Sea level variations in the Mediterranean Sea and Black Sea from satellite altimetry and tide gauges

Global and Planetary Change 34 (2002) 59 – 86 www.elsevier.com/locate/gloplacha

Sea level variations in the Mediterranean Sea and Black Sea from satellite altimetry and tide gauges A. Cazenave a,*, P. Bonnefond b, F. Mercier a, K. Dominh a, V. Toumazou a a

LEGOS-GRGS/CNES, 18 Av. E. Belin, 31400 Toulouse Cedex, France b Observatoire de la Coˆte d’Azur, GRGS, Grasse, France

Abstract Present-day sea level changes in the Mediterranean Sea and Black Sea are studied using satellite altimetry. Analysis of altimetry data from Topex – Poseidon (T/P) between January 1993 and December 1998, and from ERS-1/2 between October 1992 and June 1996 shows that the mean rate of sea level rise is 7 F 1.5 mm/year over the Mediterranean Sea and 27 F 2.5 mm/ year over the Black Sea. The geographical distribution of the observed trends is rather uniform in the Black Sea unlike the Mediterranean Sea. There we observe, over the 6 years of analysis, a quite large (20 – 30 mm/year) sea level rise in the Levantine basin. In the Ionian Sea, on the other hand, a negative sea level trend is reported during that period. In the western basin, sea level trends are significantly lower, some regions rising and others falling. An Empirical Orthogonal Function (EOF) analysis of the sea level data is presented which confirms the main features reported above. Analysis of sea surface temperature (SST) data over the two seas and over the same time span indicates that basin-scale trends are correlated with the altimetryderived sea level trends; however, the spatial variations of SST trends are smoother than sea level trends, the latter presenting subbasin fluctuations. The spatial correlation between sea level trends and SST trends suggests that at least part of the sea level change reported during the few years over the Mediterranean Sea and Black Sea is due to heating of surface layers. Moreover, the temperature and salinity increase reported since the early 1960s in the deep waters of the western Mediterranean basin and more recently since the early 1990s in the eastern basin may contribute to the observed sea level trend. However, the observed trends in the Mediterranean sea level for 1993 – 1998 may also result from the interannual/decadal variability of the upper ocean circulation that is predicted by theoretical circulation models. In the Black Sea, apart from a possible steric contribution, change in regional hydrology, in particular, a decrease in river runoff, could be responsible for observed sea level changes. Finally, we also present results of long-term sea level trends (multidecadal time scale) using tide gauge records. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mediterranean Sea; Black Sea; Altimetry

1. Introduction The Mediterranean Sea is a semi-enclosed basin that communicates with the Atlantic Ocean through * Corresponding author. Tel.: +33-56133-2922; fax: +3356125-3205. E-mail address: [email protected] (A. Cazenave).

the Straits of Gibraltar and with the Black Sea through the Turkish Straits. It is subdivided into the western basin and the eastern or Levantine basin with the straits of Sicily as boundary. Fig. 1 presents a sketch of the main features of the Mediterranean Sea general circulation (from Roussenov et al., 1995). On a yearly average, evaporation over the sea exceeds precipitation but water loss ( f 100 cm/year) is compensated

0921-8181/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 ( 0 2 ) 0 0 1 0 6 - 6

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Fig. 1. (a) Sketch of the Mediterranean Sea circulation (from Roussenov et al., 1995). (b) Sketch of the Black Sea circulation (from Oguz et al., 1995).

by water inflow from the Atlantic Ocean through the Straits of Gibraltar (e.g., Bethoux and Gentili, 1999). However, the year to year water budget of the Mediterranean Sea may not be exactly balanced. The Black Sea is a nearly closed sea having limited interaction with the Mediterranean Sea through the Turkish Straits. Circulation in the Black Sea is pre-

dominantly cyclonic and mostly driven by surface fluxes (evaporation and precipitation) and freshwater input from the Danube, Dnieper and Dniester rivers. Wind stress only plays a minor role in the Black Sea circulation. Unlike the Mediterranean Sea, there is excess precipitation over evaporation in an average year, a situation balanced by outflow from the Black

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Sea to the Aegean Sea through the Turkish Straits. Fig. 1b shows a schematic map of the Black Sea circulation (from Oguz et al., 1995). Measurement of variations in sea level with complete basin coverage would provide constraints on the water mass balance and thermal expansion of seawaters in response to climate change. Satellite altimetry, operational for nearly one decade, can provide important information on mean sea level changes on time scales ranging from f 1 month to several years with high spatio-temporal coverage. Quite importantly, this technique gives access to the geographical variations of the sea level changes which may certainly not be uniform over the basin. In this paper, we presents results on sea level variations over the Mediterranean Sea on time scales ranging from intraseasonal to interannual using several years of altimeter data of the Topex– Poseidon, ERS-1 and ERS-2 satellites. We also presents sea level change results for the Black Sea. Finally, we briefly discussed interdecadal sea level trends over the two seas based on tide gauge data.

