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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C07006, doi:10.1029/2009JC005904, 2010
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Possible atmospheric origin of the 7 May 2007 western Black Sea
shelf tsunami event
Ivica Vilibić,1 Jadranka Šepić,1 Boyko Ranguelov,2 Nataša Strelec Mahović,3
and Stefano Tinti4
Received 12 October 2009; revised 15 January 2010; accepted 15 February 2010; published 8 July 2010.
[1] The paper examines the possibility that tsunami event recorded in the western Black
Sea on 7 May 2007 was triggered by a traveling atmospheric disturbance. As
meteotsunamis are favored by specific synoptic conditions, we inspected available ground
and sounding observations, European Centre for Medium‐Range Weather Forecasts
reanalysis fields, and satellite‐based products and compared them to the documented
Mediterranean meteotsunamis. We found an atmospheric disturbance traveling toward
30° (NNE) with amplitude of 2–3 hPa and propagation speed of about 16 m/s, passing
through few tens of kilometers wide pathway over the region affected by the tsunami. This
disturbance occurred in the lower troposphere, but it was capped by instable convective
cell that preserved gravity disturbance’s coherence over a region at least 150 km long. An
ocean modeling study showed that such a disturbance is capable of generating large
tsunami waves and strong currents over the shallow regions, following the observations
over the region where maximum sea level oscillations have been documented. Therefore,
this event has a potential to be classified as a meteotsunami, the first of such kind in the
Black Sea.
Citation: Vilibić, I., J. Šepić, B. Ranguelov, N. S. Mahović, and S. Tinti (2010), Possible atmospheric origin of the 7 May 2007
western Black Sea shelf tsunami event, J. Geophys. Res., 115, C07006, doi:10.1029/2009JC005904.
1. Introduction
[2] Although tsunamis in the Black Sea are not as frequent and intense as in the nearby Marmara and Aegean
Seas, they can be quite destructive especially if accompanied by a strong nearshore earthquake [Yalçiner et al.,
2004]. Most of the Black Sea tsunamis are of seismic origin and have been reported along the northern Crimean and
Caucasus coastlines [Pelinovsky, 1999; Solov’eva and
Kuzin, 2005], and less along the Turkish [Altinok and
Ersoy, 2000] and Bulgarian [Ranguelov and Gospodinov,
1995] coasts. Several destructive and tsunamigenic earthquakes occurred off the western coastline during the last
millenniums. Of these events the strongest tsunami was
reported in 543 A.D., causing large flooding in Varna Bay
and the Balchik area with runup exceeding 2–4 m [Nikonov,
1997]. Just a decade later, in 555 A.D., a nonseismic tsunami has been reported in the southwestern Black Sea by the
ancient Byzantine chronicler Nikomedius [Ranguelov,
1996], giving reliability to a hypothesis that submarine
1
Institute of Oceanography and Fisheries, Split, Croatia.
Geophysical Institute, Bulgarian Academy of Sciences, Sofia,
Bulgaria.
3
Meteorological and Hydrological Service, Zagreb, Croatia.
4
Department of Physics, University of Bologna, Bologna, Italy.
2
Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2009JC005904
landslides or the atmosphere may create destructive tsunami
waves in the Black Sea. In addition, paleostudies indicate
that major tsunamis may occur along the western Black Sea
over a geological timescale [Ranguelov, 2003].
[3] However, no tsunamigenic earthquake has been
reported to occur on 7 May 2007 in the Black Sea area,
although tsunami waves of 2–3 m height have been
observed in the morning hours along the northern section of
the Bulgarian Black Sea coastline. Two hypotheses have
been developed in order to detect the source of these nonseismic waves [Ranguelov et al., 2008]: (1) a submarine
landslide occurred about 50 km off the western coast, along
the shelf break perimeter, generating the tsunami waves
which were amplified along the double‐beam waveguide,
and (2) an atmospheric high‐frequency disturbance traveled
along the shelf and generated long ocean waves through the
Proudman resonance mechanism [Proudman, 1929]. The
former hypothesis has been explored by Ranguelov et al.
[2008], who concluded that “submarine mass movements
taking place within a certain delimited source area off
Bulgaria may have generated tsunamis compatible with the
observations.” The latter hypothesis will be elaborated in
this paper—the tsunami waves generated through such a
mechanism are known as meteotsunamis [Monserrat et al.,
2006], although these events have so far not been documented to occur in the Black Sea giving such large effects.
