Since 2000, and for the following 20 years, hydrological
data of the Mediterranean Sea, with a particular focus on the western and
central Mediterranean sub-basins, have been acquired to study the
hydrodynamics at both coastal and open sea scales. In total, 1468 hydrological casts were realized in 29 oceanographic cruises planned due to scientific purposes linked with funding research projects but were also sometimes driven by sea conditions and type of vessel. After accurate quality assurance and control, following standard procedures, all hydrological data were included in four online public open-access repositories in SEANOE (SEA scieNtific Open data Edition), available from
The Mediterranean basin is an east–west-oriented, semi-enclosed sea connected with the North Atlantic Ocean through the narrow and shallow Strait of Gibraltar. Its circulation, although characterized by complex patterns, can simply be described as made of three main active overturning circulation cells, i.e., a shallow one and two deep ones (Tanhua et al., 2013). The Mediterranean Sea is one of the few basins in the world where deep convection and water mass formation take place (Siedler et al., 2001, p. 422). Morphology, climatology, and hydrodynamics, with also reduced timescales in the turnover of most of the oceanographic processes of the global ocean, make the Mediterranean Sea a “small and accessible ocean”, where changes can be more easily studied (Bethoux et al., 1999).
Within this context, the Italian National Research Council in Oristano and La Spezia (hereafter CNR), organized over 29 scientific cruises in 21 years in the Mediterranean, mostly in the western Mediterranean and the Strait of Sicily, with a particular focus on the acquisition of vertical profiles of temperature, salinity, dissolved oxygen, fluorescence, and, more recently, pH and turbidity. The cruises were always planned based on funding projects' objectives, but then further activities linked to other research interests and/or scientific collaborations were added. Therefore, for each cruise and dataset there is a duality between “fit for purpose”, i.e., the projects that prompted us to organize the campaigns, and “fit for use”, i.e., the possibility of reusing the data for purposes other than the original aims, such as data also acquired in areas outside of the projects' interest. This duality was also behind the sampling plan that could be on a regular or mostly regular grid (cruises 2000–2004 and 2014), which is generally suitable for calibrating and validating ocean numerical models, and/or along transects (after 2004) for studies on general or local circulation, and, if there was some repetition over the years, also for studies on hydrological variability (for example Gibraltar–Sardinia, Balearic Islands–Sardinia, and Sardinia–Tunisia). Finally, the campaigns were sometimes linked with the maintenance of the instruments installed on deep-moored chains, as in Tuscany–Corsica and Sicily–Tunisia. Some cruises, or parts of them, also show an apparently regular spatial resolution, e.g., during a cruise in 2012 with the transect from the Balearic Islands to Sardinia, and apparently, scattered casts, e.g., in the southern Tyrrhenian Sea–Sardinian Channel, are due to different scientific purposes.
Regardless of the sampling plan, the data acquired over the 2000–2020 period allowed us to characterize the different water masses (Ribotti et al., 2004; Puillat et al., 2006; Santinelli et al., 2008; Belgacem et al., 2020) and mainly, but not only, the ocean circulation in the seas around the island of Sardinia (Sorgente et al., 2016; Pessini et al., 2018, 2020) and to monitor changes with time along repeated transects (Schroeder et al., 2008) or single casts (Durante et al., 2019). In addition, we could calibrate and validate ocean numerical models at different spatial scales, from open ocean to coastal regions (Sorgente et al., 2003), and also for issues of marine environmental management (oil spill combat; Gonnelli et al., 2016; Sorgente et al., 2016).
The paper is organized as follows: in Sect. 2 (methods), cruises, vessels, and data are described, as well as the quality assurance and check procedures applied to the datasets. Subsequently, two examples of their use are given in Sect. 3, where the results obtained from data analyses on the diffusion of the Western Mediterranean Transient are given and a still-open issue on the thermohaline staircases in the Western Mediterranean is described.
All the conductivity, temperature, and depth (CTD) casts acquired during the 29 cruises in the Mediterranean Sea between 2000 and 2020.
List of cruises carried out from 2000 to 2020. Areas with at least three casts are listed. Ocean parameters are listed with acronyms, so P stands for pressure, C for conductivity, T for temperature, O for dissolved oxygen, F for fluorescence, Tu for turbidity, CD for CDOM (chromophoric dissolved organic matter), and pH is just pH.
Continued.
