Surface Salinity Distribution Research Articles

Patos Lagoon outflow within the Río de la Plata plume using an airborne salinity mapper: Observing an embedded plume

Major river systems discharging into continental shelf waters frequently form buoyant coastal currents that propagate along the continental shelf in the direction of coastal trapped wave propagation (with the coast on the right/left, in the northern/southern hemisphere). The combined flow of the Uruguay and Paraná Rivers, which discharges freshwater into the Río de la Plata estuary (Lat. ∼36°S), often gives rise to a buoyant coastal current (the ‘Plata plume’) that extends northward along the continental shelf off Uruguay and Southern Brazil. Depending upon the prevailing rainfall, wind and tidal conditions, the Patos/Mirim Lagoon complex (Lat. ∼32°S) may also produce a freshwater outflow plume that expands across the inner continental shelf. Under these circumstances the Patos outflow plume can be embedded in temperature, salinity and current fields that are strongly influenced by the larger Plata plume. The purpose of this paper is to present observations of such an embedded plume structure and to determine the dynamical characteristics of the ambient and embedded plumes. We describe selected results of coincident airborne remote sensing and shipboard in-situ surveys of the salinity distribution and extent of the Plata and Patos/Mirim Lagoon plumes conducted under contrasting winter (2003) and summer (2004) conditions. The surveys were carried out in the context of a comprehensive multi-disciplinary study of the Plata plume and its response to prevailing seasonal weather conditions. The objective was to map the surface salinity distribution of the Plata plume at synoptic scales under representative winter and summer conditions. Additionally, the airborne survey included finer-scale mapping of specific features including the Río de Plata estuarine front and the Patos Lagoon plume, with the objective of determining the distribution and behavior of the plumes in the estuaries and on the continental shelf. The airborne survey was conducted with an aircraft carrying an infrared and microwave radiometer system, the Naval Research Laboratory's (NRL) Salinity, Temperature and Roughness Remote Scanner (STARRS). A series of broad-scale flights over the continental shelf off Argentina, Uruguay and Brazil were made using STARRS to determine the spatial extent of the Plata plume, and detailed mapping flights were undertaken in the vicinity of Río de la Plata Estuary and Patos/Mirim outflows to observe associated frontal features. The results of the airborne surveys were compared with shipboard hydrographic data obtained from a conductivity, temperature and depth (CTD) profiler. The combined ship and aircraft data were used to estimate parameters of dynamical classification schemes. These schemes were used to characterize the gross behavior and dynamics of the ambient Plata plume and embedded Patos plume. The Plata plume was highly asymmetric with along-shelf development towards the north and it behaved dynamically like a buoyant coastal boundary current, with an approximately geostrophic across-shelf momentum balance. The Patos plume, on the other hand, maintained its integrity as a relatively symmetric, ageostrophic, frictionally dominated plume with significant across-shelf, and modest along-shelf, development. The dynamical implications of the embedding of the smaller scale Patos plume within the larger-scale ambient Plata plume were explored, and it was concluded that the ambient plume could exert a significant dynamical influence on the behavior of the embedded plume.

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

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.

Phytoplankton production and nutrient distributions in a subtropical estuary: Importance of freshwater flow

The relationships between phytoplankton productivity, nutrient distributions, and freshwater flow were examined in a seasonal study conducted in Escambia Bay, Florida, USA, located in the northeastern Gulf of Mexico. Five sites oriented along the salinity gradient were sampled 24 times over the 28-mo period from 1999 to 2001. Water column profiles of temperature and salinity were measured along with surface chlorophyll and surface inorganic nutrient concentrations. Primary productivity was measured at 2 sites on 11 dates, and estimated for the remaining dates and sites using an empirical regression model relating phytoplankton net production to the product of chlorophyll, euphotic zone depth, and daily solar insolation. Freshwater flow into the system varied markedly over the study period with record low flow during 2000, a flood event in March 2001, and subsequent resumption of normal flow. Flushing times ranged from 1 d during the flood to 20 d during the drought. Freshwater input strongly affected surface salinity distributions, nutrient flux, chlorophyll, and primary productivity. The flood caused high turbidity and rapid flushing, severely reducing phytoplankton production and biomass accumulation. Following the flood, phytoplankton biomass and productivity sharply increased. Analysis of nutrient distributions suggested Escambia Bay phytoplankton alternated between phosphorus limitation during normal flow and nitrogen limitation during low flow periods. This study found that Escambia Bay is a moderately productive estuary, with an average annual integrated phytoplankton production rate of 290 g C m−2 yr−1.

