1
Mediterranean Sea Level Changes over the Period 1958-2001
INTRODUCTION
Sea level has been monitored along the East Mediterranean coast since the twenties of
the 20-th century. In a parallel with Survey of Israel (SOI), other organizations such
as the Israeli Port and Railways Authority (PRA), Meteorological Service, Israel
Oceanographic and Limnological Research (IOLR) were involved in the sea level
monitoring during the different periods (Rosen and Kit, 1981).
Main aims of monitoring in SOI are to derive at the mean sea level (MSL) value and
to investigate its variations. The MSL is taken as zero of a geodetic heights level and
as a base of the geoid construction. A new goal of such monitoring was derived
during the last decades. Sea level changes are a subject of many interdisciplinary
investigations such as the greenhouse effect and the global temperature changes. A
rise in sea level will generate a need for a coastline protection and possible intrusion
of seawater into freshwater areas in a future (Woodworth, 1987). All these suggest the
necessity of a careful study of mean sea level changes.
Sea level is measured in a datum of each tide gauge station that was derived from a
local geodetic system leveling. The height system was adjusted over all the period of
sea level measurements as well sea level.
In this work we analyze all available data during 1958 – 2001 covering two 19 years
periods. Presented sea level data gathered from different sources has certainly a
different quality. We tried to estimate the reliability of such historic data.
The aims of the work are:
to compare MSL with a current datum;
to describe main tide features observed at our stations;
to distinguish tide long time periods.
1.TIDE GAUGE BENCHMARKS AND SEA LEVEL MEASUREMENTS
1.1 Benchmarks
Sea level measurements at each gauge station are performed with a reference to a
nearby benchmark called a tide gauge benchmark (TGBM). Usually, this is special
headed bolt in a substantial structure whose height is derived from a local geodetic
leveling. The local leveling is connected to the national leveling network. Below is a
short summary of the national benchmarks connected to TGBM at Yafo and Ashdod
stations in different periods.
Yafo station.
The benchmark 223/BN of height 2.350 m was fixed in December 1954. The float
operated tide gauge was installed again, about 75 m apart from the BM 223/BN and
70 m from its previous location in June 1962. The repeated leveling between 223/BN
and TGBM carried out in August 1965 and in January 1971 and revealed heights
3.172 m and 3.175 m respectively.
On June 1985 the new benchmark 55/F was fixed at the height 2.945 m with a
reference to the benchmark 223/BN. The distance between BM 55/F was about 75 m
from the 223/BN. Yafo tide gauge operated without being removed until 1989.
2
Tel-Aviv station.
In 1996, a new digital tide gauge was installed at Tel Aviv marina (5 km north to
Yafo site). The TGBM height was derived from the height transfer of the 55/F BM.
The local TGBM was fixed at the height 2.003 m.
Ashdod station.
Benchmark 17/F was fixed in August 1958. Its height 3.538 m was derived on the
basis of TGBM in Haifa. BM 17/F was destroyed during the port development in
December 1967. A new BM 17/F was fixed at the height 3.000 m. Ashdod
benchmark was removed together with tide gauge twice: for the first time in January
1968 and the second - within the port from the inner part to the entrance sector of the
port (Goldshmidt and Gilboa, 1985).
A new benchmark 718/A inside the port was fixed with a height 2.518 m in 1983 and
still exists nowdays. BM stability has been checked each year since 1990.
1.2. Sea level measurements
Before 1996 sea level measurements at Yafo and Ashdod stations were performed by
means of analog tide gauges of OTT HYDROMETRIE company (Germany). Since
1996 the analog tide gauges were replaced by float type digital instruments of the
same company.
The location of tide gauge stations at the modern period is shown on the map (Fig.1).
SOI tide gauges array includes the next stations: Ashqelon, Ashdod, Tel-Aviv and
Haifa. Two other stations at Hadera and Haifa belong to the Israel Oceanographic and
Limnological Research.
HAF - Haifa
SOI stations
IOLR stations
HDR - Hedera
TLV – Tel-Aviv
HAF
HDR
ASD - Ashdod
TLV
ASQ - Ashqelon
ASD
ELT - Eilat
ASQ
ELT
Fig. 1 Tide gauges locations along the Israeli Mediterranean sea coast.
