ESSDEarth System Science DataESSDEarth Syst. Sci. Data1866-3516Copernicus PublicationsGöttingen, Germany10.5194/essd-9-211-2017From pole to pole: 33 years of physical oceanography onboard R/V
PolarsternDriemelAmeliehttps://orcid.org/0000-0001-8667-5217FahrbachEberhardRohardtGerdGerd.Rohardt@awi.deBeszczynska-MöllerAgnieszkahttps://orcid.org/0000-0002-8108-6306BoetiusAntjeBudéusGereonCisewskiBorisEngbrodtRalphGaugerSteffenGeibertWalterGeprägsPatriziahttps://orcid.org/0000-0001-7333-7091GerdesDieterGersondeRainerGordonArnold L.https://orcid.org/0000-0001-6480-6095GrobeHanneshttps://orcid.org/0000-0002-4133-2218HellmerHartmut H.IslaEnriqueJacobsStanley S.JanoutMarkushttps://orcid.org/0000-0003-4908-2855JokatWilfriedKlagesMichaelKuhnGerhardhttps://orcid.org/0000-0001-6069-7485MeinckeJensOberSvenØsterhusSveinPetersonRay G.RabeBenjaminhttps://orcid.org/0000-0001-5794-9856RudelsBertSchauerUrsulaSchröderMichaelSchumacherStefaniehttps://orcid.org/0000-0002-8310-9743SiegerRainerhttps://orcid.org/0000-0002-9175-884XSildamJüriSoltwedelThomasStangeewElenaSteinManfredStrassVolker Hhttps://orcid.org/0000-0002-7539-1400ThiedeJörnTippenhauerSandraVethCornelisvon AppenWilken-Jonhttps://orcid.org/0000-0002-7200-0099WeirigMarie-FranceWisotzkiAndreasWolf-GladrowDieter A.KanzowTorstenAlfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, GermanyInstitute of Oceanography Polish Academy of Science, Sopot, Polandindependent researcherThünen-Institut: Seefischerei, Hamburg, GermanyMARUM – Zentrum für Marine Umweltwissenschaften, Bremen, GermanyLamont-Doherty Earth Observatory, Columbia University, New York, NY, USAInstitute of Marine Sciences-CSIC, Barcelona, SpainUniversity of Gothenburg, Department of Marine Sciences, Gothenburg, SwedenInstitut für Meereskunde, Hamburg, GermanyRoyal Netherlands Institute for Sea Research, 't Horntje, the NetherlandsUni Research Climate, Bergen, NorwayScripps Institution of Oceanography, UC San Diego, USAUniversity of Helsinki, Helsinki, FinlandCentre for Maritime Research and Experimentation, La Spezia, ItalyretireddeceasedGerd Rohardt (Gerd.Rohardt@awi.de)21March20179121122016December201622December201621February201721February2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://essd.copernicus.org/articles/9/211/2017/essd-9-211-2017.htmlThe full text article is available as a PDF file from https://essd.copernicus.org/articles/9/211/2017/essd-9-211-2017.pdf
Measuring temperature and salinity profiles in the world's oceans is crucial
to understanding ocean dynamics and its influence on the heat budget, the
water cycle, the marine environment and on our climate. Since 1983 the German
research vessel and icebreaker Polarstern has been the platform of
numerous CTD (conductivity, temperature, depth instrument) deployments in the
Arctic and the Antarctic. We report on a unique data collection spanning
33 years of polar CTD data. In total 131 data sets (1 data set per cruise
leg) containing data from 10 063 CTD casts are now freely available at
10.1594/PANGAEA.860066. During this long period five CTD types with
different characteristics and accuracies have been used. Therefore the
instruments and processing procedures (sensor calibration, data validation,
etc.) are described in detail. This compilation is special not only with
regard to the quantity but also the quality of the data – the latter
indicated for each data set using defined quality codes. The complete data
collection includes a number of repeated sections for which the quality code
can be used to investigate and evaluate long-term changes. Beginning with
2010, the salinity measurements presented here are of the highest quality
possible in this field owing to the introduction of the OPTIMARE Precision
Salinometer.
Introduction
Our oceans are always in motion – huge water masses are
circulated not only by winds but also by global seawater density gradients.
