Temperature-salinity Relationships Research Articles

Compensation of horizontal temperature and salinity gradients in the ocean mixed layer

Establishment of the temperature-salinity relationship in the ocean has concerned oceanographers for decades because of its importance for understanding ocean circulation. High-resolution measurements in the ocean mixed layer are used to show that temperature and salinity gradients on horizontal scales of 20 meters to 10 kilometers tend to compensate in their effect on density. These observations support the notion of a horizontal mixing in the mixed layer that depends on density gradient.

Spice and the Demon

One of the oldest mysteries of oceanography is how temperature and concentration interact in the midlatitude ocean. Measurements as a function of depth reveal a remarkably tight and predictable relation between temperature and salinity below the surface. In his Perspective, Schmitt discusses results presented in the same issue by Rudnick and Ferrari that offer some clues as to the solution of the puzzle. The new measurements suggest that internal mixing rather than atmospheric forcing may explain the temperature-salinity relation. Furthermore, the process of salt fingering--convective cells that transport heat and at different rates--appears to play an important role.

Mean and Near-Inertial Ocean Current Response to Hurricane Gilbert

The three-dimensional hurricane-induced ocean response is determined from velocity and temperature profiles acquired in the western Gulf of Mexico between 14 and 19 September 1988 during the passage of Hurricane Gilbert. The asymmetric wind structure of Gilbert indicated a wind stress of 4.2 N m−2 at a radius of maximum winds (Rmax) of 60 km. Using observed temperature profiles and climatological temperature–salinity relationships, the background and storm-induced geostrophic currents (re: 750 m) were 0.1 m s−1 and 0.2 m s−1, respectively. A Loop Current warm core ring (LCWCR) was also located to the right of the storm track at 4–5 Rmax, where anticyclonically rotating near-surface and 100-m currents decreased from 0.9 m s−1 to 0.6 m s−1 at depth. The relative vorticity in the LCWCR was shifted below the local Coriolis parameter by about 6%. In a storm-based coordinate system, alongtrack residual velocity profiles from 0 to 4 Rmax were fit to a dynamical model by least squares to isolate the near-inertial content over an e-folding timescale of four inertial periods (IP ≈ 30 h). Observed frequency shifts in the mixed layer currents ranged from 1.03 to 1.05f in agreement with both the backrotated velocity profiles at 1.04f relative to the storm profile (where maximum correlation coefficients were 0.8) and the predicted frequency shift from the mixed-layer Burger number. This frequency was increasingly blue shifted in the upper 100 m to 1.1f, decreasing toward f within the thermocline. Near-inertial currents rotated anticyclonically by 90°–180° in the upper ocean, providing the velocity shear for layer cooling and deepening observed on the right-hand side of the track. A summation of the first four baroclinic modes described up to 77% of this near-inertial current variability during the first 1.75 IP. However, the variance explained by this modal summation decreased to a minimum of 36% after 2.9 IP following passage due to phase separation between the first baroclinic mode and higher-order modes in the mixed layer. Although the response was complicated by the LCWCR, the evolving three-dimensional current structure can be described by linear, near-inertial wave dynamics.