[4] Meteotsunamis or meteorological tsunamis are observed
to occur regularly at certain places in the World Ocean, and
even have specific local names: “rissaga” in the Balearic
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Islands [Ramis and Jansà, 1983; Tintoré et al., 1988];
“marubbio” in Sicily [Candela et al., 1999]; “milghuba” in
Malta [Drago, 2008], “abiki” in Nagasaki Bay, Japan
[Hibiya and Kajiura, 1982], and “Seebär” in the Baltic Sea
[Metzner et al., 2000]. These waves are also documented in
the Yellow Sea [Wang et al., 1987], the Adriatic Sea [Vilibić
et al., 2004; Vilibić and Šepić, 2009], the Aegean Sea
[Papadopoulos, 1993], the English Channel [Douglas,
1929; Haslett and Bryant, 2009], the Great Lakes [Ewing
et al., 1954], Florida [Churchill et al., 1995], the northwestern Atlantic coast [Mercer et al., 2002], the Argentine
coast [Dragani et al., 2002], and the New Zealand coast
[Goring, 2005]. Meteotsunami wave heights can be as high
as 6 m (Vela Luka, Adriatic Sea; Daytona Beach, Florida) or
5 m (Ciutadella inlet, the Balearic Islands; Nagasaki Bay,
Japan) or less, and the waves can cause substantial damage
to coastal infrastructures and produce human injuries and
losses [Donn and Ewing, 1956; Hibiya and Kajiura, 1982;
Monserrat et al., 2006; Vučetić et al., 2009]. The most
“problematic” regions are low‐tidal basins such as the
Mediterranean Sea, since the coastal infrastructure is not
adapted to such large sea level oscillations.
[5] The source of meteotsunami waves is a high‐
frequency atmospheric gravity disturbance visible in the
ground air pressure series. Air pressure gradient is particularly important as it directly affects the ocean (enters directly
into the ocean equations), and it may surpass 5 hPa over
5 min during the strongest events [Vilibić et al., 2008].
Moreover, each destructive meteotsunami event also needs a
nondissipative atmospheric disturbance traveling over a
shelf with the speed similar to the speed of the tsunami
waves (Proudman resonance). Then, the generated open‐
ocean waves are amplified topographically close to the
coast, especially within a harbor or a bay with large
amplification factor [Monserrat et al., 2006]. However,
nondissipative atmospheric gravity disturbances are occurring rarely and only during specific conditions, which
include ducting of the atmospheric disturbance in the lower
troposphere below an unstable layer [Lindzen and Tung,
1976; Monserrat and Thorpe, 1992, 1996]. Such conditions are usually related to typical synoptic conditions,
which include temperature inversion in the first kilometer
above the ground, overtopped by the flow of warm and dry
air, and capped with an instable layer where the wind speed
equals the speed of disturbance [Ramis and Jansà, 1983;
Vilibić and Šepić, 2009]. The disturbance is more efficiently
maintained over large distances if capped not only by
instable layer itself but also by a convective system that
encompasses much of the upper troposphere [Belušić and
Strelec Mahović, 2009; Šepić et al., 2009a].
[6] In this paper all of these conditions will be evaluated
using available ground and sounding observations, satellite‐
derived products, and atmospheric reanalysis fields, in order
to assess the possibility that the atmospheric processes were
responsible for the generation of the strong tsunami waves
that were observed along the western Black Sea coast on
7 May 2007. These analyses are part of section 3, preceded
by an overview of the available material, data, and methods
in section 2. Section 4 deals with the results obtained from a
barotropic numerical ocean model forced by a traveling
atmospheric disturbance, while further discussion and conclusions are presented in section 5.
2. Material and Methods
[7] Not so many in situ ocean and meteorological measurements along the western Black Sea coastline were
available during the tsunami event of 7 May 2007, especially concerning high‐frequency (a minute timescale) data.
Ocean observations were available at a number of locations,
including tide gauge measurements conducted at Ahtopol
and Varna and eyewitnesses’ reports. A comprehensive
report on the observations is given by Ranguelov et al.
[2008], and therefore only range values of the maximum
and minimum sea levels will be taken into account here
(Figure 1). Ranguelov et al. [2008] summarize the major
characteristics of the ocean observations as: (1) sea level
retreat was larger than sea level rise, (2) turbulence and mud
currents were reported, (3) oscillations had 4–8 min period,
and (4) maximum and minimum sea levels were +1.2 and
−2.0 m, respectively. It is also worthwhile to say that no
earthquake occurred in the region at that day.
