50
Russ E. Davis
film recorder. Wave-induced mooring motion caused the apparent current direction
to oscillate rapidly. This led to development of the Vector Averaging Current Meter
(VACM) that used electronics to frequently sample speed and direction, convert them
to vector components, average them, and record results on magnetic tape.
For many years the WHOI Buoy Group was the leader in both technical and
scientific areas. Other groups were formed around the country using methods and
equipment developed by the Buoy Group and employing personnel trained there. At
the same time, Buoy Group work motivated an interest in observations and technical
developments that made new measurements possible. It was an exciting center of
observational oceanography.
During the 1960s, Swallow-float technology gradually improved but short tracking range made ships a continued necessity. In 1968 Tom Rossby and Doug Webb
deployed two floats implementing Stommel’s idea of long-range float tracking using sound propagation in the SOFAR channel (the sound-speed-minimum waveguide
found near 1 km depth in tropical and subtropical oceans). At low frequencies sound
absorption decreases so that tracking at ranges of 1000–2000 km becomes feasible, but
the size of efficient low-frequency sound sources and batteries for extended duration
led to large floats and difficult-to-build sound projectors. In the first test (Rossby and
Webb, 1970) the floats projecting at 500 Hz were heard by a military tracking array at
1000-km ranges. Although these first floats failed after only a few days, a year later
a 380-Hz SOFAR float was tracked for 4 months (Rossby and Webb, 1971). With
tracking accuracies of O(3 km) limited by knowledge of the field of sound speed and
much better day-to-day precision, this development made much of the world’s ocean
accessible to long-range, long-term direct velocity observation.
MODE—THE FIRST OCEAN OBSERVING SYSTEM
Prior to the 1970s, technology limited observational oceanography to individual surveys, single time series, or spatial sampling with very limited duration. The development of long-lasting floats and moorings made possible the Mid-Ocean Dynamics
Experiment (MODE), which was the first large-scale observing effort that combined
diverse observing tools into a designed system. The scientific focus (The MODE
Group, 1978) was the mesoscale variability made so clear in the Aries observations
that had subsequently been described by various other observations. The approach,
led by Alan Robinson, Carl Wunsch, Francis Bretherton, and Henry Stommel, was
methodical, using a sequence of focused efforts. MODE-0 used an irregular array
to explore (a) sites for the MODE-1 array, (b) the coherence scales of mesoscale
variability, and (c) spatial variation of eddy energy, including differences between
smooth bathymetry and abyssal hills. Following this was a period of planning for the
full-scale MODE-1 array, including an intensive “summer camp” at the NCAR in
Boulder, Colorado. MODE-I was the major experiment spanning 5 months in 1973.
Observations included (a) 26 subsurface moorings supporting 83 current meters and
Russ E. Davis
film recorder. Wave-induced mooring motion caused the apparent current direction
to oscillate rapidly. This led to development of the Vector Averaging Current Meter
(VACM) that used electronics to frequently sample speed and direction, convert them
to vector components, average them, and record results on magnetic tape.
For many years the WHOI Buoy Group was the leader in both technical and
scientific areas. Other groups were formed around the country using methods and
equipment developed by the Buoy Group and employing personnel trained there. At
the same time, Buoy Group work motivated an interest in observations and technical
developments that made new measurements possible. It was an exciting center of
observational oceanography.
During the 1960s, Swallow-float technology gradually improved but short tracking range made ships a continued necessity. In 1968 Tom Rossby and Doug Webb
deployed two floats implementing Stommel’s idea of long-range float tracking using sound propagation in the SOFAR channel (the sound-speed-minimum waveguide
found near 1 km depth in tropical and subtropical oceans). At low frequencies sound
absorption decreases so that tracking at ranges of 1000–2000 km becomes feasible, but
the size of efficient low-frequency sound sources and batteries for extended duration
led to large floats and difficult-to-build sound projectors. In the first test (Rossby and
Webb, 1970) the floats projecting at 500 Hz were heard by a military tracking array at
1000-km ranges. Although these first floats failed after only a few days, a year later
a 380-Hz SOFAR float was tracked for 4 months (Rossby and Webb, 1971). With
tracking accuracies of O(3 km) limited by knowledge of the field of sound speed and
much better day-to-day precision, this development made much of the world’s ocean
accessible to long-range, long-term direct velocity observation.
MODE—THE FIRST OCEAN OBSERVING SYSTEM
Prior to the 1970s, technology limited observational oceanography to individual surveys, single time series, or spatial sampling with very limited duration. The development of long-lasting floats and moorings made possible the Mid-Ocean Dynamics
Experiment (MODE), which was the first large-scale observing effort that combined
diverse observing tools into a designed system. The scientific focus (The MODE
Group, 1978) was the mesoscale variability made so clear in the Aries observations
that had subsequently been described by various other observations. The approach,
led by Alan Robinson, Carl Wunsch, Francis Bretherton, and Henry Stommel, was
methodical, using a sequence of focused efforts. MODE-0 used an irregular array
to explore (a) sites for the MODE-1 array, (b) the coherence scales of mesoscale
variability, and (c) spatial variation of eddy energy, including differences between
smooth bathymetry and abyssal hills. Following this was a period of planning for the
full-scale MODE-1 array, including an intensive “summer camp” at the NCAR in
Boulder, Colorado. MODE-I was the major experiment spanning 5 months in 1973.
Observations included (a) 26 subsurface moorings supporting 83 current meters and