Dataset: float
Deployment: KM1713

Mixed-layer averaged O2 and optically-based POC from Profiling Floats

A profiling float (#12592; APEX, Webb Research Inc., Falmouth, USA) equipped with an oxygen optode sensor (Model 4330, Aanderaa Data Instruments, Bergen, Norway) as well as a SBE‐41 CTD instrument (Sea‐Bird Electronics Inc., Bellevue/WA, USA) was deployed at stations 2, 3, 4, and 5 for ~2 days. The profiling float was recovered by small boat operations at the end of each station. The mission was programmed to allow float to surface every 3 hours and profile from 200 m depth to surface (i.e. 8 profiles per day). At each surfacing event, the optode collected measurements in air to allow post-calibration.

Raw float oxygen optode data were corrected for pressure and salinity following Uchida et al. [2008] and Garcia and Gordon [1992], as outlined in the Aanderaa manual. Float optode oxygen data were calibrated using optode air measurements taken at the time of each float surfacing following methods described in Bushinsky et al. [2016]. Briefly, air calibration relies on the estimate of a gain factor G, such that O_{2_corrected} = G×O_{2_raw}. In practice, G is an average gain factor for each float profile determined from the ratio of the expected partial pressure of oxygen in air (pO₂) to the partial pressure of oxygen in air measured by the optode (pO_{2_optode}), i.e. g_i=pO₂/pO_{2_optode}. Optode air measurements were first filtered to remove outliers (e.g. measurements taken underwater or with high variance), and mean values per surfacing were recorded. pO_{2_optode} was calculated from optode phase and temperature as in the Aanderaa Manual. Atmospheric pO₂ was calculated as in Bushinsky et al. [2016] using float-derived water vapor pressure estimates (pH₂0) at the time of each float surfacing (Aanderaa Manual) as well as the 6-h NOAA NCEP atmospheric pressure and surface relative humidity data interpolated to the time and location of each float surfacing (https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.surface.html). An average G value of 1.24 was then calculated based on the g_i estimates from each surfacing at each station, and applied to all optode profiles. Float mixed-layer depth was calculated using the 0.125 kg m⁻³ potential density change threshold relative to 10 m depth.

Float mixed-layer averaged gain-corrected O₂ concentrations were further calibrated by multiplying mean Winkler concentrations per station to float percent deviation estimates at each station (Eq. 1). Similar procedures were applied to optode data from an identical profiling float (#12593) with same mission design deployed near station ALOHA (22.45° N, 158° W) on June 29 2017 and recovered at the end of the cruise near station 6. Exceptions included that only air calibrations were carried since no simultaneous Winkler measurements were available. Importantly, the large number of daily profiles and relative shallow depths reached by the float led to obvious biofouling of the oxygen sensor after ~ 30 days at sea. Thus, only data from the month of July 2017 are made available.