These gradients result from differences in water temperatures and salinities
and the water movement transports heat, oxygen, CO2, and nutrients
among latitudes . Measuring the ocean's temperature and salinity
is therefore essential not only to understand the ecology of the world oceans
but also the influence of the oceans on our climate.
According to the first water samples from depths down to around
1600 m were taken in the
tropical Atlantic aboard the Earl of Hallifax in 1750/1751 with a special
bucket and a thermometer . Even then, the results (a stable cold
water layer beneath the warm surface) hinted at an inflow of deep water from
the polar regions . Until the 1970s, measurements of ocean
temperatures and salinities were conducted primarily using reversing mercury
thermometers and Nansen water bottles . Due to the usually
limited number of Nansen bottles and thermometers on board, the number of
depth levels which could be sampled was also limited, which resulted in a
rather coarse vertical resolution of temperature and salinity. With the
development of submersible electrical instruments for temperature and
salinity (conductivity) measurements in the 1950s, high-resolution
measurements of temperature and salinity profiles became possible
. During the 1970s and 80s, the use of CTDs (conductivity,
temperature, depth instruments) replaced the formerly used method almost
completely. Numerous manufacturers produced a variety of sensors and
instruments. For example, in 1974 Neil Brown formed Neil Brown Instrument
Systems, Inc. and manufactured the Mark III CTD
Later it was
manufactured by EG&G Ocean Instruments and after that by General Oceanics.
.
R/V Polarstern is a research icebreaker operated by the Alfred
Wegener Institute (AWI) in Bremerhaven (Germany), which has operated since
1982 in Antarctica (austral summer) and the Arctic (northern
summer)
See also the description in about
Polarstern history and cruise characteristics.
. The first CTD used
on Polarstern was the aforementioned Neil Brown, Mark IIIB CTD. It
was deployed for the first time on cruise leg ANT-II/3, during the ship's
second trip to Antarctica in November/December 1983. This giant step for AWI
oceanographers, which was supervised by Gerd Rohardt , ended
abruptly when that same probe accidentally “flew” overboard a month later
during ANT-II/4. Despite many efforts to regain it, the probe was lost, which
is why the respective data set of this leg contains CTD as
well as Nansen-bottle-derived data. The latter was only possible due to the
fact that guest researcher Manfred Stein (Institut für Seefischerei,
Hamburg) had brought Nansen bottles and reversing thermometers as a backup
for his ME-OTS-CTD on board during leg ANT-II/2 (no data). This anecdote
clearly demonstrates that 1983 was still a transition period for hydrographic
observations to electronic devices.
Despite this rather unfortunate start, a Neil Brown, Mark IIIB CTD was
successfully used on Polarstern until 1996. Starting in 1992, a
Sea-Bird SBE 911plus CTD was in use by Kees Veth (Royal Netherlands Institute
for Sea Research, data included here). A year later, Gereon Budéus was
the first AWI researcher to use the SBE 911plus on Polarstern,
testing the behavior of the probe in cold conditions. The instrument has been
used routinely on Polarstern since then, in parallel with Neil Brown
equipment. On four cruise legs (1995–1999) Polarstern was equipped
with the direct successor of the Mark IIIB, called the ICTD and manufactured
by Falmouth Scientific. Additionally, during two legs (1986–1987), guest
researchers deployed a ME-OTS-CTD. The SEA-BIRD SBE 911plus is probably the
most widely used CTD type currently, and has been the only type used on
Polarstern since 1999 (see also Fig. 1).
Overview of the period of deployment of different CTD types onboard
Polarstern, with first line denoting the years. Sea-Bird CTD sondes
were here combined into a single bar.
Picture of a typical CTD/rosette system used by the
Alfred Wegener Institute (picture by Gerd Rohardt).
In the following, we describe a data compilation of 33 years (1983–2016) of
CTD measurements from R/V Polarstern. In Sect. 2 we provide details
on the CTD types used, the parameters measured, and on data processing. A
focus is set on the improvement of the salinity measurements over time and
the reasons thereof. In Sect. 3 we describe
the data sets in respect to composition, extent, access, and quality.
Methods
A CTD directly measures conductivity, temperature, and pressure of water
during its down- and up-cast, resulting in a profile from the water surface to
the bottom and back. Derived variables are salinity, density, and water depth.