Dynamics of Turbidity in the Tweed Estuary

Results are presented of turbidity measurements made in the Tweed Estuary during 14–29 September 1993. The observations covered a spring-neap period of very strong and relatively weak tides, which included strong wave activity at the mouth of the estuary, the aftermath of a strong freshwater inflow event and a minor inflow event which coincided with neap tides. Turbidity levels between the mouth and the limit of saline intrusion during this period were observed to lie in the range 2–30ppm. Temperature–salinity relationships, based on rapid sampling throughout the estuary, often indicated conservative mixing between riverine and coastal waters. Turbidity–salinity relationships throughout the estuary were approximately linear for most of the lower salinity range (<30) although the relationships varied throughout a tidal cycle. At the highest salinities, in the lower 2·5km of estuary, significantly enhanced turbidities occurred during strong, onshore swell-wave conditions. Near-mouth turbidities were very low (<5ppm) at high-water (HW) when the swell-wave height was small (<0·3m). There was no correlation between near-mouth turbidity at HW and tidal range, whereas the correlation between near-mouth turbidity and swell-wave height explained over 90% of the variance in near-mouth turbidity. The temporal trends in freshwater turbidity and freshwater inflow during the fieldwork period were fairly similar. A statistically significant relationship existed between these variables when freshwater turbidity was correlated against inflow 30h earlier. A ‘model’ of turbidity, based on 5 days of inflows and used to hindcast turbidity from 14 days of inflows between 16–29 September, explained over 80% of the variance in freshwater turbidity. Therefore, inflow exerted a significant control on the fluvial turbidity. It appears that very fine-grained particles were responsible for the observed turbidity in the central and upper reaches of the Tweed. During the flood, in the presence of strong wave activity in the coastal zone, larger sediment was rapidly winnowed in the lower reaches, close to the mouth, whereas fine silt particles remained in suspension and reached the limit of saline intrusion.

Comparison of XCTD/CTD data

XCTD (eXpendable Conductivity Temperature Depth) probes, developed recently by SIPPICAN Inc., have been used simultaneously with a CTD sonde in order to test, in the field, their performance and accuracy (interpreted as ±2 standard deviations of the XCTD-CTD differences). We have taken advantage, during the THETIS-I experiment in March 1992, of both the homogeneous and the stratified areas encountered in winter in the northern part of the western Mediterranean Sea to differentiate the errors due to the experimental conditions from those effectively due to the sensors. Although some intrinsic problems are evident, so that only seven out of the nine probes considered for comparison are usable, the accuracy specified by the manufacturer for the temperature (AT = ± 0.03°C) is reached after standard processing, while the accuracies in conductivity, salinity and potential density are AC ≈ ± 0.06 mS/cm (the specified value is AC = ± 0.03 mS/cm), AS ≈ ± 0.04 and A σ θ ≈ ±3 kg/m 3. However, when the experimental errors ( in situ natural variability, relatively rough estimation of the XCTD depth) are considered, it appears that the effective accuracies of the XCTD sensors are better than ± 0.02°C and ± 0.04 mS/cm, that is to say better than and close to the specified values of ± 0.03°C and ± 0.03 mS/cm. Occasional offsets in conductivity can further be well corrected for by using a temperature-salinity relation in some limited depth range and area where this relation is known to hold well; the conductivity-sensor accuracy then significantly improves to AC≈ ± 0.02 mS/cm resulting, for our study area, in corresponding salinity and potential density accuracies of AS≈ ± 0.03 and A σ θ ≈ ± 0.02 kg/m 3. Thus, such instruments promise to be useful tools for many experimental studies. Complementary comparisons, performed with new versions of the XCTD probes under less convenient experimental conditions, are also presented.

A deep western boundary current in the South Indian basin

A section of closely-spaced hydrographic stations was occupied for the first time across the western boundary of the South Indian Basin, above the flank of the Kerguelen Plateau. A northward-flowing deep boundary current was found, whose transport is estimated geostrophically to be 6 × 10 6 m 3 s −1. The section crossed meanders of fronts within the Antarctic Circumpolar Current, with transports estimated as 5−15 × 10 6 m 3 s −1. The source of bottom water is concluded to be the neighbouring Antarctic shelf, including the Adélie coast and a component from the Ross Sea. Changes in the deep temperature-salinity relation are consistent with an increased component of relatively salty bottom water from the Ross Sea, over the time period 1972–1992.