2. Altimetry data analysis During the 1990s, three altimeter satellites have been placed in orbit: Topex – Poseidon (T/P), ERS-1 and ERS-2 launched in August 1992, March 1991 and May 1995, respectively.

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Sea level determination by satellite altimetry relies on different techniques: radar altimetry that measures the height of the satellite above the instantaneous sea surface and the tracking data (laser ranging or radiobased signals) used to precisely compute the satellite orbit, in particular, the height of the satellite above a reference surface (usually a reference ellipsoid). Difference between these two heights provides the instantaneous sea level above the reference ellipsoid. This quantity is the sum of two contributions: the geoid plus the oceanographic signal. Mean sea level change is part of the latter. For the geoid, it is usual to approximate it through a mean sea surface determined from altimetry data averaged over a time span longer than the studied mean sea level variations. In this study, we used T/P altimetry data available by the AVISO/Altimetry project of CNES (Centre National d’Etudes Spatiales) in charge of processing and distributing observations of the T/P mission. This data set covers six complete years from January 1993 through December 1998. Table 1 summarizes the various geophysical and environmental corrections applied to the T/P altimeter range measurements. These corrections include the ionospheric, dry and wet tropospheric corrections, solid Earth and ocean tides, ocean tide loading, pole tide, electromagnetic bias (EMB), instrumental corrections (altimeter bias and onboard oscillator drift) and inverted barometer (IB) correction. Quantitative values used for these corrections are those provided by AVISO with the

Table 1 Geophysical corrections applied to T/P and ERS-1 data

Altimeter range data Orbit . Dry troposphere . Inverse barometer . Wet troposphere . Ionosphere Sea state bias

. . . .

Ocean tide Loading tide Solid earth tide Pole tide Instrumental bias and oscillator drift

Topex – Poseidon

ERS-1

GDR-M products version C NASA JGM3 orbits . from ECMWF . from ECMWF . from TMR radiometer . from dual-frequency altimeter ranges for Topex data, from Doris for Poseidon data updated BM4 formula from Gaspar et al. (1994) for Topex and Poseidon

sea level anomalies from AVISO precise D-PAF orbits . from ECMWF . from ECMWF . from ATSR-M radiometer . BENT model

. . . . .

CSR3.0 model applied applied applied applied

.5.5% of significant wave

height for phases C, E and F; BM3 formula from Gaspar et al. (1994) for phase G . CRS3.0 model . applied . applied . applied . applied

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reprocessed data. Here, the standard IB correction is applied, which assumes that sea level adjusts locally to atmospheric pressure perturbations. In principle, this correction is valid for open ocean at periods longer than 10 days. However, it has been shown that for a semi-enclosed sea such as the Mediterranean, the

barometric response of the sea is more complicated than assumed by the simple IB equilibrium because of dynamical control at the straits (e.g., Le Traon and Gauzelin, 1997). However, the study by Le Traon and Gauzelin (1997) indicates that departure from IB equilibrium is mostly significant at periods shorter

Fig. 2. (a) Topex – Poseidon track coverage. (b) ERS-1/2 track coverage.

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Fig. 3. (a) Time evolution of the Mediterranean mean sea level from T/P data between January 1993 and December 1998. (b) Residual Mediterranean mean sea level variations from T/P (seasonal signal removed). The thick solid line is the 30-day running mean residual curve.

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Fig. 4. (a) Time evolution of the Black mean sea level from T/P between January 1993 and December 1998. (b) Residual Black mean sea level variations from T/P (seasonal signal removed). The solid line is the 30-day running mean residual curve.

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Fig. 5. Map of the Mediterranean sea level trends (linear variation with time) from T/P for 1993 – 1998. 65

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Fig. 6. Map of the Mediterranean sea level trends (linear variation with time) from ERS-1/2 between October 1992 and July 1997.

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than f 200 – 300 days. As the main purposes of this study are seasonal and interannual sea level variations, the standard correction should be sufficient. The response of the Black sea level to atmospheric pressure also significantly departs from an IB response (Le Traon, pers. commun.). However, as for the Mediterranean, we applied the standard correction since we are also focusing on seasonal and interannual fluctuations. For the reference mean sea surface, we considered the OSU95 model (Yi, 1995). The data have been edited as described in Cazenave et al. (1998). More details on the T/P data processing can be found in Cazenave et al. (1998). For the ERS-1 and ERS-2 data, we used the data set displayed by AVISO and regularly updated. This data set is an upgraded version of the products made available by the French Data Processing and Archiving Facility (D-PAF) of European Space Agency (ESA). Upgrades concern, in particular, the ERS orbits, the sea state bias and ocean tide corrections. The ERS-1 data considered here cover the periods October 1992 through November 1993 and March 1995 through April 1996. Gaps in the data set correspond to the 3-day repeat orbit and the Geodetic Mission for which the upgraded data were available. One year (June 1996 through July 1997) of ERS-2 data is also included. The geophysical and environmental corrections applied to generate sea level measurements are summarized in Table 1. As ERS-1 and

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ERS-2 have similar orbits, the ERS-1/ERS-2 altimeter data can be used to construct a single homogeneous sea level time series. However, they are available for the Mediterranean Sea only. For data editing, a procedure closely similar to that used for T/P data has been followed. Finally, the OSU95 mean sea surface has also been used. Fig. 2a and b shows track coverage over the Mediterranean Sea and Black Sea, respectively, for T/P and ERS-1/ERS-2.

3. Observed mean sea level variations To generate mean sea level time series over the Mediterranean Sea and the Black Sea, we have spatially averaged the sea level data over each sea separately during an orbit cycle (10 days for T/P and 35 days for ERS-1/2). 3.1. Mediterranean Sea Fig. 3a shows the time evolution of the Mediterranean mean sea level constructed with the T/P data. The mean sea level curve is dominated by a strong annual signal of f 8 cm in amplitude, which is discussed in Section 5. The seasonal fluctuation has a sawtooth shape with a progressive increase and a sharp decrease. Maximum signal occurs in autumn. Fig. 3b shows the residual mean

Fig. 7. Location map of sea level time series shown in Fig. 8 (stars and numbers).

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Fig. 8. Individual sea level time series from T/P (locations shown in Fig. 7). (a) Western Mediterranean Sea, (b) Central Mediterranean Sea, (c) Eastern Mediterranean Sea and (d) Black Sea.

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Fig. 8 (continued).

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Fig. 8 (continued).

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Fig. 8 (continued).

sea level after removing the seasonal signal (annual and semiannual components) and smoothing (a lowpass filter with a 30-day cutoff has been applied). Large intraseasonal fluctuations dominate the residual sea level time series. These fluctuations are highly correlated with mean surface pressure variations (1-month averages over the whole sea) on time scales in the order of 100– 200 days. Let us recall that we have applied the simple inverted barometer correction that may not be exactly valid for a period shorter than 200 days. Thus, the reported correlation between sea level and atmospheric pressure at intraseasonal time scale may simply reflect the dynamical contribution of the sea level response not accounted for by the standard IB correction. In Fig. 3b, we noted large sea level variations in winter of 1995 – 1996 and 1996 – 1997. A mean sea level rise superimposed to the intraseasonal variations is apparent in Fig. 3b. Linear regression to the data indicates that the Mediterranean

mean sea level has been rising over 1993– 1998 at a mean rate of 7.0 F 1.5 mm/year. 3.2. Black Sea The time evolution of the mean sea level for the Black Sea is presented in Fig. 4a. The annual signal is only 4 cm in amplitude (compared to 8 cm for the Mediterranean Sea) and more irregular from year to year. We noted however that while an annual signal is clearly present in 1993, 1994 and 1995, it has been significantly attenuated in 1996, 1997 and 1998. The residual mean sea level (seasonal signal removed, 30-day smoothing) is presented in Fig. 4b. The residual mean sea level of the Black Sea appears much more energetic than in the Mediterranean Sea, large fluctuations of f 100 mm peak to peak are being observed. The residual curve suggests that these fluctuations are quasi-periodic, as indeed confirmed by the power spectrum of the residual time

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Fig. 9. Annual cycle of the Mediterranean and Black Sea mean sea level from T/P. (a) Amplitude map. (b) Phase map.