The CTDs onboard Polarstern were typically deployed in
combination with a water sampler construction, holding 12, 24, or 36 bottles
(named rosette or carousel, depending on the manufacturer; see Fig. 2). The
CTD is mounted inside the frame of the water sampler in a way that the
sensors measure the undisturbed water during the down-cast. The down-cast CTD
profile is displayed on board in realtime to allow the CTD operator to
choose the water layers from which water samples for subsequent chemical and
biological analyses are to be taken during the up-cast.
Due to the mounting technique, the measurements taken during the up-cast are
not from undisturbed water but are influenced by water parcels from deeper
layers which are dragged upwards by the CTD/rosette. Therefore, mostly only
the down-cast CTD profile is used and archived (for details, see Sect. 2.5).
Instruments and specifications
Five different CTD types have been used onboard Polarstern from
1983 until the present. As the instruments have changed, so have the range, accuracy,
stability, resolution, and response of the sensors. Table 1 shows in detail
the manufacturers' specifications of the instruments, and the periods of use
are shown in Fig. 1 (Sea-Bird probes combined). The table also
indicates the accuracy limits officially adopted for the World Ocean
Circulation Experiment (WOCE). Using the OPTIMARE Precision
Salinometer (OPS) has provided accuracies even better than those required
by WOCE (see Sect. 2.6). However, we would like to stress here that regular
servicing and calibration is required to keep the instrument at least within
the accuracy given by the manufacturer.
Laboratory calibration of instruments
In order to obtain precise hydrographic data, frequent calibrations of the
sensors and careful inspection and preparation of the instruments (CTD, water
sampler and bottles) is necessary. From 1983 until 1986, Neil Brown Mark IIIB
CTDs were calibrated by the manufacturer. Each sensor had its own electronic
board with the calibration stored on it. Changing a sensor thus required
installing the corresponding electronic board as well. When Ray Weiss from
Scripps Institution of Oceanography (SIO) participated on Polarstern
cruise ANT-V/3 in 1986, he suggested including the AWI-CTDs into the SIO
calibration process. Since that time, the AWI-CTDs have been calibrated by SIO
before and after each campaign. The first calibration revealed that the AWI
Mark IIIB showed the same behavior as the SIO Mark IIIB: (a) the
pressure sensor showed strong hysteresis depending on the maximum pressure and
(b) the temperature readout showed a step-like discontinuity near
0 ∘C which further depended on the direction of the temperature
change, i.e., whether the temperature increased or decreased (R. Williams,
SIO, personal communication, 1986). Because a temperature correction of such
a behavior is fairly complicated, a few years later SIO modified the
electronic boards, shifting the discontinuity from about 0 to
+3 ∘C.
The Falmouth Triton ICTDs from AWI were also shipped to SIO for calibration.
This continued support made the change from Mark IIIB to ICTD much easier,
and underlined the advantages of the new instrument: the SIO calibration
confirmed that the pressure showed negligible levels of hysteresis and that the
temperature correction was only small, with no stepwise behavior from -2 to
30 ∘C.
Sensor types and the manufacturers' specifications of CTDs used on
board Polarstern.
Instrument andPeriodSpecificationsPressureTemperatureConductivitymanufacturerof useWOCE accuracylimits±3 dbar±0.001 ∘C±0.003 mScm-1Multisonde*1986Sensor:Strain gauge bridgePlatinum resistanceSymmetric electrode cellME-OTS-CTDtoRange:0 to 6000 dbar-2 to 35 ∘C5 to 55 mScm-1Meerestechnik1987Accuracy:0.35 %f.s.±0.005 ∘C±0.005 mScm-1Elektronik,Stability:–±0.001 ∘Cmonth-10.002 mScm-1month-1TrappenkampResolution:0.2 dbar0.001 ∘C0.001 mScm-1Response:–60 ms–Mark IIIB1983Sensor:Strain gauge bridgePlatinum ThermistorFour-electrode cellNeil BrowntoRange:0 to 6500 dbar-3 to 32 ∘C1 to 65 mScm-1Instruments1996Accuracy:±6.5 dbar±0.005 ∘C±0.005 mScm-1later: EG&GStability:0.1 %month-10.001 ∘Cmonth-10.003 mScm-1month-1Marine Instruments/Resolution:0.1 dbar0.0005 ∘C0.001 mScm-1General OceanicsResponse:–––Triton ICTD1995Sensor:Precision-machined SiPlatinum ThermistorInductive cellFalmouth ScientifictoRange:0 to 7000 dbar-2 to 35 ∘C1 to 70 mScm-1Product line1999Accuracy:±0.01 %f.s.