Reminiscences of MODE
21
meters, SOFAR floats, and STD/hydrographic capabilities described in the previous
section, there were many other instruments that promised additional types of information but were unready or not suitable for sustained large-scale deployment. Notable
was John Swallow himself with a version of his original float that was equipped with a
transponder. Tracking still required a dedicated ship, but was simpler and worked over
longer ranges. There were two varieties of vertically profiling float, the first sensing
the electric field associated with motion across the lines of the Earth’s magnetic field,
and the second measuring distance to previously planted acoustic transponders on the
ocean floor. Comparison of ascending and descending profiles exposes the tides and
internal waves, and allows qualitative inferences about the lower frequency velocities
that were of most interest to MODE. There were stationary pressure gauges deployed
on the sea floor that, after the tidal signal had been filtered out, could provide estimates
of the changes with time of the pressure differences between pairs, and hence map
the geostrophic velocity near the bottom. Unfortunately, the depth of the sea floor at
each location cannot be determined with sufficient accuracy to provide a steady-state
value for the velocity. There was also an inverted echo sounder, placed on the bottom
and recording the transit time for an acoustic pulse reflected back from the ocean
surface. This transit time responds predominantly to the changing depth of warmer
water as the thermocline moves up and down. Apart from issues of data recovery,
which still required retrieval by a ship, it was a simple and inexpensive instrument. It
was of particular interest to me because a similar approach had become common in
the atmospheric context. There a simple, ground-based dish antenna and sound source
were used to monitor the height of the top of the well-mixed boundary layer above
a land surface, using backscattering from temperature fluctuations in the inversion
layer just above.
I was a member of the Theoretical Panel, with a project to develop a numerical
model of a 480 km × 480 km square domain of the ocean, above bottom topography
that was, or could be, representative of the area proposed for the MODE field program.
My essential assistant in this was Mike Karweit. We called it a meso-scale model
(Bretherton and Karweit, 1975) because it could resolve phenomena on the scale of
the Rossby radius of deformation, which in the region of interest was around 40 km.
It solved the quasi-geostrophic equations for a stratified fluid on a beta-plane, with
an adjustable coefficient of friction at the ocean floor and a pseudo-viscosity that
imposed on each Fourier component a decay rate proportional to its wave number
to the fourth power. What was novel was the embedding in the surrounding ocean.
All other existing models simulated an entire ocean basin with rigid boundaries,
including western boundary currents as well as interior regions, and had a flat or
highly simplified representation of the ocean floor. Given the limited computing
power available at the time it was very difficult to do justice to both the boundary
and the details of interior regions simultaneously. The artifice we used was to assume
that, over the square domain of interest, the velocity fields in each layer could be
represented by a Fourier series in each horizontal direction. This implies that the
velocity at each point on the bounding surface of the domain is supposed equal
21
meters, SOFAR floats, and STD/hydrographic capabilities described in the previous
section, there were many other instruments that promised additional types of information but were unready or not suitable for sustained large-scale deployment. Notable
was John Swallow himself with a version of his original float that was equipped with a
transponder. Tracking still required a dedicated ship, but was simpler and worked over
longer ranges. There were two varieties of vertically profiling float, the first sensing
the electric field associated with motion across the lines of the Earth’s magnetic field,
and the second measuring distance to previously planted acoustic transponders on the
ocean floor. Comparison of ascending and descending profiles exposes the tides and
internal waves, and allows qualitative inferences about the lower frequency velocities
that were of most interest to MODE. There were stationary pressure gauges deployed
on the sea floor that, after the tidal signal had been filtered out, could provide estimates
of the changes with time of the pressure differences between pairs, and hence map
the geostrophic velocity near the bottom. Unfortunately, the depth of the sea floor at
each location cannot be determined with sufficient accuracy to provide a steady-state
value for the velocity. There was also an inverted echo sounder, placed on the bottom
and recording the transit time for an acoustic pulse reflected back from the ocean
surface. This transit time responds predominantly to the changing depth of warmer
water as the thermocline moves up and down. Apart from issues of data recovery,
which still required retrieval by a ship, it was a simple and inexpensive instrument. It
was of particular interest to me because a similar approach had become common in
the atmospheric context. There a simple, ground-based dish antenna and sound source
were used to monitor the height of the top of the well-mixed boundary layer above
a land surface, using backscattering from temperature fluctuations in the inversion
layer just above.
I was a member of the Theoretical Panel, with a project to develop a numerical
model of a 480 km × 480 km square domain of the ocean, above bottom topography
that was, or could be, representative of the area proposed for the MODE field program.
My essential assistant in this was Mike Karweit. We called it a meso-scale model
(Bretherton and Karweit, 1975) because it could resolve phenomena on the scale of
the Rossby radius of deformation, which in the region of interest was around 40 km.
It solved the quasi-geostrophic equations for a stratified fluid on a beta-plane, with
an adjustable coefficient of friction at the ocean floor and a pseudo-viscosity that
imposed on each Fourier component a decay rate proportional to its wave number
to the fourth power. What was novel was the embedding in the surrounding ocean.
All other existing models simulated an entire ocean basin with rigid boundaries,
including western boundary currents as well as interior regions, and had a flat or
highly simplified representation of the ocean floor. Given the limited computing
power available at the time it was very difficult to do justice to both the boundary
and the details of interior regions simultaneously. The artifice we used was to assume
that, over the square domain of interest, the velocity fields in each layer could be
represented by a Fourier series in each horizontal direction. This implies that the
velocity at each point on the bounding surface of the domain is supposed equal