Conductivity, Temperature, Depth Data Research Articles

Evaluation of calibrated salinity from profiling floats with high resolution conductivity‐temperature‐depth data in the East/Japan Sea

The East/Japan Sea is an ideal region for the assessment of the quality of calibrated profiling float salinity data due to the availability of recent shipboard conductivity‐temperature‐depth (CTD) observations, dense spatial occupation by profiling floats, and its unique water mass structure. More than 5000 salinity profiles obtained from the profiling floats have been corrected with respect to two calibration methods, and their qualities have been evaluated using 787 high‐resolution CTD profiles. In this paper, we suggest not only a method for the real‐ and non‐real‐time quality control of the float salinity data acquired in the East/Japan Sea (EJS) but also the uncertainty in the quality‐controlled salinity estimated from a statistical comparison with independent high‐resolution measurements. After applying the scientific quality control (Wong et al., 2003, hereafter referred to as WJO), the salinity data exhibit 1 standard deviation uncertainty of 0.003 psu which is comparable to the manufacturer's accuracy. However, the scientific calibration in the EJS provides a significant bias error of about 0.002 psu in the float salinity profiles adjusted to historical CTD data over the last 7 years. The WJO method could produce a bias error in the calibrated salinity for an open ocean where recent hydrographic data are insufficient and if significant salinity trends exist. Since the deep salinity change in the North Atlantic is two times larger than that in the EJS, the bias problem of the calibrated float salinity could be more serious in the North Atlantic.

Estimating salinity to complement observed temperature: 1. Gulf of Mexico

This paper and its companion [Thacker, W.C., Sindlinger, L., 2007-this issue. Estimating salinity to complement observed temperature: 2. Northwestern Atlantic. Journal of Marine Systems. doi:10.1016/j.jmarsys.2005.06.007.] document initial efforts in a project with the goal of developing capability for estimating salinity on a region-by-region basis for the world oceans. The primary motivation for this project is to provide information for correcting salinity, and thus density, when assimilating expendable-bathythermograph (XBT) data into numerical simulations of oceanic circulation, while a secondary motivation is to provide information for calibrating salinity from autonomous profiling floats. Empirical relationships between salinity and temperature, which can be identified from archived conductivity–temperature–depth ( CTD) data, provide the basis for the salinity estimates. The Gulf of Mexico was chosen as the first region to explore for several reasons: (1) It's geographical separation from the Caribbean Sea and the North Atlantic Ocean makes it a “small ocean” characterised by a deep central basin surrounded by a substantial continental shelf. (2) The archives contain a relatively large number of CTD data that can be used to establish empirical relationships. (3) The sharp fronts associated with the Loop Current and its rings, which separate water with different thermal and haline characteristics, pose a challenge for estimating salinity. In spite of the shelf and the fronts, the relationship between salinity and temperature was found to be sufficiently regular that a single empirical model could be used to estimate salinity on each pressure surface for the entire Gulf for all seasons. In and below the thermocline, root-mean-square estimation errors are small — less than 0.02 psu for pressures greater than 400 dbar, corresponding to potential density errors of less than 0.015 kg/m 3. Errors for estimates nearer to the surface can be an order of magnitude larger.

Super-deep CTD measurements in the Izu-Ogasawara trench and a comparison of geostrophic shears with direct measurements

CTD (Conductivity-Temperature-Depth) data at five stations across the Izu-Ogasawara Trench at 34°N were examined. Geostrophic velocity was in accordance with the directly measured currents. Above the trench floor, potential temperature increased at a rate of 0.6 m°C/1000 db from 8000 db to 9417 db, and salinity increased from 8300 db to the bottom. Potential density was almost constant at 7100–8700 db, and it increased to the bottom. Above the eastern and western flanks, inversion of potential density was indicated in the bottom layers with an increase of potential temperature and a decrease of salinity, suggesting geothermal heating and outflow of ground water.