±0.002 ∘C±0.002 mScm-1continued byStability:±0.002 %f.s.month-1±0.0002 ∘Cmonth-1±0.0005 mScm-1month-1Teledyne RDResolution:0.0004 %f.s.0.00005 ∘C0.0001 mScm-1InstrumentsResponse:25 ms150 ms5 cm at 1 ms-1SBE911plus1992Sensor:Paroscientific DigiquartzThermistorThree-electrode cellSea-BirdtoRange:0 to 6800 dbar-5 to 35 ∘C1 to 70 mScm-1ElectronicspresentAccuracy:±0.015 %f.s.±0.001 ∘C±0.003 mScm-1Stability:±0.0015 %f.s.month-1±0.0002 ∘Cmonth-1±0.003 mScm-1month-1Resolution:0.001 %f.s.0.0002 ∘C0.00001 mScm-1Response:15 ms65 ms65 msSBE191997Sensor:Strain gaugeThermistorThree-electrode cellself-recordingtoRange:0 to 10 000 psi-5 to 35 ∘C0 to 70 mScm-1Sea-Bird2003Accuracy:0.15 %f.s.±0.01 ∘C±0.01 mScm-1ElectronicsStability:–––Resolution:0.015 %f.s.0.001 ∘C0.001 mScm-1Response:–––
* operated by guest institutes; f.s.: full scale; –: no
data.
The long lasting collaboration between AWI and the calibration laboratory of
SIO ended after completely switching over to Sea-Bird SBE911plus because
Sea-Bird Electronics themselves performed high-level calibration of their
instruments. In general, ever since the SBE 911plus was introduced, the CTD
operators' job on board became much easier. The SBE 911plus featured dual
sensors (two for both temperature and conductivity) and software,
which displayed the sensor differences. This allowed identifying and changing
sensors which became faulty. Replacing faulty sensors early prevents losing
valuable data. With the introduction of dual sensors and the use of special
software, in situ calibrations were still executed (see Sect. 2.4), but the
number of samples could be reduced.
Water samplers
With the exception of the self-contained probe SBE19, all CTDs were used in
combination with a water sampler. The Neil Brown Mark IIIB was combined with
a General Oceanics (GO) rosette. The GO rosette required taking numerous
samples for checking conductivity measurements and also using reversing
thermometers to verify that bottles were closed at the desired depth. The
reason is that GO used a non robust mechanical release to close the water
samplers. Often the mechanics failed, which resulted in the
closure of two or more samplers during one release command. This problem was
solved with the introduction of the ICTD because Falmouth Scientific (FSI)
supplied a new release module which confirmed successful or non-successful
release commands. Later the complete GO hardware was replaced by a release
unit from FSI, which used a release system similar to the one used in the
SBE32 carousel water sampler, confirming the release command and thus making
water sampling more reliable. This positive development (1992 onwards)
affected the in situ calibration, rendering the usage of reversing
thermometers obsolete.
In situ calibration
Laboratory calibration of instruments (see Sect. 2.2) is crucial to maintain
the sensors and obtain comparable results. It is not sufficient, however to
anticipate how a sensor behaves at sea under tough environmental conditions,
especially during deep casts. Also sensor drift is not necessarily a continuous
process. For this purpose in situ calibrations are essential.
Temperature: the Mark IIIB CTD was equipped with one temperature (and one
conductivity) sensor only. Therefore reversing thermometers attached to the
bottles of the water sampler had to be used to verify the quality of the
temperature data. The Triton ICTD was equipped with a redundant temperature
sensor which allowed for much better control of temperature data than the
reversing thermometers. Lastly, the SBE911plus features double sensors, both
for temperature and conductivity measurements, allowing the plotting of the
difference between both sensors versus depth, which eases identification of
individual sensor problems and pressure effects. Additionally, a SBE35 Deep
Ocean Standards Thermometer was attached to the water sampler, recording the
temperature every time a water sample was taken. However, the comparison of the CTD and SBE3plus temperature values to the SBE35 temperature values is only possible if the water temperature is relatively
stable, i.e., if the values do not vary much.
