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Relationship between atmospheric circulation types and
storm surges over the Greek seas.
Tolika K., Anagnostopoulou C., Krestenitis Y.
Using a flexible automatic circulation type classification scheme, an attempt is made to study
and analyze the atmospheric circulation over Greece, during the days when storm surges were
detected. This classification scheme provides daily circulation type calendars (with five
anticyclonic and seven cyclonic types). The center of this classification was chosen to be at
the center of the Aegean Sea. Sixteen cases of storm surges were observed over Greece from
2005 until 2012 in different parts of the domain of study. The first results showed, that in all
cases, during the day of the storm surge, only cyclonic types are observed over Greece, with
the cyclonic type C (centered over Greece) to prevail with a percentage of 38%, followed by
the cyclonic type Cne (at the northeast of Greece) with a percentage of 25%. One day before
the storm surge event, the percentages of the prevailing cyclonic type do not seem to change
much (C (38%) and Cne (31%)). However, it is worth mentioning the two days before the
event, the cyclonic circulation type Cne is the one presenting the highest percentage (44%).
Tolika K.1*, Anagnostopoulou C.1, Krestenitis Y.2
1 Department of Meteorology and Climatology, School of Geology, Aristotle University of Thessaloniki, 54124, Greece
2 Laboratory of Maritime Engineering & Maritime Works, School of Civil Engineering, Aristotle University of
Thessaloniki, 54124, Greece
*corresponding author e-mail: [email protected]
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1 Introduction
A storm surge is defined as a change in sea level that is caused by a storm. According to
Pirazzoli (1999) it corresponds to the difference between the observed sea level and the
predicted tidal level at the same time. The storm surge is generally associated with a low
pressure system and high wind speeds and is considered as a deadly and costly natural
disaster especially in areas were tropical cyclone occur (such as the United States) triggering
these storm surges (Neddham and Keim 2011). However, even in regions where the
atmospheric depressions are not so intense, a storm surge could result to modification of the
coastal morphology, an alteration of the coastal environment, the displace of sediment and
erosion processes as well as the destruction of crops and forests or the damaging of habitats
(Neddham and Keim 2011).
In the present study which took place under the frame of the national research project
THALIS- (www.thalis-ccseawavs.web.auth.gr), an attempt is made to analyze the
atmospheric conditions and characteristics over Greece during days that a storm surge had
occurred during the last ten years and affected the several coastal areas in the domain of
study.
2 Data and Methodology
2.1 Data
Sixteen cases of storm surges that occurred during the period from 2005 to 2012 which where
primarily studied by Harisi (2013) were used to identify the main characteristics of the
atmospheric circulation during those days. Table 1 shows the dates of the storm surge day
(SS-Day) and Fig. 1 shows the locations of the areas that where documented to have the
greatest impacts of those storm surges. The selection of the specific episodes was based both
on the extreme wind speeds and the sea wave height.
Fig.1 The geographical locations of the areas affected by the storm surges
2.2 Methodology
An automatic classification was applied for the computation of the daily calendars of the
circulation types. This classification includes twelve (12) circulation types and the scheme is
flexible all over the Mediterranean (Anagnostopoulou et al. 2009). The 1000hPa geopotential
data (2.5 o x 2.5o spatial resolution) from the NCEP/NCAR were employed, covering a large
spatial window over the whole European region, for the development of the daily circulation
type’s calendar. The center of the classification was chosen to be at the center of the domain
of study (Greece) (25oW and 37.5oN).
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Table 1. The dates and the areas affect by the
storm surge.
a
65.0
62.5
60.0
57.5
55.0
52.5
50.0
47.5
45.0
42.5
40.0
37.5
35.0
32.5
30.0
27.5
25.0
22.5
20.0
-20.0 -15.0 -10.0
Anw
Ane
A
Asw
-5.0
0.0
5.0
10.0
15.0
Ase
20.0
25.0
30.0
35.0
40.0
45.0
50.0
b
65.0
62.5
60.0
57.5
55.0
52.5
50.0
47.5
45.0
42.5
40.0
37.5
35.0
32.5
30.0
27.5
25.0
22.5
20.0
-20.0 -15.0 -10.0
Cnnw
Cwnw
Cwsw
5.0
10.0
Cse
15.0
20.0
25.0
Location
15/2/2005
Kavala
2
4/12/2008
Aitoliko
3
21/12/2009
Samos
4
28/12/2009
Tinos
5
1/1/2010
Halkida, Halkidiki
6
2/1/2010
Lesvos
7
4/1/2010
Mesologi
8
6/1/2010
9
7/1/2010
10
20/10/2010
11
19/2/2011
Messinia - Crete
12
7/1/2012
Rio - Crete - Tinos
30.0
35.0
13
6/2/2012
Arkadia
14
19/4/2012
Fokida
15
20/4/2012
Santorini
16
29/10/2012
Samos - Ftiotida
Volos
Xios- Zakinthos Hleia
Cssw
0.0
Date
1
Cne
C
-5.0
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40.0
45.0
50.0
Fig. 2. The circulation types used in the study
Parga
3 Results
On the first step of the analysis, after computing the daily circulation type calendar for the
time period 1958-2012, we investigated which circulation types occur during a storm surge
day. As it was expected during all the sixteen storm surge episodes the prevailing type over
Greece is cyclonic. More specifically in 38% of the cases the cyclonic type that may have
generated the storm surge is the circulation type C, centered exactly over the domain of study.