Over 29 oceanographic cruises were realized in the Mediterranean basin between May 2000 and October 2020 (Table 1 and Fig. 1) by the CNR in Oristano jointly with the CNR in Spezia, except for the years 2016 and 2019 when no cruises were carried out. The strict collaboration, from 2000 till 2018, with the CNR institute in La Spezia, which has much longer experience with ocean cruises and studies, has offered the choice of some sampling plans like the transects in the Strait of Sicily and Corsica Channel, where they moored deep oceanographic chains for tens of years.
In total, 1468 hydrological vertical profiles were acquired in 21 years in the Mediterranean Sea, covering areas spanning from the Strait of Gibraltar in the western sub-basin to the south of Crete in the eastern part of the Mediterranean Sea.
In addition to the projects' objectives and further research interests, two more aspects have influenced, even heavily in the last 4 years, data
acquisition, i.e., sea conditions and type of vessel (described in the next
subsection). Until 2017 (Table 1), the availability of large research CNR
vessels like
Different research vessels (R/Vs) were used during several measuring campaigns, each one with characteristics and facilities that influenced both sampling plans and data acquisition. Between May 2000 and December 2017, the CNR used two multidisciplinary research vessels designed in Italy that permitted researchers to work all over the Mediterranean and in all seasons. These vessels are the 61.30 m long R/V
In March 2010, the 42.35 m long R/V
During the years 2018–2020, the 35.7 m long R/V
In May 2014, the 21.5 m long patrol boat
For all the adopted vessels, the hydrological instruments were mounted on a
General Oceanics Rosette system with
Since the cruise of Medgoos12, in April 2006, the Chelsea Aqua 3 fluorometer has been used, whereas, due to sensor malfunction, no fluorescence data were collected during the Medgoos11 and the two cruises of Bonifacio2010-Sic and Bonifacio2010-Cor. Turbidity was acquired only during Bonifacio2011 by a Seapoint turbidity meter and by a WET Labs Environmental Characterization Optics (ECO)-NTU during Piattaforme2018 and Ichnussa2020. The pH/redox sensor (SBE 27) was available only during the cruise of Piattaforme2018, but it had been calibrated 1 year before. During cruises from Medgoos5 to Medgoos7, temperature data were checked on board at defined depths against inverted thermometers, i.e., model RTM 4002 by Sensoren Instrumente Systeme GmbH (SiS), installed in correspondence with the Niskin bottles numbered 1, 3, 5, and 7 of the Rosette sampler.
Since the cruise of Medgoos2 in March–April 2001 (see Table 1), redundant or secondary sensors have always been used for a data quality assessment (as defined in Bushnell et al., 2019) for measurements of temperature, salinity, and dissolved oxygen (this is from Medgoos5 in October 2022; Table 1). These secondary sensors were used to evaluate the stability of primary ones on board and during the following visual data quality check.
Pre-cruise and post-cruise calibrations of first and redundant sensors of temperature, salinity, and dissolved oxygen were performed by CNR technicians at the NATO Centre for Maritime Research and Experimentation (CMRE) in La Spezia (Italy). These two calibrations permitted us to obtain a slope correction, used in the configuration file of the SBE Seasoft™ suite of programs, improving the data quality. The post-cruise calibration was not performed on data acquired during the cruise of Piattaforme2018. After their acquisition, data were pre-processed by the SBE Data Processing™ software with the updated configuration file to correct coarse errors. Data from the redundant sensor were used instead of the primary data in case of malfunction (Ribotti et al., 2020).
But, despite their double calibrations, during a cruise all sensors can
drift, thereby reducing their data quality. For conductivity and dissolved
oxygen sensors, their stability was checked through onboard comparisons
with data from water samples. On board R/V
Finally, Chl
During all cruises, dissolved inorganic nutrients were also acquired from water samples, and analyses were carried out once back in the laboratory. Data from Medgoos8 in May 2004 and Ichnussa2017 in October–November 2017 are available online (Belgacem et al., 2019) and described, in their quality check procedures, in Belgacem et al. (2020).
Furthermore, in the framework of the MEDESS4MS project (see info at
All CTD data listed in Table 1 are available in the SEANOE (SEA ScieNtific Open data Edition) data repository
(Ribotti et al., 2019a, b, c, 2022), where each dataset is identified per cruise and provided in the Ocean Data View (ODV; Schlitzer, 2022) ASCII file format, with missing values set to
Methodologies before, during, and after any acquisition, instruments, and personnel changed a little during the 21 years of CTD acquisitions, apart from during the two coastal cruises of SeriousGame2014-1 and SeriousGame2014-2 in 2014, due to different adopted instruments. This makes the data in the datasets highly coherent and comparable.
