Measuring Currents Directly

At this point, it would be useful to make another short diversion away from our Gulf Stream theme, to briefly consider current measurement techniques in general.

Surlace buoys that could be tracked visually or by radar (Figure 4.26(a). left) were in use before neutrally buoyant floats were developed, but drifting buoys in general only came into their ow n w ith the advent of electronic acoustic and radio (racking s\stems and. later, satellite navigation I Figure 4.26(a), middle and righti. Surlace buoys are inevitably affected by the wind. In order to reduce its effect, they generally have most of their volume below water and in addition are equipped with some type of drogue, at depth

Sofar floats and surface buoys may. theoretically, be tracked until their batteries run down, which may he as long as several years. Other simpler types of drifters are not tracked at all. The current flow is simply deduced from the time and place at which they are found, either at sea or on the shore. In the nineteenth and early twentieth centuries, the 'drift bottle" was commonly used. Today, a variety of cheap "drifters' (e.g plastic cards) are used for small-scale local studies in estuaries, coastal waters, or semi-enclosed seas such as the North Sea and the Irish Sea. Each drifter carries a message asking the finder to inform a central agency of the position and time at which it was found, so that its trajectory may be deduced. Of the large number ot drifters released in any one experiment only a small proportion will be returned, and of those that are returned some may have been washed ashore manv weeks earlier.

Argos satellite navigation communicator light

GPS receiver

sea temperature sensor compliant tether sea temperature sensor

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