Spatial and temporal aspects of phytoplankton blooms in complex ecosystems off the korean coast from satellite ocean color observations

Complex physical, chemical and biological interactions off the Korean coast created several striking patterns in the phytoplankton blooms, which became conspicuous during the measurements of ocean color from space. This study concentrated on analyzing the spatial and temporal aspects of phytoplankton chlorophyll variability in these areas using an integrated dataset from a Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Advanced Very High Resolution (AVHRR) sensor, and Conductivity Temperature Depth (CTD) sensor. The results showed that chlorophyll concentrations were elevated in coastal and open ocean regions, with strong summer and fall blooms, which appeared to spread out in most of the enclosed bays and neighboring waters due to certain oceanographic processes. The chlorophyll concentration was observed to range between 3 and 54 mg m-3 inside Jin-hae Bay and adjacent coastal bays and 0.5 and 8 mg m-3 in the southeast sea offshore waters, this gradual decrease towards oceanic waters suggested physical transports of phytoplankton blooms from the shallow shelves to slope waters through the influence of the Tsushima Warm Current (TWC) along the Tsushima Strait. Horizontal distribution of potential temperature (θ) and salinity (S) of water off the southeastern coast exhibited cold and low saline surface water (θ 12°C; S>34.4) at 75dBar, corroborating TWC intrusion along the Tsushima Strait. An eastward branch of this current was called the East Korean Warm Current (EKWC), tracked with the help of CTD data and satellite-derived sea surface temperature, which often influenced the dynamics of mesoscale anticyclonic eddy fields off the Korean east coast during the summer season. The process of such mesoscale anticyclonic eddy features might have produced interior upwelling that could have shoaled and steepened the nutricline, enhancing phytoplankton population by advection or diffusion of nutrients in the vicinity of Ulleungdo in the East Sea.

Sea‐ice melting processes inferred from ice–upper ocean relationships in the Ross Sea, Antarctica

Sea‐ice melting processes are inferred from various summer sea‐ice and upper ocean data obtained in the Ross Sea in January 1999. Using spatially (30 km) averaged continuous data, an ice concentration–water temperature plot (CT‐plot) shows that the temperature at a depth of ∼7 m increases as ice concentration decreases in the ice interior region. The CT‐plot is explained by a simple ice–upper ocean coupled model in which sea‐ice melting is caused only by heat input through open water. The bulk heat transfer coefficient between ice and ocean (Kb) is estimated to be 1.2 × 10−4 m s−1. These findings are supported by the relationship between ice concentration and heat content in the mixed layer estimated from conductivity‐temperature‐depth (CTD) data. Salinity at a depth of ∼7 m, salt deficit in the mixed layer associated with sea‐ice melt estimated from CTD data, and their relationships with ice concentration also suggest that melting is mainly caused by atmospheric heat input through open water and that local balance of salinity nearly hold. The time evolution of ice concentration calculated from the ice–upper ocean coupled model with the estimated Kb corresponds with that derived from the Special Sensor Microwave Imager (SSM/I), confirming the model's applicability and the value of Kb.

Circulation and water masses beneath the northern Ronne Ice Shelf, Antarctica

We present oceanographic data from beneath the northern Ronne Ice Shelf. The data were collected during the austral summer of 2002–2003 from four sites located near the ice front in the Ronne Depression. They consist of conductivity‐temperature‐depth (CTD) profiles and time series from moored instruments that vary in length from 9 to 20 weeks. A strong, tidally modulated inflow of relatively fresh water was found at the eastern margin of the Ronne Depression. This low‐density inflow powers high basal melt rates that are responsible for a substantially thinned area of ice shelf. A northward flow of Ice Shelf Water along the western margin of the depression (the Antarctic Peninsula coast) was inferred from the CTD data. From the new CTD and current meter data, and from published results from cruises along the ice front, we suggest that the flows at the margins of the Ronne Depression establish east‐west density gradients that drive an anticyclonic circulation within the depression. The barotropic component of the circulation forms a gyre of strength 5 × 105 m3 s−1 and occupies a bowl in the field of water column thickness in the northern portion of the depression. All water masses sampled had temperatures below the surface freezing point and are therefore classified as Ice Shelf Water. The relatively complex nature of the oceanographic regime in the Ronne Depression is overlain by a seasonal variability that is hinted at by the available time series, probably explaining the apparent absence of inflowing HSSW at the time of the measurements.

