In situ phase-domain calibration of oxygen Optodes on profiling floats

Comparison of profiles from oxygen Optodes deployed on profiling floats with ship-based bottle casts taken at the time of deployment shows typical low biases of approximately 0 to −40 μmol kg⁻¹. Most proposed methods to correct these biases use linear or multiplicative corrections of the derived variable $O_2$. Some of these methods depend on specific reference data such as deployment casts or air measurements. Here, we describe a versatile in situ method to recalculate $O_2$ directly from Optode phase and temperature by recalibrating two coefficients of the modified Stern–Volmer equation. This method may be used to calibrate most floats deployed with Optodes to date, as well as present floats, including those equipped with air measurement capability. Reference data can be taken from historic ship casts, climatology, deployment casts, and/or air measurements, depending on availability.

In situ calibrations were performed on 147 Optodes floats deployed on UW floats between 2004 and 2015 using only World Ocean Database (WOD) references. Median differences to World Ocean Atlas (WOA) 2009 climatology were reduced from ∼6% to ∼1%. Deployment casts were used to estimate error for eight Argo floats deployed in the Indian and Pacific Oceans; the aggregate error was reduced from 8% to 0.3%.

Comparison of six pairs of Optodes deployed on the same float showed relative errors after in situ calibration of $0.1\pm 0.6\mu \mathrm{mol}{\mathrm{kg}}^{-1}$. WOD-calibrated surface air oxygen values for nineteen Optode floats with air-measurement capability were compared with expected oxygen levels from NCEP surface level pressures and showed typical errors of $<\pm 2\%$.

Using data from eight floats with deployment casts, comparison of phase-domain linear correction with oxygen-domain linear correction showed a difference of less than $\sim \pm 2\%$. Comparison of surface gain correction with deployment casts found gain-corrected values below the depth of the oxygen minimum to be consistently low, with residuals of approximately −0.5 to −4.5%.