[8] As seismic origin of the tsunami was unrealistic due to
the absence of earthquakes, Ranguelov et al. [2008] assessed
a possibility that a submarine landslide generated the
observed tsunami. They also introduced a possibility that
meteorological forcing was the generator of the event. As
our study is focused on the latter option, we tried to analyze
synoptic situation and available ground, satellite, and radio
sounding measurements and to compare it to the characteristics documented for other meteotsunamis [Monserrat
et al., 2006; Belušić and Strelec Mahović, 2009; Vilibić and
Šepić, 2009]. For that purpose air pressure weekly charts
have been analyzed for a number of locations (Figure 1), but
their resolution and quality did not allow for high‐resolution
digitizing (e.g., at the 5 min resolution) and in‐depth analysis on the minute timescale. However, sudden variations in
air pressure are captured by a number of barograms, indicating the propagation of the atmospheric disturbance visible in ground air pressure.
[9] We also used the profile of the radio sounding carried
out at the Istanbul station (taken from University of Wyoming
Web site http://weather.uwyo.edu/upperair/sounding.html),
measuring vertical profiles of wind, temperature, and
humidity (dew‐point temperature). Moreover, the Richardson
number Ri was derived from these measurements as
Ri ¼
N2
ðdu=dzÞ2
ð1Þ
where N is the Brunt‐Väisälä (BF) frequency, u is the wind
speed, and z is the height. The BF frequency was calculated
as the moist BF frequency [Durran and Klemp, 1982] on the
levels with high relative humidity (here taken as higher than
90%) and as the dry frequency otherwise. A layer was
considered unstable if its Richardson number was lower than
0.25. Unstable layers are favorable for the reflection and
ducting of the lower troposphere gravity disturbances [Šepić
et al., 2009a], providing that the wind speed at the unstable
level equals the propagation speed of the disturbance
[Lindzen and Tung, 1976].
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Figure 1. Geographical map of the western Black Sea with the position of the barograph stations (circles) and ocean observation sites (diamonds, maximum and minimum sea levels are given in brackets
after Ranguelov et al. [2008]). Sea level values at Ahtopol tide gauge are taken from the chart record.
[10] Synoptic conditions have been analyzed using temperature, wind, relative humidity and geopotential values at
500, 700 and 850 hPa from the European Centre for
Medium‐Range Weather Forecasts (ECMWF) reanalysis
data. Further, the minimum Richardson number between
300 and 600 hPa has been spatially examined, in order to
map the areas favorable for conductive tunneling of the
atmospheric gravity disturbances. Notice that as for formula
(1) the BF frequency was computed as the moist BF frequency in areas with high relative humidity and as dry
frequency elsewhere, the only difference being that here a
lower threshold was taken—70% opposed to 90%—to
account for coarser vertical resolution data.
[11] Meteosat Second Generation satellite images were
used for the recognition of convective clouds that passed
over the western Black Sea area. For the detection of the
cloud characteristics, 10.8 mm infrared channel images were
used. Cloud motion vectors, defining the speed and direction of the convective clouds, were computed from the
satellite images, comparing similar‐looking equally sized
areas in the precursor and target images up to a certain
distance from the grid point, by using a standard cross‐
correlation technique applied to rectangular targets [Kidder
and Vonder Haar, 1995; Belušić and Strelec Mahović,
2009]. The optimal matcher is the one for which the
cross‐correlation coefficient between the target and the
matcher is maximal, and the shift between them defines
the cloud motion vector. It should be pointed out that the
velocity determined in this way is associated with the cloud
top movement, and not with the movement at the instability
level relevant for the propagation of the surface air pressure
disturbance. However, since the atmospheric motion vectors
define the velocities of the features in the atmosphere, they
can be regarded as a proxy for the velocity of the perturbation if caused by the considered atmospheric system.
[12] Finally, a two‐dimensional ocean numerical model
was applied to the western shelf of the Black Sea. The
model was forced by a moving air pressure disturbance
only, with speed and direction as determined from the
sounding and satellite measurements. The details of the
model are given in section 4.1.