In 21 years, projects and scientific interests led to the choice of areas and sampling plans that could change in relation to vessel characteristics and sea conditions. Nevertheless, repetitions of casts were realized, like in the Sea of Sardinia, where the same sampling strategy was repeated for seven cruises in 5 years, or the sampling transects between Sardinia and the Balearic Islands, across the Strait of Sicily and the Corsica Channel, and along the western Mediterranean (see in Table 1), mainly for oceanographic studies like circulation, hydrology, and their variabilities. In the following subsections, a process and an oceanographic event, observed from the data described in this paper, are mentioned to suggest further scientific research interests and opportunities for using these data.
In the first subsection, we show the step structures observed during the first four Medgoos cruises between 2000 and 2002 in the Sea of Sardinia but also during other cruises in the western Mediterranean. In the following subsection, we describe studies on the Western Mediterranean Transient that were detected in the datasets acquired between 2005 and 2017.
The first seven cruises named Medgoos have been organized to cover the Sea of Sardinia either partially or totally. Casts were realized on a regular 25 km
grid from the shelf to the abyssal plain, which are some hundreds of kilometers from the coast. The integration of numerical models' solutions and satellite images permitted the study of the circulation in the Sea of Sardinia (Ribotti et al., 2004; Puillat et al., 2006). It is mainly characterized by fresher modified Atlantic Water (AW) in the upper 200 m, warmer and saltier Levantine Intermediate Water (LIW) below and till 800 m depth, moving northward along the Sardinian slope, and the Western Mediterranean Deep Water (WMDW) between 800 m and the abyssal plain. Due to baroclinic instabilities of the Algerian Current, surface and intermediate waters are dynamically affected by the presence of deep anticyclonic Algerian eddies (AEs) along the Sardinian coast, involving surface and intermediate waters till 1500 m depth (Puillat et al., 2006) and reaching a latitude around 40
Some of the steps observed offshore of northwestern Sardinia, between 500 and 1500 m, through the profiles of absolute salinity and conservative temperature at the interface of intermediate–deep waters in the period of March 2001–January 2004.
During the first cruise conducted in March 2001, the staircase structure was
less evident than in 2002 and 2004. Each step is defined by a layer with
values of the stability ratio (
Staircase systems were also found in the central Algero-Provençal Basin during the Bonifacio2011 (November 2011) and Ichnussa2012 (January 2012) cruises and were well identified from the analysis of the profiles of absolute salinity and the conservative temperature profile, as reported in Fig. 3.
Profiles of absolute salinity and conservative temperature of two casts where the steps were observed along the transect between the Balearic Islands and Sardinia at 500–1500 m depth in the period of November 2011–January 2012.
These staircases are organized in seven- to nine-step structures, with interfaces 60–110 m thick and
The features of these dynamical structures confirm that they are related to salt fingering that originated in the presence of intermediate waters, both when close to the continental slopes (NW Sardinia) and when in the middle of the Algero-Provençal area. In both cases, their presence is probably dependent on the absence of eddies that, if deep, can erode the step structure, which is in agreement with what observed in June 2017 by Taillandier et al. (2020) in the Algerian Basin along the 38
In the Mediterranean, the deep circulation is organized in two deep cells, located in the western and eastern sub-basin, respectively, and is constituted by denser waters than those above whose thermohaline features can differ between the two sub-basins (Malanotte-Rizzoli et al., 1997; Sparnocchia et al., 1999; Wu et al., 2000; Bethoux et al., 2002; Rixen et al., 2005; Marty and Chiavérini, 2010; Bensi et al., 2013; Ingrosso et al., 2017; Send and Testor, 2017).