3
An identical tide gauges were installed at the Tel-Aviv and Ashqelon Marinas and at
the port of Ashdod. Fig 2a shows the THALES float type tide gauge at the Ashqelon
Marina. Sea level record changes have been monitoring in digital form with 5-min
sampling and 1 cm resolution. Sea level has been checked relatively to the nearby
Bench Marks at each station once a week.
Fig.2a Float type tide gauge
at the Ashqelon Marina
Fig. 2b Radar tide gauge
at the Haifa port
A new microwave radar tide gauge was installed at Haifa station in 2001 (Fig. 2b).
The KALESTO OTT radar sensor uses microwaves of 24,125 GHz frequency and
5 mW power with a minimum measuring interval 5 sec.
The characteristic of data used in the sea level analysis is given in the Tab.1.
Tab.1
of Sampling
Responsible
Years
Station
1958-1960
1961-1984
Ashdod
Ashdod
Category
data
Tables
Tables
1962-1989
Yafo
Analog records Digitized (max – min)
SOI
1990-1995
Ashdod
Analog records Digitized (max – min)
SOI
1996 –2001
Tel –Aviv
Digital records
SOI
Monthly mean
Hourly mean
15 or 5 min sampling
2. TIDE WAVES APPEARING ALONG ISRAELI COAST
PRA
PRA
4
In order to clarify what is a difference between tide appearance along coast shore,
records from different stations were compared.
Comparison between Ashqelon, Ashdod and Tel-Aviv records shows that tide waves
appear at the same phase and almost the same amplitude in all stations. At the Fig.3
tide records with 5-min sampling from Tel-Aviv and Ashdod station over the period
of September- November 1999 are presented. The differences between records are
defined as a high frequency noise. A mean value and standard deviations of
differences are estimated as about 1 cm. Those comparisons justify arranging the data
from all considered stations in one array.
à
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à
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çùåï
Tel-Aviv
Tide, m
0.4
0.2
0.0
-0.2
Ashdod
Tide, m
0.4
0.2
0.0
Differences, m
-0.2
0.2
Ashdod - Tel-Aviv
0.0
mean = 1.0 cm
st. dev. = 1.1 cm
-0.2
260
270
280
290
300
310
Julian days (16 Sep-13 Nov), 1999
Fig.3 Comparison between Tel-Aviv and Ashdod records.
An example of 1996 yearly record at Tel-Aviv station is given on Fig. 4. The
spectrum of that record is shown in the bottom part of Fig.4. The groups of the
semidiurnal, diurnal and low frequencies constituents are revealed.
Sea level
Tel-Aviv 1996
0.6
a level, m
0.4
0.2
0.0
5
Fig..4 Yearly (1996) record at Tel-Aviv station and spectrum.
The comparison between calculated astronomical tides and measured sea level at the
Tel-Aviv station are given on Fig.5a and Fig.5b. It is evident that calculated diurnal
and semidiurnal spectral groups are similar to the measured ones, but in the case of
low frequencies distinctions are seen. Such differences in monthly periods can be
explained through meteorological factor influence mainly in the winter period.
Astronomical tide 2000
60
Sea level, cm
40
20
0
-20
-40
01
03
04
06
08
10
Spectral amplitude, cm
month
30000
semidiurnal
20000
monthly
diurnal
10000
0
1.0
2.0
frequency, 1/day
Fig.5a Calculated astronomical tide
3.0
4.0
12
6
Sea level
Tel-Aviv 2000
80
Sea level, cm
60
40
20
0
-20
-40
01
03
04
06
08
10
12
Spectral amplitude, cm
month
30000
semidiurnal
20000
monthly
diurnal
10000
0
1.0
2.0
3.0
4.0
frequency, 1/day
Fig.5b Yearly 2000 record at Tel-Aviv station
3. ACCURACY ESTIMATION OF ANALOG RECORDS
For the accuracy estimation of previous analog record, sea levels at Yafo and Ashdod
were compared. Comparisons between one-hour data at Ashdod with records at Yafo
usually show a good agreement over a monthly period. It means that semidiurnal and
low frequency constituents are appeared in the same phases and amplitudes at both
stations. Only full complete month records were included in a consideration. Fig.6
shows monthly mean values compiled from the records of Ashdod station and
differences between Ashdod and Yafo over the period 1962 – 1984. The missing data
at Ashdod station were complemented by the Yafo data. All the data reffered to the
Yafo datum. A systematic mistake in noticed differences results from different
benchmark height determination. Two periods of systematic errors are evident during
1962 – 1984. During the first period 1962-1967 the TGBM heights were determined
more accurately and the average of difference between month’s mean in Ashdod and
Yafo was -0.2 cm. During the second period 1968 – 1983 the average of difference
assumes value –4.8 cm. As mentioned above, the Ashdod benchmark together with
the tide gauge were removed to the other place in January 1968 and a new benchmark
was fixed in 1983. Such distinction in the mean differences probably results from a
mistake in a TGBM height determination at the Ashdod station in 1968 (Goldsmith
and Gilboa, 1985). The main reason of random errors is probably due to mistakes at
the sea level adjustment in a time of a recording paper changing. During the first
period the standard deviation amounted to as much as 2.8 cm and during the second
one it was 4.8 cm.
