Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 18

Figure 1.9 Deploying a glider
into the Irish Sea. The
conductivity and temperature
sensors are just visible
underneath the left wing. The
black tube on top is a turbulent
dissipation instrument, with
the shear sensors visible
forward of the glider nose.
(Photo courtesy of the
National Oceanography
Centre, UK.)
fixed depth and attached by a line to a small, low-drag surface unit which relays
the drogue's position. This arrangement ensures that the measured current is that of
the drogue and is not significantly influenced by variations of current with depth.
A more radical solution, which avoids the surface connection, is to use neutrally
buoyant floats which are tracked acoustically. Currents in the deep ocean have been
determined in this way (Swallow, 1955 ; Dasaro et al., 1996 ) but the requirement to set
up a dedicated acoustic tracking system has limited applications and the method
has been little used in the shelf seas. Most recent Lagrangian measurements in shelf
seas have been based on satellite tracked drifters attached to large subsurface drogues.
While they are used less frequently than Eulerian measurements, Lagrangian measure-
ments provide an important complement to measurements from fixed moorings and
are of particular value in relation to studies of dispersion.
Drifting floats with buoyancy control and equipped with CTD sensors, called
Argo floats, are used for profiling in the deep ocean. These devices descend to a
predetermined depth where they drift with the current for a period before returning
to the surface. The profile data, along with position information, are then relayed to
shore via a satellite link and the next descent begins. Argo floats are now present
throughout much of the world ocean and are greatly enhancing the ocean database
for the deep ocean. As presently configured, they are not suitable for shallow waters,
but a further development of the profiling float in the form of a 'glider' seems likely to
be a valuable tool for the shelf seas. Instead of descending vertically, the glider
(Rudnick et al., 2004 ) is equipped with wings and a rudder, as can be seen in
Fig. 1.9 , which enable it to glide down on a fixed compass bearing. After increasing
its buoyancy at the end of the descent, the device glides upwards with a similar
component of horizontal speed. Before launch, the glider's track is programmed to
pass through a series of way points which, if required, can be updated during the
periods when it is at the surface to download data. Gliders can carry a CTD plus
additional sensors, such as fluorometers or turbulence probes (e.g. Fig. 1.9 ); the glider
endurance is limited by the battery power available for sensor buoyancy changes,