Observations and simulations of an unsteady island wake in the Firth of Forth, Scotland

Beamer Rock, a 50-m-wide island in the Firth of Forth, produces a distinctive von Karman vortex street wake, the characteristics of which depend on the speed and direction of the tidal flow. ADCP (acoustic Doppler current profiler), CTD (conductivity–temperature–depth) and aerial photograph data were collected from the region during flood and ebb tidal flow under neap and spring conditions. Good agreement was found between the observations and the results from a fixed-grid depth-averaged numerical tidal model. The island wake parameter correctly predicted the unsteady nature of the wake, and the Strouhal number (defined in terms of flow past a circular cylinder) was found to give excellent predictions of the wake wavelength when scaled on the island width. Contrary to published results, the study shows that it is possible to accurately simulate an unsteady island wake using a relatively coarse fixed-grid numerical model.

Salinity estimation using the T‐S relation in the context of variational data assimilation

A salinity adjustment scheme using statistics of the temperature‐salinity (T‐S) relationship is proposed in the framework of variational data assimilation. The scheme not only considers the mean T‐S diagram but also takes the variance of the T‐S relations into consideration. To test performance of the scheme, a 1‐D mixed layer model of the Mellor‐Yamada type and a conductivity‐temperature‐depth (CTD) data set in the western tropical Pacific are used in two experiments. The CTD data set contains intensive temperature and salinity observations. In the experiments, first, the mean T‐S diagram and its variance are obtained from temperature and salinity observations over the assimilation period; then, temperature and salinity profiles are adjusted by minimizing a cost function that contains four terms: the temperature observation term, the background terms for both temperature and salinity, and a term measuring the distance between the salinity and the inferred salinity from the temperature via the mean T‐S diagram. A simpler scheme, which first adjusts the temperature profile by assimilating temperature data and then replaces the model salinity profile using the mean T‐S diagram and the newly updated temperature profile, is also tested for comparison. The proposed scheme shows good skill in reconstructing salinity variations at most levels and performs slightly better than the simpler scheme with overall improvements of 4–10%. The applicability of the method is also discussed.

Direct velocity measurements in the Subantarctic Front and over Campbell Plateau, southeast of New Zealand

Flow variability in the Subantarctic Front (SAF) and over the Campbell Plateau, southeast of New Zealand, is examined using data from a current meter mooring array along with conductivity‐temperature‐depth (CTD) and satellite altimetry data. These data are part of an ongoing study of ocean climate variability in the region. Contrasting flow regimes were found with very energetic and highly variable flows in the SAF flowing along the Subantarctic Slope and much weaker flows over the Campbell Plateau. Currents up to 75 cm s−1 were observed in the core of the SAF, while flows over the plateau were typically less than 10 cm s−1. Estimates of the total bottom referenced transport from three CTD sections across the SAF varied between 48 and 68 Sv. Transports were also estimated using TOPEX/Poseidon altimetry sea truthed with the CTD data. The resulting 8‐year time series exhibited high variability resulting from current meanders and eddy activity. One event in the record appeared to be the result of a cyclonic eddy passing through the mooring array. This event was modeled using an idealized geostrophic eddy. The simulation reproduces the main features observed in the current meter records and the surface topography from the TOPEX/Poseidon altimeter. Inspection of the TOPEX/Poseidon data suggests that large cyclonic and anticyclonic eddies occur in the frontal region with a frequency of around 9 yr−1.

Data on bottom water in Prydz Bay, Antarctica, revised

We report on salinity biases in conductivity‐temperature‐depth (CTD) data collected offshore Prydz Bay that affect conclusions about possible bottom water formation. The role of the Prydz Bay region (68–81°E) in bottom water production around Antarctica is an important unresolved issue [Smith and Treguer, 1994; Nunes Vaz and Lennon, 1996; Whitworth et al., 1998]. Wong, Bindoff, and Forbes [1998] identified a new, dense water type over the abyssal plain northwest of the Prydz Bay Channel at ˜72.5°E. Its spatial pattern was such that the highest sea floor salinities, ˜0.03 psu above historical measurements, were observed nearest the channel and decreased westward, away from Prydz BayExamination of the calibrated CTD data bottle data analyzed in Wong et al. [1998] shows a systematic bias in salinities off the continental shelf, most strongly in the stations to the west of Prydz Bay. The bias appears directly attributable to an undetected deterioration in conductivity cell performance toward the end of the observation program. The change in cell behavior is consistent with a cracked conductivity cell. This deterioration introduced an artificial curvature into the salinity profiles at depths exceeding 2000 m and a wider spread of the bottom salinity values to the west and north of Prydz Bay.

