Towed CTD

  • Deploying SeaSoar
    Bringing in Seasoar
    SeaSoar at night

    SeaSoar

    Seasoar is a vehicle that is towed behind the ship and undulates between the surface and up to 400 metres depth. It changes depth by altering the angle of a pair of wings, in much the same way as a plane changing the angle of its flaps. During JR311 we are primarily interested in the upper ocean so we expect to sample between the surface and 200 metres but the exact depth will be chosen after the first few hours of sampling during which we will see exactly how deep the mixed layer is; we may need to go a little deeper if the mixed layer depths are greater than we expect or shallower if the thermocline is closer to the surface. The time lapse video below shows the deployment of Seasoar which involves slowing the ship to 6 knots whilst the vehicles is lowered into the water. The deployment and recovery takes about an hour each and requires many crew and technicians to ensure that Seasoar is safely deployed and recovered without hitting the ship, something not easily achieved in a heavy sea. For this reason we tend to use Seasoar for days at a time; during JR311 our longest deployment was for more than 5 days when we mapped the newly formed eddy in the ACC.

    Seasoar is equipped with many oceanographic sensors to measure a variety of water properties. A pair of Seabird 49 conductivity-temperature-depth (CTD) mounted on the nose measure the key water properties from which we can derive salinity and ultimately density. Additional sensors measure dissolved oxygen, chlorophyll, phycoerythrin (a green pigment found in zooplankton and bacteria), bioluminescence, plankton abundance and size using the Laser Optical Plankton Counter, turbidity, water clarity, light adaption of phytoplankton and productivity, light colour and intensity and coloured dissolved organic matter (CDOM).

    A key operational consideration when using Seasoar, or any towed CTD, is the trade off between how much distance you can cover and at what horizontal resolution. Because Seasoar needs water to flow over its wings to generate lift, the ship needs to maintain a speed of around 9 knots when towing; anything slower and it will sink whereas much faster may snap the tow cable. For a fixed towing speed, this means that we can only increase the horizontal resolution of our measurements by decreasing the depth to which Seasoar dives. The shallower, the dive, the less time is required for the dive and ascent to be completed and therefore the horizontal distance between the position at which Seasoar reaches the surface is reduced. Conversely, if we send Seasoar to it’s maximum profiling depth of 400 m, there will be several kilometres between each dive cycle which is too far apart to resolve submesoscale fronts that may be only 1 km wide.

    A close up of the MVP 'Fish'
    The 'fish' going overboard

    Moving Vessel Profiler

    An alternative to Seasoar is the Moving Vessel Profiler (MVP). As opposed to the vertical undulations described by Seasoar as it ‘flies’ through the water, MVP displays more of a ‘saw-tooth’ pattern throughout its dive cycle. This is because the MVP ‘fish’ which houses the oceanographic sensors free-falls at 4 metres s-1 to the required depth, which can be up to 800 metres, before being winched back to the surface to continue the dive cycle. The biggest advantage of the MVP over Seasoar is the option to obtain measurements whilst moving more slowly as the fish does not rely on wings to generate lift in the same way as Seasoar. The result is that we can both reduce the depth of our measurements and the tow speed to get better horizontal resolution than that we can achieve with Seasoar. During JR311 we have been typically towing MVP at 5 knots and as a result obtaining vertical profiles every 500 metres or less in the horizontal.

    The instrument package on MVP is more modest that that available on Seasoar, with sensors available to measure conductivity, temperature and depth (CTD), fluorescence (both chlorophyll and fluorescein), and light colour and intensity which is important for biological productivity.

    Phil checking the dye hose
    A drogue released

    Towed CTD survey strategy

    After positioning the ship within a submesoscale front, we will first release an inert dye solution at a depth of between 50-100 metres. The purpose of the dye is to reveal the circulation within the surface mixed layer; the evolution of submesoscales is quite subtle and difficult to detect using standard measurements of temperature and salinity for example. By observing where the dye goes and how it disperses throughout the surface mixed layer, we will gain crucial information on how the presence of submesoscales impacts on currents near the surface. A triplet of drogued drifters will also be deployed at the start of dye pumping and used to both mark the dye patch and tell us about how water parcels are moving relative to each other. All of the drifters are equipped with a strobe and transmit positions by Iridium every 10 minutes so that the bridge knows exactly where they are. It’s critical that the ship doesn’t run over a drifter that’s trailing a 35 metre rope as subsurface lines and propellers don’t mix!

    A highlight of JR311 has turned out to be witnessing the birth of a mesoscale eddy in the Antarctic Circumpolar Current (ACC). We originally started to survey a frontal meander with Seasoar as the front in this region is almost 30 km wide, therefore requiring the faster towing speed of Seasoar to complete short transects across the front in a short time. After releasing the dye and drifters however, we noticed from satellite observations of sea surface temperature that the meander had become ‘pinched’ so that it’s head had been closed off into a circular eddy, somewhat like a giant whirlpool that measures almost 100 kilometres across. We followed drifters as they circled around the eddy, at first within a cold, narrow current that was bordered by a strong and abrupt density front. As we moved around the eddy to the southern boundary, this abrupt front evolved into a series of narrow jets and fronts, precisely the transition to submesoscales that we aimed to capture with observations!