Corrected salinity and seawater temperature ship data from expedition ARTofMELT, Arctic Ocean, 2023

Femke den Ouden, Ben Kopec, Jeff Welker

The ARTofMELT expedition was carried out in the Fram Strait pack ice during the spring transition (May to June) of 2023, with the Swedish icebreaker Oden serving as research platform.

The ship data and Ferrybox collected onboard the Oden during the expedition is not processed or checked for quality. After analysis with salinity and seawater temperature variables, we found considerable inconsistencies in performance of various instruments and inaccurate data.

This dataset contains the most accurate seawater temperature variable and corrected salinity. This correction is based on a comparison of all underway salinity and temperature sources against independent, discrete measurements from conductivity, temperature, and depth (CTD), vertical microstructure profiler (VMP), and remotely operated vehicle (ROV) profiles to assess there accuracy and established quality control (QC) methods.

MarineHydrographyArcticArctic OceanARTofMELTARTofMELT 2023 expeditionIcebreaker OdenSalinityHydrographySeawater temperature

Name

Version

Citation

References

Anhaus P, Schiller M, Planat N, Katlein C Nicolaus M (2023) Hydrographic data from remotely operated vehicle (ROV) surveys during the ARTofMELT2023 expedition [Dataset publication series]. PANGAEA. https://doi.org/10.1594/PANGAEA.962344

EuroGOOS DATA-MEQ working group. (2010). Recommendations for in-situ data Near Real Time Quality Control. Coriolis data centre. https://doi.org/10.13155/36230

Holthusen LA, Planat N, Asmussen M Muchowski J (2025) Oceanographic vertical microstructure profiler (VMP) data from expedition ARTofMELT, Arctic Ocean, 2023 — subset with CTD data. (Version 1) [Data set]. Bolin Centre Database. https://doi.org/10.17043/oden-artofmelt-2023-vmp-ctd-1

Muchowski J, Planat N, Holthusen LA, Asmussen M (2025) Oceanographic conductivity, temperature and depth (CTD) data from expedition ARTofMELT, Arctic Ocean, 2023. (Version 1) [Data set]. Bolin Centre Database. https://doi.org/10.17043/oden-artofmelt-2023-ctd-1

SeaDataNet (2010) Data Quality Control Procedures (Version 2.0). European Commission, DG-Research. https://www.seadatanet.org/

Swedish Polar Research Secretariat, Ship-Based Research Support (2024) ARTofMELT 2023 - Meteorological, Oceanographic, and Ship Data Collected Onboard Icebreaker Oden (Version 1) [Data set]. Swedish Polar Research Secretariat. https://doi.org/10.48515/xtgq-wt93

Swedish Polar Research Secretariat, Ship-Based Research Support (n.d.) ARTofMELT 2023 - Ferrybox Data Collected Onboard Oden [Unpublished dataset].

Data description

• Number of files: 3
• File format(s): comma-separated values (csv), JPEG (jpg)
• Total data size: 6.6 MB

ARTofMELT_salinity_seawatertemperature_corrected.csv Data

• File size: 6.0 MB
• Number of columns: 14
• Number of rows: 58055
• Temporal resolution: from the Ferrybox 1 minute, from the Kofferdamm 2 minutes
• Variables
  • DateTime date and time (yyyy-mm-dd hh:mm:ss)
  • Latitude latitude (decimal degrees)
  • Longitude longitude (decimal degrees)
  • Salinity_Corrected final salinity product (PSU)
  • Ferrybox_SBE45_Salinity_Clean cleaned salinity from Ferrybox (PSU)
  • Ferrybox_QC_flag quality control flag of Ferrybox salinity (1 = bad, 2 = good)
  • Kofferdamm_SBE45_Salinity_Clean cleaned salinity from Kofferdamm (PSU)
  • Kofferdamm_QC_flag quality control flag of Kofferdamm salinity (1 = bad, 2 = good)
  • Ferrybox_flow_main main flow through Ferrybox (L/min)
  • Ferrybox_SBE45_Temperature raw seawater temperature from Ferrybox (°C)
  • Ferrybox_SBE45_Salinity raw salinity from Ferrybox (PSU)
  • Kofferdamm_SBE45_Temperature raw seawater temperature from Kofferdamm (PSU)
  • Kofferdamm_SBE45_Salinity raw salinity from Kofferdamm (PSU)
  • Inlet_Temperature raw seawater temperature from inlet (°C)

FigureS1.jpg Figure S1

• File size: 305 kB

Salinity and temperature comparison between all sensors of the ship and discrete point measurements.

FigureS2.jpg Figure S2

• File size: 291 kB

QC flagged data time series.

Comments

Measurement systems

Surface seawater onboard Oden was supplied through a flow-through intake system at approximately 8 m below the surface. The seawater intake fed three primary hydrographic measurement locations:

• the Ferrybox
• the Kofferdamm
• the main laboratory.

Ferrybox

The Ferrybox (4H-JENA) is an automated system designed for high-quality surface observations, including debubbling, temperature control and stabilized internal flow, which reduces noise from ship motion and air entrainment.

Kofferdamm

The Kofferdamm is a protected intake chamber between watertight bulkheads, while the main laboratory receives water farther downstream in the intake line where internal heating may affect temperature measurements.

