Reminiscences of MODE
19
First was the internally recording moored current meter. Under the leadership
of Nick Fofonoff and others at the Woods Hole Oceanographic Institution, batteryoperated instruments were available that recorded internally the velocity (magnitude
and direction) of the water relative to the instrument, sampled at whatever interval
was appropriate for the application. For work in the deep ocean, a string of such
instruments was attached to a cable suspended from a buoy a few hundred meters
beneath the ocean surface and moored to ballast dropped onto the ocean floor. The
subsurface mooring was to reduce motion induced by surface waves, and a buoyant
polymer cable was used over most of its length to reduce the stresses on all parts
of the system. Once deployed from a research vessel, a complex operation requiring
an experienced crew, the mooring was left unattended for up to a year until it was
revisited. Upon return, an acoustically activated release severed the connection to
the ballast and the remainder of the mooring rose buoyantly to the surface. The
development phase had been plagued by repeated, seemingly inexplicable, losses of
the entire string. These were eventually resolved when a section of severed cable was
recovered with a shark tooth embedded within it, leading to its replacement by steel in
the upper part of the rig. There were clearly many potential uses for a capability like
this. An early candidate was a spatial array of such strings of current meters, deployed
specifically to map a limited area comprehensively in three-dimensional space and
time.
A second development was the SOFAR float, due to Tom Rossby at the University of Rhode Island. He had adapted the concept of the Swallow float to tracking
over basin scale distances, using pulses of low-frequency sound emitted in the SOFAR
(SOund Fixing And Ranging) waveguide. This waveguide surrounds the depth of the
minimum speed of propagation that is found throughout most of the world ocean.
In the region of interest its center is at a depth of about 1000 m. The low frequency
of the sound reduces absorption by molecular processes. Higher speeds above and
below refract the waves back towards the central depth so that they spread in two
dimensions rather than three. Both these effects contribute to potential transmission
over very long distances. To achieve this transmission requires a powerful transmitter
at a depth close to the axis of the waveguide, and a sensitive receiver comparably
located. In their original configuration, Rossby’s floats were packed with batteries to
power the transducer, and used listening stations deployed by the U.S. Navy for other
purposes. By differencing the arrival times of a pulse at several listening stations, the
horizontal position could be located several times a day to within less than 1 km, a
capability that exceeded that of ship navigation on the ocean surface above. With a
lifetime of months to a year or more, a fleet of such floats could potentially provide an
effectively continuous record of the absolute velocity at a known reference level, and
remove a key ambiguity in the hydrographic method for determining the circulation.
A third substantial improvement in capability was the STD (Salinity, Temperature, Depth). This instrument provided continuous measurements of these three
variables on board ship as it was lowered on a cable from a winch as in a traditional
hydrographic bottle sounding, except that a multi-core conducting cable is required.
19
First was the internally recording moored current meter. Under the leadership
of Nick Fofonoff and others at the Woods Hole Oceanographic Institution, batteryoperated instruments were available that recorded internally the velocity (magnitude
and direction) of the water relative to the instrument, sampled at whatever interval
was appropriate for the application. For work in the deep ocean, a string of such
instruments was attached to a cable suspended from a buoy a few hundred meters
beneath the ocean surface and moored to ballast dropped onto the ocean floor. The
subsurface mooring was to reduce motion induced by surface waves, and a buoyant
polymer cable was used over most of its length to reduce the stresses on all parts
of the system. Once deployed from a research vessel, a complex operation requiring
an experienced crew, the mooring was left unattended for up to a year until it was
revisited. Upon return, an acoustically activated release severed the connection to
the ballast and the remainder of the mooring rose buoyantly to the surface. The
development phase had been plagued by repeated, seemingly inexplicable, losses of
the entire string. These were eventually resolved when a section of severed cable was
recovered with a shark tooth embedded within it, leading to its replacement by steel in
the upper part of the rig. There were clearly many potential uses for a capability like
this. An early candidate was a spatial array of such strings of current meters, deployed
specifically to map a limited area comprehensively in three-dimensional space and
time.
A second development was the SOFAR float, due to Tom Rossby at the University of Rhode Island. He had adapted the concept of the Swallow float to tracking
over basin scale distances, using pulses of low-frequency sound emitted in the SOFAR
(SOund Fixing And Ranging) waveguide. This waveguide surrounds the depth of the
minimum speed of propagation that is found throughout most of the world ocean.
In the region of interest its center is at a depth of about 1000 m. The low frequency
of the sound reduces absorption by molecular processes. Higher speeds above and
below refract the waves back towards the central depth so that they spread in two
dimensions rather than three. Both these effects contribute to potential transmission
over very long distances. To achieve this transmission requires a powerful transmitter
at a depth close to the axis of the waveguide, and a sensitive receiver comparably
located. In their original configuration, Rossby’s floats were packed with batteries to
power the transducer, and used listening stations deployed by the U.S. Navy for other
purposes. By differencing the arrival times of a pulse at several listening stations, the
horizontal position could be located several times a day to within less than 1 km, a
capability that exceeded that of ship navigation on the ocean surface above. With a
lifetime of months to a year or more, a fleet of such floats could potentially provide an
effectively continuous record of the absolute velocity at a known reference level, and
remove a key ambiguity in the hydrographic method for determining the circulation.
A third substantial improvement in capability was the STD (Salinity, Temperature, Depth). This instrument provided continuous measurements of these three
variables on board ship as it was lowered on a cable from a winch as in a traditional
hydrographic bottle sounding, except that a multi-core conducting cable is required.