Evidence of a Barrier Layer in the Sulu and Celebes Seas

Variability in the surface isothermal and mixed layers of the Sulu and Celebes Seas is examined using the conductivity–temperature–depth data from the Navy's Master Oceanographic Observational Data Set (MOODS). Vertical gradient is calculated to determine isothermal layer depth with a criterion of 0.05°C m−1 for temperature profiles and mixed layer depth with a criterion of 0.015 kg m−4 for density profiles. When the isothermal layer depth is larger than the mixed layer depth, the barrier layer occurs. This study shows that the barrier layer occurs often in the Sulu and Celebes Seas. In the Sulu Sea, the barrier layer has seasonal variability with a minimum occurrence (38%) and a minimum thickness (3 m) in May and a maximum occurrence (94%) and a maximum thickness (36.5 m) in September. In the Celebes Sea, the barrier layer thickness changes from a maximum (49.7–62.0 m) in March–April to a minimum (9.6 m) in June. Possible mechanisms responsible for the barrier layer formation are discussed. In the Sulu Sea, the barrier layer may be formed by both rainfall and stratification; in the Celebes Sea, a rain-formed mechanism seems a major factor.

Evaluation of the Princeton Ocean Model Using South China Sea Monsoon Experiment (SCSMEX) Data

The Princeton Ocean Model (POM) has been implemented in the South China Sea for hindcast of circulation and thermohaline structure. A two-step technique is used to initialize POM with temperature, salinity, and velocity for 1 April 1998 and integrate it from 1 April 1998 with synoptic surface forcing for 3 months with and without data assimilation. Hydrographic and current data acquired from the South China Sea Monsoon Experiment (SCSMEX) from April through June 1998 are used to verify, and to assimilate into, POM. The mean SCSMEX data (Apr‐Jun 1998) are about 0.58C warmer than the mean climatological data above the 50-m depth, and slightly cooler than the mean climatological data below the 50-m depth, and are fresher than the climatological data at all depths and with the maximum bias (0.2‐0.25 ppt) at 75-m depth. POM without data assimilation has the capability to predict the circulation pattern and the temperature field reasonably well, but has no capability to predict the salinity field. The model errors have Gaussian-type distribution for temperature hindcast, and non-Gaussian distribution for salinity hindcast with six to eight times more frequencies of occurrence on the negative side than on the positive side. Data assimilation enhances the model capability for ocean hindcast, if even only conductivity‐temperature‐depth (CTD) data are assimilated. When the model is reinitialized using the assimilated data at the end of a month (30 Apr; 31 May 1998) and the model is run for a month without data assimilation (hindcast capability test), the model errors for both temperature and salinity hindcast are greatly reduced, and they have Gaussian-type distributions for both temperature and salinity hindcast. Hence, POM gains capability in salinity hindcast when CTD data are assimilated.

Subantarctic oceanography around New Zealand: Preliminary results from an ongoing survey

An ongoing observational program focused on variability in the subantarctic currents and water masses around southern New Zealand was initiated in May 1998. This paper describes the preliminary results deduced from CTD (conductivity, temperature, depth) data collected during three hydrographic surveys, and moored current and temperature records between 1998 and 1999. An extensive archived data set has also been analysed to provide a hydrographic climatology of the region. The low‐frequency circulation within the subantarctic zone is described, revealing previously unreported flow features: persistent but weak anticyclonic and cyclonic circulations over the Campbell Plateau and a strong cyclonic flow around the western edge of the Bounty Trough. Flow within the Subantarctic Front (SAF) is strongly constrained by the New Zealand bathymetry; diverting to follow the south‐eastern flanks of the Campbell Plateau and crossing to the Bounty Plateau, before separating to join the basin‐scale circulation. The flow features deduced from recent data are consistent with the hydrographic climatology as well as mid‐depth float trajectories. Subantarctic Mode Water (SAMW) is observed year‐round over an extensive fraction of the subantarctic zone: along the equatorward side of the SAF, over the Campbell Plateau, and flowing within the cyclonic circulation around the Bounty Trough. There is a marked cooling and freshening of SAMW between the deep water along the western flanks of Campbell Plateau and waters further east, consistent with a blocking of the eastward flow carrying warm and salty SAMW by the plateau. Earlier ideas that a substantial volume of SAMW is formed over the Campbell Plateau by deep vertical mixing (Heath 1981) are not substantiated by our data, which include seasonal observations of the upper water column.

