Graduate student Leah Johnson, from the University of Washington is our guest blogger and writes about Chief Scientist Craig Lee’s moorings in the Davis Strait.
The long term monitoring of Davis Strait ensues from the dedication of several participating labs and scientists. The Integrative Observing Platforms (IOP) lab from the Applied Physics Lab, University of Washington is one of these groups. IOP has been involved with this campaign from its inception, and is largely responsible for the deployment and maintenance of a moored array of instruments that span across the strait (i.e. ‘moorings’).
The entire array is carefully designed, with each instrument placed strategically in order to capture the spatial and temporal variability of fluxes connecting the Arctic and subpolar North Atlantic. Each mooring is equipped with it’s own suite of instruments and floats, attached together by a synthetic line and anchored to the bottom of the ocean. The line and instruments are held upright by the floats and are therefore situated vertically in the water column, well below the surface of the sea (diagram – see description of instruments below).
Deployment and recovery of these moorings are just as carefully engineered as the design itself. The longer moorings are maneuvered through an A-frame on the ship’s stern. Each instrument is raised off deck by the A-frame and carefully lowered into the water so as not to collide with the often rocking and swaying ship. Instruments are handled one at a time; starting with the top of the mooring and working successively down the line. During deployments, the ship moves at a steady and slow pace, allowing the instruments and floats to extend horizontally along the sea surface. The final step is to attach an anchor to the end of the line. When the anchor is released from the A-frame, it plunges to the depths of the ocean, bringing the mooring line with it. The instruments remain suspended in the water column below the surface of the ocean for the next two years, recording data all the while. When it is time for recovery, the science team sends a signal to the acoustic release at the bottom of the mooring and commands it to let go of the anchor. The entire line of instruments and floats rise to the surface and are hauled onboard the ship, along with the two years of stored information about the ocean interior across the Strait.
The moored array encompasses a broad suit of instruments that tell the story of the annual fluxes through Davis Strait. Below is a summary of instruments incorporated in this array:
Ocean Physics: Each mooring line is equipped with a ‘CTD’ that measures the water’s conductivity (salinity), temperature, and depth (pressure). In addition to measuring these tracers, each line is equipped with current meters (RCM-8 and ADCPs) that determine both the magnitude and direction of the water flow. Continuous measurements of current velocities, temperature and salinity supply the information needed to construct an annual budget for heat and salt transport through the Strait.
Marine Mammals and Fish: Measurements along the array extends beyond physics and branches out in search for marine life. Several lines are equipped with a marine mammal receiver that records sound traveling though the ocean. This data is processed to differentiate sounds of marine mammals and help identify the presence of certain species throughout the year. In addition to the sound receiver, several moorings are equipped with a fish tag detector. This detector will identify and count fish that have been tagged by collaborators at the British Institute of Oceanography.
RAFOS-Seaglider communication: Seagliders are autonomous underwater vehicles (or AUVs) that maneuver independently through the water and collect measurements of temperature, salinity and depth. Seagliders orient themselves at the sea surface via GPS, which can be hazardous in the presence of sea ice. The IOP lab has surpassed this limitation by programming seagliders to communicate with RAFOS sound sources underwater that provide a spatial reference. RAFOS are included in the moored array, allowing seagliders to survey the array line during times of ice cover and develop a more complete spatial image of the temperature and salinity fluctuations through the strait.