Simulation of temperature, salinity and suspended matter distributions induced by the discharge into the East China Sea during the 1998 flood of the Yangtze River

A three-dimensional model simulated the distributions of current, water temperature, salinity and suspended matter in the Yangtze estuary area and the East China Sea caused by the discharge of freshwater from the Yangtze (Changjiang) River for 3 years from May 1996 to April 1999. The calculation was conducted using a longitude–latitude mesh of 5 min × 5 min, in assimilating the daily average discharge from the Yangtze with the meteorological conditions on the sea surface obtained from the atmospheric global circulation model (European Centre for Medium-range Weather Forecasts, ECMWF) and the daily average precipitation over the sea surface obtained from the TRMM (Tropical Rainfall Measuring Mission) satellite data. The calculated water temperatures were in excellent agreement with the measurement data observed by the marine meteorological buoy robot (ST22001). For the surface salinity distribution on the east–west transection line across the East China Sea and the cross-sectional distribution of salinity in the Tsushima Strait, the calculated values were in excellent agreement with the observed data. During the Yangtze flood in 1998, turbid water was discharged in repeated pulses from the Yangtze and penetrated the East China Sea. This behavior of suspended matter discharged from the Yangtze was clearly identified by the 3D model results in salinity, suspended matter and current distributions. The model results revealed the fact that the freshwater and suspended matter originating from the Yangtze crossed the East China Sea and reached as far as the coasts of Kyushu, the Japan Sea and the Pacific coast of mainland Japan.

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.

Mesoscale hydrological variability induced by northwesterly wind on the French continental shelf of the Bay of Biscay

On the French continental shelf of the Bay of Biscay the variability of the surface salinity distribution has been mainly investigated at the seasonal and inter-annual scales. Here, new mesoscale features such as lower-salinity lenses observed in model results are investigated by hydrological measurements acquired during 8 cruises (1997-2000). These lenses are 50-80 km wide and ~30 m thick and occur during westerly to northerly wind events that push offshore the less saline water of river plumes. These water masses detached from the coast are replaced with upwelled saltier water at the coast, so coastal upwelling is often observed at the same time along Landes and southern Brittany coasts. We show that in addition to the influence of seasonal and inter-annual variability of the wind and river outflows, short term meteorological variability may drive mesoscale structures on this continental shelf.

Response of the Pacific and Atlantic oceans to interannual variations in net atmospheric freshwater

The response of the Pacific and Atlantic Oceans to net atmospheric freshwater (evaporation minus precipitation (EmP)) between 1988 and 2000 was studied using a global ocean general circulation model developed in the Massachusetts Institute of Technology. Evaporation estimates from the Goddard Satellite Surface Turbulent Fluxes and precipitation estimates from the Global Precipitation Climatology Project were used. Model simulations showed that the spatial distribution of the average sea surface salinity (SSS) changes during the 1988–2000 period resembled that of average EmP changes, because SSS changes were primarily associated with anomalous vertical mixing forced by the anomalous EmP. The spatial distribution of average near‐surface temperature anomalies, however, was different from those of average EmP and SSS anomalies. Analyses indicated that temperature changes in the subtropical North and South Pacific resulted from anomalous heat advection which, in turn, resulted from changes in the subtropical gyre circulations. Temperature changes in the tropical Pacific were associated with anomalous heat advection due to changes in the South Equatorial Current (SEC). Furthermore, the changes in the subtropical gyres and the SEC resulted largely from anomalous EmP in the subtropical Pacific. Temperature changes in the Atlantic, however, were largely associated with vertical mixing changes due to anomalous EmP. Observed interannual variations of SSS in the Western Pacific Warm Pool were simulated successfully. A large difference between simulated and observed SSS, however, was found in the central equatorial Pacific. Simulated SSS and temperature varied at interannual and longer timescales in most of the Pacific and Atlantic Oceans. Large changes in SSS and temperature due to anomalous EmP in the Pacific and Atlantic strongly suggest that EmP variability can play an important role in ocean and climate variabilities at interannual and longer timescales.