Hydrophysical experiment “Megapolygon‐87” in the northwestern Pacific subarctic frontal zone

The purpose of this paper is to familiarize readers with major results and findings of the biggest multi‐institutional Soviet hydrophysical experiment, “Megapolygon‐87,” conducted in the northwestern Pacific Subarctic Frontal Zone in June‐October 1987. Six large‐scale, high‐spatial‐resolution conductivity‐temperature‐depth (CTD) surveys (1653 profiles) and numerous moorings (an array of 177 buoys deployed within a 500 km × 500 km area) with current meters at four depths provided unique observations of frontal structure, mesoscale and submesoscale eddies, and their evolution and interaction with each other. We discuss errors induced in the velocity records by surface waves and show how they can be corrected to a reasonable value. Joint analysis of current meter and CTD data allowed detection of a number of interesting processes and objects in the study area. Among them are meandering of the Subarctic Front around its branching point with formation of chains of cyclonic and anticyclonic mesoscale eddies and strong baroclinicity of velocity fields with significant difference between the thermocline and abyssal flows. Statistical characteristics of corrected “Megapolygon‐87” velocities are compared with preceding measurements in the region.

Diagnostic analysis of the 3D ageostrophic circulation from a multivariate spatial interpolation of CTD and ADCP data

A multivariate (MV) optimal statistical interpolation method is applied to conductivity–temperature–depth (CTD) and ship-mounted acoustic doppler current profiler (ADCP) data from quasi-synoptic oceanographic surveys. MV analysis aims to improve the spatial interpolation of any particular variable (e.g., dynamic height) by including in the analysis observations of other physically related variables (e.g., current). The version used in this work also provides estimates of the non-divergent and irrotational components of the flow. The method is tested in a sharp frontal region to the north of the Western Alborán gyre. After deriving the optimal analysis parameters, we first show that MV statistical dynamic height analysis errors are significantly smaller than those derived from univariate (UV) analysis. In our region, this translates in a more realistic shape for the geostrophic relative vorticity and the vertical velocity field. The latter peaks at about 45 m/day (as given by the quasi-geostrophic omega equation), with a tendency for light water to be upwelled upstream of the gyre while denser water is downwelled downstream of the gyre. For the horizontal velocity we show the existence of large (up to 40 cm/s) ageostrophic velocities. These are mainly non-divergent and can be explained by the cyclostrophic acceleration induced by the anticyclonic gyre. The irrotational velocity component is of the order of 10 cm/s towards the dense side of the front. The robustness of the method is checked by means of several tests that evaluate the sensitivity of results with respect to the synopticity of the data, the analysis parameters, the reference level and the presence of tidal or inertial currents.

Retrospective Analysis of the Salinity Variability in the Western Tropical Pacific Ocean Using an Indirect Minimization Approach

Abstract Empirical orthogonal functions of the combined variability of temperature and salinity have been used as basis functions for the indirect reconstruction of salinity from observations of temperature alone. The method employs a weighted least squares procedure that minimizes the misfit between the reconstructed temperature and the observed temperature, but also constrains the variability of the reconstructed salinity to remain within specified bounds. The method has been tested by fitting to temperature profiles from the Tropical Atmosphere Ocean array along 165°E in the western equatorial Pacific Ocean (8°N–8°S) for the 1986–97 period. Comparisons of the reconstructed salinity field with sea surface salinity and conductivity–temperature–depth data and of the reconstructed dynamic height with TOPEX/Poseidon observations of sea level demonstrate the reliability of the method. The reconstructed data successfully capture the upper-ocean variability at annual to ENSO timescales. The impact of neglectin...