Conductivity: for the in situ calibration of the conductivity sensor
(Mark IIIB and Triton ICTD) or the conductivity double sensors (SBE911plus),
water samples were taken and measured on board with the laboratory
salinometer Guildline Autosal 8400a/b and, from 2010, with the OPS (see Sect. 2.6). The samples were taken from deep
(> 3000 m) and shallow depths (ca. 500–1000 m) regularly
during the CTD deployments in order to reveal pressure effects of the
conductivity sensor and its temporal shift.
Data processing
The data processing procedures were substantially dependent on the
development of the CTD and the computer generation. In 1983, CTD data were
recorded on nine-track magnetic tape. The station data (location, water
depth, date, and time) were noted on a sheet of paper. An HP 9825B computer
was used to visualize the temperature and salinity profile on a connected
plotter. The data processing was performed at the institute. Due to the fact
that, for safety reasons, the magnetic tapes always came back to Bremerhaven
with Polarstern, the data processing often only started several
months after the end of the cruise leg. Later (around 1986), EG&G – who
took over the production of the Mark IIIB in 1984 – transferred the FORTRAN
code of the data acquisition and processing routines of the Woods Hole
Oceanographic Institution (WHOI) for use on PCs. A similar
software package was also provided for the ICTD from Falmouth Scientific.
This made the data acquisition and visualization as well as the transfer of
raw data to AWI much easier. The substructure of the software for applying
the SIO calibration came from R. Williams and F. Delahoyde (personal
communication, 1990).
Sea-Bird Electronics provided the data acquisition software SEASAVE and
developed a package especially for their pumped CTD SBE911plus, SBE
DataProcessing. This software became the primary tool for CTD data processing
at the AWI. Also, the raw data were routinely stored on the onboard computer
and transferred to the AWI in an automatic workflow.
The data processing workflow can be divided into four parts, as explained in
the following subsections:
Data cropping and handling
Data recording started before the actual profile began (starting point at the
lowering of the CTD/rosette to the water surface). Thus, one of the first
tasks was the truncation of the unused beginning (the depth of the first
“used” data point depends on the wave height). Converting the raw file into
readable engineering units was the next step as well as the separation
between the down- and up-cast, if both had been saved in one file.
Afterwards, the station information was added to the data file. In the past
this information was manually edited from handwritten station protocols. With
the inauguration of the DSHIP electronic station book
(http://www.werum.de/en/platforms/DSHIP.jsp) station details were
directly merged with the CTD data.
Correction of measurement errors
Physical properties of the sensors and environmental influences on them, as
well as disturbances of the data transmission between sensors and recording
units on deck, can create measurement errors. These were reduced using
suitable software in the following ways:
Spikes: spikes in the pressure measurements resulting, for example, from winch
cable or slip ring problems, were removed. The procedure is called “par” in
Sea-Bird's SBEDataProcessing software package.
Response time/time lag correction: salinity was computed from conductivity,
temperature, and pressure. The response time of a temperature sensor,
however, is higher than the response time of the pressure and conductivity
sensors. If left uncorrected, this would result in salinity spikes in layers
with strong gradients. A precise correction for this time lag would require a
constant lowering speed of the CTD, which is not possible on a moving ship.
Sea-Bird solved this problem by pumping water with a constant speed through
the temperature and conductivity sensors. For Mark IIIB and ICTD the time lag
was adjusted/corrected by minimizing the salinity spikes and evaluated
visually based on profile plots.
Pressure hysteresis: Mark IIIB strain gauge pressure sensors did not
respond linearly to increasing pressure and additionally exhibited a lagged
response during decreasing pressure. This behavior also depended on the
maximum pressure. A laboratory calibration (see Sect. 2.2) revealed this
behavior and provided the coefficients for the software to apply the
correction. However, the software was rather tricky because it only used
hysteresis correction for the maximum pressure (6500 dbar) to
calculate the correction for all profile depths (R. Williams and
F. Delahoyde, personal communication, 1990). A second calibration up to
1500 dbar was recorded to verify the algorithm of the software. ICTDs
did not show this behavior and only a minor offset had to be applied. A
Digiquatz® pressure sensor from
Paroscientific was used in the SBE911plus. This sensor was stable, operating
without hysteresis, so no frequent calibration was necessary.
