This time series has been collected by the Italian Consiglio Nazionale delle Ricerche (CNR) in a station located at about 3500 m depth in the central Tyrrhenian Sea (39° 46.85 N, 011° 53.00 E) over the period 2003-2016. The dataset contains 21 hydrological profiles (Pressure (dbar), Temperature (ITS-90, °C) and Salinity) performed on average every six months, with a lack of data in the period 2007-2010. The dataset is highly homogeneous in technical characteristics and quality, since the instrument used to collect most of the data is the same CTD probe (SBE911plus) and data quality procedures have been performed. The calibration of the probe was performed at the NATO-CMRE Centre of La Spezia up to 2014, and by the manufacturer (SeaBird) or by the Oceanographic Calibration Center of OGS in Trieste thereafter. To verify the post calibration performance of the probe, redundant sensors were often used for both temperature and salinity measurements, and on board salinity analysis of the water samples was performed using a Guideline Autosal or a Portasal salinometer. Raw data are post-processed and averaged at 1 dbar. Until 2014, the data were collected by the Italian R/V URANIA, those of 2015 by the Italian R/V MINERVA UNO, and those of 2016 by the Spanish R/V Angeles Alvarino. This is the longest time series (fourteen years) of temperature and salinity of thermohaline staircases due to double diffusion in the salt finger regime ever observed. Parameters included in the .csv data file: - Cruise name (Cruise) - Time as julian days (Time_julian day) - Latitude - Longitude - Pressure in dbar (Pressure_dbar) - Practical Salinity (Salinity) - Temperature ITS-90 in °C (Temperature_ITS-90_°C)

Hydrological data have been collected during 7 oceanographic cruises, named medgoos, organized in the Sardinia Sea and in the Sardinia Channel between May 2000 and January 2004 (table 1) by the Italian Consiglio Nazionale delle Ricerche (CNR). Tab.1 The 61,30 meters long R/V Urania of CNR was used during all cruises. Data of conductivity, temperature, dissolved oxygen (DO) and Chlorophyll-a fluorescence (Chl-a) were acquired by a SBE911 plus CTD probe (Sea-Bird Inc.) equipped with a 24 Niskin bottle rosette for water column samples collection. The CTD data were quality checked and processed by Seasoft software. Then salinity was checked against the on-board analyses of water samples by a Guildline-Autosal salinometer. The same for DO data checked against Winkler titration analyses. Chl-a data were not calibrated, so they are reported as Relative Fluorescence Units. During cruises from medgoos4 to medgoos7 acquired temperature data were checked at defined depths against inverted thermometers installed in correspondence of the Niskin bottles number 1, 3, 5, 7 of the rosette sampler. Pre-cruise and post-cruise calibrations of the sensors were performed at the SACLANT Center (now NATO-CMRE) of La Spezia (Italy). Redundant sensors were often used for both temperature and salinity measurements, apart during the medgoos1 and medgoos2 cruises. The vessel was positioned using an integrated navigation system consisting of two DGPS antennas managed by Andrews Hydrographics software installed on HP 386, 33 MHz PCs. The data set is provided per cruise as ODV Spreadsheet files in TXT format: - Cruise name - Station number - Type of acquisition (here C) - Date in mon/day/yr and Time in hh:mm - Coordinates in Longitude [degrees_east] and Latitude [degrees_north] - Bot. Depth [m] - Ship name - Pressure, Digiquartz [db] - Conductivity [mS/cm] - Fluorescence, Seatech - Temperature [deg C]; IPTS-68 in medgoos1; ITS-68 in medgoos2, 6; ITS-90 in medgoos3, 4, 5, 7 - Oxygen, Beckman/YSI , WS = 3 [ml/l] - Depth [salt water, m] - Practical Salinity from Conductivity [PSS-78]

The sampling plan of the ICHNUSSA2015 proposes the route already covered in previous cruises since 2000 to acquire data for the activities of Cal/Val of oceanographic ecosystem or for the validation of forecasting numerical models at different spatial scales. Then the aim is also to study the inter-annual variability of biogeochemical and physical properties of the water masses in crucial areas for understanding the circulation and exchange between basins, in particular the transport of heat and salt in the western Mediterranean. In particular, we want to study the exchange between western and eastern Mediterranean and the waters recirculating or formed in the Algerian-Provençal basin (like the new deep water in the Gulf of Lions) and along the transect Sardinia-Balearic islands. Then we want to monitor characteristics and distribution of the new western Mediterranean deep water and its possible effects on the general circulation of the Mediterranean. The ICHNUSSA2015 is designed to approximately replicate the stations of previous cruises like those named medgoos, MedCO or MedOc and the most recent ICHNUSSA2012 and 2013. CTD Casts At every station, pressure (P), salinity (S), potential temperature (θ) dissolved oxygen concentration (DO) and Fluorescence were measured with a CTD-rosette system consisting of a CTD SBE 911 plus, and a General Oceanics rosette with 24 Niskin Bottles (12 liters each). Temperature measurements were performed with a SBE-3/F thermometer, with a resolution of 0.00015 °C/bit at -1 °C or 0.00018°C/bit at 31 °C, and conductivity measurements were performed with a SBE-4C sensor, with a resolution of 3 x 10-4 S/m. Dissolved oxygen was measured with a SBE-43 sensor (resolution 4.3μM). The vertical profiles of all parameters were obtained by sampling the signals at 24 Hz, with the CTD/rosette going down at a speed of 1 m/s. The data were processed on board, and the coarse errors were corrected thanks to the sampling and analyzing on board o f oxygen and salinity of the deepest station. The rosette is equipped with a sonar altimeter which intercept the bottom 70-50 meters before getting to it. The altimeter is used just for safety, to avoid the rosette to touch the bottom, and for more precision in measuring depth. Oxygen and Salinity Determination Salinity samples were collected, stored and analyzed with a Guildline Portasal Salinometer, standardized with IPSO standard 34.999 PSu Water and at controlled temperature. Also dissolved oxygen samples were collected and analyzed with the Winkler method,using a computer controlled potentiometric end-point titration procedure. Samples were taken from the Niskins bottle with the recommendedprecautions and following the Winkler procedure. The dataset is provided as an ODV txt file and has been processed in accordance to the SeaDataNet's Data QC rules; for each station (66 station) the file contains the following parameters: DEPTH, PRESS, TEMP, PSAL, DOXY and FLUOZZ.