Use of TOPEX/Poseidon Sea Level Data for Ocean Analyses and ENSO Prediction: Some Early Results

Abstract In this study, two sets of Pacific Ocean analyses for 1993–96 were analyzed. Both analyses were produced with the assimilation of subsurface temperature data from expendable bathythermographs and tropical atmosphere–ocean moorings. In addition one analysis also assimilated sea level data from TOPEX/Poseidon. Sea level variability in the two analyses agreed well with each other, and both agree with tide gauge and altimetry data for 1993–95. However, beginning in late 1995 through 1996, large sea level differences of 5–8 cm were found in the tropical western Pacific between the two analyses. Furthermore, large sea level discrepancies were also found between dynamic height estimated from TAO temperatures and tide gauge–altimetry observations in the same region during 1996. These discrepancies are consistent with the sea level differences between the two model based analyses. Historical conductivity–temperature–depth data along 165°E near the equator were also analyzed and it was found that salinity ...

Use of the Temperature–Salinity Relation in a Data Assimilation Context

Abstract A data analysis using conductivity–temperature–depth (CTD) measurements in the western tropical Pacific is carried out to get an estimate of the timescale over which temperature–salinity (T–S) relationships are preserved. Results show that the T–S preservation holds for periods on the order of a few weeks. A new method for assimilating upper-ocean temperature profiles with salinity adjustments into numerical ocean models is then proposed. The approach would use a T–S relation that is more local in space and time than is the climatological T–S relation used in previous studies. The assimilation method avoids convective instability as the temperature data are introduced. The CTD data and instantaneous fields from an ocean model are used to test the assimilation method by combining one profile with another. These tests recover the salinity profiles and the 0–500-m dynamic height very well (differences are smaller than 1 dyn cm). By contrast, analyses that used a climatological T–S relation did not p...

Spectral light absorption and attenuation measurements from a towed undulating vehicle

Measurements from a nine-wavelength light absorption and attenuation instrument mounted on a towed, undulating vehicle (SeaSoar) capable of rapidly profiling the water column – complete up-down cycles in 1.5–12 min depending on the maximum sampling depth – are used to characterize strong spatial variations in the distributions of upper-ocean bio-optical properties. Water sampled from adjacent to the conductivity and temperature sensors located in the nose of the vehicle is pumped through the 25-cm dual optical flow tubes of a Western Environmental Technology Laboratories (WET Labs) ac-9 instrument mounted on top of SeaSoar. A three-stage algorithm for post-processing the optical data to insure high-quality measurements concurrent with conductivity–temperature–depth data is described. After synchronizing the optical, navigational and conductivity–temperature–depth data streams, the method relies on finding the optimal time delay between when a water parcel is sampled first by the temperature and conductivity sensors and then by the absorption and attenuation optics. After applying the calculated time-dependent lag, a correction for the dependence of light absorption on temperature and salinity and a scattering correction to absorption are made. The final processed optical data from SeaSoar compare well with the same parameters sampled by a slowly lowered vertical profiling package deployed from a nearby stationary vessel. A 2-h, 30-km long cross-shelf section consisting of 184 vertical profiles separated by 150–200 m reveals strong horizontal variations on short spatial scales (1 km or less) of the vertical distributions of bio-optical properties.

Use of Sea Level Observations to Estimate Salinity Variability in the Tropical Pacific

Abstract The equatorial sea level analysis of the National Centers for Environmental Predictions deviates by as much as 8 cm from independent TOPEX/Poseidon (T/P) observations. This may be due to the model’s underestimation of salinity variability. Therefore, methods are developed to improve the model’s salinity field through T/P data assimilation and use of sea surface salinity (SSS) observations. In regions where temperature is well known, salinity estimates are made with the use of climatological temperature–salinity (T–S) correlations. These estimates are improved by combining T–S with SSS observations and corrected with dynamic height, which provides information on salinity variability. Tests with independent conductivity temperature depth data show that the combination of T–S with SSS significantly improves salinity estimates. In the western Pacific, the maximum root-mean-square (rms) estimation error of 0.55 psu is reduced to 0.42 psu by the use of SSS in the salinity estimate. Correction with dyna...