Effects of channel regulation on salt intrusion and residual circulation of Keelung River

AbstractA vertical (laterally integrated) two‐dimensional numerical model was applied to study the hydrodynamic characteristics, salt‐water intrusion and residual circulation in the Danshuei River estuarine system. The cross‐sectional profiles measured in 2001 and 1990 respectively represent the conditions after and before channel regulation in the Keelung River. The model was re‐verified with the available hydrological data measured in 2001. Detailed model re‐verification has been conducted with water surface elevations, tidal current, and salinity distributions measured. The overall performance of the model is in qualitative agreement with the available field data. The model was then used to investigate the change in tidal ranges, salt‐water intrusion, and residual circulation as a result of channel regulation in the Keelung River. The model simulations indicate that more tidal energy propagates into the estuarine system before channel regulation because of the substantial increase in river cross‐sections. The residual circulations before channel regulation are greater than those after channel regulation and result in the limits of the salt intrusion before channel regulation being extended farther inland than those after channel regulation. This may show that channel regulation for flood control in the Keelung River did not contribute to the expansion of the mangrove areas and the disappearance of freshwater marshes at the Kuan‐Du wetlands. Copyright © 2005 John Wiley & Sons, Ltd.

Water properties and transport of the Leeuwin Current and Eddies off Western Australia

Hydrological data from the western coast of Australia to 110°E are used together with direct velocity observations to investigate the water mass distribution and the associated circulation. High spatial variability reflects advective features related to the turbulent character of the Leeuwin region. The correlation between the sea surface temperature and the sea surface salinity distributions and the upper layer circulation depicts a southward warm and fresh Leeuwin Current along the coast with several branchings and three anticyclonic eddies offshore. The strongest eddy includes a thick surface mixed layer of 220 m and perturbs the hydrological structure down to more than 2000 m. The net transports through the four sections are northward in the upper layers with larger transports through the southern sections, which imply a westward transport just north of Abrolhos Islands. In the intermediate and deep-water layers, the net transport is southward across the southern section and divergent, implying an eastern inflow around 29°S. The data confirm that Subantarctic Mode Water is spreading north while the Antarctic Intermediate Water appears to recirculate to the south along the eastern rim of the Perth Basin.

Application of classical thermodynamic principles to the study of oceanic overturning circulation

Stationary deep-reaching overturning circulation in the ocean is studied by means of classical thermodynamic methods employing closed cycles in pV-space (p, pressure; V, volume). From observed (or computed) density fields, the pV-method may be used to infer the power required for driving a circulation with a given mass flux, or, if the available power is known, the resulting mass flux of the circulation may be assessed. Here, the circulation is assumed to be driven by diapycnal mixing caused by internal disturbances of meteorological and tidal origin and from transfer of geothermal heat through the ocean bottom. The analysis is developed on the basis that potential energy produced by any of these mechanisms is available for driving a circulation of the water masses above its level of generation. The method also takes into account secondary generated potential energy resulting from turbulence developed by the ensuing circulation.Models for different types of circulation are developed and applied to four types of hemispheric circulation with deepwater formation, convection and sinking in an idealized North Atlantic. Our calculations show that the energy input must exceed 15 J kg—1 for a cycle to the bottom to exist. An energy supply of 2 TW would in that case support a constant vertical mass flux of 3.2 G kg s—1 (3.1 Sv). Computed mass fluxes reaching the surface in the subtropics, corresponding to the same energy input, range between 2.3–5.2 G kg s—1, depending on the type of convection/sinking involved. Much higher flux values ensue with ascending water masses reaching the surface at higher geographical latitudes.The study reveals also that compressibility of sea water does not enhance the circulation. An incompressible system, operating within the same mass flux and temperature range, would require about 25% less energy supply, provided that the circulation comprises the same water masses. It is furthermore shown that the meridional distribution of surface salinity, with higher values in the tropics and lower values in regions of deep-water formation, actually enhances the circulation in comparison with one of a more uniform surface salinity. With a homohaline North Atlantic, operating within the same temperature range as presently observed, an increase of 66% of power supply would be required in order that the mass flux of the overturning circulation should remain the same.

