5.8 Freak Wave Problem
221
of the above described facts and the analysis of similar events presented
by Mallory (1974), it is possible to specify the main hydrometeorological
conditions, under which the occurrence of abnormal waves is most probable
in this region. They are as follows:
1. The appearance of waves is most likely in the Agulhas Current in the
area with the maximum of its velocity (3-5 knots), coinciding approximately
with a 200m isobath and deeper. The probability of the appearance of a wave
rapidly decreases approaching the coast from the 200 m isobath.
2. Approximately a day before the appearance of abnormal waves, a cyclone with atmospheric pressure of 980-995 hPa is observed on the synoptic map over Marion Island. The isobaths on the map are elongated southwestward to the south of Africa. A cold atmospheric front propagates with
a speed of about 15 knots to the north-east along the coast.
3. Several hours before the appearance of the abnormal wave a strong
persistent north-easterly wind begins blowing along the south-eastern coast
of Africa increasing the maximum current velocity to 4-5 knots.
4. There is a rapid change of wind direction while passing the cold atmospheric front. The change of the north-easterly wind with a force of 6 knots
to the south-easterly wind with a force of 6-7 knots occurs within 4 hours.
5. The local south-westerly wind rapidly increases the wind wave, on
which the south-west direction swell is superimposed propagating opposite
to the Agulhas Current.
The spatial distribution of the speed of the Agulhas Current. At
present there are no theories explaining the abnormal wave phenomenon,
appearing under the above described conditions. The propagation of a cold
atmospheric front results in so-called "frontal swell waves", which in combination with wind waves can generate waves of a greater amplitude (Ivanenkov
et al., 1 977). However, the appearance of abnormal waves cannot be explained
by these factors alone.
Some other factors will be investigated in this monograph, in particular,
swell transformation in the Agulhas Current, which is a necessary condition
for freak wave generation. According to Smith (1976) the appearance of large
waves in the Agulhas Current was explained by a local wave pattern generated
by wave reflection from the current. However, the problem of large-scale wave
transformation in a current was not considered.
Abnormal waves are typically observed on the ocean surface, where the
depth approximately coincides with a 200 m isobath. The latter passes parallel to the coastline. It is the boundary of the continental shelf, where the
depth increases sharply to 3-4 km (see Fig. 5.28). That is why it was initially assumed that this abrupt depth change was actually the cause of the
abnormal wave formation (Mallory, 1974). But, in reality, the bottom relief
in a 200m depth can influence only waves with lengths more than 500 m.
It should be noted that typical horizontal dimensions of abnormal waves are
much shorter.
221
of the above described facts and the analysis of similar events presented
by Mallory (1974), it is possible to specify the main hydrometeorological
conditions, under which the occurrence of abnormal waves is most probable
in this region. They are as follows:
1. The appearance of waves is most likely in the Agulhas Current in the
area with the maximum of its velocity (3-5 knots), coinciding approximately
with a 200m isobath and deeper. The probability of the appearance of a wave
rapidly decreases approaching the coast from the 200 m isobath.
2. Approximately a day before the appearance of abnormal waves, a cyclone with atmospheric pressure of 980-995 hPa is observed on the synoptic map over Marion Island. The isobaths on the map are elongated southwestward to the south of Africa. A cold atmospheric front propagates with
a speed of about 15 knots to the north-east along the coast.
3. Several hours before the appearance of the abnormal wave a strong
persistent north-easterly wind begins blowing along the south-eastern coast
of Africa increasing the maximum current velocity to 4-5 knots.
4. There is a rapid change of wind direction while passing the cold atmospheric front. The change of the north-easterly wind with a force of 6 knots
to the south-easterly wind with a force of 6-7 knots occurs within 4 hours.
5. The local south-westerly wind rapidly increases the wind wave, on
which the south-west direction swell is superimposed propagating opposite
to the Agulhas Current.
The spatial distribution of the speed of the Agulhas Current. At
present there are no theories explaining the abnormal wave phenomenon,
appearing under the above described conditions. The propagation of a cold
atmospheric front results in so-called "frontal swell waves", which in combination with wind waves can generate waves of a greater amplitude (Ivanenkov
et al., 1 977). However, the appearance of abnormal waves cannot be explained
by these factors alone.
Some other factors will be investigated in this monograph, in particular,
swell transformation in the Agulhas Current, which is a necessary condition
for freak wave generation. According to Smith (1976) the appearance of large
waves in the Agulhas Current was explained by a local wave pattern generated
by wave reflection from the current. However, the problem of large-scale wave
transformation in a current was not considered.
Abnormal waves are typically observed on the ocean surface, where the
depth approximately coincides with a 200 m isobath. The latter passes parallel to the coastline. It is the boundary of the continental shelf, where the
depth increases sharply to 3-4 km (see Fig. 5.28). That is why it was initially assumed that this abrupt depth change was actually the cause of the
abnormal wave formation (Mallory, 1974). But, in reality, the bottom relief
in a 200m depth can influence only waves with lengths more than 500 m.
It should be noted that typical horizontal dimensions of abnormal waves are
much shorter.