RV Investigator Voyage IN2023_V07 Triaxus (undulating CTD) Data

This record describes the data (CTD, Dissolved Oxygen, Transmissometer, PAR, ECO Triplet, Nitrate, Fluorescence and LOPC) collected using the towed TRIAXUS platform on Investigator voyage IN2023_V07, titled: "SWOT-ACC: smaller scales of the Antarctic Circumpolar Current in a meander south of Tasmania." The voyage took place between November 15, 2023 and December 20, 2023 (AEST), departing from Hobart and returning to Hobart.

The primary focus of this voyage was to enhance our ability to observe and comprehend the effects of small-scale motions (ranging from 10 to 100 km) on the dynamics of the Antarctic Circumpolar Current (ACC). The groundbreaking Surface Water Ocean Topography (SWOT) satellite mission will provide unprecedented measurements of sea surface height over a wide area with substantially improved spatial resolution compared to traditional altimeters. While SWOT will offer unparalleled coverage in both space and time, it is crucial to obtain in situ subsurface ocean measurements to validate the satellite observations and establish the connection between small-scale variability in sea surface height and subsurface ocean circulation and dynamics. 

The objective was to gather comprehensive multidisciplinary measurements along a swath traversing a standing meander of the ACC. This meander represents a region of intense eddy activity, cross-front exchange, and vibrant small-scale motions. The experiment's goals encompass characterizing the small-scale variability within this meander, establishing a correlation between small-scale variations in sea surface height and subsurface ocean structure, quantifying the contribution of small-scale processes in facilitating the transport of heat and carbon across the ACC, and investigating the interconnectedness of physical, biogeochemical, and biological processes at small spatial and temporal scales.
5 Triaxus tows were conducted. Pressure, conductivity, temperature, and dissolved oxygen data were gathered from duplicate sensors using a Seabird SBE9plus CTD. Also carried on the Triaxus hull were a cosine Photosynthetically Active Radiation (PAR) sensor, a transmissometer, an ECO Triplet (optical backscatter, CDOM fluorometer and chlorophyll fluorometer), a Laser Optical Plankton Counter (LOPC) and SUNA nitrate sensor.
Triaxus hull number 2 was utilized throughout the entire voyage, completing 5 tows with a total towing time of 121.75 hours. The vehicle was generally towed between 5-7 knots depending on the conditions. Overall, it performed well during the entire voyage. 
The following issues were encountered: 
• There were 5 instances (Tow1) when the Triaxus software crashing and Seasave stopping. Fortunately, after multiple restarts, it returned to normal operation once payloads and undulations were reset. Additionally, on two occasions, the software failed to maintain the designated undulation schedule and required a restart to execute undulations correctly. 
• During tow #3, halfway through, the secondary DO sensor #3534 was replaced with #1794. The DOs were in specification however this brought the reading closer together. During the beginning of Tow 3, the Suna data for Triaxus was not displaying on Grafana, leg-2 was thus aborted.
• On several of the tows, the fairing became loose from the cable and thrashed in the current. Eventually, the method outlined in IN2023_V06 for securing the fairing was adopted. This involved using regular-sized cable ties and hose clamps spaced approximately 300mm apart along the fairing to keep it securely attached to the sea cable. 
• During Triaxus tow 4 on December 5th a malware attack was encountered, corrupting the network share drive containing CTD data. The tow was promptly aborted at 0830 UTC, and the vehicle was safely recovered without incident. For more detailed information on the malware attack, please refer to the DAP report. 
On each tow only data from legs where the Triaxus was vertically undulating regularly were processed. Data from deployment into the water, turns, and retrieval were also gathered but not further processed except to provide surface pressure references for the undulation legs.
Pressure, conductivity, temperature, and dissolved oxygen sensor data were converted to calibrated units. Spikes and out-of-range values were removed, and primary sensor data were compared to secondary sensor data. PAR, transmissometer, ECO Triplet and SUNA nitrate data were combined into the dataset. LOPC data was gathered but is not processed or published with this report.
Published data have been grouped into 1-decibar bins. Data from the primary CTD sensors were assessed to be more reliable, so the final dataset uses the primary CTD sensor values in the default data variables (i.e., variables without “_2” appended).
On this voyage all tows were made up of legs that were sufficiently short to be recorded in single files. As such, a “leg” is the same as a “section” for all tows in this voyage.
The published data consist of:
• Along-track time-series data for each leg
• Synthetic interpolated vertical casts for each undulation peak and trough of each leg
• Interpolated sectional plots of various sensor data taken from along-track time-series data for each leg

The collected data were processed (quality-controlled), and subsequently archived by the CSIRO National Collections and Marine Infrastructure (NCMI) Information and Data Centre (IDC).

Additional information regarding this dataset may be contained in the Voyage Summary and the Data Processing Report.

Organization: Australian Ocean Data Network