3. Atmospheric Observations
3.1. Synoptic Conditions
[13] The synoptic situation over Europe on 7 May 2007
12:00 UTC (Figure 2) reveals flow characteristics similar to
these observed during most of the Adriatic and Balearic
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Figure 2. Synoptic charts as obtained from the ECMWF reanalysis fields of 7 May 2007 12:00 UTC:
(a) surface air pressure, (b) temperature and winds at 850 hPa level, (c) geopotential height and winds at
500 hPa level, and (d) minimum Richardson number Ri at heights between 600 and 300 hPa.
meteotsunamis. Ramis and Jansà [1983] listed them for the
Balearic Islands as (1) the Mediterranean air mass is present
over the surface, with weak depression to the west of the
Balearic Islands, (2) warm and dry air blows along the
850 hPa level, and (3) an unstable layer is present at the top
of the warm air. Such a vertical stratification is favorable for a
waveguide mechanism [Lindzen and Tung, 1976; Monserrat
and Thorpe, 1996]. Similar conditions are satisfied during
most of the Adriatic meteotsunamis [Vilibić and Šepić, 2009],
although, at least one of such events was provoked by a
wave‐CISK (Convective Instability of the Second Kind,
Belušić et al. [2007]) generated air pressure disturbance.
However, Belušić and Strelec Mahović [2009] found that
convective systems were present during all of the wave‐duct
destructive Adriatic meteotsunamis, indicating that convective system’s instability, largely present in the upper troposphere and overtopping the warm African air, is the best
conductor for the trapped gravity disturbances traveling in
the lower troposphere.
[14] The mean sea level pressure field of 7 May 2007 at
12:00 UTC (Figure 2a) indicates a surface low pressure
region over the western Black Sea and eastern Balkans,
stretching from a deep Scandinavian low toward the Eastern
Mediterranean. The low, situated between the Azores and
Siberian Highs, traveled from the Adriatic Sea (6 May 2007,
12:00 UTC, not shown) toward the northeast at the leading
side of a deep upper‐level trough (Figure 2c), reaching the
western Black Sea on 7 May 2007. Simultaneously, the
inflow of the warm African air, as seen in the 850 hPa level
temperature field (Figure 2b), was extending over the
Eastern Mediterranean and Turkey, with a thermal front
over the western Black Sea margin. At this level a southwesterly wind was blowing with speed of 10–15 m/s, but
without a pronounced jet‐like structure. However, strong
southwesterly jet‐like winds with speed of 15–25 m/s may
be seen at the 500 hPa level (Figure 2c), stretching from the
Central Sahara up to the Black Sea. The mid‐troposphere
instability layer (Figure 2d) is encompassing western Black
Sea, denoting the regions where meteotsunamis are allowed
to occur [Šepić et al., 2009b]. Although the center of the
Black Sea instability layer is positioned more to the northeast of the region attacked by the tsunami, one should be
aware that the tsunami appeared around 06:00 UTC (i.e., 6 h
before the time of the plotted reanalysis fields). Therefore,
the instability area moved for about 400 km during these
6 h, which implies that the center of the instability was just
above the affected area at 06:00 UTC on 7 May 2007.
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Figure 3. Vertical atmosphere structure (air temperature, solid line; dew point temperature, dashed line;
wind speed; and Richardson number Ri) obtained from radio sounding on 7 May 2007 in Bucharest at
(a) 00:00 UTC and (b) 12:00 UTC, and in Istanbul at (c) 00:00 UTC and (d) 12:00 UTC.
Figure 4. Ground air pressure series at a number of eastern Bulgarian barograph stations as digitized
from barograms of 7 May 2007. Vertical line marks the disturbance which presumably generated
meteotsunami waves.
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Figure 5. Color‐enhanced Meteosat 9 IR10.8 mm satellite images, measuring cloud top temperature,
taken on 7 May 2007 every 30 min from 04:30 UTC (up) till 7:00 UTC (bottom).
