MicroCAT/SeaCAT Sensor Calibration and Data Quality Control: Lessons Learned from 10 Years of WHOI-Hawaii Ocean Time-Series Site (WHOTS) Mooring Deployments

The WHOTS surface mooring has been measuring atmospheric surface forcing and upper ocean variability at Station ALOHA (22° 45'N, 158°W) with high temporal resolution since mid-2004. The subsurface instrumentation has included Sea-Bird SeaCATs (SBE-16) and MicroCATs (SBE-37) measuring temperature, conductivity and pressure in the upper 155 m, with vertical separation between instruments ranging from 5-20 m. The mooring is replaced every year, and visited during near-monthly Hawaii Ocean Time-series (HOT) cruises. A compilation of the best practices for sensor calibration and data quality control are presented here based on data processing from 10 annual deployments. The inspection of the internal clock and temperature data from more than 160 instruments (and pressure data from more than 50) confirmed the stability and reliability of these sensors. The majority of the conductivity sensors experienced a positive drift for reasons still under investigation. For some of them, especially those close to the surface, the drift turned negative after several months and/or they suffered sudden offsets apparently due to fouling by marine organisms. The availability of CTD profiles conducted within 1 km from the WHOTS mooring during HOT cruises and during mooring replacement cruises allowed us to evaluate and correct conductivity sensor drifts and to compare them with drift estimates based on pre- and post-deployment calibrations. The differences between these two drift estimates from 44 instruments had a standard deviation of 0.0016 S/m/year, corresponding to 0.012 g/kg/year in salinity. This estimate must be considered part of the uncertainty when correcting conductivity drift with pre- and post-deployment calibrations, although drift-correction using CTD profiles is recommended. Experimental MicroCATs currently installed in the WHOTS mooring as a collaborative work with Sea-Bird are part of the active research being conducted to understand and control the conductivity drift.