The second type is Cne (a cyclonic type centered at the northeast of Greece (see Fig. 2))
(25%) which center is at the northeast of Greece. Moreover, we wanted to investigate also the
atmospheric conditions during the two previous days of the storm surge. The day before the
event (SSDay – 1) it was found that also the cyclonic types C and Cne are the most frequent
ones. Finally, two days before the storm surge the cyclonic type Cne presented the greatest
frequency of occurrence reaching 44% (Fig. 2).
Frequency of occurence (%) of the cyclonic types
45
40
35
30
%
25
20
15
10
5
0
C
Cnnw
Cwnw
SSDay -2
Cwsw
SSDay - 1
Cssw
Cse
Cne
SSDay
Fig. 2. The frequency of occurrence (%) of the cyclonic types of the classification scheme used during the storm surge
day (SSDay) the two days before the event.
Aiming on a more thorough analysis of the atmospheric characteristics of the occurring
circulation types during the storm surge days we also computed the anomaly field of the
prevailing type at the 1000hPa level and compared it with the mean anomaly field of the same
type for the period 1958-2012, as derived from the classification scheme. Generally, it was
noted that the cyclonic type at the ss-day is much deeper than its mean field, and the negative
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anomalies (due to the fact that it is a cyclonic type) have much lower values. More
specifically two characteristic cases are presented below.
3.1 The storm surge at the 15/2/2005.
According to Harisi (2013) the storm surge episode was mainly observed in Kavala area, with
very strong southern winds that reached 30m/sec. The daily circulation type calendar showed
that the prevailing type during that day is the circulation type C, centered over the Aegean
Sea. The same type occurred the previous day of the event (SSday-1) while the day before,
the cyclonic type Cnnw was observed over the domain of study. Figure 3 illustrates the
anomaly patterns of the prevailing circulation types during the selected days, the mean fields
of these circulation types, as well as the spatial distribution of the computed differences of the
aforementioned patterns.
13_2_2005 Circulation type Cnnw
FEB Mean Circulation type Cnnw Diff (SSDay-2 - MeanType) FEB Circulation type Cnnw
60
60
60
50
50
50
40
40
40
30
-10
0
10
20
30
40
50
30
-10
14_2_2005 Circulation type C
0
10
20
30
40
50
FEB Mean Circulation type C
60
30
-10
60
60
50
50
50
40
40
40
30
-10
0
10
20
30
40
50
30
-10
15_2_2005 Circulation type C
10
20
30
40
50
FEB Mean Circulation type C
60
50
40
40
0
10
20
30
40
50
30
-10
10
20
30
40
50
30
-10
0
10
20
30
40
50
Diff (SSDay - MeanType) FEB Circulation type C
60
50
30
-10
0
0
Diff (SSDay-1 - MeanType) FEB Circulation type C
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
-4
-4.5
-5
-5.5
-6
-6.5
-7
60
50
40
0
10
20
30
40
50
30
-10
0
10
20
30
40
50
Fig. 3. The anomaly patterns (1000hPa) of the circulation types occurring during the 15/2/2005 (SSDay) as well as two
days before the event (first column). The anomaly patters of the mean field of each circulation type for the period 1958 2012 (second column). The differences of the circulation type during the selected days and their mean fields (ssday –
mean field).
It is worth mentioning that during the day of the storm surge, but also during the two
precedent days, the predominating circulation types are deep depressions with much more
intense negative anomalies than their mean fields. On the 13/2/2005 a Cnnw type is found at
the northwest of Greece with the center of its negative anomalies over central Europe. The
next day, this depression moves towards the south, and it changes to circulation type C, since
the center of the depression has moved over the Greek region. Finally, during the day of the
storm surge the center of the circulation type C move a little further to the south, but still
preserves its deep depression characteristics. The differences maps that derived for this storm
surge event confirm the intense character of the cyclonic types. Negative differences cover a
large part of the European region with the greatest negative values located mainly at the
center of its circulation type.