3.2. Sounding Profiles and Ground Observation
[15] Direct measurements of the vertical atmospheric
structure are unfortunately not available for the area affected
by the tsunami. However, two closest sounding stations,
Istanbul and Bucharest, reveal the structure already found
during the Adriatic and Balearic meteotsunamis [Monserrat
and Thorpe, 1992; Vilibić and Šepić, 2009]. Low ground
winds and temperature inversion may be found from 400 to
900 m at Bucharest on 7 May 2007 both at 00:00 and
12:00 UTC (Figures 3a and 3b). Dryer air may be found up to
3000 m, overtopped by a thick layer of moist and instable
air (3000–6000 m). The instability layer was accompanied
by a SSE wind with the speed of 20–22 m/s. Temperature
inversion on heights of 200–700 m at Istanbul on 7 May
2007 (Figure 3c) was very strong before the tsunami event,
vanishing 12 h later (Figure 3d). Dry air with relative
humidity between 20% and 50% extended up to 5000 m,
where humid and unstable air may be seen, being characterized by negative Ri. Weaker S to SSW winds in the lower
troposphere (<10 m/s) at 00:00 UTC increased in the following hours, reaching 16 m/s at the minimum Ri layer
(∼5500 m). Vertical wind gradient was quasi‐constant from
the region of zero wind recorded at the surface up to the
altitude of the instability layer, which is a known favorable
condition for the ducting of lower‐troposphere gravity disturbances [Monserrat and Thorpe, 1996].
[16] Ground air pressure was measured at synoptic stations by analog barographs having weekly charts. Figure 4
displays air pressure series measured at a number of barograph stations on 7 May 2007, possessing enough quality
records to be digitized with 10 min resolution. Such a resolution is adequate to follow eventual disturbances over the
area but is not sufficient to perform any in‐depth analysis, as
the processes are occurring on a minute timescale. One can
see that only weak high‐frequency air pressure oscillations
have been recorded in the southern part of the Bulgarian
coastline (Ahtopol, Burgas) as well as in the inland Bulgaria
(Shumen). However, strong negative air pressure oscillation
of about 2 hPa may be seen at Emine between 5:10 and
5:50 UTC. Similar oscillation occurred at Varna between 6:10
and 6:50 UTC, and at Kaliakra between 6:20 and 7:00 UTC
but with amplitude of about 3 hPa. Also, noteworthy oscil-
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3.3. Satellite‐Based Analyses
[17] Color‐enhanced satellite images in 10.8 mm infrared
channel (Figure 5) recorded during the tsunami event clearly
depict the approach of the frontal zone to the western Black
Sea around 04:00 UTC on 7 May 2007, with a number of
embedded convective cells traveling and developing over
the region. A particular attention should be drawn to the
second convective cell that approached the area affected by
the tsunami at 06:00 UTC (the cells are marked by 1 and 2 in
Figure 5). A narrow cloudless corridor oriented perpendicularly to the propagating direction may be seen, indicating
strong mesoscale activity and a gravity disturbance capped
by the second convective system eventually preceded by a
gust front, which altogether could cause the sudden pressure
change above the sea.
[18] Cloud motion vectors (Figure 6) indicate the cloud
top velocity of the second convective system are close to
18 m/s at the beginning of the tsunami event (06:00 UTC).
The direction of the cloud tops over the affected area was
toward NE (30°–35°). The speed was rising a bit in the next
hour, but remaining below 25 m/s—lower than in the rear
part of the frontal zone, where the convective cells traveled
more to the north, with speeds surpassing 25 m/s. One
should be aware that the velocities of the cloud tops are
larger than the velocities at the cloud instability layer (ca.
5000 m), as measured at both Bucharest and Istanbul
sounding stations (Figure 3). However, the propagation
direction of the convective cells as estimated from satellite
images follows the estimates from the ECMWF reanalysis
fields, and therefore it may be taken as relevant during the
assessment of the atmospheric gravity disturbance propagation over the area affected by the tsunami.
4. Ocean Numerical Modeling
Figure 6. Cloud motion vectors computed by using 15 min
subsequent satellite images and plotted every hour on 7 May
2007 from 05:00 UTC (up) till 07:00 UTC (bottom).
lations on a minute timescale are found at the Kaliakra
barogram (not shown), introducing a noise in the record
(significantly widening the pen‐recorded line); they may
indicate the occurrence of infra‐gravity waves that may also
generate resonant long ocean waves, if they possess coherence over an ocean area. Nevertheless, the latter cannot be
proved without precise microbarograph measurements, with
subminute sampling resolution [Monserrat et al., 1998]. The
oscillation can be traced also in Karnobat and Shabla, but
with lower amplitudes. Rough estimates of the air pressure
disturbance speed may be assessed if assuming the propagation direction toward NNE (30°): the disturbance traveled
the distance of 90 and 130 km between Emine and Kaliakra
and between Karnobat and Kaliakra in about 100 and
130 min, respectively, yielding to the disturbance speed of
about 15–17 m/s.
