Physical and Biochemical Structure Measured by APEX-EM Floats

Technology for measuring the deep ocean currents (including temperature and salinity) is now available using the recently Autonomous Profiling Explorer floats with electromagnetic sensors (APEX-EM) recently developed by Teledyne Webb Research. The float acquires temperature and salinity profiles using a Seabird sensor. In addition, the APEX-EM profiling floats were equipped with chemical and bio-optical sensors: Aanderaa Optode sensor measuring dissolved oxygen, and the WET Labs ECO Puck sensor which combines measurement of chlorophyll fluorescence (proxy of phytoplankton abundance), measurement of backscatter (as proxy of particle concentration) and colored dissolved organic matter (CDOM) fluorescence (incl. fluorophoric oil components). The ECO Puck was specifically designed to minimize space and power requirements for applications in autonomous measuring platforms, and is rated for sampling to 2000 m depth. Such technological advancements bringing together the physical and biochemical measurements have relevance on the response to subsurface oil spills such as Deepwater Horizon. Here we explore the impacts of wind-driven current on the biochemical response during the passage of a cold front in early May 2017 using profiles from multiple floats and shipboard data. During the frontal passage, surface friction velocity associated with the wind stress approached values of 0.6 m/s that forced strong near-inertial currents in the upper ocean of 0.75 m/s. The associated shear forced layer deepening of more than 20 m over the two days of frontal passage. The biochemical response indicated a dissolved oxygen and chlorophyll fluorescence maxima at depths of 90-100 m during the event. This deep chlorophyll maximum tends to correspond to the oxycline lying between the 24-25 isopycnal in temperature and salinity space. During this period the floats resolved these responses as continuous measurements were acquired over time scales of a few hours. Thus, this float technology enables investigators to look more closely at the upper ocean response to strong forcing events that impacts biochemistry and hydrocarbon dispersion. Note that sampling rates can changed by updating mission profiles from continuous to profiling modes between specific depths to once every five to ten days to depths of up to 2000 m by changing the piston counts on the missions.