The EP/CC-Yoyo is a moored profiler designed for special applications. It is originally developed to monitor changes caused by winter convective events in the polar regions (Greenland Sea).
Basic demands were
- deep profiles (typically 4000 m)
- many profiles (typically 360, i.e. 1 profile each day for 1 year)
- self contained vehicel (internal data storage to avoid problems with
ice in arctic regions)
- measure C,T and p with precision high enough to indicate convective events
in polar regions
- light construction for easy handling.
These requirements have a number of implications with respect to the construction.
First, the vehicel has to be pressure resistant to withstand 400 bar, leading to a basic weight of the instrument. Floation used to balance the instrument in the water has also to be resistant to this pressure.
Second, the energy supply must suffice for the many and deep casts. It is not practical to use a construction in which all the energy that is needed for the vertical movement is carried within the vehicel itself. This energy, say in form of batteries, would be associated with increasing weight that would have to be balanced in water, thus increasing also the volume of the vehicel. By a big volume, however, the energy needed to achieve a certain speed is increased.
Third, the speed of the vehicle has to be high enough to result in a total measurement time for which the energy can be carried within the vehicle.
We chose a design for the EP/CC-Yoyo that utilizes the potential energy of weights (made from lead) for the power required for downward movement. The system is devided into two main parts: The vehicel moving down and up, and the control unit at the top of the mooring line, containing the weights and supplying them in preselected time intervals.
The control unit therefore contains one weight for each profile (360 weights). Each weight amounts to 690 g in air, of which about 470 g are used to drive the downward movement of the vehicle. Such a weight is placed onto the vehicle at the time a cast shall start, and it is removed when it reaches the ocean bottom. The vehicle itself is balanced to posess some buoyancy and to swim up by itself, finally reaching the control unit again. The upward speed is much slower than the downward speed (0.7 to 1.0 m/s), and measurements are performed only during the downcast.
The vehicle is long and slim to achieve this high speed with the mentioned weight (11 cm diameter, 230 cm long). The instrument and the floatation modul are separate devices. We found it important to use a reliable well proven instrument which can be protected against biofouling to maintain a high accuracy of the measurements. A modified Seacat is used, with extended memory, Digiquartz pressure sensor, and special lightweight titanium housing. The floatation modul contains buoyant material with controlled compressibility. Hereby the compressiblity of the entire vehicle is adjusted to that of sea water, leading to a constant speed throughout the water column.
The two parts, control unit and vehicel, are synchronized by sufficiently accurate clocks within each unit and need no communication.
EP/CC-Yoyos have been successfully moored in the Greenland Sea, providing time series which show the variability of possible winter ventilation processes. The winters 2000/2001 and 2001/2002 reveal different ventilation mechanisms, and the latter shows that in contrast to presently proposed concepts winter ventilation can result in an increase in temperature and salinity, and that no ice formation is needed. This is due to the special conditions in the Greenland Sea with subsurface advection of Atlantic Waters.