Compression and thermal effect: the ICTD with its inductive conductivity
sensor had a known pressure dependency (compression of the cell ceramics),
which was corrected by SBE software. In addition a thermal mass
correction
A cell which is lowered from a warm
into a cold layer needs some time to reach the same temperature as the water.
That means that heat from the cell is transferred into the water and the
water becomes slightly warmer resulting in higher conductivity.
was
applied for the ICTD and the SBE911plus conductivity cell.
Creation of a uniform profile
Monotonic increasing pressure: as a ship is always pitching and rolling,
the constant lowering speed of the winch is superimposed by the ships motion.
Rejecting all records with pressure reversals is thus one of the standard
procedures in CTD data processing, and was also applied on
Polarstern data.
Averaging: the SBE911plus CTD sampled with a frequency of 24 Hz.
A typical lowering speed of 0.8 ms-1 resulted in a vertical
resolution of around 3 cm. This sample rate was needed to apply the
time lag correction reliably and also to guarantee that, although lots of
records were rejected, a monotonic increasing pressure record could be
created. In the end, the profile was smoothed by averaging on 1 dbar levels
(i.e., P, T and C were averaged between ≥ 1.5 and < 2.5;
between ≥ 2.5 and < 3.5 dbar, and so on). As this will not
necessarily result in an averaged pressure record for 2.0, 3.0,…dbar (more probable in 1.97, 3.05,…dbar), a
linear interpolation was applied for temperature and conductivity, so that
the values could be centered on exactly 2.0, 3.0,…dbar. Only
after this procedure was the salinity calculated.
Final correction and validation
Drift, stability, and pressure dependency: the physical characteristics of
sensors change continuously through time. This behavior becomes visible as a
slight change of their sensitivity. The order of this change is given by the
manufacturer (“stability”; see Table 1). But the stability depends on the
environmental conditions as well. For example, by conducting many deep casts,
an additional sensor drift could be induced due to an a priori unknown
pressure dependency. Also marine growth inside the conductivity cell will
change the drift. Additionally, Polarstern CTDs were deployed even
in rough weather conditions meaning that the instruments could bump against
the ship's hull or experienced hard impacts on deck. These events could
result in a visible step-like change. The station log sheets (which
essentially contain descriptions of special occurrences), the pre-, post-,
and in situ calibration helped to
reconstruct the history of a sensor during a cruise and to identify which
T–C sensor pair should be used. General plotting software can be used to
visualize the in situ calibrations versus pressure or versus time to
investigate the dependency (drift), and to then apply and verify the
corrections.
Validation: all profiles were imported into Ocean Data View
which provides various plots (profiles, scatter, and sections)
for a visual inspection. When a suspicious profile was found, the processing
steps mentioned above were repeated from the necessary level onwards.
Additionally, these profiles were compared to profiles from previous cruises.
The working database included a number of regularly repeated transects, which
allowed consistency checks and quality confirmation.
OPTIMARE precision salinometer
Since 1985, laboratory measurements of salinity have been conducted on water
samples taken with a rosette/carousel multi-bottle sampler to cross-validate
the in situ CTD measurements. These laboratory salinity measurements were
taken with salinometers. Salinometer measurements have several advantages
compared to in situ measurements. For one, the salinometer measurements are
controlled directly with the primary standard International Association for the Physical Sciences of the Oceans (IAPSO) Standard Seawater, which
means that the salinometer is closer to the primary standard. Furthermore,
the SBE911plus salinity sensor (SBE4) is calibrated using a bath
of nearly constant salinity and varying temperatures, leading to different
conductivities. The salinometer, however, is calibrated by using
different salt concentrations, which makes the salinometer measurements more
accurate for salinities varying around the typical open-ocean value of 35 PSU.
Map showing all sites where CTD data were collected with
Polarstern from 1983 to 2016.
A Guildline Autosal 8400a/b salinometer was in use until 2010. Since then, it has been replaced by a new laboratory salinometer, the OPS developed by AWI
scientists and engineers and manufactured by
OPTIMARE
Optimare Sensorsysteme GmbH & Co. KG
. The highly
accurate OPS lab measurements have been in use since June 2010 to
cross-calibrate in situ salinity data measured by the CTD. As a result,
beginning with campaign ANT-XXV/1 (2010/06) the accuracy of the salinity
measurements improved tremendously and the resulting data sets are of the
highest quality possible for these kinds of measurements.