Instruments

Each location was equipped with a SeaBird SBE45 thermosalinograph (TSG), providing high-frequency temperature and salinity measurements. Two independent temperature sensors, the SBE38 thermometer in the intake pipe and a PT100 platinum resistance thermometer mounted directly at the seawater inlet, also provided temperatures; the latter best reflects the true intake temperature, although it responds more slowly to rapid changes. Together, these instruments enabled cross-validation of temperature and salinity across multiple points in the intake system.

Data comparison and corrections

Because the Ferrybox was used for the first time during ARTofMELT and occasionally experienced unstable inlet flow, flow to other consumers was sometimes compromised and the dataset required thorough evaluation before use. To understand instrument performance, all temperature and salinity measurements were compared internally (Swedish Polar Research Secretariat, 2024, n.d.) and against 50 independent “cast” measurements collected by CTD (Muchowski et al., 2025), VMP (Holthusen et al., 2025), and ROV (Anhaus et al., 2023) deployments at ~8 m depth along the cruise track, visualised in Figure S1.

The inlet PT100 temperature consistently agreed best with the cast data, whereas the Kofferdamm and main lab SBE45 temperatures showed unrealistically high values, particularly before 29 May, which coincided with abnormally stable salinity not reflected in the cast measurements. These patterns indicate insufficient flow to the Kofferdamm and main lab sensors during this period. After 29 May (Kofferdamm) and 5 June (main lab), the agreement with casts and the Ferrybox improved substantially. Based on this assessment, the inlet PT100 temperature and the Ferrybox salinity were identified as the most reliable measurements for further analysis.

To remove instrumental artefacts, a thermosalinograph (TSG) Quality Control (QC) procedure was applied following established recommendations (EuroGOOS DATA-MEQ working group, 2010; SeaDataNet, 2010).

The SBE45 sensor in the Ferrybox is sensitive to air entrainment, variable flow, micro-ice, pressure fluctuations, intermittent icing, and heat contamination inside the flow-through loop. These issues produce high-frequency spikes and steps that cannot represent real ocean variability at the temporal and spatial scale of the surface intake measurements.

Because the cast data show that natural salinity variability along the cruise track is relatively smooth, artefacts can be reliably distinguished from real signals. The QC approach therefore combined several complementary filters.

Firstly, unrealistic values were removed by applying a physically plausible range, rejecting salinities outside 31-35 and temperatures outside −3 to +7 °C; these limits were chosen based on the cast measurements and the known hydrography of the region.

Secondly, salinity data collected during periods of low flow (<5 L/min, 5-8 L/min is stable flow), or when the SBE45 temperature deviated by more than 1 °C from the inlet PT100 (SeaDataNet, 2010) were excluded, as both conditions indicate unstable measurement environments.

Thirdly, the data were examined for spikes and short-lived anomalies by identifying rapid salinity changes greater than 0.3 min-1, which exceed typical natural variability of 0.1-0.2 min-1. Individual point outliers were also removed when they deviated more than 0.3 from their neighbours within a 3 and 11 min window or from the broader trend estimated by a 61 min running median. Together, these filters removed artefacts caused by sensor noise and flow instability while preserving true oceanographic variability.

After QC filtering, two substantial gaps remained in the Ferrybox salinity time series (04 June 17:57 to 07 June 15:16 and 13 June 00:00 to 14 June 23:59). During this part of the cruise, the Kofferdamm salinity closely matched both the Ferrybox data and the cast measurements, indicating reliable flow and sensor behaviour. Therefore, the Kofferdamm salinity, after undergoing the same QC procedure, was used to fill these specific gaps. The QC flags and final corrected salininty product are visualised in Figure S2. Despite that the original salinity data required substantial filtering for quality, we are confident that the final corrected salinity record is reliable, because it closely matches the cast measurements from 8 m depth and we applied established methods.

When calculating the temperature difference between the seawater inlet and downstream sensors described above there was a mismatch between the 1 minute resolution of the Ferrybox and the 2 minutes resolution of the Inlet. To resolve this, we applied a selective forward-fill strategy for the Inlet_Temperature. This means the temperature value from a given timestamp (e.g., 12:00) was carried forward to fill the next minute's gap (e.g., 12:01). This is a reasonable physical assumption, as the bulk seawater temperature at the inlet does not change significantly over a one-minute interval. This step ensured that a T_in value was available for every 1-minute Ferrybox measurement, allowing for a robust dT calculation across the entire Ferrybox dataset. This forward-fill was not applied to the Kofferdamm data. The QC calculations for the Kofferdam data were performed only on the timestamps where original data existed, with the gaps being ignored.

Project

ARTofMELT 2023 expedition. Analysis and correction was done during the MSc graduation research of Femke den Ouden at the department of geosciences at Utrecht University on continuous surface seawater isotopes from ARTofMELT, supervised by Jeff Welker and Ben Kopec.

Publisher

Bolin Centre Database

License

Creative Commons Attribution 4.0 International License (CC BY 4.0)

First name

Femke

Last name or organisation

den Ouden

Email address

fdenouden@protonmail.com

Address

Department of Earth Sciences; Utrecht University

Postal code

Postbus 80.115

City

Utrecht

Province or state

Utrecht

Country

The Netherlands

GCMD science keyword

Earth science > Oceans

GCMD location

Ocean > Arctic Ocean

Dataset language

English

Begin date

2023-05-07

End date

2023-06-13

DOI

Time