneration in the middle portion of the estuary and to the production of and advective contributions of phytoplankton from both the freshwater and seaward ends.

Tidal impacts on riparian salinities near estuaries

Groundwater-dependent riparian biota is known to be sensitive to changes in soil and groundwater salinity in estuarine systems. The groundwater flow and salinity behaviour in a phreatic aquifer adjoining a partially penetrating, tidal. estuary is investigated through two-dimensional numerical experiments for a lateral cross-section, which explore the influence of factors, such as aquifer and soil materials, tidal amptitudes, and regional groundwater hydraulic gradients. The density contrast between estuarine water and the fresh groundwater drives saltwater penetration of the aquifer even in the case of a marked groundwater hydraulic gradient towards the estuary. We show that tidal fluctuations in estuaries can significantly affect the groundwater salinity distribution in adjacent density-stratified phreatic aquifers. This has consequences for the expected distribution of salinity-sensitive biota in the hyporheic zone as well as vegetation and fauna dependent on water in the riparian soil and aquifer. The shape of the dense saltwater wedge propagating into the adjacent groundwater system is also modified by the estuarine tidal signal, although this effect appears to have only minor influence on the maximum distance penetrated into the aquifer (i.e., location of the 'toe' of the wedge). Tide-induced changes to riparian groundwater salinity are advection-driven, as evidenced by the modified time-averaged groundwater flow dynamics. (c) 2006 Elsevier B.V. All rights reserved.

The characteristics of distribution and change of the seawater temperature and salinity of Cai river estuary and Northern Part of Nha Trang bay in the dry and rainy season

In this paper, some results of analyzing the hydrographic characteristics of the seawater temperature and salinity are presented. The received results showed that: in dry season, the influence of the Cai river water has is limited in Cai river estuary with the approximate transferable distance from the river mouth to the open sea of about 1 km. The isohaline 32%o could be defined as the separate boundary of the Cai river water; In rainy season, due to the river water discharges are high, the influence of Cai river water could be transferred to the open sea and island areas. The immerge of the Cai river water in the open sea areas in rainy season has changed the vertical structure of salinity and temperature in the northern part of Nhatrang bay. In both seasons, the Cai river water have influenced in the surface water layers 0 - 2m and the water layers deeper than 2m are influenced by the sea waters with the salinity of higher than 32%o.