4.1. The Model
[19] A two‐dimensional nonlinear shallow‐water model
has been used to reproduce the ocean waves generated over
the western Black Sea shelf. The model is based on the
momentum equations containing the air pressure forcing
term and the continuity equation:
@u
@u
@u
@ guðu2 þ v2 Þ1=2 1 @P
þ u þ v fv ¼ g ;
C 2 ðh þ Þ
@t
@x
@y
@x
@x
ð2Þ
@v
@v
@v
@ gvðu2 þ v2 Þ1=2 1 @P
þ u þ v þ fu ¼ g ;
@t
@x
@y
@y
@y
C 2 ðh þ Þ
ð3Þ
@ @
@
þ ½ðh þ Þu þ ½ðh þ Þv ¼ 0;
@t @x
@y
ð4Þ
where t is time, u and v are the vertically averaged velocity
components in the x and y directions, g is the acceleration of
gravity, z is the sea level elevation, h is the undisturbed
water depth, f is the Coriolis parameter, r is the water
density, P is the air pressure, and C is the Chezy’s friction
coefficient:
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C ¼ 18 log ð0:37h=z0 Þ ½m1=2 =s;
ð5Þ
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Figure 7. Bathymetry and domain of the ocean numerical model. Isobaths of 50 and 20 m are marked
with dashed and dotted line, respectively.
where z0 is the roughness scale. An explicit leapfrog scheme
[Imamura, 1996] has been used to solve equations (2)–(4).
[20] The model domain (Figure 7) encompassed the
western Black Sea shelf; the corresponding grid was taken
from the General Bathymetric Chart of the Oceans (GEBCO)
bathymetry database (available from http://www.gebco.net).
This grid has 30″ resolution (i.e., 680 m × 925 m (for longitude and latitude). The size of the computational domain
was 259 × 299 grid cells. According to a numerical stability
criterion, the time step was taken to be Dt = 1 s and the
bottom roughness scale z0 was set to 0.001 m. A radiation
condition was used at all open boundaries.
[21] The model was forced by a moving air‐pressure
disturbance that was introduced into the model in the following way: (1) artificial traveling disturbance has been
constructed, with linear increase of air pressure of 3 hPa
over 6 min, and constant air pressure values before and after
the disturbance, (2) speed and direction of the moving air
pressure disturbance were set to be constant over the whole
domain with the values assessed from the measurements
(i.e., 16 m/s and 30°), respectively, (3) the time interval
elapsed from the passage of the air pressure front above each
grid point was calculated, and (4) the air pressure data were
interpolated linearly in time at each grid point shifted in time
according to the determined time interval.
4.2. Model Results
[22] The response of the ocean to the moving air pressure
disturbance is noteworthy (Figure 8), with maximum sea
levels of about 1 m occurring west of Kaliakra up to Balchik
and in Burgas Bay. The latter maximum is not realistic as
high‐frequency air pressure oscillations were weak in this
region, and therefore not introduced properly into the idealized forcing. However, the first region of the modeled sea
level maxima is coinciding with the observations (Figure 1),
except for the coastline northeast of Kaliakra, where noteworthy sea level oscillations have been observed but not
modeled in Shabla. The model also reproduced well some
other observed properties of the tsunami waves, such as sea
withdrawal rate being larger than sea level rise rate (Figure 9).
Minimum sea level values have been modeled in Kavarna
(−1.7 m), Dalboka (−1.3 m), and Balchik (−1.2 m), exactly
at places where minimum sea levels have been observed
[Ranguelov et al., 2008]. Model is slightly underestimating
both maximum and minimum values in that area, but this is
expected as the coastal bathymetry is too coarse for coastal
tsunami modeling [Poisson et al., 2009] and the model is
forced by idealized nondispersive disturbance propagating
with constant speed and direction, which is far from the real
situation [Monserrat and Thorpe, 1992; Šepić et al., 2009a].
Moreover, the model may underestimate the sea level
oscillations due to lack of infragravity air pressure forcing
(i.e., on timescale of a few minutes and spatial scales of a
kilometer) that may generate resonant sea level oscillations
on a periods of several minutes. Indeed these oscillations
have been reported (oscillations of 4–8 min, [Ranguelov et
al., 2008]), but not reproduced well by the model in this
study.
