Moorings
Fig. 1: Schematic representation of a mooring
Moorings are autonomous systems, which remain in the ocean for several years to measure a variety of oceanographic properties at a distinct location with high temporal resolution. By this means it is possible to obtain measurements to detect the exact course of temporal variations even if there is no ship in the area.
A mooring consists of the ground weight, a wire, and floats which keep the wire upright in the water column. On the wire instruments are fixed to measure e.g. temperature, salinity current speed and velocity, sea ice thickness and underwater sound. All data are recorded internally. After the end of the observation period, the wire is released from the ground weight by means of an acoustic signal from Polarstern’s Posidonia system and the floats tow the wire with the instruments to the surface. This ascend can be surveyed as well by the Posidonia system. At the sea surface a satellite transmitter is activated and one can receive the signal of it on board. If the mooring cannot be spotted by eye, the satellite signals helps to detect it. In case of reception problems on board, the position is obtained by email from Bremerhaven.
In open water normally no problems occur, however if there is fog even recovery in open water can be a challenge. In the ice, either the floats find their way through the floes up to the surface or one has to locate them by acoustic means under the ice and break them free. Then the ship must approach the mooring very carefully not to crash them between the floes. The last few meters are made the mammy chair, from which the connection to the ship is achieved by a rope.
However, even in open water the recovery of a mooring can be a challenge if the wind is blowing as strong as it happens frequently in higher latitudes. Then, high skill is required from the ship’s officers to bring the huge ship very smoothly to the relatively small mooring without damaging it.
Fig. 2: Deployment of a mooring in the ice (anchor first) and in open water (anchor last)
In Fram Strait moorings were maintained since August 1997 in the context of the VEINS, ASOF-N, DAMOCLES and ACOBAR projects. Mainly current meters have been used which also record temperature. Moorings are exchanged every year during the summer season. Since 1989 time long-term measurements are performed in the Atlantic sector of the Southern Ocean in the Weddell Sea and since 1996 along the meridian of Greenwich between 53°S and the continental slope. After a recording period of two to three years these moorings are exchanged during southern summer. Adittionally moorings with sound sources are deployed in the Weddell Sea for acoustically tracking the RAFOS floats.
Fig. 3: Acoustic ranging of a mooring from the ship
Fig. 4: Recovery of a mooring under sea ice by means of the acoustic Posidonia ranging system
Fig .5 : Recovery of a mooring in sea ice (Photo Eberhard Fahrbach)