Interpreting the colour of an estuary

This paper explores the possibility of using water colour to quantify the concentration of coloured dissolved organic matter (CDOM), and through it, dissolved organic carbon (DOC) and salinity in a turbid estuary in which suspended sediments also influence water colour. The motivation of the work is that the method could be applied to water colour measurements made remotely from an aircraft (or, in larger estuaries, a satellite) enabling near-synoptic mapping of surface salinity and DOC distributions. The paper describes observations at 29 stations distributed along the salinity gradient of the Conwy estuary in North Wales. At each station, surface water samples were collected and analysed for salinity, concentrations of DOC, chlorophyll and suspended particles and absorption spectra of CDOM, or yellow substance. Profiles were made of both upwelling and downwelling irradiance in four narrow band channels, and these were used to calculate irradiance reflection and attenuation coefficients. Results show that spectrally averaged light absorption in the estuary is caused principally and equally by mineral suspended solids and yellow substance, with water and chlorophyll in third and fourth place. The CDOM is strongly correlated ( R 2=0.99) in a negative sense with salinity, and more weakly correlated with DOC. There is a linear relationship between CDOM and the ratio of reflection coefficients in the red (670 nm) and blue-green (490 nm) parts of the spectrum, which could be applied to remote sensing; the slope and intercept of the relationship are however different to those found in less turbid water bodies. It is shown that the change in slope and intercept are consistent with the presence, in the Conwy estuary, of suspended particles which influence the water colour. A method is described and tested for inverting water colour measurements in a turbid estuary to give estimates of CDOM in the presence of suspended particles. The solution, which has not been adjusted to fit the data, produces profiles of CDOM, DOC and salinity, which behave reasonably but which currently have a limited accuracy. RMS differences between measured and optically derived parameters for the entire data set are 0.27 m −1 (CDOM), 4 PSU (salinity) and 67 μM (DOC) although better accuracy is obtained on individual surveys. The fact that there is little bias between predicted and observed parameters indicates that much of the scatter is caused by random measurement error and that the approach is fundamentally sound.

Measuring the salinity of the Clyde Sea from remotely sensed ocean colour

This paper describes a novel method of deriving surface salinity from remotely sensed ocean colour. The method is based on two important observations of optical properties in regions of freshwater influence (ROFI). The first is the strong effect that a form of dissolved organic matter (yellow substance) has on ocean colour when present in relatively high concentrations. The second is the close relationship between salinity and yellow substance originating from fresh water runoff. In this paper these relationships are demonstrated for the Clyde Sea, Scotland, and applied to SeaWiFS imagery for the derivation of yellow substance and surface salinity. The empirical relationships demonstrated in this study allow the satisfactory prediction of yellow substance and salinity in the Clyde Sea from remotely sensed ocean colour. The r.m.s. difference between the observed and predicted parameters are 0.19 m −1 and 1.1 for yellow substance and salinity, respectively, over a range of salinity from 16 to 34. Salinity maps created from the satellite-retrieved ocean colour identify features in the surface salinity distribution that to date have only been observed by in situ instrumentation. For example, the salinity distribution derived in this study is suggestive of an anticlockwise circulation in the Clyde Basin, driven both by the input of fresh water from the River Clyde and the intrusion of more saline water from the North Channel. The movement of deep water in the Clyde Sea has been well documented (Proc. R. Soc. Edinb. 90B (1986) 67), however, the residual circulation of surface water remains uncertain. The ability to obtain synoptic views of salinity such as those presented in this paper therefore provides great potential in furthering the understanding of Shelf Sea and coastal dynamics.

