AIDJEX Bulletin #40 - Polar Science Center - University of Washington

Another source for large variations on scales of a few hours is inertial
oscillation of the ice cover and upper ocean. We found at the manned stations
that it was not uncommon for the apparent direction at 30 m to swing full
circle in one inertial period. Thus, the extreme scatter exhibited in Figures
11 and 12 for the 30 m direction is expected. It does not show up in the 2 m
direction because the water at that level is oscillating in phase with the ice.
An interesting aspect of these oscillations is that their onset is apprently
about two months earlier than was observed at the manned camps the previous
summer. Presumably, the oscillations are damped when the ice is thick, but
occur freely when the ice can no longer support internal stress gradients.
For useful results it was clear that some sort of filtering of the current
data was required, and as a first attempt we applied a ''cosine bell''
running mean; i.e., each smoothed sample was calculated by averaging the corresponding
unsmoothed sample with the 12 preceding and succeeding samples, all
with the proper cosine weighting. '?he effect in the frequency domain is a
low-pass filter with little energy content at periods shorter than 12 hours.
The filter attenuates most of the energy at the inertial period, which is
12.6 hours at 72"N.
The filter was applied to the zonal and meridional velocity components.
These were obtained by subtracting the corrected magnetic heading from the
current direction, then adding the magnetic declination at the buoys' posit
ions.
Results of the calculations described above are shown in Figures 13
through 16. We have reconverted the smooth components to speed and bearing
and have shown them compared with the ice speed and bearing as determined from
the smoothed satellite data. The reference frame is chosen such that the
actual current at either level is obtained from the vector addition of the ice
velocity and the measured current. In other words, if the water at 30 m were
still, the 30 m current would, (ideally) have the same speed as the ice and its
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bearing would be 180" out of p%&e with that of the ice,
With this in mind, the speed plots show many of the characteristics we
have seen at the manned camp; i.e., the 30 m speed is usually close to and
shows many of the same fluctuations as the ice speed. The 2 m speed also follows,
but at a reduced magnitude, indicating that the water at 2 m is following
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