The deep silica tongue in the North Atlantic

The distribution of dissolved silica and the variation of the temperature-salinity relationship are important tracers of the large-scale mid-depth flow of water in the Atlantic. A prominent feature of the silica distribution near 24°N is a westward-pointing silica tongue, analogous to the salty Mediterranean tongue concentrated at shallower levels. Earlier advection-diffusion studies showed how the structure of the salt tongue could be consistent with the expected general circulation of the subtropical gyre. The expected circulation pattern of the mid-depth levels is that of the Stommel and Arons model, with the deep interior flow to the north and east; however, an advection-diffusion model is unable to reproduce the deeper silica tongue for any value of horizontal diffusion with eastward flow, nor, it is argued, can reasonable spatial variations in horizontal diffusivity be responsible. Thus, while the Stommel and Arons circulation would have the deep interior flow to the east and north, evidence is presented here for a westward flow of water in this depth range (2000–3500 m). Such interior flow would tend to increase the deep western boundary current transport as it flows south. The explanation for this is suggested to be related to the existence of deep-water formation in the northern North Atlantic, which entrains mid-depth water and forces the area-averaged vertical velocity to decrease with height.

Conversion among North Atlantic surface water types

Climatological data sets are used to estimate the amount of water changing temperature and salinity at the sea-surface owing to air-sea exchanges. Considering this conversion as a function of sea-surface temperature and salinity leads to the definition of a conversion vector, which represents mass sources and sinks for each water type. The vector representation shows graphically the way the shape of the temperature-salinity relation is driven at the surface by the air-sea heat and fresh water fluxes.

On the Density Ratio Balance in the Central Water

An equation for the conservation of density ratio on isopycnal surfaces is derived. It is shown that a vertical shear can modify the density ratio if salinity (and temperature) gradients exist along isopycnals. The mixing processes of turbulence, salt fingering and lateral isopycnal mixing are evaluated for possible balances with the shear term in the “Rp, = constant” Central Waters. It is found that turbulence cannot provide a balance and that isopycnal mixing can balance only with a particular vertical variation in the mixing rate. Salt firm however, provide a simple balance between the shear term and the vertical variation of flux divergence, since beat and salt are transported at different rates. The “conservation of density ratio” should provide a useful constraint on mixing estimates from hydrographic data in both double-diffusive and non-double-diffusive regimes and could be applicable to modelling studies of the temperature-salinity relation in the thermocline.

Trends in Atlantic equatorial current variability

Approximately twice-monthly expendable bathythermograph (XBT) sections between Europe and Brazil, are used to characterize trends in the equatorial geostrophic surface currents orthogonal to the sections between September, 1980 and May, 1984. Using mean temperature-salinity relationships the upper layer temperature profiles are converted to density and used to compute 0/300 db dynamic height. Applying a second derivative method, at and near the equator, geostrophic surface currents are computed along each quasimeridional XBT section and time/space series of the equatorial currents are developed using spline interpolations in both time and space. Equatorial currents are mapped as time series of dynamic height and geostrophic current.

Water types and volumetric considerations of the South-East Atlantic upwelling regime

A preliminary investigation was carried out to establish the characteristic temperature-salinity (t/s) relationships of the upwelling, mixing and pure oceanic domains of the South-East Atlantic Ocean. The upwelling and oceanic regimes have disparate and non-overlapping t/s characteristics, whereas the mixing regime shows characteristics of both the other regimes. Upwelled water is derived from South Atlantic Central Water, which is heated at the sea surface to a maximum of 23°C. A volumetric analysis for the same geographic area shows that, for the area as a whole, South Atlantic Subtropical Surface Water is dominant at the sea surface and that, furthermore, the volumes of Central Water available for upwelling are invariant with bivariate t/s class. This suggests that the origin of upwelled water, and the intensity of upwelling, may be estimated from salinity readings only.

Mean temperature-salinty relationships in Australian waters and their use in water mass analysis

Within a network of 2.5� (latitude) × 5� (longitude) bins in Australian waters (5�-45�s.; 105�-160�E.), mean temperature-salinity (TS) relationships have been obtained. They consist of a series of mean salinities at 2.5-degree intervals between 5 and 27.5�C. From the TS curves, at temperatures between 10 and 25�C, a region of strong meridional TS gradients is identified, centred west of the Australian mainland at 15�s. This marks the northernmost penetration of South East Indian Central Water as specified by a salinity maximum in the TS curves. The waters to the east of Australia display a uniform pattern with an upper salinity maximum associated with Subtropical Lower Water. In the whole region of study the presence of Antarctic Intermediate Water is observed as a lower salinity minimum. The TS curves are shown to be tight enough throughout the regions for useful geopotential values to be computed from temperature profiles and a mean TS relationship. Errors introduced into the calculation by this technique are less than or equal to the error involved using actual temperature and salinity data.