3.2 The storm surge at the 19/2/2005.
In the second episode that we have decided to present the low pressure system that triggered
the storm surge at the west – south west of Greece, it moved further closer to the domain of
study and then during the specific day of the event it turned to a depression exactly over the
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Greek region. At it can be seen from figure 4, the two previous days of the storm surge (17
and 18 February 2011) the prevailing circulation type is Cwsw. During the day of the event
(19 February 2011) according to the classification scheme, the circulation type that influenced
Greece is C, which has the greatest frequency of occurrence during the days of the storm
surge. As in the previous case, it is also noted that these atmospheric low pressure systems
(Cwsw and C) showed much more intense negative anomalies classifying them as deep
depressions.
17_2_2011 Circulation type Cwsw
Diff (SSDay-2 - MeanType) FEB Circulation type Cwsw
FEB Mean Circulation type Cwsw
60
60
60
50
50
50
40
40
40
30
-10
0
10
20
30
40
50
30
-10
18_2_2011 Circulation type Cwsw
0
10
20
30
40
30
-10
50
FEB Mean Circulation type Cwsw
60
60
50
50
50
40
40
40
0
10
20
30
40
50
30
-10
60
60
50
50
40
40
30
-10
0
10
20
30
40
0
10
20
30
40
50
30
-10
30
-10
50
FEB Mean Circulation type C
19_2_2011 Circulation type C
10
20
30
40
50
Diff (SSDay-1 - MeanType) FEB Circulation type Cwsw
60
30
-10
0
0
10
20
30
40
50
Diff (SSDay - MeanType) FEB Circulation type C
60
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
-4
-4.5
-5
-5.5
-6
-6.5
-7
50
40
0
10
20
30
40
50
30
-10
0
10
20
30
40
50
Fig. 4.The anomaly patterns (1000hPa) of the circulation types occurring during the 19/2/2011 (SSDay) as well as two
days before the event (first column). The anomaly patters of the mean field of each circulation type for the period 19582012 (second column). The differences of the circulation type during the selected days and their mean fields (ssday –
mean field).
The composite maps of the differences derived for these three days showed that two days
before the storm surge event the negative differences cover almost the entire European region.
During the SSDay -1, the negative differences are more centered over Greece while during
the storm surge day the center of the negative difference is shifted over Cyprus, though still
covering the domain of interest (Fig. 4). From the study of Harisi (2013) it is noted that the
main regions that were affected from this storm surges are area several areas in Messinia as
well as Crete. The winds were also very strong with velocities that reached 20m/sec. The sea
waves resulted to the sinking of numerous fishing boats and caused damages to many coastal
settlements.
4 Conclusions
The examination and the links between storm surge events and the associated atmospheric
conditions through a circulation type approach is the main motivation of the present study. In
other words we made an attempt to investigate the association of the circulation types and the
occurrence of the storm surges.
Sixteen cases of storm surges were observed over Greece from 2005 until 2012 in different
parts of the domain of study (Harisi 2013). Overall it can be concluded that the prevailing
circulation types are the cyclonic ones. The most frequent ones during the day of the storm
surge are cyclonic types C and Cne. However, since the previous two days seem to play an
important role to the generation of the storm surge it was found that the day before the event
also C and Cne are the most frequent types, while two days before the prevailing type is Cne.
Finally, the circulation types are found to be much deeper during these three-day period and
their negative anomalies are much lower than the equivalent one of the mean field of the
circulation type.
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As a future work the authors plan to investigate further the dynamical characteristics of the
cyclonic conditions resulting to a storm surge in Greece, by examining over geopotential
levels as well as the wind field (direction and velocity) during the days of the events. Also, it
would be interesting to see if the relationship of the atmospheric conditions and the storm
surge can be successfully simulated by a regional climate model and if this relationship
remains the same in a future period.
Acknowledgments This research has been co-financed by the European Union (European Social fund - ESF) and
Greek national funds through the Operational Program “Education and Lifelong Learning” of the National
Strategic Reference Framework (NSRF) - Research Funding Program: Thales. Investing in knowledge society
through the European Social Fund. (www.thalis-ccseawavs.web.auth.gr)
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Harisi SA (2013) Sea level rise due to storm surge events over the Greek coastal area. Post Graduate Thesis, Civil
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