We report on data from an oceanographic cruise in the Mediterranean Sea on
the German research vessel
Repository reference:
The Mediterranean Sea is a marginal sea, as it is partly isolated from the
Atlantic Ocean through the narrow Strait of Gibraltar. It consists of two
sub-basins, the western (WMed) and the eastern (EMed) Mediterranean, which
communicate through the broad (145 km) and shallow (maximum depth 550 m)
Sicily Channel. Due to its dimensions (2 500 000 km
The WMed and EMed show distinct differences, both in their hydrography and circulation. Different attempts to schematize surface as well as intermediate and deep paths of the circulation have been made in the past, based on observational evidence (see e.g. Malanotte-Rizzoli et al., 1997, 1999; Robinson et al., 2001; Roether et al., 2007; Schroeder et al., 2012).
The heat and freshwater budgets in the Mediterranean Sea are negative with a
net loss of about 5 W m
The water mass formation cycle is characterized by the inflow of low-salinity
Atlantic Water (AW) in the upper 100 m of the water column and with
identification values of
The principal scientific objective of the cruise is threefold:
to add knowledge to the understanding of the dispersion of LIW water
masses from the eastern basin of the Mediterranean Sea to the Strait of
Gibraltar to investigate the mesoscale variability of the upper water columns of
the two basins of the Mediterranean Sea to continue the documentation and to contribute to the understanding of
the evolution of the deep water masses in the EMed since the appearance of
the Eastern Mediterranean Transient.
According to the Med-Ship program (CIESM Monographs 43, 2012) the cruise
supports the investigation of its relevant objectives which are, amongst
others, engaged in the determination of changes and of long-term variability
of hydrographic parameters in the Mediterranean Sea. Although most of the
data of this campaign are not “full water column observations”, this survey
is a valuable contribution to improve the database of the Mediterranean Sea
for a better understanding of the variability on multiple timescales and for
numerical model evaluations.
To our knowledge, just one further campaign exists which covers an
east–west transect through the whole Mediterranean Sea, but with a much lower
sampling rate (
List of parameters from
The survey was carried out on the German RV
The most recent campaign (April 2011), including an east–west transect
through the whole Mediterranean Sea, was the RV
During the cruise, altogether 37 full depth standard hydrographic stations (Fig. 1) were collected with a 24 Hz sampling Sea-Bird SBE 911plus CTD, fastened to a 12-bottle SBE 32 Carousel Water Sampler. The instrument was equipped with double conductivity and temperature sensors and two SBE 43 dissolved oxygen sensors. Specifications for the CTD sensors are given in Table 2.
Cruise track of P468 with CTD stations marked in red and UCTD stations marked in blue.
CTD instrument and sensors used. Owners of instruments are either the University of Hamburg, Germany (IFM-CEN) or the National Institute of Oceanography and Geophysics (OGS), Italy.
At almost all stations water samples were taken at 12 pre-defined depths
along the water column for oxygen analysis and three of which also for
salinity analysis. The salinity samples were analysed on board using a
Guildline Autosal Salinometer. The batch no. of the standard seawater samples
is 38H11 and they have a K15-factor of 1.07631 (24
Temperature and salinity CTD data were post-processed by applying standard
Seabird software and MATLAB routines. At this stage spikes were removed,
1 dbar averages were calculated and the downcast profiles of temperature and
salinity were corrected with regression analysis. Data from the double
sensors were correlated, and the salinity measurements were additionally
corrected by comparison with the discrete salinity water samples to improve
the level of precision. Since the corrections to the parameters were
negligible, the data quality was excellent. Overall accuracies are within
expected ranges: 0.002
Dissolved oxygen samples were analysed on board by means of the Winkler
potentiometric method. The dissolved oxygen CTD data were treated in the same
way as for temperature and salinity. A comparison between CTD oxygen sensors
and the discrete water samples was carried out also for this parameter. The
accuracy of the data reached approximately 2
All procedures fit the guidelines of the GO-SHIP Repeat Hydrography Manual (McTaggart et al., 2010)
Underway measurements of pressure, temperature and conductivity profiles were made with an Oceanscience UCTD system in order to increase the spatial resolution of the survey, but without having to perform several additional time-consuming CTD casts. Altogether, we took 378 casts (Fig. 1). Initially we used three probes (s/n 0068, 0155 and 0183), but after the loss of a probe right at the beginning of the cruise, we decided to use only the tow-yo (Ullman and Hebert, 2014) deployment procedure in the following, where no spooling on the probe's spindle was carried out. The sampling strategy included a sampling distance between UCTD casts of approximately 6 nm or about 1 h keeping a ship's speed of 6 knots. The ship reduced speed to 2–3 knots, while the probe was falling for a maximum of 480 s. The ship enhanced speed again to 6 knots during the recovery of the probe. We reached maximum depths of around 850 m, minimum depths of 500 m. The average depth was approximately 650 m. Specifications for the UCTD sensors are given in Table 3.
UCTD sensors used.
Temperature and salinity distribution along a west–east section
through
TS diagram determined by CTD data. The inner panel shows the location of CTD stations. The colours correspond to the colours of the profiles.
The data are logged internally and are downloaded to a computer after recovery of the instrument. No processing is done internally. Since the probes are not georeferenced, ship navigation data were used. During processing one has to account for mainly two factors which cause inaccuracies: different probes show different offsets to the CTD, and the accuracy of results depends on the variable descent rate during deployment. For the correction of the offset we used data from CTD stations, as we carried out a UCTD measurement at each CTD position. Additionally, we run one CTD station with the remaining two UCTD probes installed at the CTD rosette. For each probe we determined the mean deviation from the UCTD with respect to the CTD casts and corrected all UCTD results accordingly. To account for the variable descent rate, we carried out the steps suggested by Ullman and Hebert (2014). Hence, we corrected for a descent-rate-dependent alignment of temperature and conductivity, for the effect of viscous heating and for the conductivity cell thermal mass.
Underway current measurements were taken with a vessel-mounted 75 kHz Ocean
Surveyor (ADCP) from RDI in narrow band mode covering approximately the top
600–800 m of the water column. The bin size was set to 8 m. The instrument
was controlled by computers using the conventional VMDAS software under a MS
Windows system. Pinging was set as fast as possible. No interferences with
other used acoustical instruments were observed. The ADCP data were
post-processed with the software package ossi14 (ocean surveyor sputum
interpreter) developed by the Leibniz Institute of Marine Sciences (GEOMAR,
Fischer, 2011), Kiel, which also corrects for the
misalignment angle. The misalignment angle was calculated at approximately
The temperature-salinity (TS) diagram (Fig. 3) gives an overview of the TS characteristics in the
whole Mediterranean Sea during April 2014. The results are comparable to
those we found already in April and June 2011 on cruises with
RV
Velocity distribution of the upper 400 m of the water column along
a west–east section through
The salinity distribution (Fig. 2) furthermore reflects the spatial development of LIW from east to west. The LIW signal is stronger in the EMed than in the WMed and can be identified by the salinity maximum in the 100–600 m depth layer.
Data are published in the information system PANGAEA;
We would like to thank Captain Matthias Günther, his officers and the
crew of RV
The ship time of RV