A Low-Cost Digital XBT System and its Application to the Real-Time Computation of Dynamic Height

An XBT interface is described for use with Commodore and other 6502 based microprocessors. This interface takes the form of a single circuit board mounted inside the microcomputer and is completely software controlled. The application of this digital XBT system to the real-time computation of density and dynamic height, using historical or recent temperature-salinity relationships, is also described. Comparison between XBT and CTD measured temperatures from the Northeast Atlantic yield a mean temperature difference of −0.08°C and an rms temperature difference of 0.33°C for the upper 800 m. Examples of dynamic topography maps and a temperature section computed using this technique are also presented and comparison between objectively analyzed XBT and CTD dynamic topographies demonstrates the reliability of the method for mapping the baroclinic flow.

Forma da correlação T-S de massas de água das regiões costeira e oceânica entre o Cabo de São Tomé (RJ) e a Ilha de São Sebastião (SP), Brasil

Characteristics of the structure and sea water properties on the continental shelf and waters adjacent to the continental slope, were studied based on the temperature-salinity (T-S) relationships. The water sampling was carried out with standard methods in a network of hydrographie stations worked in five-time intervals (5-10 days), from January 1970 through February 1971. The classification review of the water masses used under regional conditions confirm that the Subtropical-Deep (STDW), Subtropical (STW) and South Atlantic Central (SACW) Water Masses are different nomenclature for the same water mass; the latter is suggested to indicate a water mass with a thermohaline index given by (20.0 oC; 36.36 ‰), observed underneath the Tropical Water Mass. This salinity value is slightly higher than the modified thermohaline index of the SACW (36.20 ‰), and can be used with great simplification in volumetric studies in the area. Polynomial expressions of the mean T-S curve, whose coefficients were computed with the least squares method, were given for practical applications. From these applications, alternative methods for the determination of the thermohaline derivative and constant density ratio (Rρ = αdT/βdS) were obtained.

Currents, fronts and eddy fluxes in the Canary Basin

Hydrographic data from two cruises in the Canary Basin ( Meteor 57, July 1981; Poseidon 86, April 1982) are analysed with respect to current distribution and lateral heat flux in the Azores-Madeira region. The first part of the data base consists of long transects of XBT and G.E.K. measurements between Cape Finisterre (North West Spain) and the northern Canary Basin, where several year-long current meter records exist. Further information is obtained by thermosalinograph surface data and by expendable current profilers (XCP). Geostrophic currents are derived from XBT profiles, using the tight temperature-salinity relationship in the depth range of the Warmwassersphäre. The results compare well with the G.E.K. and XCP current observations. The second part consists of CTD data from an eddy resolving, box-shaped CTD survey (500 × 500 km 2) centered at the mooring location (33°N, 22°W), The observations are supplemented by satellite-buoy trajectories. Horizontal parameter distribution is shown in terms of objectively contoured maps. Bands of spatially enhanced energetic structures, seen in the long transects are further resolved by the box survey as a deep jet-like current system cross the Canary Basin in a west-east direction. Associated with this Azores Current is a frontal zone with near-surface temperature and salinity steps of order 2 K and 0.3 practical salinity units. The dynamic topography field can be decomposed into a linear background field, a Rossby wave and a mesoscale eddy field. We find that major contributions to the meridional eddy heat flux are confined to the vicinity of the Azores current frontal zone. It is shown that the principal balance in the temperature equation is between heating by the mean horizontal advection terms and cooling by the eddy flux divergence.