Resulting data sets
In total 131 data sets (1 data set per cruise leg) containing data from
10 063 CTD casts have been produced on Polarstern in the course of
33 years (22 November 1983 to 14 February 2016, Fig. 3) and are archived in
the PANGAEA (Data Publisher for Earth and Environmental Science,
www.pangaea.de) database. The data sets can be accessed at
http://doi.pangaea.de/10.1594/PANGAEA.860066. This link
leads to the central page which contains all meta-information of
the respective cruise legs (name of leg, start/end, area, link to cruise
report), the number of CTD casts, the CTD type used, the overall quality of
the data, the link to a map displaying all CTD stations, and the link
to the data set of the specific leg.
Quality code details for Polarstern CTD data sets in PANGAEA.
QualityDescriptionCommentPossible usecode(example)AHighest accuracy andSBE911plus with double sensors;Investigate long-term changes ofquality possiblepre- and post-calibration applied,temperature and salinitysalinity samples measured during the cruiseBWithin WOCE accuracySBE911plus without double sensors, Mark IIIBInvestigate long-term changes of temperatureand quality limitsor ICTD; pre- and post-calibration applied,salinity samples measured during the cruiseCAccuracy and qualityWithout pre- and post-calibration,Hydrography for the specific cruise onlyof the data is ratherno salinity samples, or no detailedlow or unknowndocumentation of data processing
When clicking on the link to a data set of one cruise leg (see, e.g.,
, 10.1594/PANGAEA.733664) the data set page contains
metadata, a Google map of all sample sites, and on the bottom the actual
data. On the top of the page the citation of the data set is given, followed
by the citation of the respective cruise report (if available). The CTD type
used is indicated in the “Method” column of the Parameter(s) overview table
of the page. The data table opens by clicking on “View dataset as html”.
Here, the position, date/time (at maximum depth) of sampling, and the water
depth precede the actual data. The “Elevation” is the bathymetric depth
relative to sea level and is therefore negative. It can be used, for
example, to extract information on how close to the seafloor the CTD
measurements ended (comparing water depth of the last measurement with the
elevation). The “Number of observations” is the number of measurements
included in one averaging step (see Sect. 2.5.3). With programs like Ocean
Data View and Pan2Applic the data can be
visualized easily (for more information, see
https://wiki.pangaea.de/wiki/ODV). With respect to CTD type, the 131
data sets are composed of 27 legs with Mark IIIB CTD data, 4 legs with ICTD
data, 2 legs with ME-OTS data, 5 legs with data from a Sea-Bird
self-recording CTD, and 93 legs with the SBE911plus. Most of the data sets
(1992 onwards) contain additional measurements of oxygen concentration, light
transmission/attenuation, and/or chlorophyll fluorescence.
Several remarks on the best use of Polarstern CTD data
If available, the respective cruise report is linked to the data set. It
contains valuable information on the cruise itinerary, the scientific
purpose, and on the quality of the CTD data or the calibration applied.
We defined a column on the overall quality of the data of each leg in
called “Quality code”. Here we use flags “A”, “B”, and “C” to
classify the data with A being high quality data (see Table 2 for details).
In general, the number of decimals in the data sets is at least n+1,
with n being the last significant decimal. This was done deliberately, as
we experienced that for calculations (in models), the actual (unrounded)
number of the last significant decimal can be essential.
You can search for specific parameters, regions, etc., in the data
sets described here using the www.pangaea.de search engine and adding
“PSctd”. You can then define a geographic bounding box in the map (right
side of search page) to search for specific regions (e.g., Arctic data) and
press “apply”. Or you can try “PSctd + parameter:oxygen” to get all
data sets with oxygen measurements. We also added an overview in .xls format
of these additional measurements at
http://doi.pangaea.de/10.1594/PANGAEA.860066 (under “Further
details”) which contains information (where available) on whether or not
these measurements were calibrated, and during which campaign which
additional measurements were taken.
To download several or all data sets at once, you can either use the Data
Warehouse integrated into PANGAEA, or you can use a program especially
designed for this purpose called PanGet. Data Warehouse: log in to PANGAEA
(or create an account) at www.pangaea.de, then search for “PSctd”
(or, for example, “PSctd + oxygen”). On the top right corner
you can click on Data Warehouse (above the Google map). Here you can choose
which parameters to download, followed by clicking on “Start Data Warehouse
Query”. Please be aware that downloading all files requires over
1.5 GB which might take some time to download. How to download files
with PanGet is described at https://wiki.pangaea.de/wiki/PanGet.
