What do we measure and how?
During JR311 we will be measuring a huge variety of oceanographic parameters including water properties, currents, turbulence, and biogeochemical tracers. The primary activity throughout the cruise will be towed conductivity-temperature-depth (CTD). CTD sensors provide the most important measurements of water properties; temperature, salinity (which is derived from measurements of conductivity) and pressure are the three variables that combine to determine the density of seawater. Horizontal changes in density lead to the generation of oceanic currents, so understanding where and why density changes throughout the ocean allows us to understand why the water moves, transporting both the water itself but also anything within it. Measurements with CTDs are usually accompanied by observations of chlorophyll, oxygen and other properties important to marine life. Currents beneath the ship are routinely measured over the top 600m by an acoustic Doppler current profiler mounted in the hull of the ship.
During SMILES the CTD sensors will be mounted on a variety of vehicles. We will use SeaSoar for larger scale surveys of 150-200 km length, of the SAF followed by the Moving Vessel Profiler (MVP) during smaller-scale surveys that will be repeated for periods of 4 days or so whilst surveying an area on the scale of 10 km centred on 3 instrumented drifters. Additional activities will involve the release of a number (8 at present) of Argo floats, up to 20 satellite tracked surface drifters, several deep CTD profiles and a few slow transects (at approximately 1 knot) during the small-scale surveys when we will tow-yo a free falling microstructure profiler (the MSS).
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!
The two large scale surveys will target an instability in the subantarctic front, apparent as a meander or eddy that has developed in SAF as it flows along the Scotia Ridge. The first survey will take place during days 1-9, and the second during days 17-25 after completion of the first two Lagrangian surveys. The exact position will be determined at the time of cruise by examining satellite data and operational forecasts. On departing Stanley, we plan to deploy Seasoar as soon as possible following the MSS, UCTD and MVP testing and complete a long survey to the south of the SAF (as far as 60°S). Following the first leg, we will return to the main SAF as identified by the data from the initial transect, recover Seasoar, and deploy a number of Argo floats in addition to a CTD profile with water samples for calibration purposes. On recovery of the CTD, we will complete the large-scale survey over the following 6 days.
To measure the properties of the surface mixed layer and the submesoscales that we expect to find throughout it, we will first use SeaSoar. Seasoar is a vehicle that is towed behind the ship and undulates between depths that we choose (to a maximum of 400m). It changes its depth by changing 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 10m (which is as close to the surface as we can get before surface waves may roll Seasoar over) and 200m 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.
Following the large-scale SAF frontal surveys, we aim to map the evolution of a number of submesoscale fronts over a period of 3-5 days. The target areas will to the north and south of the SAF. For each Lagrangian survey, the submesoscale front will first be identified using towed CTD.
After positioning the ship within the submesoscales front, we will first release an inert dye solution at a depth of between 50-100 m. The purpose of the dye is to reveal the circulation within the surface mixed layer; the evolution of submesoscales is quite and 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 can gain crucial information on how the presence of submesoscales impacts on currents near the surface. A drogued drifter will be deployed at the start of dye pumping and a second drifter at the end. We anticipate the dye release to take 10 minutes once pumps are engaged. The ship will maintain its position on dynamic positioning but the subsurface current will ensure the initial drifter is displaced relative to the second drifter. All of the drifters will be 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 35m rope as stray lines and propellers don’t mix!
The survey will then begin following deployment of the Moving Vessel Profiler (MVP), another towed CTD that rather than ‘flying’ through the water like the Seasoar, drops vertically downwards under its own weight before being winched back to the surface. We aim to tow the MVP repeatedly in bow-tie patterns (see below) around the main drifter. Each leg will be approximately 10 km such that each complete survey will last no more than 3-4 hours. The precise length of each leg will depend on the depth of the mixed layer; shallower mixed layers will require shorter legs and each individual survey being completed in approximately 2 hours. The figures below show results from a previous experiment in the Pacific and from which you can see how the density surfaces slump over throughout the course of a 24 hour period.