Observations of planktonic bioluminescence in the euphotic zone of the California Current

The distributions of bioluminescence, temperature, salinity, oxygen. pH, and chlorophyll a were measured at 10 m intervals, to a depth of 100 m at a station (33°46′N; 119°36′W) in the California Current from 17 to 20 July 1982. The distribution of bioluminescence showed a marked day-night change which was consistent over the sampling period. The nighttime maximum was at the surface, and the daytime maximum was between 30 and 40 m. The shapes of the day and night distributions were independent of the absolute intensity of bioluminescence and were also insensitive to advection, as inferred from changing temperature-salinity relationships. The nighttime depth distribution broadened during a period of high wind Day to night differences in the color spectrum at the depth of maximum bioluminescence suggest that the luminescent organisms differed from day to night.

Estimation of dynamic heights from temperature profiles in the Tasman Sea

Three methods chosen to estimate dynamic height from temperature profiles in the Tasman Sea were: (1) simple linear regression of dynamic height relative to 1300 dbar (D0/1300) on temperature at 250 m (T250); (2) simulating salinity profiles from mean temperature-salinity (T-S) relationships to use with standard- depth temperatures such as Nansen casts to 1300 m; and (3) simulating salinity profiles from mean T-S relationships to use with expendable bathythermograph profiles to 450 m; and then linear regression of D450/1300 on T450 for the dynamic height below 450 m. The approximate dynamic heights, relative to 1300 dbar, were compared with the true dynamic heights (calculated in the normal way) at the 702 stations examined. Root-mean-square differences were shown to be 7-8, 24 and 5-5 dyn cm, respectively, for the three methods.

The Subpolar Mode Water of the North Atlantic Ocean

Abstract The warm waters of the subtropical and subpolar basins of the North Atlantic have tight regional temperature-salinity relationships, and are conventionally called the regional “Central Waters.” A volumetric census of the temperature-salinity characteristics of the North Atlantic by Wright and Worthington (1970) shows that waters characterized by certain segments of the T-S relationships have large volumes compared to those of other segments: volumetric “Mode Waters.” Such Mode Waters appear as layers with increased vertical separation between isopycnals-pycnostads. The present study reports on the existence of pycnostads in the central and eastern North Atlantic. These Subpolar Mode Waters are formed by deep winter convection in the subpolar North Atlantic, and participate in the upper water circulation of the northern North Atlantic. The seasonal outcropping of the pycnostads occurs within and adjacent to the North Atlantic Current, the Irminger Current, the East and West Greenland Currents, and...

Oceanographic and ecologic significance of diatoms in surface sediments of the Bering and Okhotsk seas

Distribution of diatom assemblages in surface sediments of the northernmost Pacific and its marginal seas shows a close correlation to the distribution of major water masses. The water masses, defined mostly by temperature-salinity relations, are the result of the interaction of a complex of processes—including precipitation, river runoff, ice formation and melting, and upwelling and mixing by divergence, wind stress, and tidal currents. At least seven assemblages of diatoms can be defined. They represent the Alaskan Stream, the inner continental shelf, productivity in sea-ice, the East Kamchatka Current, central “dichothermal” waters of the Okhotsk Gyre, outer shelf productivity in the Sea of Okhotsk, and high-temperature, high-salinity oceanic waters in the southern Sea of Okhotsk. Variations in dominance of these assemblages in older sediments can be used to derive detailed information on paleo-hydrography.

Form of the Temperature-Salinity Relationship in the Central Water: Evidence for Double-Diffusive Mixing

Abstract Ingham (1966) reported that the temperature-salinity relationships in the Central Waters were much better described by a curve of constant density ratio (Rρ = αΔT/βΔS) than by a straight line. His result is quantitatively verified and a simple, but powerful, double-diffusive mechanism is proposed to explain the observed constancy of Rρ in the main thermocline. The mechanism is based on the evidence from theory, experiment and observation that the intensity of salt-finger convection is a strong function of Rρ. This dependence, plus the fact that more salt than heat is transferred by the fingers, causes any deviation from a constant Rρ to be the site of convergence or divergence of the vertical salt flux that acts to remove the perturbation in Rρ. A linear treatment of the mechanism shows that Rρ can be “diffused” with an effective diffusivity that is much greater than the diffusivities of heat or mass. A few numerical examples illustrate the predicted effects of salt fingering on the T-S relation,...