Compiled Ashdod and Yafo records reffered to Yafo datum
30
Sea level, cm
20
10
0
-10
-20
-30
7
Fig 6. Comparison between the month mean sea levels of Yafo and Ashdod.
4. SEA LEVEL DATA OVER THE PERIOD 1958 – 2001
4.1 Sea level data over the period 1990 – 2001
The next part of data belongs to the latest period of sea level recordings.
It falls into two periods of observations conducted by SOI. The first was performed at
the Port Ashdod during 1990 – 1995 and the second – at Marina Tel-Aviv during
1996 – 2001.
A figure of monthly mean values at Ashdod & Tel-Aviv stations during 1990 –2001
shows a gradual increase of Mediterranean sea level in the last decade (Fig.7).
Ashdod - Tel-Aviv Sea Level
1990 - 2000
40
Monthly mean Sea level (cm)
30
20
10
0
-10
-20
-30
-40
1991
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
Year
Ashdod
Tel-Aviv
Fig.7 Monthly mean values at Ashdod and Tel-Aviv stations during 1990-2000
The red line shows missing data.
2002
8
4.2 Sea level changes over the period 1958-2001
The monthly mean changes over all the period 1958 – 2001 are presented in Fig. 8.
The series was compiled from the different sources that are shown below.
1) Ashdod: 1958-1984. The first displacement of the tide gauge and TGBM were in
1968. So, the data of the period 1958-1967 can be considered reliable. The sea level
during the period 1968 – 1984 was refered to the Yafo datum.
2) Yafo: 1962-1989. The last tide gauge displacement was in 1962, before the
considered period. Sea level measurements were realized with reference to the
benchmark 223/BN till 1985. The error in the 55/F height determining (in 1985) did
not exceed 2-3 mm.
3) Ashdod: 1990-1995. Based on new leveling and new tide gauge installation.
4)
Tel-Aviv: 1996-2001. Based on Yafo-Tel-Aviv high precision (about 1 cm)
leveling.
Fig.8 shows sea level time series referring to Yafo datum. It is evident that a mean
value depends on the period of averaging. The mean value of the all the considered
period is estimated at 5.9 cm., whereas averaging over the last 18.6 years period
yields value of 3.3 cm.
East Mediterranean Sea level changes
40
Sea level, cm
30
20
10
0
-10
-20
-30
-40
1958 1961 1964 1967 1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000
Year
Spectral amplitude, cm
1500
Spectrum
1000
500
0
1.0
2.0
3.0
4.0
5.0
frequency, 1/year
Fig.8 East Mediterranean Sea level during 1958 – 2001.
All the monthly mean values were reffered to the Yafo datum.
6.0
9
Missing data was provided by the maximum entropy method. This method is based on
the signal extrapolation using spectrum characteristics in complex plan (Press et al.,
1988). Two examples of this method are illustrated in the Appendix.
The spectral analysis revealed periods of about 20 years, eight years, one year, half
year and about a third year.
One year averaged &
eight years smoothed
East Mediterranean Sea level
(1958-2001)
16
12
Sea level, cm
8
4
0
-4
-8
58
61
64
67
70
73
76
79
82
85
88
91
Years
Fig. 9 Yearly mean sea level changes and their low-pass filtering.
94
97
00
10
5. METEOROLOGICAL FACTORS
5.1 Air pressure and wind influences.
Among meteorological factors air pressure and wind have the most effect on sea level
changes. In order to estimate contribution to those factors, we used the data of Sde
Dov meteorological station, located at the Tel-Aviv coast. The air pressure effect is
expressed by the following relation (Pugh, 1987):
h=-0.993 Pa
where h is sea level change in cm and Pa is atmospheric pressure change at the
coast station relative Standard Atmosphere of 1013 mb.
The relation between wind parameters and sea level changes is complicated .
It involves a space gradient of atmospheric pressure. Good correlation was shown
between wind velocity and sea level changes on the Scotian shelf (Sandstrom, 1980).
Following are brief discussions of the wind influence on sea level changes.
Comparisons were made on the basis of three hours vector velocity measurements at
the Sde Dov station. Fig. 10 shows the difference between measured sea level at TelAviv station and astronomical tide (top) (Rosen, 2000) and North and West
components of wind velocity and its smoothed values with low-pass filtration
(bottom). It is clearly seen the correlation between wind velocity components and sea
lavel variation during the period 25 November – 06 December 2000. The graph of
sea level changes has a similar form as the North component with about two days
delay between them.
October-December 2000
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
Differences (Tel-Aviv - calculated)
North component of wind
12
8
4
0
-4
-8
West component of wind
12
8
4
0
-4
-8
31
04
08
12
16
November
20
24
28
02
06
10
14
December
Fig.10 The residual part of sea level changes and the two components of
wind with low-pass smoothing for October – November 2000.
11
Spectral amplitude, cm
10
8
October - December 2000
6
Differences (Tel-Aviv - calculated)
4
2
Spectral amplitude, m/sec
Spectral amplitude, m/sec
0
300
200
North component of wind
100
0
300
West component of wind
200
100
0
1.0
2.0
frequency, 1/day
Fig.11 Spectral characteristics of time series are shown on Fig. 10.
Spectral analysis distinguishes between the constitution of North and West wind
velocities. The North component includes changes with a longer period than the West
one and thus it is responsible for long period sea level changes. It can be seen from
the spectrum presented on the Fig. 11. Similar relations are demonstrated in Fig. 12
and Fig. 13. From these illustrations it follows that wind velocity of periods from
some days gives rise to the sea level.
12
August-October 2000
Sea lev diff, m
0.2
Differences(Tel-Aviv-calculated)
0.1
0.0
-0.1
-0.2
-0.3
North component of wind
8
Velocity, m/sec
4
0
-4
-8
-12
West component of wind
8
Velocity, m/sec
4
0
-4
-8
-12
03 07 11 15 19 23 27 31 04 08 12 16 20 24 28 02 06 10 14 18 22 26
August
September
October
Fig.12 The residual part of sea level changes and the two components of
wind with low-pass smoothing for August - October 2000.
Spectral amplitude, cm
12
10
August-October 2000
8
6
Differences (Tel-Aviv - calculate)
4
2
0
Spectral amplitude, cm
800
600
North component of wind
400
200
Spectral amplitude, cm
0
800
600
West component of wind
400
200
0
1.0
frequency, 1/day
Fig.13 Spectral characteristics of time series are shown on Fig. 12.
2.0
13
5.2 Pressure and wind velocity changes over the period 1964-1999.
The time series was compiled from available data of two coast stations: Naharia
(1964-1970) and Sde Dov (1971-1999). Fig. 14-16 shows monthly mean values of
pressure and two components of wind velocity. At the bottom part of each figure
associated spectrum is presented. In all the time series, stromg yearly and semiyearly
periods were isolated. Besides, a third year period in the pressure is clearly seen as
well as North component of wind velocity. A resonable small-amplitude in long
periods of North component cannot be a reason for the strong sea level changes of
twenty years period sea level.
Air pressure
1964 - 1999
pressure, mb
1020
1016
1012
1008
1004
66 68
70
72
74
76 78 80
Spectral amplitude, mb
82
84
86 88
90
92
94
96 98
00
year
1000
800
600
400
200
0
0
1
2
3
count/year
4
5
Fig. 14 Monthly mean changes of air pressure during 1964 – 1999 and spectrum.
6
14
X-component (N->S)
of wind velocity
1964 - 1999
3
wind velocity, m/sec
2
1
0
-1
-2
-3
-4
66 68
70
72
74
76 78
80
82
120
84
86 88
90
92
94
96 98
00
year
Spectral amplitude, cm
100
80
60
40
20
0
0
1
2
3
count/year
4
6
5
Y-component (W->E)
of wind velocity
1964 - 1999
4
wind velocity, m/sec
3
2
1
0
-1
-2
66 68
70
72
74
76 78
Spectral amplitude, m/sec
160
80
82
84
86 88
90
92
94
96 98
00
year
140
120
100
80
60
40
20
0
0
1
2
3
count/year
4
5
6
Fig. 16 Monthly mean North and West components of wind during 1964 – 1999 and
spectrum.
15
Discussion
Sea level changes over forty four years constitute a series of different frequencies
fluctuations. Yearly and semiyearly periods are due mainly to the astronomical tide.
The period of about 1/3-year is conditioned probably by meteorological factor
influence. Comparison between calculated astronomical tide and measured sea level
shows significant difference in the winter high winds period.
The low frequencies changes are clearly visible from the showed yearly mean sea
level changes over all the considered period. The yearly MSL reached the maximum
at the beginning of sixties. After that, it falls sharply to the minimum value in the
middle seventies. Such lowering MSL (about 10 cm) in the middle seventies was
observed also at UK tide gauge stations (Woodworth, 1987). Sea level gradual rise
is observed since 1973.
The period of about twenty years cannot be explained on the basis of the ‘nodal tide’
(18.6 years – period of the precession of the moon’s nodes). The nodal tide account
for about 10% of semidiurnal tide (Pugh, 1987) or 4-5 cm at our stations.
In addition to the meteorological influence on sea level a water salinity and
temperature, and rainfall effect (Woodworth, 1987). So, long periods sea level
variations call for further investigation.
Results
Tide level changes caused by influences from the Moon and the Sun are in
the same phase and amplitude along the coast between Tel-Aviv and Ashqelon.
This justifies linking together data from different stations into a continuous time
series of forty years.
Spectral analyses of the tide level changes revealed semidiurnal and diurnal
periods and long periods of about 20 years, eight years, yearly and half-yearly and
third of a year.
The influence of meteorological factors on the sea level changes (of some days and
and clearly defined third of year periods) was pointed out.
The average tide level relative to the Yafo datum is about 5.9 cm.
Time series averages during a yearly period indicates a gradual rise of about 15 cm
of the sea level since 1973.
16
Appendix
As mentioned previously, missing data was provided by the maximum entropy
method.
In order to filling short gaps in data forward and backward, extrapolations across the
gaps were performed. The gap in data was filled by linear combination of two
extrapolations if both agree tolerably well.
Ashdod
Sea level
Tide, cm
Tide, cm
20
0
hypothetical
gap
0
40
m=60
Tide, cm
40
Tide, cm
20
-20
-20
20
0
hypothetical
gap
20
0
-20
-20
40
bridged
gap
20
0
Tide, cm
40
Tide, cm
Sea level
Tel-Aviv
40
40
bridged
gap
20
0
-20
-20
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
January 2000
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
February 2000
Fig.17. Examples show data processing by means of the maximum entropy method.
Acknowledgments
This work has been carried out at the Research Division of the Survey of Israel (SOI).
Klara Shpitalnik carried out the work with the analog data. The field work was carried
out by Aaron Nahary and Iliya Perelman. We would like to thank them for their work
and devotion. Thanks are also due to Yossi Melzer, Aviel Ron, Joseph Forrai,
Gershon Steinberg and Ron Adler for fruitful discussions, critical comments and
corrections.
17
REFERENCES
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comparison with other Mediterranean tidal data. IOLR rep. H8/85, Haifa, 28 pp.
Manual of sea level measurement and interpretation V.1, 1985. Intergovernmental
Oceanographic Comission, UNESCO.
Pugh D.T., 1987. Tides, surges and mean sea-level. John Wiley & sons, Chichester,
472 p.
Rosen, D. and Kit, E., 1981. Evalution of the wave characteristics at the
Mediterranean coast of Israel. Israel Journal of Earth Science, 30, 120-134.
Rosen, D., 2000. The forecast at the Israeli Mediterranean coastline for year 2000.
IOLR REPORT H01/2000.
Press W. H., Flannery B. P., Teukolsky S. A., Vetterling W. T.,1988, Numerical
recipes. Cambridge University Press, Cambridge. 818 p.
Sandstrom, H, 1980. On the wind – induced sea level changes on the Scotian shelf.
Journal of Geophysical Research, 85, 461-468.
Woodworth P. L., 1987. Trends in U.K. mean sea level. Marine Geodesy,
11, 57-87.