LETTERS TO THE EDITORS
Prints of Tides
To the Editors:
David Cacchione and Lincoln Pratson have proposed that internal
tidal waves can be a principal factor that determines the angle of
oceanic continental slope ("Internal Tides and the Continental
Slope," March–April). The authors argue that the
continental slopes we see today are close to what one would expect
if the semi–diurnal period is the same as the critical period.
That is, incessant forcing of semi–diurnal tides over the past
hundreds of millions of years has nudged the slope in such a way
that the semi–diurnal tides are critical. Since these waves
have highest orbital velocities, they have potential for intense
sediment suspension activity. This idea is well and good, but one
might also ask the question: Can the slope be determined by some
other geological process, while the observed coincidence of critical
and semi–diurnal frequencies is determined by mixing near the
slope that has a feedback effect on oceanic density structure?
We had the opportunity of examining atmospheric data, including our
own measurements, taken near mountain slopes under fairly calm
weather conditions. At night, the air layer over the slopes becomes
strongly stable, thus supporting internal wave activity. The figure
shows the measured dominant period as a function of the critical
wave period (the data–handling procedure is discussed in
Monti, P., et al., 2002, Journal of the Atmospheric
Sciences 59(17):2513–2534), and the results strongly
suggest that the wave activity near mountain slopes is also strongly
correlated with the critical period. The sediment transport and
tides are insignificant in the atmospheric case, and the background
stratification appears to be adjusting in response to turbulent
mixing so that the dominant internal waves near the surface are near
critical (and hence maintaining condition for most effective
turbulent mixing). If this deduction is to be extended to oceans,
one may surmise that, over the years, the near–slope
stratification could have been swayed by the internal tide in such a
way that the latter is critical and an effective mixer! The sediment
suspension perhaps is a pleasant offspring of this phenomenon,
rather than a dominant mechanism that determines the slope angle.
The true chicken (slope) and egg (stratification) story will unravel
as more observations and analyses become available.
H. J. S. Fernando
M. Princevac
Arizona State University