Structure and influence of tropical river plumes in the Great Barrier Reef: application and performance of an airborne sea surface salinity mapping system

Input of freshwater from rivers is a critical consideration in the study and management of coral and seagrass ecosystems in tropical regions. Low salinity water can transport natural and manmade river-borne contaminants into the sea, and can directly stress marine ecosystems that are adapted to higher salinity levels. An efficient method of mapping surface salinity distribution over large ocean areas is required to address such environmental issues. We describe here an investigation of the utility of airborne remote sensing of sea surface salinity using an L-band passive microwave radiometer. The study combined aircraft overflights of the scanning low frequency microwave radiometer (SLFMR) with shipboard and in situ instrument deployments to map surface and subsurface salinity distributions, respectively, in the Great Barrier Reef Lagoon. The goals of the investigation were (a) to assess the performance of the airborne salinity mapper; (b) to use the maps and in situ data to develop an integrated description of the structure and zone of influence of a river plume under prevailing monsoon weather conditions; and (c) to determine the extent to which the sea surface salinity distribution expressed the subsurface structure. The SLFMR was found to have sufficient precision (1 psu) and accuracy (∼3 psu) to provide a useful description of plumes emanating from estuaries of moderate discharge levels with a salinity range of 16 to 32 psu in the open sea. The aircraft surveys provided a means of rapidly assessing the spatial extent of the surface salinity distribution of the plume, while in situ data revealed subsurface structure detail and provided essential validation data for the SLFMR. The combined approach allowed us to efficiently determine the structure and zone of influence of the plume, and demonstrated the utility of sea surface salinity remote sensing for studying coastal circulation in tropical seas.

On sensitivity of ocean circulation to sea surface salinity

Sensitivity of global ocean circulation to details of sea surface boundary conditions has been revisited in a set of numerical experiments. The focus is on the role of sea surface salinity (SSS) asymmetries between different ocean basins. The results indicate that the basin-wide inter-basin salinity contrasts are the key elements of success for many idealized ocean models that have realistically mirrored ocean circulation despite strongly simplified sea surface conditions. We suggest that exact knowledge of the spatial distribution of sea surface salinity may not be crucial to the simulation of the thermohaline ocean circulation (THC) in many process-oriented climate studies. Therefore, simulations based on limited sea surface data, for example in paleoclimate studies, can yield acceptable circulation patterns, as long as the inter-basin surface salinity contrasts are retained. Possible feedbacks of both concurrent northern and southern freshwater impacts and the building up of inter-basin sea surface salinity contrasts are discussed.

On the role of inter‐basin surface salinity contrasts in global ocean circulation

The role of sea surface salinity (SSS) contrasts in maintaining vigorous global ocean thermohaline circulation (TOC) is revisited. Relative importance of different generalizations of sea surface conditions in climate studies is explored. Ocean‐wide inter‐basin SSS contrasts serve as the major controlling element in global TOC. These contrasts are shown to be at least as important as high‐latitudinal freshwater impacts. It is also shown that intra‐basin longitudinal distribution of sea surface salinity, as well as intra‐ and inter‐basin longitudinal distribution of sea surface temperature, is not crucial to conveyor functionality if only inter‐basin contrasts in sea surface salinity are retained. This is especially important for paleoclimate and future climate simulations.

Strategies to Improve Information Extraction from Multivariate Geophysical Data Suites

This paper discusses strategies for improving interpretation outcomes from large, multivariate data suites. It is shown that a systematic approach using simple spatial analysis techniques implemented in a geographic information system reveals information that is not readily evident using conventional, manual interpretation procedures. A combination of simple exploratory analysis, principal components analysis and unsupervised classification is used to represent the landscape as a series of geophysical/terrain units. A comparison of these classified units with the distribution of surface salinity reveals that the classification is not just an arbitrary division of the landscape, but one clearly related to the properties of the landscape relevant to salinity. This result is important for two reasons. First, it demonstrates a fast, systematic, robust and repeatable process for successfully extracting information from the multivariate data suite. This result would not be achievable using the conventional interpretation approach. Second, it provides a useful summary and clear representation of multivariate geophysical data, which when applied appropriately could significantly improve the utility of the data for land management planning.