A CTD file downloaded from PANGAEA can easily be imported into Ocean Data
View using Pan2Applic . Open the downloaded file
in Pan2Applic, click on “Convert ⇒ Ocean Data View” and click
“OK”, then choose “Select data (2:)” and “Select geocode (3:)” and
press “OK”. The data are now loaded into ODV and you can, for example, visualize it with different modes at
“View ⇒ Layout templates”.
For a detailed geographical search of the available data we created a
Google .kmz file containing all CTD casts. When clicking on a single cast, a
small window opens up displaying metadata details and a link to the data in
PANGAEA. The .kmz file can be found under “Further details” at
http://doi.pangaea.de/10.1594/PANGAEA.860066 or at
http://hdl.handle.net/10013/epic.50376.d001.
Mean potential temperature (left) and mean salinity (right) of the
Weddell Sea Bottom Water calculated from nine repeated CTD sections at the
tip of the Antarctic Peninsula (T. Kanzow, personal communication, 2016). The
thin curves – magenta and cyan – include the seasonal effect. In the thick
red and blue curves, the seasonal influence is eliminated. Linear regression
lines are shown in black: dashed lines with seasonality included and solid
lines with seasonality removed. The CTD type used is shown as follows: NB,
Neil Brown; FSI, Triton ICTD; and SBE, SBE911plus.
All data are accessible via
http://doi.pangaea.de/10.1594/PANGAEA.860066. The data sets are freely
available and can be directly downloaded. A moratorium is still in place for
the latest campaign (PS96), but the data are available upon request.
Conclusions
Even small changes in sea-water density might affect vertical
layering of water masses in the ocean . Especially at low
temperatures, small salinity changes affect the density much more than
temperature changes of the same order . Therefore precise
salinity measurements are needed, especially in polar regions. Based on
repeated measurements, long-term changes of water mass properties can be
studied (see, e.g., ). Figure 4 shows the mean potential
temperature and mean salinity of the Weddell Sea Bottom Water from nine
repeated CTD sections at the tip of the Antarctic Peninsula. While the
temperature shows similar errors of the mean, the errors of the mean salinity
have become much smaller since 2005, which coincides with the use of the
SBE911plus CTD on Polarstern. This illustrates clearly that when
analyzing long-term trends from CTD data, the CTD type has to be taken into
account. Additionally, in situ onboard calibration, regular servicing and
laboratory calibration, data processing procedures, and experienced operators
are required for precise data. CTD data therefore are of the highest value
only if they come with proper documentation. One ambitious project analyzing
and describing the complete set of available Arctic CTD data in respect to
quality is currently taking place and will hopefully be published soon
.
The authors declare that they have no conflict of
interest.
Acknowledgements
We would like to thank Wolfgang Cohrs for creating Fig. 3. Many thanks to the numerous students who joined
the CTD watches. Without their help it would have been impossible to run CTDs
in 24 h shifts. Sometimes it was necessary to use external CTD operators and
we thus appreciate the help of the Bremerhaven firms OPTIMARE and FIELAX, who
both did an excellent job. Well-prepared instruments are the foundation for
precise measurements. We therefore extend our gratitude to all CTD
technicians, first and foremost to Ekkehard “Ekki” Schütt, who was not
only dedicated to his job but also had the uncanny talent of identifying
impending failures before they happened. Many thanks also to our current
technicians: Matthias Monsees, Rainer Graupner and Carina Engicht. Last but
not least, we are indebted to the crew of R/V Polarstern. During
these 33 years crew members changed, of course, but it was always an
exceptional team determined to make every campaign a success. Additionally,
we would like to also thank the German Bundesministerium für Bildung und
Forschung (BMBF) for placing Polarstern at the disposal of science.
This ship has enabled polar research in conditions with up to force 8 winds.
It even allowed for CTD transects during extremely heavy sea ice conditions
with the ships powerful thrusters keeping the water surface ice-free while
getting the CTD/rosette into the sea and back on deck safely.
The article processing charges for this open-access
publication were covered by a Research Centre of the
Helmholtz Association.
Edited by: G. M. R. Manzella
Reviewed by: L. Rickards and E. Viazilov
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