Metadata Report for BODC Series Reference Number 1088839

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Access Policy

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

Instrument Description

CTD Unit and Auxiliary Sensors

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus, that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus.

Specifications

Specifications for the SBE 9 plus underwater unit are listed below:

Further details can be found in the manufacturer's specification sheet.

BODC Processing

Fifty three profiles were provided by the originator with CTD002 and CTD016 being omitted, as these were aborted. Data from the auxiliary sensors were also not provided. The data were received in a structured matlab format and converted into BODC internal format (QXF). The following table shows how the variables within the matlab file were mapped to appropriate BODC parameter codes:

The reformatted data were visualised using the in-house EDSERPLO software. No data values were edited or deleted. Quality control flags were applied to data as necessary. Overall no quality issues with very few flags added.

References

Fofonoff, N.P. and Millard, R.C., 1983. Algorithms for computations of fundamental properties of seawater. UNESCO Technical Papers in Marine Science, No.44, 53pp.

Originator's Data Processing

Sampling Strategy

Fifty five CTD profiles were performed during the cruise. At stations where Vertical Microstructure Profilers (VMPs) were deployed, the CTD deployment duration had to approximately match the VMP cast duration, within 1.5 hours. There were several problems which occurred during the deployment of the CTDs. CTD002 was aborted at 1000 m and CTD016 at 1700 m due to termination problems related to shortage within the electric cable between the CTD and the ship, and 50 m of cable was chopped twice to solve the problem. CTD049 was aborted to catch the VMP, and during CTD050 the was a wire out offset variable throughout cast.

Data processing

The files were produced by Seasave and initial data processing was performed using Sea-Bird processing software. Firstly, the raw data were converted into physical units using 'data conversion', the surface soak was removed from the data and the surface pressure offset obtained from the first 30 readings was applied. Temporal shifts were then applied to align the sensor readings using 'align CTD'. Corrections for the thermal mass of the cell were made using 'cell thermal mass' and the output from the align CTD, in order to minimise salinity spiking in steep vertical gradients due to temperature/conductivity mismatch.

After the Sea-Bird processing a further set of processing using Mstar programs were applied. The data were firstly averaged to 1 Hz, then the practical salinity and potential temperature were calculated. The downcast data was extracted, gaps were interpolated and the data were then averaged to 2 dbar. All profiles were visually checked to detect any possible anomalies, and profiles were also plotted on top of each other in order to detect any possible sensor drift. A number of small localised spikes were detected in the conductivity/salinity profiles and these were removed and replaced by null values.

An electric problem caused large spikes in the data (all sensors) from station 43 onwards, the problem was not solved at the time but the profiles were cleaned during post-processing.

Field Calibrations

Salinity

Between five and seven Niskin bottles were sampled for salinity at each station. Surface and bottom bottles were chosen as well as a couple of bottles at tracer cluster depths. The salinity differences between bottle salinities and salinities from sensor 1 showed a strong jump between stations 38 and 41. The reason for this sudden failure of sensor 1 was not known. In contrast, there was no failure or jump in salinities from sensor 2. The salinity differences between bottle salinities and salinities from sensor 2 all fell within ± 0.002. No clear trend or pattern in pressure or time dependence was identified. Density (θ) profiles were used to detect possible sensor drift, and all θ characteristics of bottom water sampled during the cruise were found to fall within the same narrow band. There was a change noted in the salinity from the beginning to end of the cruise, with fresher salinities at the beginning, however, this was thought to be a real change as it did not appear to be a constant shift and the difference was also consistent with previous bottom water studies. Based upon the analyses conducted, it was concluded that data from sensor 2 with no calibration should be used as if the calibration was applied it would be less than the targeted accuracy.

Project Information

Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) project document

DIMES is a US/UK field program aimed at measuring diapycnal and isopycnal mixing in the Southern Ocean, along the tilting isopycnals of the Antarctic Circumpolar Current.

The Meridional Overturning Circulation (MOC) of the ocean is a critical regulator of the Earth's climate processes. Climate models are highly sensitive to the representation of mixing processes in the southern limb of the MOC, within the Southern Ocean, although the lack of extensive in situ observations of Southern Ocean mixing processes has made evaluation of mixing somewhat difficult. Theories and models of the Southern Ocean circulation have been built on the premise of adiabatic flow in the ocean interior, with diabatic processes confined to the upper-ocean mixed layer. Interior diapycnal mixing has often been assumed to be small, but a few recent studies have suggested that diapycnal mixing might be large in some locations, particularly over rough bathymetry. Depending on its extent, this interior diapycnal mixing could significantly affect the overall energetics and property balances for the Southern Ocean and in turn for the global ocean. The goals of DIMES are to obtain measurements that will help us quantify both along-isopycnal eddy-driven mixing and cross-isopycnal interior mixing.

DIMES includes tracer release, isopycnal following RAFOS floats, microstructure measurements, shearmeter floats, EM-APEX floats, a mooring array in Drake Passage, hydrographic observations, inverse modeling, and analysis of altimetry and numerical model output.

DIMES is sponsored by the National Science Foundation (U.S.), Natural Environment Research Council (U.K) and British Antarctic Survey (U.K.)

For more information please see the official project website at DIMES

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: