Geophys. J. Int. (2008) 175, 70–82
doi: 10.1111/j.1365-246X.2008.03892.x
Coastal sea level trends in Southern Europe
Marta Marcos∗ and Michael N. Tsimplis
National Oceanography Centre, Southampton SO14 3ZH, UK. E-mail: mimm@noc.soton.ac.uk
Accepted 2008 June 23. Received 2008 June 23; in original form 2007 October 25
GJI Marine geoscience
SUMMARY
Low frequency sea level variability in the Mediterranean Sea and in the Atlantic Iberian coast is
investigated by use of tide gauge records. The five tide gauge records that span most of the 20th
century show positive trends between 1.2 and 1.5 ± 0.1 mm yr−1 and negative accelerations
between −0.3 ± 0.3 and −1.5 ± 0.4 mm yr−1 century−1 . Sea level trends obtained from
the 21 longest records (>35 yr) are smaller in the Mediterranean (0.3 ± 0.4 to −0.7 ±
0.3 mm yr−1 ) than in the neighbouring Atlantic sites (1.6 ± 0.5 to –1.9 ± 0.5 mm yr−1 ) for
the period 1960–2000. Decadal sea level trends in the Mediterranean are not always consistent
with global values, in particular for the 1990s, during which the Mediterranean has shown
enhanced sea level rise of up to 5 mm yr−1 compared to the global average (mostly attributed
to higher warming). The atmospheric and steric contributions to the observed sea level trends
for 1960–2000 are also examined. The atmospherically induced sea level is obtained from
a barotropic model forced by wind and atmospheric pressure. The atmospheric contribution
accounts for 20–50 per cent of the observed yearly sea level variability and introduces negative
trends of –0.2 to –0.9 mm yr−1 . The steric sea level, obtained from T and S climatologies, has
negative trends ranging from −2.1 ± 0.6 to −0.1 ± 0.3 mm yr−1 . Other shorter tide gauge
records (>7 yr) are used to quality check longer series and to explore their consistency with
the long-term records and identify short but apparently consistent tide gauge records.
Key words: Time series analysis; Sea level change; Europe.
1 I N T RO D U C T I O N
Sea level is an important environmental parameter because of its
impacts on the coastal zone as well as an indicator of climate change.
The Fourth Assessment Report of the Intergovernmental Panel for
Climate Change (IPCC AR4) (2007) recognises the variability of
sea level at regional scales (Cazenave & Nerem 2004).
Coastal sea level is measured by tide gauges referenced to a point
on land. Thus the measurement includes land movements that can
significantly contaminate the sea level signal (Emery & Aubrey
1991; Zerbini et al. 1996). Land movements can be due to different processes: long-term changes like glacial isostatic adjustment
(GIA) due to the ongoing viscous response of the solid earth following the removal of the great ice loads following deglaciation, local
subsidence either natural (e.g. sedimentary loading of delta) or anthropogenic (like coal mine collapses or extraction of gas or water)
or fast changes caused by for example seismic activity (Emery &
Aubrey 1991). Zerbini et al. (1996) used the tide gauge network in
the Mediterranean referred to a global reference system to determine the rates of vertical crustal motion. They found that at most
stations these rates are of the order of ±1 mm yr−1 , a value which is
small compared to decadal rates of sea level change during most of
∗ Now at: IMEDEA, Mallorca, Spain.
70
the time periods but of the same order of magnitude with the longterm trends. In the NW Mediterranean for example the rates of
crustal movements decontaminated for PGR are 0.5 mm yr−1 (with
positive values meaning land uplift). They also found that the crustal
motion due to tectonics in this region is smaller than the postglacial
rebound (PGR) and the response to surface loads. Fenoglio-Marc
et al. (2004) used differences in sea surface heights measured by
tide gauges and TOPEX/POSEIDON altimeter to estimate vertical
land motion in the Mediterranean region. They obtained smaller
rates in the Northwestern Mediterranean (0.5 ± 0.9 mm yr−1 ) and
the largest in the Italian Peninsula and the eastern basin (−3.0 ±
1.6 mm yr−1 in Antalya, for example), with an average accuracy of
2.3 ± 0.8 mm yr−1 . More recently Woppelmann et al. (2007) have
used the longest GPS series available globally to determine vertical
land movements directly from measurements.
Changes in sea level are also caused by meteorological forcing, variations in the density structure, mass addition and changes
in oceanic circulation. Instrumental errors, unrecorded changes or
updates in the vertical referencing system and changes in the configuration of the proximity of the instrument, like dredging, also
affect the quality of the tide-gauge records and should be taken into
account when sea level trends are estimated.
Each tide gauge record integrates all the above forcing factors,
reference point movements and instrumental problems and thus tide
gauge based estimates of sea level trends are, strictly speaking, local
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Journal compilation Coastal sea level trends in Southern Europe
in character, although they have been routinely used to estimate far
field long-term changes on the assumption that such larger scale
changes are reflected in most of the available tide gauge data, that
is, they are a significant part of the signal. Semi-enclosed basins,
like the Mediterranean Sea, pose additional problems, because of
their restrictions in their communication with the open ocean.
In the Mediterranean Sea coastal sea level derived from the
longest tide gauges indicates a rate of sea level rise for the 20th
century of 1.1–1.3 mm yr−1 (Tsimplis & Baker 2000). For the period 1960–1990 an increase in the average atmospheric pressure
over the basin caused negative sea level trends (Tsimplis & Baker
2000; Tsimplis & Josey 2001). During the same period, sea level
was rising in the Atlantic stations although with a lower rate than
before 1960. From the 1980s onwards sea level rise appears to have
increased globally (Holgate & Woodworth 2004; Holgate 2007),
while in the Mediterranean fast sea level rise was observed in the
late 1990s (Cazenave et al. 2001; Fenoglio-Marc 2001). Despite the
above general statements which are derived, as customarily done
in sea level research, on the basis of the longest tide gauges available and in spite of the well known bias in their spatial distribution
(Tsimplis & Spencer 1997), there are several other tide gauges in
the Mediterranean Sea providing of information regarding local sea
level variability.
In this work, we estimate the sea level trends from the available
tide-gauge records in the Mediterranean Sea and in the Atlantic
Iberian coasts. Sea level trends are estimated on the basis of records
longer than 35 yr. These estimated sea level trends include contributions by long-term changes in the meteorological forcing (Tsimplis
& Josey 2001) as well as changes in the steric contribution (Tsimplis
& Rixen 2002). For the period 1960–2000 and for each tide gauge
we separate the direct atmospheric forcing and steric contributions
from the other forcing factors and present the residual trends. The
main objective of the paper is to explore the low frequency behaviour
(interannual and decadal variations and long-term trends) of all the
tide gauge records available in the region spanning at least 7 yr in
recent decades, using their most updated version. The ultimate aim
of this work is to provide the potential user of tide gauge data in this
region with an assessment of the quality of the time-series and with
estimates of sea level trends in the different areas of study. This is
done through a detailed analysis of the longest records (>35 yr) and
71
their intercomparison with shorter ones. Additionally, the causes of
this variability are also investigated for the period 1960–2000, when
atmospheric and steric corrections are available. The improvement
of this research in respect to previous works is the large number of
tide gauge records involved and the application of the corrections
in order to explain the physical mechanisms.
2 D ATA S E T S
2.1 Tide gauge data
Monthly sea level values with benchmark datum history (Revised
Local Reference, RLR) from the Permanent Service for Mean
Sea Level (PSMSL) database (Woodworth & Player 2003) which
cover the last decades of the 20th century were considered. Because of the lack of long-term reliable measurements in the Eastern
Mediterranean (Fig. 1) the measurements in Alexandria (Egypt)
were also included in the analysis. However, this station is not an
RLR station and this must be kept in mind during the analysis.
The PSMSL performs quality checks and provides comments on
the quality of the records. A well known problem with the station of Marseille is the high values found during the beginning of
the 1950s, which are not reproduced in any of the nearby records
and that are claimed to be due to bad operation of the instrument
(http://www.pol.ac.uk/psmsl). Therefore this part (1951–1953) of
the record was removed. For Cascais (Portugal) 20 yr of quality
controlled data (1985–2005) have been kindly provided by the local authorities (Instituto Geográfico Português) and added to the
PSMSL long record after they were quality checked.
Among the 68 selected sea level records 21 span more than 35 yr
(marked with circles on Fig. 1 and listed in Table 1). The remaining
47 records are longer than 7 yr (Table 2) and span the last decades
of the 20th century.
The shorter tide gauges used in this study together with their
period of operation and their percentage of data gaps are listed in
Table 2. The shorter records are useful in three capacities. First,
they can be used for quality checks on the longer records. Second,
if they are consistent with the long records they can be used for
filling in gaps, where needed, of the longer record. Third, their
consistency or inconsistency with the longer records behaviour can
Figure 1. Location of the tide gauges. Black dots correspond to the longest time-series (>35 yr) which are labelled, while red squares are the shorter records
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2008 The Authors, GJI, 175, 70–82
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M. Marcos and M. N. Tsimplis
Table 1. Tide gauges records in Southern Europe longer than 35 yr.
Latitude (◦ , Minutes) Longitude (◦ , Minutes)
Area
S.Jean de Luz
Santander
Coruña
Vigo
Cascais
Lagos
Atlantic
43 24 N
43 28 N
43 22 N
42 14 N
38 41 N
37 06 N
01 41 W
03 48 W
08 24 W
08 44 W
09 25 W
08 40 W
1942–1996
1944–2001
1943–2001
1943–2001
1882–2005
1908–1999
55
58
59
59
112
91
39.55
3.02
1.98
1.13
7.44
21.65
50.55
62.13
63.63
85.11
47.10
47.35
2.1 ± 0.3
2.0 ± 0.2
1.4 ± 0.2
2.5 ± 0.2
1.3 ± 0.1
1.5 ± 0.1
Strait
36 32 N
36 00 N
35 54 N
36 43 N
38 20 N
43 18 N
44 24 N
06 17 W
05 36 W
05 19 W
04 25 W
00 29 W
05 21 E
08 54 E
1961–2001
1943–2001
1944–2002
1944–2001
1960–1997
1885–2004
1884–1997
40
58
58
57
37
119
113
2.44
8.19
6.21
20.98
3.07
2.71
21.71
74.18
53.18
26.03
54.73
24.24
55.39
50.41
4.0 ± 0.3
−0.4 ± 0.2
0.5 ± 0.1
2.4 ± 0.2
−0.3 ± 0.2
1.2 ± 0.1
1.2 ± 0.1
45 26 N
45 39 N
45 05 N
45 18 N
43 30 N
43 30 N
42 40 N
31 13 N
12 20 E
13 45 E
13 38 E
14 32 E
16 23 E
16 26 E
18 04 E
29 55 E
1909–2000
1905–2006
1955–2004
1930–2004
1952–2004
1954–2004
1956–2004
1944–1989
91
99
47
72
50
48
46
45
5.89
6.00
1.39
15.07
2.94
0.34
0.89
4.35
101.16
66.09
49.76
68.21
48.73
47.06
43.08
40.08
2.5 ± 0.1
1.2 ± 0.1
0.6 ± 0.2
1.1 ± 0.1
0.7 ± 0.2
0.5 ± 0.2
0.9 ± 0.2
1.9 ± 0.2
Cadiz
Tarifa
Ceuta
Malaga
Alicante
Marseille
Genova
Venice
Trieste
Rovinj
Bakar
Split I
Split II
Dubrovnik
Alexandria
W. Med.
Adriatic
E. Med.
Period
Number years Per cent gaps Variance (cm2 ) Trend (mm yr−1 )
Station Name
Note: The percentages of data gaps, the variances and the trends are computed on the basis of monthly values. Errors in sea level trends correspond to
standard errors.
give early indications on whether they can be suitable for long-term
monitoring of sea level.
Shorter records can be contaminated by decadal signals arising
from a variety of forcing factors (Tsimplis & Spencer 1997). Thus
an effort was made to identify shorter records which can be considered as reliable. This is done by comparing the short records with
a nearby long-term reference record using a two-step process. The
first step consists of calculating the correlation between the shorter
records and the chosen reference station for each area, over the
period where both tide gauges were operating and after the trends
have been removed (Table 2). The common period between each
short series and the corresponding reference station is used to compute the covariance matrix from which the correlation coefficient
is obtained. Statistically significant correlation indicates that these
records confirm the signals observed on the reference stations and,
in addition, can be used for filling in gaps in the reference stations.
Thus they provide some redundancy in the monitoring network.
The second step, which is only performed where the correlation
is statistically significant, involves the comparison of the trends of
the differences between the short station and the reference station
over the common period. Where the trend of their difference is
not statistically significant then we can conclude that the shorter
stations can be expected to provide, in the future, additional good
long-term monitoring points for the Mediterranean Sea. In addition
the confidence to the longer stations improves.
The comparison between reference stations and shorter records is
a very reliable quality check. Thus the discussion of the suitability of
short records is included in the Appendix although the identification
of the consistent short records and the rejection of the others is also
a result of this work.
The inconsistency amongst the shorter records, and by comparison with the longer records, prohibits their further use. Thus the
shorter records will not be used in the following discussion, except
where needed to confirm or question parts of the longer records.
Our assessment of which of the short-terms tide gauges can be
considered reliable can be found in Table 2.
2.2 Methodology
Sea level trends are estimated by a linear robust fit and their uncertainties are defined as the standard errors. This alternative to a
simple linear regression minimizes a weighted sum of squares, but
the weight given to each data point depends on how far the point is
from the fitted line by using a bisquare weighting function, resulting
in a smaller sensitivity to outliers. The variance-covariance matrix
of the coefficient estimates is computed as V = inv(X∗ X)∗ σ 2 where
X is the vector of times and σ is the root mean square error between
the prediction and the data. The standard errors are derived from
the matrix V. Prior to the trend computation the monthly time-series
have been deseasonalized. To do this, the mean monthly values for
each calendar month have been computed to obtain the mean seasonal cycle, and this cycle has been subtracted from the monthly
time-series. Only complete years were used to compute the mean
seasonal cycle in order to avoid biases.
Acceleration is estimated by a second order least-square fit regression on time as the coefficient of the quadratic term. Sea level
accelerations during the 20th century have been computed for the
five longest tide gauges. Again uncertainties correspond to the standard errors.
For the longest tide gauge records yearly time-series have been
produced for the period 1960–2000 (Table 2). Lagos and Alexandria
have been discarded for this period because of lack of data during
the 1990s decade, that is, they cover less than 75 per cent of the
period 1960–2000. A yearly value was assigned only if at least 11
months of the year were available; otherwise the value is taken as a
gap. In order to compute variances and correlations the time-series
have been previously detrended. Correlations presented are always
significant to the 99 per cent confidence level.
2.3 Atmospherically induced sea level variations
The meteorological contribution to sea level has been quantified
using the output of a barotropic oceanographic model. In the
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Journal compilation Coastal sea level trends in Southern Europe
73
Table 2. From left- to right-hand columns: tide gauge stations with shorter records, the area where they are located, their position (in latitude and longitude),
period of operation, percentage of data gaps, reference station for computation of correlations, common period with the reference station, correlations and
trends of the difference between the short series and the reference station.
Station name
Area
Latitude
Longitude
(◦ , Minutes) (◦ , Minutes)
Period
Per cent gaps Reference st. Common period Correlation Relative trend Quality
Bilbao
Santander 3
Gijon 2
Coruña 3
Villagarcia
Vigo 2
Setroia
Atlantic
43 20 N
43 28 N
43 34 N
43 22 N
42 36 N
42 14 N
38 30 N
03 02 W
03 48 W
05 42 W
08 24 W
8 46 W
08 44 W
08 54 W
1992–2005
1992–2005
1995–2005
1992–2005
1997–2005
1997–2005
1976–1996
10.71
3.57
9.85
8.93
7.41
4.63
8.73
Cascais
1992–2005
1992–2005
1995–2005
1992–2005
1997–2005
1997–2005
1976–1996
0.49
0.54
0.49
0.66
0.57
0.58
0.91
−0.7 ± 1.2
−0.9 ± 1.1
4.1 ± 1.9
−1.9 ± 1.3
3.0 ± 2.9
−2.4 ± 2.7
−0.4 ± 0.5
G
G
Q
Q
Q
G
G
Huelva
Bonanza
Algeciras
Ceuta 4
Malaga 2
Strait
37 8 N
36 38 N
36 07 N
35 54 N
36 43 N
6 50 W
06 20 W
05 26 W
05 19 W
04 25 W
1996–2005
1992–2005
1991–2001
1991–2001
1992–2005
10.00
8.33
0.00
5.30
5.95
Ceuta
1996–2002
1992–2002
1991–2001
1991–2001
1992–2002
0.68
0.79
0.80
0.95
0.83
3.3 ± 3.7
8.9 ± 1.6
5.1 ± 1.2
−0.3 ± 1.2
5.9 ± 1.3
G
G
G
G
G
Almerı́a
Cartagena
Valencia
Barcelona
L’Estartit
Sete
Nice
Valleta
W. Med.
36 50 N
37 36 N
39 20 N
41 21 N
42 03 N
43 24 N
43 42 N
35 54 N
02 29 W
00 58 W
00 20 W
02 10 E
03 12 E
03 42 E
07 16 E
14 31 E
1977–1997
1977–1987
1992–2005
1992–2005
1990–2001
1996–2004
1978–2004
1988–2004
10.71
15.91
15.48
9.52
0.00
1.85
13.27
14.71
Marseille
1977–1997
1977–1987
1992–2004
1992–2004
1990–2001
1996–2004
1978–2004
1988–2004
0.61
0.66
0.57
0.77
0.87
0.86
0.71
0.61
−0.9 ± 0.6
1.2 ± 1.4
9.3 ± 1.6
1.7 ± 1.5
−0.6 ± 1.7
−0.3 ± 2.7
0.2 ± 0.4
−1.9 ± 0.9
G
G
Q
G
G
G
G
G
Koper
Luka Koper
Zadar
Sucuraj
Bar
Adriatic
45 34 N
45 34 N
44 07 N
43 08 N
42 05 N
13 45 E
13 45 E
15 14 E
17 12 E
19 05 E
1962–1991
1992–2003
1994–2004
1987–2004
1964–1991
4.44
8.33
5.30
1.39
5.06
Trieste
1962–1991
1992–2003
1994–2004
1987–2004
1964–1991
0.93
0.79
0.85
0.85
0.85
−0.1 ± 0.4
−8.9 ± 1.8
−0.9 ± 1.8
1.8 ± 0.9
1.5 ± 0.5
G
Q
G
U
U
Preveza
E. Med.
Levkas
Posidhonia
Patrai
Katakolon
Kalamai
North Salaminos
Piraievs
Khalkis South
Khalkis North
Skopelo
Thessaloniki
Kavalla
Alexandropoulis
Khios
Siros
Leros
Soudhas
Rodhos
Izmir
Hadera
Antalya
38 57 N
38 50 N
37 57 N
38 14 N
37 38 N
37 01 N
37 57 N
37 56 N
38 28 N
38 28 N
39 07 N
40 37 N
40 55 N
40 51 N
38 23 N
37 26 N
37 05 N
35 30 N
36 26 N
38 26 N
32 28 N
36 50 N
20 46 E
20 42 E
22 57 E
21 44 E
21 19 E
22 08 E
23 30 E
23 37 E
23 36 E
23 36 E
23 44 E
23 02 E
24 25 E
25 53 E
26 09 E
24 55 E
26 53 E
24 03 E
28 14 E
26 43 E
34 53 E
30 36 E
1969–2006
1969–2006
1969–2006
1969–2006
1969–2006
1969–2001
1984–2000
1969–2002
1977–2001
1969–2006
1999–2006
1969–2001
1969–2006
1969–2006
1969–2006
1969–2006
1969–2006
1969–2001
1969–2006
1995–2004
1992–2005
1985–2002
28.07
16.67
36.40
14.04
11.62
25.25
11.27
23.77
22.33
10.96
19.79
12.12
18.20
13.38
12.94
45.61
22.59
5.56
29.17
21.67
10.71
18.52
Antalya
1985–2002
1985–2002
1985–2002
1985–2002
1985–2002
1985–2001
1985–2000
1985–2002
1985–2001
1985–2002
1999–2002
1985–2001
1985–2002
1985–2002
1985–2002
1985–2002
1985–2002
1985–2001
1985–2002
1995–2002
1992–2002
–
0.57
0.80
0.53
0.74
0.66
0.69
0.72
NS
0.54
0.63
0.52
0.74
0.26
0.46
0.81
0.36
0.49
0.51
0.54
0.36
0.62
–
−4.0 ± 1.1
−2.4 ± 0.8
−29.0 ± 2.2
4.0 ± 0.8
−4.2 ± 0.8
0.6 ± 1.0
−3.6 ± 0.9
–
−7.7 ± 1.0
−6.1 ± 0.8
8.2 ± 0.9
0.6 ± 0.9
−10.5 ± 2.0
−3.0 ± 0.9
−0.1 ± 0.9
−3.8 ± 4.9
−7.5 ± 0.7
−6.6 ± 0.8
−4.4 ± 1.1
−3.5 ± 3.0
−0.9 ± 1.4
–
Q
Q
Q
Q
Q
G
Q
Q
Q
Q
Q
G
Q
Q
G
U
Q
Q
Q
Q
G
–
Notes: Uncertainties correspond to standard errors. Trends significantly different from zero are highlighted in boldface. NS means not significant. Last
column is a quality flag of the record: G, good; Q, questionable; U, unknown.
framework of the Hindcast of Dynamic Processes of the Ocean
and Coastal Areas of Europe (HIPOCAS) project (Guedes Soares
et al. 2002), atmospheric pressure and wind fields were produced by
a dynamical downscaling of the reanalysis of NCEP/NCAR for the
period 1958–2001 (Garcı́a-Sotillo et al. 2005). These fields were
used to force a barotropic version of the Hamburg Shelf Circulation Model (HAMSOM) model covering the Mediterranean Sea
and the Eastern Atlantic coast, with a spatial resolution of 1/4◦ ×
1/6◦ in latitude and longitude, respectively. The comparison between the HIPOCAS sea level hourly output and the tidal residuals
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Journal compilation at coastal sites is very good with correlations between 0.8 and
0.9 (Marcos et al. 2005). Atmospherically induced sea level trends
derived from HIPOCAS data set have already been computed in
previous works (Tsimplis et al. 2005; Gomis et al. 2008). The comparison of HIPOCAS with another 2-D barotropic model in the
region did not reveal any artificial drift of the model (Pascual et al.
2008).
HIPOCAS monthly values have been used to correct sea level
observations for the atmospheric contribution. For each tide gauge
station, data from the closest point of the HIPOCAS grid has been
74
M. Marcos and M. N. Tsimplis
subtracted from the observations for the period covered by
HIPOCAS data (1958–2001).
the shorter records would make them consistent with the long
records.
2.4 Steric sea level
2.5 Postglacial rebound
Steric sea level variability has been estimated from temperature and
salinity climatologies by integration from the surface to 300 m of
the specific volume anomaly caused by density changes. The steric
component of each tide gauge has been computed at the closest gridpoint. In the Atlantic stations the Ishii climatology was used (Ishii
et al. 2003). This consists of 1◦ × 1◦ gridded temperature anomalies
given as yearly means for the upper 700 m from 1945 to 2005 (Ishii
et al. 2003). This database was preferred instead of the alternative
of Levitus et al. (2000) simply because it spans a longer time period.
Differences in the steric sea level from the various databases in the
Mediterranean are in general small at basin scales both at annual
and interannual scales (Fenoglio-Marc, personal communication,
2007). In the Mediterranean the MEDAR database was preferred
(Rixen et al. 2005), consisting of 1-yr temperature and salinity fields
from 1945 to 2002 with a spatial resolution of 0.2◦ .
It is still a subject of discussion how the steric signals propagate
between the open ocean and coastal sites. Previous works assume
that tide gauge records are representative of the steric changes in
a basin wide area (Miller & Douglas 2004) while others have used
close gridpoints to account for local steric sea level variations at the
coast (Plag 2006; Marcos & Tsimplis 2007). Here we have corrected
for the steric effects only those tide gauges whose atmospherically
corrected record is significantly correlated with the steric signal
corresponding to the closest gridpoint deeper than 300 m. We have
not used the simply closest gridpoint to avoid very local effects (e.g.
river runoff) affecting our estimation of steric signal.
It was hoped at the beginning of the study that the removal of
the direct atmospheric forcing and the steric contribution from
The PGR effects have been corrected by means of the Glacial
Isostatic Adjustment model ICE-5G (VM2) (http://www.pol.
ac.uk/psmsl/peltier/index.html, Peltier 2004). The model does not
provide uncertainties for vertical land movements associated to this
effect, so the error bars for this correction have been neglected. In
the Mediterranean the effect of the isostatic compensation due to
PGR is of the order of ±0.3 mm yr−1 (Zerbini et al. 1996).
3 R E S U LT S
In the first part of this section we discuss sea level trends as directly
derived from the observations and for the longest period for each
tide gauge. Then we discuss trends for the common period 1960–
2000 and the contribution of the atmospheric and steric forcing to
sea level trends. The next section discusses accelerations of sea
level for the longest records. Finally, the last part of this section
discusses the change in the decadal trends and helps us link the
long-term trends with those dominating the last four decades of the
last century.
3.1 Sea level trends
The observed sea level trends and the variance of the 21 longest
records are shown in Table 1, together with their location, period of
operation and percentage of data gaps. Yearly time-series for these
records are plotted in Figs 2(a)–(c), where nearby stations have been
grouped into three different areas: Atlantic, Western Mediterranean
Figure 2. Yearly observations and linear sea level trends for the longest sea level records in Southern Europe. (a) Atlantic area and Gibraltar, (b) remaining
stations in Gibraltar and Western Mediterranean and (c) Adriatic and Eastern Mediterranean.
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Table 3. From left- to right-hand columns: Name of the stations, values of variance of yearly observations, percentage of variance explained by the atmospheric
contribution, observed trends, atmospherically induced trends, trends of atmospherically corrected series (observation minus atmospheric contribution) and
steric sea level trends for the period 1960–2000; residual (observations minus atmospheric and steric contributions) trends.
Variance (cm2 )
Per cent variance atm.
Observed trend
Atm. trend
Atm. residual
Steric trend
Residual
Residual + PGR
S.J. de Luz
Santander
Coruña
Vigo
Cascais
8.84
13.45
12.42
19.00
7.64
19.73
20.92
31.18
25.41
30.66
1.7 ± 0.7
1.6 ± 0.5
1.9 ± 0.5
1.7 ± 0.6
0.0 ± 0.4
−0.7 ± 0.2
−0.4 ± 0.1
−0.3 ± 0.2
−0.2 ± 0.2
−0.5 ± 0.1
2.1 ± 0.6
2.1 ± 0.5
2.2 ± 0.4
2.0 ± 0.5
0.6 ± 0.3
−0.1 ± 0.3
−0.3 ± 0.3
−0.4 ± 0.3
−0.4 ± 0.3
−0.7 ± 0.5
2.7 ± 0.6
2.4 ± 0.6
2.5 ± 0.5
2.5 ± 0.5
1.2 ± 0.7
2.6 ± 0.6
2.4 ± 0.6
2.6 ± 0.5
2.6 ± 0.5
1.2 ± 0.7
Cadiz
Tarifa
Ceuta
Malaga
19.78
13.45
8.09
21.28
14.82
−11.73
9.81
14.17
4.5 ± 0.7
−1.3 ± 0.5
0.6 ± 0.4
4.1 ± 0.7
−0.6 ± 0.1
−0.6 ± 0.1
−0.6 ± 0.1
−0.6 ± 0.1
5.0 ± 0.6
−0.8 ± 0.5
1.0 ± 0.4
4.5 ± 0.6
−1.3 ± 0.6
−2.0 ± 0.6
−2.1 ± 0.6
−1.8 ± 0.6
7.1 ± 0.8
0.9 ± 0.7
3.0 ± 0.8
6.9 ± 0.8
7.0 ± 0.8
1.0 ± 0.7
3.2 ± 0.8
7.1 ± 0.8
Alicante
Marseille
Genova
2.14
6.94
6.81
36.53
40.51
46.76
−0.7 ± 0.3
−0.0 ± 0.4
−0.3 ± 0.4
−0.7 ± 0.1
−0.7 ± 0.2
−0.9 ± 0.2
0.0 ± 0.2
0.7 ± 0.2
0.6 ± 0.2
−0.7 ± 0.3
−0.1 ± 0.4
−1.3 ± 0.3
1.0 ± 0.4
1.1 ± 0.4
2.0 ± 0.5
1.0 ± 0.4
1.1 ± 0.4
2.1 ± 0.5
Venice
Trieste
Rovinj
Bakar
Split I
Split II
Dubrovnik
14.92
9.00
8.64
13.17
10.31
9.33
8.50
29.34
48.56
44.89
44.57
42.69
45.72
42.82
0.3 ± 0.4
0.3 ± 0.4
−0.2 ± 0.4
0.2 ± 0.5
−0.2 ± 0.5
−0.4 ± 0.4
0.2 ± 0.4
−0.9 ± 0.2
−0.8 ± 0.2
−0.8 ± 0.2
−0.8 ± 0.2
−0.9 ± 0.2
−0.9 ± 0.2
−0.9 ± 0.2
1.2 ± 0.3
0.9 ± 0.3
0.7 ± 0.3
0.9 ± 0.4
0.6 ± 0.3
0.4 ± 0.3
0.9 ± 0.3
−1.6 ± 0.3
−1.6 ± 0.3
−1.6 ± 0.3
−1.6 ± 0.3
−1.6 ± 0.3
−1.6 ± 0.3
−1.7 ± 0.3
2.6 ± 0.4
2.4 ± 0.3
2.2 ± 0.3
2.4 ± 0.4
2.0 ± 0.4
2.0 ± 0.4
2.5 ± 0.4
2.8 ± 0.4
2.6 ± 0.3
2.3 ± 0.3
2.6 ± 0.4
2.1 ± 0.4
2.2 ± 0.4
2.6 ± 0.4
Station name
Notes: Those stations where the steric and the atmospherically corrected series are significantly correlated at 99 per cent confidence level are highlighted in
boldface; residual-PGR trends are the previous value corrected for PGR. Steric trends are calculated using MEDAR database for the Mediterranean stations
and Ishii for the Atlantic ones. Trends are in mm yr−1 and uncertainties correspond to standard errors.
and Adriatic–Eastern Mediterranean. The corresponding linear
trends are also plotted for each time-series (Fig. 2).
3.1.1 The Atlantic Iberian coasts
On the Atlantic coasts the two longest stations, Cascais and Lagos
(Fig. 1), cover almost one century of data and display sea level
trends of 1.3 ± 0.1 and 1.5 ± 0.1 mm yr−1 . The rest of the records,
which have up to 60 yr of data covering the more recent decades,
have linear trends ranging from 2.0 ± 0.2 mm yr−1 in Santander to
2.5 ± 0.2 mm yr−1 in Vigo, with the exception of Coruña which
shows a trend of 1.4 ± 0.2 mm yr−1 . Marcos et al. (2005) detected
a reference jump in the Coruña tide gauge in 1963 which artificially
decreased the observed trend by up to 0.9 mm yr−1 for the period
1943–2001. By correcting the value at this station the trend becomes
2.4 mm yr−1 for the period 1943–2001, which is consistent with
nearby tide gauges.
For the period 1960–2000, the observed, atmospheric and steric
sea level trends are compared in Table 3. Note that Lagos and
Alexandria are not listed because they have significant gaps over
this period. Yearly variances and the percentage explained by the
atmospheric contribution are also listed. For 1960–2000 (Table
3) sea level trends in the Atlantic vary between 1.6 ± 0.5 and
1.9 ± 0.5 mm yr−1 in the Bay of Biscay and the Northern Spanish coast, and 0.0 ± 0.4 mm yr−1 in Cascais, mainly caused by a
continuous sea level drop since 1980 at this station. The variance
of the observed yearly series range between 7 and 19 cm2 , being
larger at the Northern Spanish coast. The meteorological contribution to the sea level variability explains between 20 and 30 per cent
of the yearly variance, with atmospherically induced trends varying
between −0.2 ± 0.2 mm yr−1 in Vigo and −0.7 ± 0.2 mm yr−1
in Sant Jean de Luz. The atmospherically corrected series, that is,
the observations minus the meteorological contribution, have very
consistent trends in the Northern Spanish coast ranging between
2.0 ± 0.5 and 2.2 ± 0.4 mm yr−1 , again with the exception of
Cascais that presents a lower value of 0.6 ± 0.3 mm yr−1 .
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Journal compilation The steric contribution to sea level trends for the period 1960–
2000 is also negative in the Atlantic sector (Table 3). The steric
trends decrease (increase in magnitude) southwards, varying from
−0.1 ± 0.3 mm yr−1 in Sant Jean de Luz to −0.7 ± 0.5 mm yr−1 in
Cascais. Only in Sant Jean de Luz the steric signal is significantly
correlated with the atmospherically corrected time-series with a
correlation larger than 0.3. The residual trend at this station is
2.6 ± 0.6 mm yr−1 . At the other stations the residual trends after
the steric sea level corrections are around 2.6 ± 0.6 mm yr−1 with a
smaller value for Cascais of 1.2 ± 0.7 mm yr−1 for 1960 onwards.
Thus for this area the atmospherically corrected trends are 2.1 ±
0.5 mm yr−1 with the steric correction slightly increasing this value
to around 2.6 ± 0.6 mm yr−1 .
3.1.2 The Strait of Gibraltar
Close to the Strait of Gibraltar the tide gauges at Cádiz, Tarifa,
Málaga and Ceuta, span between 40 and 60 yr (Table 1). Each
tide gauge shows significantly different trend (Fig. 2) respectively
4.0 ± 0.3 mm yr−1 in Cádiz, −0.4 ± 0.2 mm yr−1 in Tarifa, 2.4 ±
0.2 mm yr−1 in Málaga and 0.6 ± 0.1 mm yr−1 in Ceuta. The
lack of coherence in the trends cannot be explained by the different
periods of operation as Fig. 2 indicates. In Cádiz, the trend is continuously present during the whole period, while in Málaga the trend
appears after 1980. Data gaps at both stations are only 20 per cent
thus the discrepancy cannot be attributed to missing observations
either. Problems with the Cádiz tide gauge were reported in Ross
et al. (2000), who corrected for what they considered a spurious
downward trend of 14 mm yr−1 in 1988–1993.
The discrepancies do not disappear or reduce when the common
period 1960–2000 is considered (Table 3), with trends ranging from
−1.3 ± 0.5 mm yr−1 in Tarifa to 4.5 ± 0.7 mm yr−1 in Cádiz. The
atmospherically induced trends for this period in the area are −0.6 ±
0.1 mm yr−1 for all the tide gauges and the steric trends are between
−2.1 ± 0.6 and –1.3 ± 0.6 mm yr. When the atmospheric correction
76
M. Marcos and M. N. Tsimplis
is applied the sea level trends show the same discrepancies ranging
between −0.8 ± 0.5 and 5.0 ± 0.6 mm yr−1 . For Tarifa tide gauge
the steric signal is correlated with the atmospherically corrected
sea level (with a correlation larger than 0.3 again). When the steric
correction is applied together with PGR the residual trend is 1.0 ±
0.7 mm yr−1 . In summary, no regional trend can be defined for this
region due to the large scatter of the results from the observations.
In the quality control section we identified the contrast between
Ceuta and the short tide-gauges located at the European side of
the Strait of Gibraltar. We note that the same differences occur in
respect of the Malaga and Cadiz and only Tarifa appears to have a
trend smaller than Ceuta.
The anomalously large trend in Cádiz has been checked against
the two closest stations Huelva and Bonanza, also located in the
Atlantic side of the Strait and thus expected to be subjected to
the same forcing. The linear trend of the differences between each
of these series and the Cadiz sea level, are 20 and 12 mm yr−1
for their common periods 1996–2001 and 1992–2001, respectively.
This indicates that the observed trend in Cádiz is due to either a
large local subsidence or a malfunctioning of the instrument, but in
any case cannot be considered as representative of this area.
The tide gauge in Málaga has been compared with the shorter
series in Málaga2, Algeciras and Almerı́a for their common periods. The correlation of Málaga with Malaga2 and Algeciras is
0.9 and lower (0.6) with Almeria. The trends of the difference of
these stations with Malaga are −6.5 ± 1.6 mm yr−1 for Málaga2,
−7.7 ± 1.2 mm yr−1 for Algeciras and −6.7 ± 0.6 mm yr−1 for
Almerı́a, indicating that the long tide gauge trend in Málaga is spurious. Thus, one must utilise the two remaining long tide gauges of
Tarifa and Ceuta which are not, as noted, consistent in their trends.
Thus we cannot provide a sea level trend estimate for this area. This
is problematic because the Strait of Gibraltar is a crucial area for
sea level observation for the whole of the Mediterranean Sea. On
one hand the Atlantic inflow has been found to be significantly correlated with the across Strait component (Tsimplis & Bryden 2000)
and, on the other hand the along and across Strait slopes are considered to be relevant in determining the maximal or sub-maximal
character of the exchange. Knowledge of the slopes can be an efficient way of monitoring the Mediterranean Sea. However, accurate
sea level measurements are essential for the correct estimation of
these slopes.
3.1.3 The Western Mediterranean
The longest series in the Western Mediterranean are Alicante,
Marseille and Genova, the two last ones starting by the end of
the 19th century (Table 1). The variance in Marseille and Genova
is very similar (between 50 and 55 cm2 ) and they show the same
trend for their entire period of 1.2 ± 0.1 mm yr−1 . Alicante shows
less variability with a variance of 24 cm2 and a sea level trend of
−0.3 ± 0.2 mm yr−1 .
For the period 1960–2000 the observed trends become coherent
among the three series with values between 0.0 ± 0.4 and −0.7 ±
0.3 mm yr−1 (Table 3). The atmospheric contribution with trends of
−0.7 ± 0.2 and −0.9 ± 0.2 mm yr−1 accounts for 36–46 per cent
of the total yearly variance of the series. In general, the variability
explained by the atmospheric forcing in the Mediterranean is larger
than in the Atlantic (Table 3). By applying the atmospheric correction the sea level trends become positive and more coherent in the
area with values between 0.0 ± 0.2 mm yr−1 in Alicante and 0.6 ±
0.2 and 0.7 ± 0.2 mm yr−1 in Genova and Marseille respectively.
Thus we consider the atmospherically corrected trends of this area
to be slightly positive with zero values in the western part of the
basin.
Steric trends range between −1.3 ± 0.3 mm yr−1 in Genova,
−0.1 ± 0.4 mm yr−1 in Marseille and –0.7 ± 0.3 mm yr−1 in Alicante. Only the steric signal corresponding to Marseille tide gauge is
correlated with the meteorological residual time-series. The timeseries in Marseille corrected for the atmospheric effects and for
both the atmospheric and steric effects are plotted in Fig. 3 (upper
plot) for the period 1960–2000. The sea level trend corrected for
the atmospheric, steric and PGR effects in this station is 1.1 ± 0.4
mm yr−1 . The discrepancy between the steric signal in Marseille
and Genova is problematic. These stations are highly correlated
and the atmospherically corrected trends are consistent. The addition of the steric signal would make them inconsistent by about
0.8 mm yr−1 , with Genova showing higher trends of about 2.0 ±
0.5 mm yr−1 . This value appears consistent with the values at the
Atlantic coasts. However, it contradicts the values in Marseille and
Alicante which are significantly lower. In addition, only Marseille
appears to be correlated with the steric sea level signal. Thus we
consider that the lower trends values in Marseille are representative
of the area in respect of the steric correction.
3.1.4 The Adriatic Sea
The Adriatic Sea is a region where seven stations have been operating continuously during the last 50 yr, including two, Venice and
Trieste, covering more than one century (Table 1). All the available
long-term tide gauge records are of good quality with low percentages of data gaps, between 1 and 6 per cent, except for Bakar
which, although of good quality, has about 15 per cent. Interannual
oscillations appear to be very coherent among all the time-series
(Fig. 2c) and the same holds for observed trends, which vary between 0.5 ± 0.2 and 1.2 ± 0.1 mm yr−1 . The only exception is
Venice with a trend of 2.5 ± 0.1 mm yr−1 during the 20th century due to local subsidence caused by water extraction which
was reduced after 1960. As a result this tide gauge is unsuitable
for regional or global trend studies (Woodworth 2003). Monthly
variability is also very similar amongst stations (40–70 cm2 ) with
the exception again of Venice, which presents larger variance
(Table 1).
For the period 1960–2000 sea level trends are consistent between
the stations with values between −0.4 ± 0.4 and 0.3 ± 0.4 mm
yr−1 (Table 3). Both the atmospheric and steric contributions to sea
level trends are negative in the area. The corrections applied to Trieste time-series are plotted in Fig. 3 (lower plot). The atmospheric
trends are −0.8 ± 0.2 mm yr−1 , while the steric trends vary between
−1.6 ± 0.3 and −1.7 ± 0.3 mm yr−1 (Table 3). The atmospheric
contribution accounts for almost 50 per cent of the total yearly
sea level variability in the Adriatic, except in Venice where only
30 per cent is explained by meteorological effects. However, this
could be partly explained by the influence of local wind conditions
on sea level which are not reproduced in most atmospheric reanalyses accurately (Wakelin et al. 2000) as well as the location of Venice
tide gauge inside a lagoon.
The atmospherically corrected series have trends ranging between 0.4 ± 0.3 and 0.9 ± 0.3 mm yr−1 , with the exception of
Venice. In three of the tide gauges located in the Northern Adriatic,
Trieste, Rovinj and Bakar, the atmospherically corrected series are
correlated with the steric signal. It is worth mentioning that due
to the shallowness of this basin, the closest point to these stations
deeper than 300 m is located in the southern part of the Adriatic
Sea. However, if the closest gridpoint to each tide gauge is used for
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Figure 3. Tide gauge observations (black lines), atmospherically corrected records (dark grey lines) and atmospherically corrected records minus the steric
correction (light grey lines) in Marseille (upper plot) and Trieste (lower plot).
the steric correction the trends remain negative, although of smaller
magnitude in the northern area. The residual trends, further corrected by PGR, are 2.6 ± 0.3, 2.3 ± 0.3 and 2.6 ± 0.4 mm yr−1
in Trieste, Rovinj and Bakar, respectively. These are considered as
representative of the area. However, a significant part of these trends
of about 1.6 ± 0.3 mm yr−1 is due to steric variations. The residual
atmospheric trends are consistently around 0.8 ± 0.2 mm yr−1 . Notably, in this area the direct atmospheric forcing contributes almost
–1 mm yr−1 of sea level reduction between 1960 and 2000.
3.1.5 The Eastern Mediterranean
There is only one long tide gauge station in the Eastern Mediterranean located in Alexandria. As mentioned above, this tide gauge
is not RLR and the data stops at 1989, but has been considered
because of the lack of long-term good quality records in this region
(Table 1). During the period of operation Alexandria has few data
gaps (4 per cent) and its monthly variability (40 cm2 ) is consistent with other observations in the Adriatic. The observed sea level
trend of 1.9 ± 0.2 mm yr−1 is higher than those in the rest of the
basin. The value would be expected to be even higher if the 1990s
were covered, since the enhanced sea level rising was higher in the
Eastern Mediterranean than in the rest of the basin. For the period
1960–1989 the fraction of variance explained by the atmospheric
contribution is only 15 per cent, much smaller then than in the rest
of the basin and comparable to the Atlantic stations. The conclusion
is that despite the large number of tide gauges in operation in the
Eastern Mediterranean there are, yet, no reliable stations that can
be used for the estimation of sea level trends.
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Journal compilation 3.2 Sea level accelerations
For the five longest tide gauges, Cascais, Marseille, Genova, Venice
and Trieste, the sea level accelerations during the 20th century
are found to be negative: −0.3 ± 0.2, −0.9 ± 0.2, −0.3 ± 0.3,
−1.5 ± 0.4 and −0.8 ± 0.3 mm yr−1 century−1 , respectively
(Fig. 4). However, we noted in the Section 3.1 that Cascais presents
a downward trend from the 1980s, the period of data added to the
PSMSL original data. If the acceleration in Cascais is computed
until 1980, then a value 0.50 ± 0.36 mm yr−1 century−1 is found.
The established view is that sea level has risen globally during the
1990s at rates higher than before (IPCC 2007). As we will see below
this is also true for the Mediterranean Sea and the Atlantic stations.
Thus, we consider that the Cascais values after 1980 indicate either
a problem with the tide-gauge or a real local problem of land uplift.
Negative values in the Mediterranean are consistent with the wellknown behaviour of the sea level rise pattern in this area (Tsimplis
& Baker 2000) during the last century: from the beginning of the
20th century and up to 1960 sea level was rising between 1.2 and
1.5 mm yr−1 (Tsimplis & Baker 2000). At this time it started decreasing mainly due to an increase in atmospheric pressure, up to
1990, at rates of −1.3 mm yr−1 (Tsimplis & Baker 2000). This period is highlighted in Fig. 4 at the Marseille time-series. During the
1990s a fast sea level rise has been observed in the Mediterranean,
as can also be seen in these five tide gauges, which was stopped after 1999 (Fenoglio-Marc 2001). It is worth noting the significantly
different values of accelerations in Genova and Marseille in spite
of their proximity, and the similarity between Marseille and Trieste.
The largest deceleration value corresponds to Venice which is subjected to local subsidence and therefore not representative of the
region.
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M. Marcos and M. N. Tsimplis
Figure 4. Monthly sea level values for the five longest tide gauges and the fitted quadratic curves. The dashed square in Marseille indicates the period of sea
level drop identified from 1960 to 1990.
We do not have sufficient atmospheric data to run hindcast models
for the whole of the last century. A good proxy to the atmospherically driven part of the mean sea level variability is the North
Atlantic Oscillation index (Tsimplis & Josey 2001; Gomis et al.
2006). Thus, we check whether the negative accelerations are due
to changes in the meteorological forcing caused by the NAO. When
the effect of NAO is removed from these records by subtracting
the linear regression between winter NAO and the time-series, the
negative accelerations decrease (in absolute values) in the Mediterranean and become −0.4 ± 0.3, −0.1 ± 0.3, −1.1 ± 0.6 and
−0.1 ± 0.5 mm yr−1 century−1 in Marseille, Genova, Venice and
Trieste, respectively, while in Cascais it remains unaltered. As a
conclusion we note that for the period for which data are available we do not detect acceleration of sea level in the Mediterranean Sea or at the Atlantic coasts when the NAO contribution is
removed.
3.3 Decadal variability of the time-series
We compute the decadal rates of sea level change for the 21 long
tide gauges as linear trends for 10-yr segments of the records overlapping year to year and compare the results with the global rates
(Fig. 5). There are several studies estimating global sea level values (Holgate & Woodworth 2004; Church & White 2006; Jevrejeva
et al. 2006; Holgate 2007). Here we compare the global decadal rates
derived from the nine longest tide gauges in the world corrected for
the inverted barometer effect (Holgate 2007) with those in Southern
Europe (Figs 5a and b). Holgate (2007) used the HadSLP2 air pressure data set (Allan & Ansel 2006) for correcting the tide gauge data
for the inverted barometer effect. Although this assumption is not
totally valid for semi-enclosed seas such as the Mediterranean, here
we have used the same dataset to correct our tide gauge records for
consistency. Both in the Atlantic sites and in the Mediterranean interannual signals with periods of 10–15 yr are evident. The average
decadal rate in the Atlantic Iberian stations (Fig. 5a) does not always
match the global values and it is in general larger, except for the
period from mid-1970s to mid-1980s. During the 1920s and 1950s
the regional rate presents the largest differences with the global
values, although the former is computed with only two stations.
From 1970s onwards the agreement is better. The Mediterranean
mean decadal rate (Fig. 5b) shows good agreement with the global
values except during the 1940s and beginning of 1970s, when much
smaller values are found in the region than globally. Also during
the 1990s the largest decadal rate of sea level change is observed,
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Journal compilation Coastal sea level trends in Southern Europe
79
a
15
10
Trends (mm/yr)
5
0
–5
–10
–15
1900
1910
1920
1930
1940
1950
1960
1970
1980
1990
2000
1960
1970
1980
1990
2000
Year
b
15
10
Trends (mm/yr)
5
0
–5
–10
–15
1900
1910
1920
1930
1940
1950
Year
Figure 5. Decadal rates of observed sea level change, overlapped year to year, for the longest tide gauges separated in the Atlantic sites (a) and Mediterranean
stations (b) (grey shadowed area). Red solid line represents the mean values and the global average derived by Holgate (2007) is also plotted as the blue line.
much larger (by more than 5 mm yr−1 ) than the global values and
only comparable to the observations of the beginning of the 20th
century (although again these are computed with fewer stations).
The differences found in the last decade clearly indicate that global
estimates are not always applicable to the Mediterranean and that
these can be much larger than global values of sea level rise.
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Journal compilation The observed decadal rates of sea level change have been compared with the decadal rates of NAO (not shown). The correlation
found with the average decadal rate in the Atlantic stations was
−0.46 while in the Mediterranean is −0.60 (both significant at the
99 per cent level), indicating that the decadal fluctuations are partly
atmospherically driven.
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M. Marcos and M. N. Tsimplis
4 S U M M A RY A N D C O N C L U S I O N S
Long-term sea level variability and trends have been analysed for
21 coastal records longer than 35 yr in Southern Europe, covering
the Iberian Atlantic region and the Mediterranean Sea. We have
presented updated sea level trends for the area both in respect of
observed sea levels and after correcting them for steric and atmospheric effects. The six longest records spanning more than 90 yr
have sea level trends between 1.2 and 1.5 ± 0.1 mm yr−1 during the
20th century. The only exception corresponds to the tide gauge in
Venice, which is affected by local subsidence (Woodworth 2003).
These values are of the same order of magnitude as those found
by Zerbini et al. (1996) for crustal movement rates. The secular
sea level change in this region is therefore smaller than the global
average of 1.8 ± 0.5 mm yr−1 (IPCC 2007).
Sea level accelerations in the 20th century computed with the
longest records are negative, as a consequence of the particular
behaviour of sea level in the basin, that is, a decrease from 1960
to 1980s. This contrasts with the suggestions about global sea level
accelerations found to be positive (IPCC 2007) albeit for longer
periods. When the NAO contribution is removed from the sea level
records the accelerations become indistinguishable from zero in the
Western Mediterranean and very close to zero, but negative in the
Atlantic coast.
For the period 1960–2000 atmospheric (due to wind and pressure)
correction is available in this region. The atmospherically induced
sea level derived from the HIPOCAS data set is very well correlated (0.8–0.9) with observations and therefore can be used reliably
to correct sea level for the meteorological effects. Observed trends
in the Atlantic sites for the period 1960–2000 are in the range of
1.6 ± 0.5 and 1.9 ± 0.5 mm yr−1 , except for Cascais with 0.0 ±
0.4 mm yr−1 , and in the Mediterranean they are between 0.3 ± 0.4
and −0.7 ± 0.3 mm yr−1 . That is, sea level trends are smaller in
the Mediterranean than in the nearby Atlantic area. This behaviour
has been partly attributed to the effect of the atmospheric pressure, which increased in average for this period in Southern Europe
(Tsimplis & Baker 2000; Tsimplis et al. 2005). Its contribution
to the sea level trends is negative in the entire region, with larger
values (in absolute terms) in the Mediterranean side than in the
Atlantic. Thus, when the atmospheric correction is applied to the
observed records these differences become smaller. Sea level trends
corrected for meteorological effects vary between 2.0 and 2.2 ±
0.4 mm yr−1 in the Northern Spanish coast, between 0.0 ± 0.2 and
0.7 ± 0.2 mm yr−1 in the western basin and between 0.4 ± 0.3 and
0.9 ± 0.3 mm yr−1 in the Adriatic Sea. In terms of interannual variability the atmospheric contribution is responsible of 20–
50 per cent of the total sea level yearly variance, with higher values
in the Mediterranean that in the Atlantic sites.
The steric contribution, obtained from T and S climatologies,
varies amongst regions: maximum values are found at the Strait
of Gibraltar with −2.1 ± 0.6 mm yr−1 in Ceuta and southern
Adriatic with −1.7 ± 0.3 mm yr−1 for Dubrovnik. The overall
steric effect appears to be negative over this period, again linked
with cooler winters and negative temperature trends in the upper waters of particular basins in the winter (Tsimplis & Rixen
2002; Painter & Tsimplis 2003). However, the steric correction
is based on climatological data which in turn are produced by
extensive interpolation of non-systematic observations. Thus significant discrepancies may be introduced by this correction. It is
worth noting that most of the Atlantic and Adriatic sites as well as
Genova agree on a residual trend of around 2.0–2.5 mm yr−1 , after the atmospheric, steric and PGR corrections have been applied.
The way the steric signal propagates between the coast and the
open ocean is a subject under investigation and further research is
needed.
The atmospheric and steric corrections lead to better agreement
between the sea level trends within the Mediterranean Sea on one
hand and those at the coasts of the Iberian Peninsula on the other
hand. It was also expected that the corrections we apply would have
made trends from shorter records consistent with those estimated
for the longer ones and thus provide some confidence in their use.
However, the trends of many of the shorter than 35 yr records, even
after they were corrected, remained inconsistent with the longer
stations.
Only land movements due to ongoing GIA have been considered
in this work. Other sources of land motion have been neglected
and could partly explain the differences observed between some
sites. The approach by Woppelmann et al. (2007) is applied only
to those tide gauges with collocated GPS with time-series long
enough. In the Mediterranean only Marseille and Genova comply
with the requirements and the obtained vertical motion is −0.3 ±
0.2 mm yr−1 , that is, land subsidence in the NW Mediterranean.
This value would decrease the atmospherically corrected sea level
trend in these two stations by 50 per cent. However, it is worth
mentioning that they used GPS time-series shorter than 7 yr in their
computation and this could partly explain the discrepancies with
previous works.
Decadal changes in sea level trends in the Atlantic and the
Mediterranean Sea were not found to be consistent with published
global values, at least for parts of the records. Thus, the enhanced
sea level rise observed globally in the decade of the 1990s (Cazenave
et al. 2001; Holgate & Woodworth 2004) is larger in the Mediterranean Sea by around 5 mm yr−1 , indicating that global values are
not always appropriate to describe long-term changes in sea level in
the Mediterranean and that is can be significantly underestimated.
On the contrary, during the 1940s and 1970s the mean decadal
rate in the Mediterranean is more than 5 mm yr−1 smaller than the
global rate. Thus, we conclude that although the Mediterranean Sea
decadal sea level changes are partly due to global signals, the local
atmospheric and steric variability have dominated both the trends
and the decadal variability.
We also explored the consistency of short sea level records with
the long-term records and identified presently short but apparently
consistent tide gauges from others that are not consistent. The consistent records provide redundancy in cases where the long-term
record is interrupted. The application of atmospheric and steric corrections does not in general improve the agreement between the
short-term records and longer ones. This indicates that other factors
not covered by this study contribute to the observed trends. However, the use of shorter records is advisable for checking spurious
trends and the reliability of measurements of the longer records over
the common period of operation.
It is well known that the spatial distribution of tide gauge
records in the Mediterranean is biased towards the northern coasts
(Tsimplis & Spencer 1997). In addition we have found that most
of the tide gauges in the Eastern Mediterranean Basin (except
the Adriatic) contain spurious and inconsistent signals which
make them useless for future monitoring of sea level changes.
Significant efforts by MedGLOSS (http://medgloss.ocean.org.il/),
ESEAS (http://www.eseas.org/) and the PSMSL promise to improve in the long-term the recovery of data from the north coasts
of Africa. However not much progress appears to have been
made in this respect since Tsimplis and Spencer (1997) was
published.
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Journal compilation Coastal sea level trends in Southern Europe
AC K N OW L E D G M E N T S
The work has taken place when the author M. Marcos was visiting the National Centre for Oceanography, Southampton under a
post-doc fellowship funded by the Spanish Ministry of Education
and Science. This work has been partly supported by the VANIMEDAT project (CTM2005-05694-C03/MAR) funded by the Spanish
Marine Science and Technology Program. Global decadal rates of
sea level change have been kindly provided by Simon Holgate. The
work was partly supported by the CIRCE project. We thank Philip
Woodworth for his comments on this manuscript.
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A P P E N D I X : T H E S H O RT T I D E G AU G E
RECORDS
the Gulf of Lions close to Marseille. Therefore, these should be preferred for filling in data gaps. Also Valletta on the island of Malta,
which is located between the western and eastern basins, has a significant correlation of 0.6 with Marseille. This is in agreement with
Tsimplis et al. (2008), where correlations were computed between
Genova (next to Marseille) and the altimetric records in the entire
basin, and they revealed high values in the Northwestern Mediterranean and the westernmost part of the Levantine basin. The trends
of the difference with the reference station indicate that L’Estartit,
Sete, Nice are stations consistent with the reference station of Marseille. The other stations indicate small but statistically significant
trend differences from the reference station. Our assessment is that
most of them reflect changes in local conditions and are real with
the exception of Valencia where a very large and spurious relative
trend is found.
A.1. The Atlantic coasts
The short records in the Atlantic Iberian coast are mostly located
along the northern Spanish coast, covering periods of 8–13 yr (Table
2). Southwards Setroia has data from 1976. The comparison with
Cascais (Table 2), chosen to be the reference station in this area,
gives correlations for the monthly time-series between 0.5 in Bilbao
and Gijón2 and 0.9 in Setroia, the closest station to Cascais. Given
their high correlation Setroia can be then useful for filling in gaps in
the Cascais record. The trends in respect to the reference station
indicate statistically different from zero trends for Gijon 2 and
Coruna 3. These stations appear problematic and are not expected
to produce long-term trends consistent with the others in the area.
Villagarcia is borderline and possibly also should be considered as
unreliable (Table 2).
A.2. The Strait of Gibraltar
In the Strait of Gibraltar area five tide gauge records span periods of
9–13 yr (Table 2). In this area Ceuta has been chosen as the reference
station for comparison with the short records (Table 2). Correlations
between detrended monthly series are significant and range between
0.9 and 0.7. The highest correlation is found at the Ceuta4 station,
located in the same place as the long series. Lower correlations, but
still very high (0.7, 0.8), correspond to the stations located on the
Atlantic side of the Strait (Huelva and Bonanza). Thus, all these
short time-series in the Strait show a coherent behaviour in terms
of interannual variations. However, with the exception of Ceuta4
located at the same location to the reference Ceuta station, there are
very significant differences between Ceuta and the other stations
in this region in terms of trends. Large differences of about 5–
8 mm yr−1 appear between tide gauges at the European coast and
those at the African coast of the Strait of Gibraltar (Table 2). These
differences are attributed to other reasons rather than unsuitability
of the reference station because the trend value at Ceuta is confirmed
by Ceuta4.
A.3. The Western Mediterranean
Eight short records in the Western Mediterranean cover between
8 and 20 yr (Table 2). The correlations of the short series with
Marseille, the reference station, (Table 2) are between 0.6 and 0.9.
The highest correlation is found in Sete and Nice, both located in
A.4. The Adriatic Sea
The five short time-series in the Adriatic are all very coherent
in terms of interannual variations with correlations with Trieste,
the reference station, between 0.9 and 0.6, decreasing towards the
south as the stations are located further from the reference station
(Table 2). Luka Koper has spurious trends. Sucuraj and Bar have
statistically significant differences in their trends from the reference
station but these could reflect local changes.
A.5. The Eastern Mediterranean
Several short records are located in the Levantine basin, most of
them in the Aegean Sea and covering periods from the late 1960s
(Table 2) and other two in Antalya on the southern Turkish coast and
in Hadera in Israel. Since Antalya covers almost 20 yr of data and
Alexandria stops before the 1990s, the former has been selected
as the reference station in this region for comparison with short
records (Table 2), although it was not initially classified as one of
the long records in the basin.
When the tide gauges are correlated with the reference station
in Antalya the results vary between no significant correlation (in
Piraievs) and 0.8 in Levkas. A visual examination of the tide gauge
records in the Aegean reveals the very different behaviour they
have at intra- and interannual timescales. Therefore, these monthly
records must be considered with caution and discarded for trend
studies. Lascaratos et al. (2005) have shown that averaging of most
Aegean stations gives a time-series which correlates well with Trieste. These tide gauges have also been used successfully for studies
involving hourly data (Garret & Majaess 1984; Lascaratos & Gacic
1990; Tsimplis & Vlahakis 1994). Consequently we believe that
it is mainly a problem with the long-term benchmark maintenance
which makes these time-series spurious. This is confirmed by the
examination of the trends of the difference between each station
and the reference station (Table 2). Kalamai, Thessaloniki, Khios
and Hadera appear consistent with Antalya. All the other trend values are significantly different from that of the reference station.
While seismic and tectonic activity is frequent at this part of the
Mediterranean Sea we do not consider it as an important contributor
to the observed problems at these stations because their intercomparison indicates mean sea level differences changing sign with
time.
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