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series which shows a large peak of 45 mm amplitude and approximately 305-day period. The rate of Black mean sea level rise over the 6-year time span is estimated to 27.3 F 2.5 mm/year, a value four times larger than in the Mediterranean Sea.

4. Geographical distribution of the Mediterranea and Black sea level trends Fig. 5 presents a map of the sea level trends (linear variation with time) over 1993 –1998. The map has been constructed as follows: for each 10-day cycle, a 0.5j  0.5j grid of sea surface height is constructed for the Mediterranean Sea and Black Sea. Then, at each 0.5j  0.5j grid mesh, a sea level time series is computed from which seasonal components (annual and semiannual harmonics) and linear trend are deter-

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mined in a least-squares inversion. The estimated linear trends are then gridded to produce the map shown in Fig. 5. Fig. 5 shows that large variations occur in the rate of sea level change from one region to another. While on the average the Mediterranean sea level has been rising, two regions show evidence of sea level fall: the western Mediterranean Sea between the Balearic Islands and Sardinia, and the Ionian basin, between Greece and Sicily. In the Ionian basin, the negative trend reaches 25 mm/year. On the other hand, the southern part of the Ionian Sea shows moderate sea level rise. In the Algero –Provencß al basin along the coasts of France and Spain, a moderate sea level rise, f10 mm/year, is observed as in the northern Tyrrhenian Sea and Adriatic Sea. The largest rate of rise originates in the Levantine basin, with a strong maximum >30 mm/year located southeast of Crete

Fig. 10. Amplitude map of the annual steric sea level.

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over the Mersa– Matruh gyre. Fig. 5 also presents the geographical distribution of the Black sea level change. We noted that the trends are positive everywhere and much more uniform than in the Mediterranean Sea. The eastern region nevertheless shows a larger rate of rise than the western region. A similar procedure has been applied to the ERS1/2 data for mapping the geographical variations of the sea level trends over a similar period of time. The corresponding map is shown in Fig. 6. Comparing Figs. 5 and 6 indicates that similar trends are observed over each region, confirming that the observed features are not artefacts. However, the ERS map is globally more energetic throughout. One reason for this may be the denser coverage of the ERS satellites, which provides more detailed mapping of the spatial

variations of sea level changes. Because of its coarser spatial coverage, T/P may miss some of the shortwavelength variations of the sea level trends. Nevertheless, the larger trend magnitudes reported by the ERS satellites need further investigation. In order to check if the trend map presented in Fig. 5 is well representative of the dominant interannual sea level changes, we have plotted individual sea level time series from T/P at selected grid points. Thirtyfour and five points have been considered over the Mediterranean Sea and Black Sea, respectively. Their locations are shown in Fig. 7. Corresponding sea level time series (from January 1993 to October 1997) are plotted in Fig. 8a (western Mediterranean basin), Fig. 8b (central Mediterranean basin), Fig. 8c (eastern Mediterranean basin) and Fig. 8d (Black Sea). We

Fig. 11. Amplitude map of the non-steric annual mean sea level.

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Fig. 12. First mode of EOF analysis of the Mediterranean mean sea level: (a) spatial variation; (b) temporal variation.

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Fig. 13. Second mode of EOF analysis of the Mediterranean mean sea level: (a) spatial variation; (b) temporal variation.

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Fig. 14. Map of SST sea level drifts for 1993 – 1998.

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first note that all over the Mediterranean, intraseasonal fluctuations are highly correlated from one location to another and that the dominant interannual signal is a linear variation with time. Thus, the trend map shown in Fig. 5 represents the dominant sea level interannual signal. Similar observations can be made for the Black Sea.

5. Annual cycle of the Mediterranean and Black sea level from T/P Maps of the annual signal amplitude and phase lag (in degrees, since January 1) based on T/P data are shown in Fig. 9a and b. These are obtained by estimation at each mesh of the gridded data of the annual variation through a least-squares adjustment. In the

Mediterranean Sea, we saw three amplitude maxima (of f 10 cm) centered near 5jE, 16jE and 27jE. The phase map was quite uniform over the Mediterranean Sea, with the maximum sea level occurring in early autumn over the whole sea. The seasonal variability in sea surface height agrees well with the results of an earlier study based on 1 year of altimetry data from T/P and ERS-1 (Ayoub et al., 1998). It also agrees well with estimates based on coastal tide gauge data (Tsimplis and Woodworth, 1994). The highest seasonal variability was correlated with major features of the Mediterranean Sea general circulation: the Ionian cyclonic gyre and Ionian current, as well as over the Iarapetra and Mersa– Matruh anticyclonic gyres (which appear as a single feature in the T/P map; Fig. 9a). This agrees well with model results of the seasonal variability of the Mediterranean circulation (e.g., Roussenov et al.,

Fig. 15. Residual mean sea level (dashed curve) and SST (solid curve) (seasonal cycles removed) for the Mediterranean Sea.

