Abstract:

This is a dataset of three-dimensional simultaneous direct measurements of both physical and biochemical properties and processes throughout the water column (from the sea surface to 1200 m, occasionally extending as deep as 2000 m), from Electro-Magnetic Autonomous Profiling Explorer (EM-APEX) floats deployed during R/V Walton Smith cruise WS17121. The targeted area of the ballasted float deployments lies between the DeSoto Canyon and the northern extent of the Loop Current in the eastern Gulf of Mexico. The EM-APEX profiling float is equipped with CTD and electromagnetic current sensors and also measures the following biochemical properties: dissolved oxygen (DO), chlorophyll fluorescence as proxy for phytoplankton abundance, backscatter as proxy of particle concentration (including marine particles and oil droplets), and colored dissolved organic matter (CDOM) fluorescence as proxy of CDOM (including fluorophoric oil components). Floats were deployed in May 2017 and transmitted data for nearly two years.

Suggested Citation:

Shay, Lynn;Brewster, Jodi; Jaimes, Benjamin;Rudzin, Johna; Fennel, Katja; Gordon, Chris. 2020. Velocity and density data in the Gulf of Mexico and North Atlantic from 2017-05-02 to 2019-04-05 measured by Electro-Magnetic Autonomous Profiling Explorer (EM-APEX) floats deployed during R/V Walton Smith cruise WS17121. Distributed by: Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC), Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/n7-tad8-j468

Purpose:

When crude oil is combined with seawater this complex mixture immediately begins to change. The Deepwater Horizon oil spill was unique in that the spill originated from the deep seafloor (~1500 m) and in the volume of oil released. Underwater plumes of oil were confirmed which were not visible at the surface. Such modification processes led to differing oil mixtures, possessing different characteristics. This is the rationale why subsurface physical and biochemical measurements are critical during a spill. Such measurements are needed to understand three-dimensional dispersion as well as deep mixing events via shear instability (e.g., Richardson numbers falling below critical values) and their impacts on the biochemistry at depths to 2000 m. The EM-APEX floats from Teledyne-Webb Research are used to measure current shear and vertical density gradients at the high vertical resolution, at the same vertical positions as the biogeochemical fields.

Data Parameters and Units:

QC profiles: Latitude [degrees], Longitude [degrees], pressure [dbar], date [YYYY-MM-DD], time (UTC, [HH:MM:SS]), temperature [degrees Celsius], Salinity [psu], flag_ctd (CTD quality flag, [0=good], U (eastward velocity 1, [m/s]), V (northward velocity 1, [m/s]), Verr (standard velocity error, 1 [m/s]), DOXY (dissolved O2, [umol/kg]), CHLA (chlorophyll concentration [mg/L]), CDOM (dissolved organic matter, [mg/L]), BBP700 (backscatter at 700nm, [1/m]). The raw profiles also include ADCP data: Latitude [degrees], Longitude [degrees], pressure (CTD, [dbar]), pressure (ADCP, [dbar]), date [YYYY-MM-DD], time (UTC, [HH:MM:SS]), temperature [degrees Celsius], Salinity [psu], flag_ctd (CTD quality flag, [0=good], U1 (eastward velocity 1, [m/s]), V1 (northward velocity 1, [m/s]), Verr1 (standard velocity error 1, [m/s]), U2 (eastward velocity 2, [m/s]), V2 (northward velocity 2, [m/s]), Verr2 (standard velocity error 2, [m/s]), flag_vel (velocity quality flag, [0=good]), dP_dt (float ascension rate, [m/s]), RotP (Float rotation period, [s]), DOXY (dissolved O2, [umol/kg]), BBP700 (backscatter at 700nm, [1/m]), CDOM (dissolved organic matter, [mg/L]), CHLA (chlorophyll concentration [mg/L]), BSig (raw backscatter counts, [counts]), CSig (Raw CDOM counts, [counts]), FSig (raw fluorescence counts, [counts]). In air measurements: DOXY (dissolved oxygen [umol/kg]), pO2 (partial pressure of oxygen [Pa]), Rphase (red phase, used to determine drift), TempDegC (optode measured temperature [degrees C]), Tphase (main input product), UXT (time in seconds since 1970-01-01).

Methods:

This dataset is separated into "raw" data and "quality-controlled (QC)" data then by float number then. Raw data comes directly from the float. QC data has been visually inspected for spurious observations and interpolated from 0 db to 1200 db at 2 db intervals. Specific QC measures for CTD: Pressure must be greater than 0 db, Temperatures must be greater than 0C, Salinity must be greater than 30 psu. The temperature and salinity thresholds were determined based on the location of deployment. Specific QC measures for velocity: Pressure must be greater than 0 db, profiles must have RotP (rotation period of float) value that is less than EPNval/2, and velocity component values must be less than velocity component errors as determined by electrode pair #1. Both velocity components from electrode pair #1 and pair #2 are equal. QC velocity profiles are from electrode pair #1. Specific QC measures for biogeochemical data: Oxygen is converted from phase difference to concentration in micromole per kg following the procedure in the Argo oxygen processing cookbook case 9.2.29 (https://doi.org/10.13155/39795). The oxygen concentrations have NOT been corrected for pre-deployment drift using in-air measurements as described in Johnson et al., 2015, but in-air measurements were recorded. Low salinity values (< 25 psu) were removed as they cause anomalously high concentrations near the surface. Conversion for bbp, cdom, and chla were all performed using manufacturer-provided slope and dark counts. It should be noted that the slope for chla is known to often overestimate the pigment concentration, and bgc-Argo standard processing will divide the slope by a factor of 2; however, this is not done in this case and is left up to the user.

Instruments:

The EM-APEX profiling float is equipped with CTD and electromagnetic current sensors as well as the following chemical and bio-optical sensors: 1) the Aanderra Optode 4330F sensor for dissolved oxygen (DO), and 2) the WETlabs ECOpuck FLbb-CD, which combines measurements of chlorophyll fluorescence as proxy for phytoplankton abundance, backscatter as proxy of particle concentration (including marine particles and oil droplets), and colored dissolved organic matter (CDOM) fluorescence as proxy of CDOM (including fluorophoric oil components).

Provenance and Historical References:

Johnson, K. S., Plant, J. N., Riser, S. C., & Gilbert, D. (2015). Air Oxygen Calibration of Oxygen Optodes on a Profiling Float Array. Journal of Atmospheric and Oceanic Technology, 32(11), 2160–2172. http://doi.org/10.1175/JTECH-D-15-0101.1 Thierry Virginie, Bittig Henry, Gilbert Denis, Kobayashi Taiyo, Kanako Sato, Schmid Claudia (2018). Processing Argo oxygen data at the DAC level. https://doi.org/10.13155/39795

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