Physical and biogeochemical oceanography data from Conductivity, Temperature, Depth (CTD) rosette deployments during the Antarctic Circumnavigation Expedition (ACE).

Dataset abstract

This data set contains measurements from various sensors mounted on the Conductivity, Temperature, Depth (CTD) rosette that was deployed in the Southern Ocean during the Antarctic Circumnavigation Expedition (ACE). 63 CTD casts were carried out during three legs in the period 21st December 2016 to 16th March 2017, including one test cast and one failed cast, for which no data is available. Data include temperature, salinity, pressure, dissolved oxygen, oxygen saturation, chlorophyll-a concentration, backscatter, and photosynthetically active radiation (PAR) and reported are also the computed variables density, depth, and sound velocity. All data has been quality controlled and post-cruise calibrated, except for the oxygen data. Data is provided at 1 dbar pressure intervals for the up- and down-casts separately and as a merged bottle file when Niskin bottles were closed. This circumpolar data set provides insights into the circumpolar hydrography and biogeochemistry of the Southern Ocean during one austral summer season.

Dataset contents

For transparency, the raw files and files produced at the intermediate stages of data processing have been provided, in addition to the final processed files.

Raw data files:

• ace_ctd_raw_files.zip - includes raw files direct from instrument and XMLCON configuration files

Intermediate files:

• files output at each stage of the SeaBird processing

Processed data files:

• ace_ctd_CTD20200406CURRSGCMR - one final set of files for the complete sensor data;
• ace_ctd_BOTTLE20200406CURRSGCMR_hy1.csv - a merged bottle file extracted from the sensor data is also provided

Metadata:

• range of files describing the CTD deployments, sensors, water sampling; quality-checking and processing of the files.

Dataset license

This physical and biogeochemical oceanography dataset is made available under the Creative Commons Attribution 4.0 International License (CC BY 4.0) whose full text can be found at https://creativecommons.org/licenses/by/4.0/

Change log

v1.1
Quick summary of issues addressed in CTD DOI Update
- Resolved discrepancies between upcast and downcast MLD estimates
- ‘Bad’ datapoints in file dACE201601_002_ct1.csv which were not flagged with ‘4’ Bad measurement
- CTDFLUOR1, CTDFLUOR1Q, CTDFLUOR2, CTDFLUOR2Q ‘dark’ correction was not applied consistently in first processing and should have been applied to all fluorescence variables
- CTDFLUOR1Q, CTDFLUOR2Q quenching correction needed to be recalculated and reapplied after update to MLD and dark correction
- Limit the number of decimal places for fluorescence, PAR and backscattering variables according to the instrument sensitivity limits (which is 4 decimal places except for backscattering which is 6)
- Changed file names described in data_file_header.txt

Additional details on ‘Issues’ and resolutions
Mixed layer depth estimates
- Large discrepancy in MLD estimates from upcast and downcasts at the same station was due to differences in the ‘reference’ depth i.e. depth other than 10 m was used when there were no datapoints at 10 m.
- Note: influence of time between casts was also checked and was not the driver of the discrepancies.
- Issue was resolved by setting the MLD for any cast where the reference depth was not 10 m to NaN.

Bad data flagging
- 22 ‘bad’ datapoints for variables salinity, density, temperature and sound at the end of the downcast file dACE201601_002_ct1 were missed during the visual inspection of the first CTD processing and hence were not flagged as bad.
- The bad datapoints are now flagged as ‘4’ bad measurement

Fluorescence
- In the first processing, dark correction was only applied to the files where quenching correction was needed, and only to the quenched corrected fluorescence variable, but should have been applied to all fluorescence variables in all files. This has been corrected
- In the first processing, the upcast MLD was used as the MLD estimate in quenching correction for both the upcast and downcast file. This has been changed so that the MLD from the same cast is used i.e. downcast estimate for the downcast file and upcast estimate for the upcasts file, unless the MLD estimate is NaN (because the reference depth was not 10 m), in that case the either the downcast or upcast estimate is used - whichever exists.

Decimal places
- The number of decimal places for the fluorescence, PAR and backscattering variables far exceeded the sensitivity limits of the respective sensors - for the fluorescence and backscattering variables this was due to the additional calculations and corrections applied. For the PAR variable it was the output from the Seabird processing.

Updated files list
The following files have been updated:
Folder: ace_bottle_BOTTLE20200406CURRSGCMR (all files within)
Folder: ace_ctd_CTD20200406CURRSGCMR (all files within)
ace_ctd_mld_CURRSSRGCMR20200405.csv
ace_ctd_visual_inspection_v2.csv
README.txt
data_file_header.txt
ace_physical_biogeochemical_oceanography_ctd_change_log.txt (new file)

v1.0 - Initial release of physical and biogeochemical oceanography data set.