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1995). In the Black Sea, the annual signal (Fig. 9) varies from approximately 5 to 8 cm from west to east. We have computed the steric contribution to the observed T/P-derived annual signal in the Mediterranean Sea. To estimate the steric component of the annual sea level, we used the climatology of Levitus et al. (1994) that gives temperature and salinity on a 1j  1j grid at 19 depth levels for each month of a standard year (note that temperature and salinity fields are not available for the Black Sea). We performed the vertical integration from the 1000-m reference level to the surface. At each grid mesh, we thus estimated the annual steric contribution. The corresponding amplitude map is presented in Fig. 10. The steric contribution appears rather uniform and in the order of 50 – 60 mm. At each grid mesh, we removed the steric contribution to the observed annual signal (taking

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the phases of both steric and observed sea level signals into account). The map of annual amplitude differences is shown in Fig. 11. It represents the nonsteric annual sea level fluctuation due to sources other than thermal expansion of the Mediterranean Sea waters. This residual map more clearly depicts the seasonal sea level variability associated with the circulation pattern of the Mediterranean Sea (in particular, along the Liguro – Provencß al and Algerian currents in the western basin, the Ionian gyre and the Iarapetra gyre in the Levantine basin). In the Black Sea, the annual signal is smaller than in the Mediterranean Sea, but as noted above, the map represents an average over the 6 years (a small annual signal had been reported in years 1996, 1997 and 1998). Maximum sea level of the Black Sea occurs in summer, i.e., 3 months earlier than in the Mediterra-

Fig. 16. Residual mean sea level (dashed curve) and SST (solid curve) (seasonal cycles removed) for the Black Sea.

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nean Sea. A recent study by Stanev et al. (submitted for publication) based on T/P data until mid-1997 shows that the seasonal variation of the Black sea level is well explained by the seasonal variation of water input into the sea, in particular, runoff from the Danube, Dnieper and Dniester rivers. The observed amplitude decrease of the annual sea level variation may reflect a lesser seasonal water input into the Black Sea since 1996. Whether this results from reduced precipitation over the river drainage basin on seasonal time scale or from man-induced reduced river outflow needs further investigation.

6. EOF analysis of the spatio-temporal sea level variations in the Mediterranean Sea In this section, we applied an Empirical Orthogonal Function (EOF) analysis to the T/P-derived sea level variations over the Mediterranean Sea to show up the spatio-temporal fluctuations mapped by satellite altim-

etry in another way. The EOF analysis is based on a decomposition of a spatio-temporal varying field (here, the sea level) into a linear combination of temporal aj(t) and spatial Fj(x,y) orthogonal modes (where x, y, t are Cartesian coordinates and time) computed from the eigenvalues and eigenvectors of the covariance matrix formed from the time series of the gridded data set (e.g., Preisendorfer, 1988). The physical interpretation of the eigenvalues is related to the variance of each temporal orthogonal mode computed as a linear combination of the observed series weighted by the eigenvector components of the covariance matrix. Figs. 12 and 13 show the first two modes of the EOF decomposition of the Mediterranean sea level. The first mode is clearly annual and is maximum in autumn. The spatial and temporal variations of this mode (Fig. 12) suggest that this first mode depicts the steric contribution of the seasonal signal. The second mode is also annual but it is maximum in winter. The spatial pattern of the second mode resembles the non-steric annual sea level shown in Fig. 11. The second mode probably reflects the

Fig. 17. Location of the PSMSL tide gauge records analyzed in this study in the Mediterranean Sea.

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seasonal variability of the ocean circulation, in particular, the gyre variability. Superimposed to the temporal variation of the second mode, we clearly noted a positive slope that may be due to the sea level rise discussed in Section 4. That the seasonal variability of the Mediterranean circulation and sea level trend have same spatial pattern suggests that the latter is at least partly due to changes in the circulation (see Discussion).