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the tsunami event—turbulence, strong water currents, mud
waters, and foam in some sites (e.g., in Balchik and
Kavarna)—were reproduced as well, as the maximum currents reached 100 cm/s in front of these places (Figure 8b).
These currents were the strongest over the shallowest
region, which is extending southward of Balchik for about
20–30 km (depths lower than 15 m, with a number of shoals).
5. Discussion and Conclusions
Figure 8. Results of the ocean model simulation: (a) maximal sea level heights and (b) maximal ocean currents.
[23] Nevertheless, these “imperfections” did not prevent
the model to reproduce qualitatively the observed oscillations, indicating that the atmospheric traveling disturbance
was capable of exciting strong tsunami waves along the
western Black Sea shelf. The last observed characteristics of
[24] The sudden occurrence of tsunami waves on 7 May
2007 along the northern Bulgarian Black Sea coast with
maximal observed sea level range exceeding 3 m at some
places, and a lack of tsunamigenic earthquake in the area,
directed the research to the assessment of other possible
sources of this event. Ranguelov et al. [2008] evaluated the
possibility that a submarine landslide could have been the
source and found it plausible in terms of available
observations. However, the observations are the most critical
part of the analysis, as no observations or eyewitness reports
were documented in the area where maximum tsunami
waves have been modeled (i.e., around Emine). Therefore,
we evaluated the possibility that an atmospheric process is
the source of the observed tsunami waves (i.e., that this event
may be classified as a meteotsunami). Meteotsunami waves
are normally generated by a high‐frequency air pressure
disturbance propagating over a shallow area (though traveling wind disturbance may be generator as well [de Jong and
Battjes, 2004]), where the speed of the atmospheric disturbance is equal to the speed of the long ocean waves, activating a resonant amplification process of the ocean waves
[Monserrat et al., 2006]. The atmospheric disturbance
should be nondissipative in order to allow for significant
growth of the ocean waves, which can be found only under
specific synoptic and mesoscale atmospheric conditions.
[25] Analyzed synoptic conditions during the tsunami
event are found to be fully favorable for the generation and
ducting of an eventual traveling atmospheric gravity disturbance. The disturbance has been found as well, reaching
maximum amplitude of 3 hPa and traveling toward NNE
with the speed of 15–17 m/s (a rough estimate from the
barograph data). Moreover, the disturbance has not been
found along the southern Bulgarian coastline as well as in
the inland area a few tens of kilometers to the west of the
affected area. That is an indication that the core of the disturbance was pretty narrow, presumably not wider than a
few tens of kilometers, similarly as documented for the 2007
1st meteotsunami which occurred in the Adriatic [Šepić et
al., 2009a]. A convective system passed over the region
with velocities at cloud tops level of about 18–20 m/s,
exactly at the same times when the ground disturbance has
been observed. The sounding data indicated that the speed
and direction of the disturbance were about 16 m/s and 30°,
respectively, and according to the ducting theory this is found
to be propitious for meteotsunami generation [Monserrat
and Thorpe, 1992, 1996; Vilibić and Šepić, 2009]. The
research was thus directed to the reproduction of the observed
ocean waves—the first study of such kind in the Black
Sea—whereas similar studies were already conducted for
the Adriatic [Vilibić et al., 2004] and the Balearic [Liu et al.,
2003; Vilibić et al., 2008] meteotsunamis.
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Figure 9. Time series of the modeled sea levels at some locations along the western Black Sea coast.
[26] Unfortunately, the quality of the high‐frequency air
pressure data was unsatisfactory for the construction of a
realistic forcing based on the air pressure series; therefore,
we constructed a sharp pressure jump (3 hPa over 6 min,
following the observations at some barographs) embedded in
the constant air pressure series and traveling over the region
with constant speed and direction. The model reproduced
most of the observations, which include (1) minimum sea
levels and sea withdrawal larger than maximum sea levels
and sea level rise, (2) strong currents reproduced at places
where observed, and (3) maximum sea level rise and lowering similar to the observations at most of affected places.
On the other hand, the model overestimated the oscillations in
the Burgas Bay and along the southern Bulgarian coast,
where only a weak air pressure disturbance was observed (see
barograph records at Ahtopol and Burgas in Figure 4)—not
followed by the model which was forced by a nonchanging
sharp pressure jump across the whole computational domain.
