Subsurface drifters

It is possible to detect and measure water movements at middle depths by using neutrally buoyant floats, the weight of which can be accurately adjusted to match the density of the water so that they sink to a predetermined depth and then drift

Oceanographic Drogue

0.5 m diameter

Figure 3.1 An ARGOS satellite-tracked drifting buoy and its drogue.

0.5 m diameter

Figure 3.1 An ARGOS satellite-tracked drifting buoy and its drogue.

with the current. Such floats are tracked acoustically or by satellite and are used extensively for open ocean studies. An early example is the 'Pinger' invented by Dr J.C. Swallow of the Institute of Oceanographic Sciences in the early 1950s. This is an aluminium tube containing a battery and acoustic transmitter. It can be ballasted to be neutrally buoyant at any required depth, and emits an intermittent 'ping' sound as it drifts. The principle on which this works is that the tube, whilst being negatively buoyant at the surface, gains buoyancy as it sinks because its compressibility is less than that of seawater.

There are now several modern versions of sub-surface floating buoys. One in present use is the RAFOS float. Like the original Swallow float, it is ballasted to be neutrally buoyant at pre-selected depths and can drift for several years. The float receives acoustic signals from moored sound sources, and by timing the arrival of these signals the float position can be determined and recorded. The data are transmitted back to base by satellite when the float surfaces following an appropriate acoustic signal. In other systems the floats emit the acoustic signals which are received by automatic listening stations up to 1500 km away. Recently similar floats have been used to track eddies in the NE Atlantic off the UK (see Section 1.3.3).

The WOCE experiment uses modern floats which go a step further and are

Figure 3.2 Sub-surface drifters: (a) an ALACE float; (b) a RAFOS float of the type used in the World Ocean Circulation Experiment.

completely independent of any tracking network. The Autonomous Langranian Circulation Explorer (ALACE) developed in the late 1980s, has been designed to rise to the surface at pre-set intervals. Once at the surface, it is located by satellite and transmits its data to the laboratory directly through the satellite system. This and other surface and sub-surface floats are described in more detail in Griffiths and Thorpe (1996).

Current meter moorings

The rate and direction of flow of water can be measured by various ingenious current meters placed in fixed positions. One of the first successful and widely used instruments for this purpose was the Ekman Current Meter. This was a mechanical instrument in which the rotations of a small propeller were counted and has now been superseded by more sophisticated electronic instruments.

The Aanderaa Recording and Telemetering Current Meter (Figure 3.3) was designed in the 1960s and has been so successful it is still widely used today. It can be deployed for up to a year and is reliable and relatively cheap. The design is a simple one and is rather like a weather vane and anemometer.

Magnets attached to a rotor turned by the current generate pulses at a frequency proportional to the current speed. A large vane aligns the apparatus with the current, the direction being sensed by a magnetic compass in the base of the container. Data on the speed and direction of the current are recorded within the instrument either on magnetic tape or in modern Aanderaa meters,

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