7. Analysis of sea surface temperature We have analyzed sea surface temperature (SST) data over the Mediterranean Sea and Black Sea. The weekly Reynolds SST fields (Reynolds and Smith, 1994) available from the US National Meteorological Center via Internet were used for that purpose. The data set based on in situ and satellite observations consists of 1j  1j grids. SST time series are dominated by a seasonal cycle (annual and semiannual harmonics), which has been removed from the data in order to show up SST interannual variability for comparison to sea level. Two different data sets have been constructed with the residual SST data: a mean residual SST curve for each sea (data spatially averaged; annual and semiannual components removed) and a gridded data set of the SST trends (assuming linear drift with time) over 1993 – 1998. Fig. 14 shows a map of the SST trends over the 6-year time span. Compared to the sea level trend map (Fig. 5), we noted a similar trend tendency at basin scale in both data sets (negative SST and sea level trends in the Ionian Sea, positive SST and sea level trends in the Levantine basin). However, the SST trend map appears smoother everywhere than the sea level trend map. The latter contains shorter spatial wavelength (at subbasin scale) variability. In the Black Sea, a similar east – west contrast is observed both in sea level and SST maps. Figs. 15 and 16 present curves of mean sea level and mean SST variations (seasonal components removed) over 1993 – 1998 for the Mediterranean Sea and Black Sea, respectively. While we noted a little correlation between the two curves over the Mediterranean Sea, in the Black Sea on the other hand, a visual correlation between residual sea level and SST is clearly apparent from intraseasonal to

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interannual time scale. After mid-1996, we noticed an upward shift of the sea level curve compared to the SST curve. Recall that it is around this date that the annual sea level started to decline. The rapid increase of the Black sea level during the second half of 1996 and the decline of the annual variation after this epoch

Table 2 Mediterranean Sea, tide gauge-derived sea level drifts Record length (years)

Drift (mm/year)

Site

20 40 32 65 26 38 12 18 25 39 109 28 10 20 17 20 20 30 18 19 15 107 22 25 25 20 20 25 23 20 20 16 39 15 17 40 42 22 20 91 25 24

0.1 (1.4) 1.3 (0.6) 0.4 (0.4) 0.2 (0.4) 0.5 (0.8) 0.8 (0.1) 0.8 (3.2) 0.3 (0.2) 0.3 (0.2) 0.0 (0.6) 0.6 (0.9) 0.2 (1.6) 3.8 (2.7) 0.6 (0.4) 3.0 (0.8) 1.2 (1.8) 2.0 (0.5) 0.0 (1.0) 1.6 (1.3) 1.7 (0.4) 4.0 (0.0) 0.5 (0.5) 1.0 (0.0) 1.0 (0.2) 0.3 (0.2) 10.9 (5.9) 3.3 (3.7) 0.9 (0.0) 2.2 (0.2) 3.5 (1.0) 1.5 (1.7) 1.0 (1.6) 0.2 (0.2) 3.5 (5.1) 1.8 (2.4) 0.4 (1.1) 0.1 (0.3) 5.7 (2.6) 0.6 (0.3) 0.5 (1.3) 0.6 (1.7) 1.9 (0.7)

Alexandroupolis Alicante I Alicante II Bakar Bar Cagliari Catania Ceuta Civitavecchia Dubrovnik Genova Gibraltar Kalamai Katakolon Kavalla Khalkisn Khios Koper Leros Levkas Malaga Marseille Napolia Napolima Palermo Patrai Piraievs Portomaurizio Portsaid Posidhonia Preveza Rodhos Rovinj Siros Soudhas Splitha Splitrt Tarifa Thessaloniki Trieste Veneziaa Venezias

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Fig. 18. Interdecadal sea level trends in the Mediterranean Sea computed from tide gauge records.

may be linked together and possibly related to some abrupt change in the Black Sea water balance.

8. Analysis of tide gauge sea level time series In this section, we present an analysis of tide gauge multidecadal records from the Permanent Service for Mean Sea Level (PSMSL) in the Mediterranean Sea and Black Sea (Spencer and Woodworth, 1993). The objective here is to have an estimation of the interdecadal sea level trends and to compare them with the shorter (interannual) trends measured by satellite altimetry. We considered monthly Revised Local

Table 3 Mediterranean Sea, sea level statistics (35 stations with r < 2 mm/ year)

