Geoscience Reference
In-Depth Information
other tsunami generation mechanisms, in this case an important role is played by
resonance effects, revealed, when the propagation velocity of atmospheric pertur-
bations and their period turn out to be close to the velocity of long waves and to
the period of eigen oscillations of the acquatorium, respectively.
Like tsunamis of seismotectonic origin, meteotsunamis represent quite a rare
phenomenon. Similarly to tsunamis not being excited by each individual earthquake,
not every cyclone, atmospheric front, train of internal gravitational waves or other
atmospheric perturbation leads to the formation of meteotsunamis. A great number
of examples are known, when quite strong atmospheric perturbations were not ac-
companied by any generation of long waves. Nevertheless, only in the Pacific region
36 events have been registered, which are classified as tsunamis of meteorological
origin.
The extent, to which the parameters of tsunamis due to meteorological and seis-
motectonic causes are identical, is such that in a number of cases it is difficult to
determine the actual cause of wave generation. Thus, for example, the group of
long waves about 60 cm high with a period of 24-60 min, registered at the coast of
South Africa on May 11, 1981, was initially identified as a seismotectonic tsunami
and described in the September issue of 'Tsunami Newsletter'. Later, these waves
were classified as a meteotsunami caused by a deep cyclone and atmospheric waves
related to it.
From general arguments it is clear that an intensification of atmospheric pro-
cesses, for example, in the case of tropical cyclones should lead to perturbations
of the water layer and to the generation of long waves. The passage of cyclones is
nearly always accompanied by significant oscillations of the atmospheric pressure,
enhancement of the wind, development of storm agitation. Extreme values of pres-
sure and wind velocity in tropical cyclones reach 870 hPa (cyclone 'TIP', October
1979 ) and 82 m/s (cyclone 'LINDA', September 1997 ), respectively. Part of the en-
ergy of such intense atmospheric processes, doubtless, must be transformed into
the energy of long waves. But an analysis of synchronous measurements of ocean
level oscillations and of atmospheric pressure fluctuations reveal that the direct re-
lationship between these processes, with the exception of individual cases, is not
essential [Munk (1962); Kovalev et al. (1991); Rabinovich, Monserrat (1996)].
At the same time, there exist numerous examples of the observation of long
waves, the formation of which is unambiguously related to atmospheric processes.
Thus, for example, in [Bondarenko, Bychkov (1983)] a description is given of
the generation of long waves with a period of about 23 min, caused by internal
gravitational waves with the same period that propagated over the Caspian Sea in
the region of Svinoi island. Several cases are known of catastrophic waves arising
on the Great lakes [Donn, Ewing (1956)]. On May 5, 1952, June 26 and July 6,
1954 sharp jumps in the atmospheric pressure that propagated with velocities of
20-40 m/s led to the formation of strong long waves, which caused significant de-
struction on the coast and even the death of people.
The anomalous character of seiche oscillations in Nagasaki Bay (Kyushu island)
is renown [Rabinovich (1993)]. Oscillations of amplitudes
0
.
5 m and periods of
about 30 min in this bay represent quite a typical phenomenon. It is known by the
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