Geoscience Reference
In-Depth Information
local term 'abiki'. In a number of cases abiki waves are capable of achieving sig-
nificant amplitudes. Thus, for example, on March 31, 1979 the maximum height of
waves that caused significant damage and the death of three persons, amounted to
4.78 m. In [Hibiya, Kajiura (1982)] this event was shown to be caused by the pas-
sage of a jump of atmospheric pressure from 2 to 6 hPa over the western part of
the East-Chinese Sea. The propagation velocity of the pressure jump was about
30 m/s.
Characteristic depths of the sea in between the region where the perturbation
originates and Kyushu island lie between 50 and 150 m. The propagation velocities
of long waves, corresponding to them vary within the limits 22-28 m/s, which is
close to the propagation velocity of the atmospheric perturbation. The period of
the waves that arrived in Nagasaki Bay also turned out to be close to the period of
eigen oscillations. As a result of the double resonance the height of waves increased
by more than a factor of 100. Thus, a jump in pressure of only several hectopascal
(hPa) caused the formation of abiki waves in Nagasaki Bay several meters high.
Another well-known example consists in seiche oscillations with periods from
several minutes up to several tens of minutes, which are observed regularly in sum-
mertime off the south-east coast of Spain in the region of the Balearic islands. From
this point of view, the Ciutadella Bay, located in the north-west part of Menorca
island, is the most renown. The bay is of the order of 1 km long, about 90 m wide
with a practically flat bottom at a depth of 5 m. In certain cases seiches with typi-
cal periods of about 10 min reach heights of 4 m, here, leading to serious damage
to ships and coastal structures. This calamity has received the local name 'rissaga'
[Monserrat et al. (1991)]. An analysis of synchronous measurements of the atmo-
spheric pressure and of long waves, performed in [Rabinovich, Monserrat (1996)],
has permitted to reveal a series of cases, when strong level oscillations were caused
by perturbations of the atmospheric pressure. One of such events is the formation
of seiche oscillations in Ciutadella Bay with an amplitude of 0.87 m. Formation
of the waves resulted from the passage of a train of intense internal gravitational
waves in the atmosphere with an amplitude of about 2 h Pa and propagation velocity
of about 30 m/s. The period of atmospheric waves was of the order of 1 h, while
their length over 100 km, which essentially exceeds the period of proper oscillations
of the bay and its dimensions. These differences exclude a possible resonance re-
sponse of the bay. Most likely, formation of the long waves was a result of resonance
effects in open sea, after which they approached the coast and caused strong oscil-
lations in the bay. A fact, favouring this assumption, consists in that the velocity of
the long waves on the external shelf with depths
100 m were in good accordance
with the propagation velocity of atmospheric perturbations.
The meteotsunami phenomenon has much in common with so-called storm
surges. In the monograph of [Murty (1984)] storm surges are defined as sea-level
oscillations in the coastal zone or within internal basins with periods from several
minutes up to several days, and caused by atmospheric influence. Note that this
definition excludes wind waves and choppy sea, since they are characterized by
periods smaller than a minute. Actually, the terms meteotsunami and storm surge
denote phenomena of the same scope, which are caused by the same reason—by
influence of the atmosphere. The only formal difference between a storm surge and a
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