Reference (RLR) gauge data and have selected sites having records longer than 10 years. Fig. 17 presents the location of the 42 stations of the Mediterranean Sea fulfilling these criteria. For each station, the monthly time series have been low-pass filtered. The low-pass filtered time series was derived from the Fourier expansion of the original data set with a cutoff at the 2-year period. This procedure removes the seasonal signal that dominates the time series. Linear regression was further applied to the low-pass filtered time series to estimate the linear trend. Note the inverted barometer correction has not been applied to the tide gauge data. Table 2 displays, for each of the 42 stations, record length, sea level drift estimate and corresponding uncertainty. Some estimated drifts

Table 4 Western Mediterranean Sea, Adriatic Sea and Aegean Sea

Mean sea level drift (mm/year) 35 stations 22 stations with records >22 years 4 stations with records >60 years 3 stations with records >80 years

0.55 F 1.0 0.41 F 0.87 0.50 F 0.5 0.57 F 0.5

Mean sea level drift (mm/year) Western Mediterranean Sea: 8 stations Adriatic Sea: 10 stations Aegean Sea: 17 stations

0.72 F 0.8 0.30 F 0.7 0.62 F 1.0

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Fig. 19. Location of the PSMSL tide gauge records analyzed in this study in the Black Sea.

Fig. 20. Interdecadal sea level trends in the Black Sea computed from tide gauge records.

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present large uncertainties due to frequent interruptions of the instruments, giving rise to gaps into the data set. Out of the 42, we kept 35 stations for which the drift uncertainty is V 2 mm/year. The deleted stations are underlined in Table 2. Fig. 18 shows the sea level drifts in the Mediterranean Sea as a function of record length. The results are somewhat scattered for record lengths shorter than 20 years. Beyond the 20-year record length, however, quite stable drift values are reported. Table 3 summarizes the tide gauge statistics for the Mediterranean Sea. Results indicate that at interdecadal time scale, the Mediterranean sea level has not risen by more than f 0.5 mm/ year. We analyzed the different basins separately (Western basin, Adriatic Sea, Aegean Sea). Mean sea level trends for each basin are reported in Table 4. Observed sea level rise remains less than 1 mm/ year everywhere, the lower value being observed in the Adriatic Sea. The results reported here are in reasonably good agreement with those of previous studies (e.g., Tsimplis and Spencer, 1997; Tsimplis and Baker, 2000). A similar analysis has been conducted in the Black Sea. Fig. 19 shows the location of the 10 tide gauges with record length z 10 years, while Fig. 20 shows the sea level drifts estimated with a procedure similar to the Mediterranean Sea. Table 5 reports the sea level drift estimates for the 10 Black Sea stations. As for the Mediterranean Sea, large scatter in the sea level drift is observed for records shorter than f 30 years. Corresponding statistics are summarized in Table 6. We noted that the mean sea level rise of the Black Sea was f 2 mm/year over the last decade, i.e., a factor of 4 larger than over the Mediterranean Sea. Table 5 Black Sea, tide gauge sea level drifts Record length (years)

Sea level drift (mm/year)

Tide gauge station

79 32 10 13 66 87 33 21 14 79

0.1 (0.0) 2.1 (1.6) 29.6 (11.2) 1.0 (2.7) 2.9 (3.5) 3.1 (2.9) 3.8 (0.2) 13.4 (13.9) 9.4 (3.2) 0.9 (0.6)

Batumi Batumi 2 Erdek Ereglisi Nesebar Poti Poti 2 Samsun Trabzon Tuapse

Table 6 Black Sea, sea level statistics Mean sea level drift (mm/year) 8 stations (r < 5 mm/year) 6 stations with records >25 years 4 stations with records >40 years

3.0 F 4 2.2 F 2.5 1.8 F 2.2

With the 6 years (1993 –1998) of T/P altimetry data, we have determined the present-day sea level drift of the Mediterranean Sea and Black Sea. Results indicate sea level rises of f 8 and f 30 mm/year, respectively. We noted that during the past 6 years, the Black sea level rise has been f 4 times larger than the Mediterranean sea level rise, a ratio exactly similar to that reported over the last century with tide gauge data. That a similar ratio of sea level rise between the two seas is observed at both interdecadal and interannual time scales may be fortuitous however. We noted also that the interdecadal sea level rise based on tide gauge records ( f 0.5 and 2 mm/year for the Mediterranean Sea and Black Sea, respectively) is f 15 times lower than the interannual (6 years) signal measured by satellite altimetry. The larger altimetryderived sea level rise likely reflects long-term (from years to decades) fluctuations of the sea level rather than the ‘climatic’ signal that is possibly evidenced by the tide gauge measurements.