Deep sections (over 2000 m depth) of potential temperature
(
Potential temperature and practical salinity (
Between October 2004 (Medgoos9) and 2017 (Ichnussa2017), 15 cruises covered some deep sub-basins of the western and central Mediterranean, from the Strait of Sicily and the Tyrrhenian Sea over to the Strait of Gibraltar, with
hydrological casts (see Table 1). These data, apart from the shallow cruise
Bonifacio2010-Cor and the two SeriousGame cruises in 2014, have permitted us to follow the spreading of the new Western Mediterranean Deep Waters (nWMDW) in the western Mediterranean sub-basins since its formation in 2005 (Schroeder et al., 2008), revealing over a decade of great thermohaline variability. During winter 2004/2005, above the northwestern Mediterranean area, a little rain with intense and persistent northerly winds brought a massive heat loss in the Gulf of Lions, the highest since 1948, that led to an increase in surface salinity (Font et al., 2007). In the same years, the
propagation of the Eastern Mediterranean Transient, from the eastern to the
western basin of the Mediterranean, led to maximum values of temperature and
salinity in the intermediate layers (Gasparini et al., 2005; Lopez-Jurado et
al., 2005; Schroeder et al., 2006; Margirier et al., 2020). The concomitance
of these meteorological and oceanographic processes has created the perfect
conditions for the formation of warmer and saltier deep waters than the
pre-existing ones (Herrmann et al., 2010; López-Jurado et al., 2005), which then occupy the deepest layer close to the bottom and are known as the Western Mediterranean Transient (WMT; López-Jurado et al., 2005; Schroeder et al., 2008; Briand, 2009; Zunino et al., 2012). After the first event of 2004/2005, WMT continued with large volumes of nWMDW formed in winter 2005/2006 (Medgoos10 and Medgoos13 in Fig. 4), while in the following winters of 2006/2007 and 2007/2008, no deep water production occurred due to a mild cold season. But deep convection started again in winter 2008/2009 and in the following (i.e., 2009/2010, 2010/2011, and 2011/2012) winters, due to particularly cold weather, as in 2011/2012, in the Mediterranean Sea. In 2012, and at the bottom of the Gulf of Lions, the nWMDW had a potential temperature of 12.905
Every winter, new warmer, saltier, and denser deep waters formed, moving to
a continuous increase in the heat and salt content in the deep layer, making the deep temperature–salinity (
The four datasets described in this study are publicly available and available free of charge from the SEANOE data repository as Ribotti et al. (2019a, b, c) at
Over 1468 hydrological vertical profiles were acquired during 29 oceanographic cruises in 21 years in the Mediterranean Sea, with a particular focus on the western and central Mediterranean sub-basins, between May 2000 and October 2020. Each cruise was mainly led by the project objectives funding the research but also by scientific interests at different spatial (coastal or offshore) and temporal (from days to years) scales. The use and reuse of data with time was mainly driven by the following issues: solution of marine environmental problems, studies on ocean circulation and variability, and/or model validation. This is visible in the different spatial resolution of the sampling plans, on a regular grid of casts, or along repeated transects, sometimes with both being used during the same cruise.
During the years, and despite some improvements in sensor quality, sensors and instruments were prepared through the same procedures before and after their use and had the same quality assurance protocols, apart from the two SeriousGame cruises in 2014. CTD data followed all quality check, assessment, standard, and best practices, as defined at an international level (see Bushnell et al., 2019) and through standardized procedures for all sensors (Hood et al., 2010). Pre- and post-calibration, redundant sensors, and water samples analyzed onboard have achieved the best accuracy standard for the instruments used. The presence of the same operators on board for most of the cruise years has assured a reduced uncertainty of the measurements, further increasing the quality of the dataset.
The final results are coherent datasets of CTD data that can be used for all studies, as described in the paper for the WMT or for salt fingering.
AR led some projects and cruises, acquired funding and data, finalized the data quality procedures described in the paper, realized the datasets, and wrote the paper. MB led some cruises, finalized data quality procedures described in the paper, realized the datasets, and collaborated on the writing of the paper. RS and AC led some projects, acquired funding and data, and collaborated on the data analyses and the writing of the paper with FP and GQ.
The contact author has declared that none of the authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank captains and crews on R/V
Cruises mentioned were supported by national and European projects of which the most important are the EU projects MyOcean (contract no. 218812) and MyOcean2 (contract no. 283367), the MED programme project MEDESS4MS (agreement no. MED2S-MED11-01), the Italian MATTM project SOS-BONIFACIO (prot. DPN-2009-0001027 of 20/01/2009), the Italian MIUR project PON TESSA (agreement no. PON01_02823), the EU project COMMON SENSE (contract no. 614155), the Italian MATTM project SOS-Piattaforme & Impatti offshore (Reg. Uff. U. 0000939.17-01-2017), and the 2014–2020 Interreg V-A Italy–France Maritime project SICOMAR Plus (IAS CNR Prot. 0001156/2018 of 12 December 2018).
This paper was edited by Giuseppe M. R. Manzella and reviewed by two anonymous referees.