Profiling Float Data

A profiling float (#12592; APEX, Webb Research Inc., Falmouth, USA) equipped with an oxygen Optode sensor (Model 4330, Aanderaa), an SBE‐41 CTD instrument (Sea‐Bird Scientific), as well as a chl fluorometer and volume scattering coefficient (at 140 degrees, 700 nm) meter (ECO FLBB, WetLABS) was deployed at ~42N/158W on April 18, 2019.  The mission design consisted of ascension from 1000‐m depth to surface twice a day, at ~local noon and local midnight. The float provided its last profile on October 25, 2019, when batteries were exhausted. At each surfacing event, the optode collected measurements in air (~ 25 cm above sea surface) 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]. 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 O2_corrected = G×O2_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 (pO2_optode), i.e. gi=pO2/pO2_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. pO2_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. An average G value of 1.30 was then calculated based on the gi estimates from each surfacing at each station, and applied to all optode profiles. Gain-corrected oxygen concentration and saturation data were not calibrated against Winkler measurements, and therefore variables are noted as being "uncalibrated". Potential density was calculated using the Gibbs gsw_sigma0.m function in Matlab. Chlorophyll fluorescence was converted to chlorophyll concentrations by first subtracting a deep-value (average at ~980m depth) and applying the factory scaling factor. The volume scattering coefficient was calibrated by subtracting a dark value obtained on ship deck prior to deployment, and applying the factory scaling factor. Volume scattering coefficient was converted to particulate backscattering coefficient (bbp, the product provided in this dataset) using the approach of  Zhang et al [2009] to obtain backscattering due to seawater, and then by using the approach of Boss and Pegau [2001] to obtain bbp, assuming a X factor of 1.1 (see  Boss and Pegau [2001]). References cited:Uchida, H., Kawano, T., Kaneko, I., & Fukasawa, M. (2008). In situ calibration of Optode‐based oxygen sensors. Journal of Atmospheric and Oceanic Technology, 25(12), 2271–2281. //// García, H. E., & Gordon, L. I. (1992). Oxygen solubility in seawater: Better fitting equations. Limnology and Oceanography, 37(6), 1307–1312.  //// Bushinsky, S. M., Emerson, S. R., Riser, S. C., & Swift, D. D. (2016). Accurate oxygen measurements on modified argo floats using in situ air calibrations. Limnology and Oceanography: Methods, 14(8), 491–505. //// Boss, E. S., & Pegau, W. S. (2001). Relationship of light scattering at an angle in the backward direction to the backscattering coefficient. Applied Optics, 40, 5503–5507. //// Zhang, X., Hu, L., &He, M-X (2009). Scattering by pure seawater at high salinity. Optics Express, 17(15), 12685. https://doi.org/10.1364/oe.17.012685