9. Discussion In this study, we present new results on the sea level changes measured by satellite altimetry over a 6year time span. A mean sea level rise of approximately 7 mm/year is reported in the Mediterranean Sea. Its spatial distribution is far from uniform however. While the Levantine basin is rising at a rate of 25 –30 mm/year, the Ionian Sea is falling by 15– 20 mm/year. Some correlation is observed between sea level and SST trends at basin scale, but SST shows less spatial variability than sea level. The correlation suggests that part of the Mediterranean sea level change is steric. Several studies have reported significant change in the thermohaline circulation of the western Mediterranean Sea since the early 1960s, with a significant increase in temperature and salinity of deep and

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intermediate waters (Bethoux et al., 1990; Bethoux and Gentili, 1996). These observations are interpreted as changes in heat and water budgets across the sea surface in regions of wintertime dense water formation (i.e., in the northern part of the basin). More recently, a similar observation has been made in the eastern Mediterranean Sea where an abrupt increase in temperature and salinity of deep and intermediate waters has been detected (Roether et al., 1996; Klein et al., 1999). This abrupt change which started in the early 1990s is attributed to a transition in deep water formation from a single source, the Adriatic Sea, to a new regime presenting significant water influx from the Aegean Sea. Although not concomitant, changes in deep hydrology of the western and eastern basins may be linked together. The salinity increase is considered too large to be caused by an increase in evaporation only and might be related to a decrease of fresh water input as a result of human activities and decrease in precipitation observed since 1940 (Bethoux and Gentili, 1996, 1999). The temperature and salinity increase in deep and intermediate water layers may affect sea level in a complex way but we cannot exclude that the pattern of sea level change observed in the western and eastern basins resulted, at least partly, from the recent changes in the thermohaline circulation. On the other hand, the subbasin scale variability observed in sea level changes may be related to decadal fluctuations of the Mediterranean Sea general circulation predicted by modeling studies. In a recent numerical simulation based on a general circulation model forced by winds and heat fluxes data over 1980 –1988, Pinardi et al. (1997) showed that the Mediterranean circulation patterns present significant interannual variability. In the western basin, the interannual variability corresponds to change in the strength and meandering of the Algerian current, as well as of the Lyon gyre. The eastern Mediterranean basin is the area where the variability is the most pronounced. Strong interannual changes are reported over major currents and gyres, and especially over the Mersa– Matruh gyre. These model predictions agree remarkably well with the observations made from satellite altimetry. In a recent study (Cazenave et al., 2001), we have extended the length of the T/P time series up to the end of 1999. While the general features reported above are still fully observed, the

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magnitude of the Mediterranean sea level trends appears slightly less than for the period 1993– 1998. This strongly suggests that the observed sea level variations are part of long-term (decadal) cyclic fluctuations. In a recent study, Vignudelli et al. (1999) have demonstrated that the North Atlantic Oscillation (NAO) exerts an influence on the circulation of the northwestern Mediterranean Sea through air – sea interactions. Thus, ultimately NAO might be responsible for the recent events detected in the Mediterranean thermohaline circulation, as well as the decadal changes in the upper circulation since NAO is known to exert some control on climate of northern and southern Europe (see also Tsimplis and Baker, 2000). Tide gauge indicates that during the past decades, the Mediterranean sea level did not rise more than 0.5– 1 mm/year, i.e., a factor 10– 15 times lower than during the past few years. The 7 mm/year rate of rise reported by satellite altimetry should not be interpreted as evidence of sea level change acceleration. More likely, the recent change reflects interannual/ decadal variability of the Mediterranean surface circulation. Recent changes in the thermohaline circulation due to changes in heat and freshwater budget may also contribute to the Mediterranean sea level variations. In the Black Sea, the origin of observed sea level increase of approximately 27 mm/year over 1993 – 1998 is more hypothetical. According to the observed correlation with SST, part of this rise may be explained by heating of surface layers. In addition, possible decrease in runoff of major rivers flowing to the Black Sea, the likely cause of the quasi disappearance of the seasonal sea level cycle, would lead to a positive water balance, hence contributing to the observed sea level rise.

Acknowledgements We thank P. Woodworth and an anonymous reviewer for their comments on the manuscript. This study is a contribution to the SELF II project undertaken within the framework program Environment and Climate of the European Union. Supports from CNES, CNRS, GRGS and the Observatoire de la Cote d’Azur are acknowledged.

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