Modelling of hydrodynamics and cohesive sediment transport in Tanshui River estuarine system, Taiwan

A laterally averaged two-dimensional numerical model is used to simulate hydrodynamics and cohesive sediment transport in the Tanshui River estuarine system. The model handles tributaries as well as the main stem of the estuarine system. Observed time series of salinity data and tidally averaged salinity distributions have been compared with model results to calibrate the turbulent diffusion coefficients. The overall model verification is achieved with comparisons of residual currents and salinity distribution. The model reproduces the prototype water surface elevation, currents and salinity distributions. Comparisons of the suspended cohesive sediment concentrations calculated by the numerical model and the field data at various stations show good agreement. The validated model is applied to investigate the tidally averaged salinity distributions, residual circulation and suspended sediment concentration under low flow conditions in the Tanshui River estuarine system. The model results show that the limit of salt intrusion in the mainstem estuary is located at Hsin-Hai bridge in Tahan Stream, 26 km from the River mouth under Q 75 flow. The null point is located at the head of salt intrusion, using 1 ppt isohaline as an indicator. The tidally averaged sediment concentration distribution exhibits a local maximum around the null point.

INFLUENCE OF BATHYMETRIC CHANGES ON HYDRODYNAMICS AND SALT INTRUSION IN ESTUARINE SYSTEM1

ABSTRACT: A vertical (laterally integrated) two‐dimensional numerical model has been applied to study the hydrodynamic characteristics and salt water intrusion in the Tanshui River estuarine system. The cross‐sectional profiles measured in 1978 and 1994 are schematized for model simulations. Detailed model calibration and verification have been conducted with water surface elevations, tidal current, salinity distributions, and residual velocities measured. The overall performance of the model is in qualitative agreement with the available field data. The model was then used to study how hydrodynamics and salt water intrusion change in response to changes in bathymetry. The model simulations indicate that more tidal energy propagates into the estuarine system in 1994 because of the substantial increase in river cross‐sections. The limits of salt intrusion in 1994 extended farther inland than those in 1978. On the other hand, the extent of mangrove wetland in the lower estuary has increased over the past 20 years and is likely a result of the increased salinity in the estuary.

The salinity front of the Río de la Plata ‐ A numerical case study for winter and summer conditions

A 3‐D baroclinic model has been applied to investigate the location of the salinity front of the Río de la Plata. River run‐off, tides and mean winds for summer and winter conditions respectively were considered in order to find out the most important factors for the observed seasonal variability of this location. The results show that wind forcing is responsible for the observed variability in summer. In winter, wind forcing is not important, but the river discharge. The resulting surface salinity distributions show a surprisingly good agreement with the present knowledge from observations.

Tropical Subsurface Salinity and Tritium Distributions in the Pacific: Their Differences and Formation Mechanisms*

While high salinity water extends to the equator in the upper thermocline of the Pacific in the Southern Hemisphere (SH), it hits the western boundary (WB) farther north of the equator in the Northern Hemisphere (NH), suggesting that no interior pathway exists to the equatorial region. By contrast, high tritium water appears on the equator in the central Pacific, apparently through a NH interior pathway within the thermocline. The mechanisms of forming these salinity and tritium distributions and the causes of their difference are investigated using a realistic ocean general circulation model (OGCM). The OGCM reproduces the properties of tropical salinity distribution quite well and displays interior pathways in the NH. Analysis indicates that the observed salinity distribution is compatible with the existence of a NH interior pathway. Key to the hemispheric difference in thermocline salinity is the sea surface salinity (SSS) distribution in relation to the so-called WB (interior) exchange window, from which subducted water goes to the equatorial region through the WB region (interior ocean). In the NH, high SSSs are found only in the WB exchange window, and high salinity water thus appears to turn onto the WB before reaching the equator. In the SH, on the other hand, high SSSs are found in both the WB and interior exchange windows, and, as a result, high salinity water extends to the equatorial region through both the WB region and interior ocean. The sea surface tritium field has high values near the eastern boundary within the interior exchange window in the midlatitude North Pacific. Thus, high tritium water takes the NH interior pathway to the equatorial region after the subduction. This is demonstrated by a passive tracer experiment with a sea surface distribution resembling that of tritium. This result suggests that the apparent differences between the isopycnal salinity and tritium distributions are largely due to differences in surface distribution, raising caution about interpreting ocean circulation with tracer fields alone.