Therefore, we believe that the maximum sea level oscillations modeled in the Burgas Bay are unrealistic, as generated
by an unrealistic traveling pressure disturbance. Supportively, the modeled oscillations at Ahtopol overestimate the
observations by about three times; applying a similar ratio
between modeled and observed values at the Burgas Bay
dramatically lowers their visibility to the eyewitnesses.
Another interesting point is that the first tsunami wave
recorded at Ahtopol was not the largest as observed in the
northern part—the maximum wave occurred about 6 h after
the major disturbance and may be related to the topographic
or edge waves that come from the north or to another
weaker atmospheric disturbance that occurred at that time
just over southern part (see barographic record in Figure 4).
Conclusively, our presumptions about the source characteristics, spatial coverage, and outreach of the atmospheric
disturbance should be additionally verified by reproducing
the event by means of a mesoscale atmospheric model.
[27] Also, ocean waves were underestimated to the northeast of Kaliakra, which may be due to the changes in energy
content, speed, and direction of the atmospheric disturbance,
not embedded in the model forcing. Another serious deficit
of the model forcing (i.e., of its temporal and spatial properties which do not include any infragravity waves usually
occurring with periods up to a few minutes) is its incapacity
to reproduce properly the eyewitnessed period of oscillations which was reported to be 4–8 min [Ranguelov et al.,
2008]. However, state‐of‐the‐art mesoscale atmospheric
models are still not successful in reproducing properly these
processes [Belušić et al., 2007; Šepić et al., 2009a], and
presently no realistic forcing can be applied in such modeling
studies. The resolution of the bathymetry may also be blamed
for this model shortage; efficient bathymetry resolution for
reproduction of coastal tsunami and meteotsunami dynamics
should be an order of magnitude lower than in our computations, even down to 10 m [Vilibić et al., 2008].
[28] Summarily, we may say that the atmospheric processes had the potential to generate the tsunami waves that
were observed along the western Black Sea on 7 May 2007,
and that the event may be classified as a meteotsunami. This
is a new fact, not mentioned and never modeled before for
this region, which increases the probability of tsunami
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expectations and influences the tsunami risk evaluation to
the population and infrastructure. Another novelty in the
paper may be found in the analyses of meteorological satellite
images, where a method of computation of cloud motion
vectors was applied to the event, and resulted in estimates of
disturbance velocities that may possibly be used in an
operative system in the future. This study may be a clue for
assessing other less destructive tsunami‐like events which
are still not classified regarding their source, as lots of
cataloged tsunami events are still marked as unknown, and some
of them have only recently been recognized as meteotsunamis (e.g., the middle Adriatic meteotsunami which
occurred on 21 June 1978 with 6 m waves observed in the
Vela Luka Bay was classified as a tsunami of “unknown”
type in several tsunami catalogs [Tinti et al., 2004; Vučetić
et al., 2009]). Nevertheless, submarine landslide is still an
option for the generation of tsunami waves in the Black Sea
[Ranguelov et al., 2008].
[29] A further element of discussion is the question whether
(1) possible Black Sea meteotsunamis are connected with
changes in the atmospheric circulation above Europe due to
recent climate changes [Giorgi and Coppola, 2007], allowing for meteotsunamis to appear nowadays in this region, or
(2) the Black Sea meteotsunamis are more infrequent than
the Balearic and Adriatic meteotsunamis, possessing larger
return period and therefore having occurred in the past, but
not being recorded by observations or historical chronicles.
The understanding of the meteorological conditions and of
rarity of these events is a very intriguing question, which
may be answered through long‐term examination of the
reanalyzed atmospheric fields. However, such a study should
be accompanied with high‐resolution atmospheric models
capable of reproducing very specific conditions in the
atmosphere. In any case, meteotsunamis should be included
in any Black Sea tsunami assessment study, as being possible
for the region.
[30] Acknowledgments. Sounding data were taken from the University
of Wyoming Web site (http://weather.uwyo.edu/upperair/sounding.html),
while the synoptic fields were analyzed using ECMWF reanalysis fields.
The comments raised by two anonymous reviewers are appreciated. The
work was supported by the Ministry of Science, Education and Sports of
the Republic of Croatia (Grant 001‐0013077‐1122). The method for calculating Cloud Motion Vectors was developed within the framework of the
project “CEI Nowcasting System” sponsored by the Austrian Ministry for
Education, Science and Culture.
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