Low-frequency current variability at the Straits of Crete,

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 12, NO. Cll, PAGES 25,5-25,2, NOVEMBER 15, 1997 Low-frequency current variability at the Straits of Crete, eastern Mediterranean M.N. Tsimplis, A.F. Velegrakis,: A. Theocharis, 3 and M.B. Collins: Abstract. Current meter data from six moorings are analyzed to study current variability at the straits of the Cretan Arc which connect the south Aegean Sea with the eastern Mediterranean. The three layers in which the current meters were deployed show different current characteristics. The upper layer shows evidence of the Asia Minor current in the Rhodes Strait and a clockwise eddy in the Karpathos Strait. Water in the middle layer moves slowly into the south Aegean. The bottom current meters in Kassos and Antikithira Straits recorded currents, steady in amplitude and direction, with speeds equal to or higher than those observed in the upper layer. The observations suggest a significant outflow (.5 Sv) of Cretan Dense Water (CDW) to the Levantine and Ionian Seas. Rotary empirical orthogonal functions are used to extract and describe the coherent part of the variability. Forty-four percent of the total variability is described by the first three empirical orthogonal functions. Less than 2% of the current variability of the western strait is coherent with variability of the eastern straits. The coherent part of the variability refers mainly to periods longer than 2 days. The current meter data are compared with atmospheric pressure, showing good correlation during the first period of the measurements, when a major atmospheric pressure low passed over the Aegean. Introduction The straits to the east and west of Crete (Cretan Island Arc Straits ) connect the eastern Mediterranean Sea to the Cretan Sea (South Aegean) (Figure 1). In spite of being a comparatively small peripheral basin of the eastern Mediterranean, the Cretan Sea plays a significant role in the hydrology of the eastern Mediterranean, as it has been recognized as one of the major formation sites of both intermediate and deeper water masses [Georgopoulos et al., 1989]. After 1987 dense waters of Aegean origin contribute considerably to the deep and bottom layers of the eastern Mediterranean [Theocharis et al., 1992; Roether et al., 1996]. The central part of the Cretan Sea comprises a deep basin with depths reaching down to 26 m, bounded to the north by Cyclades Plateau at a depth of 4 m and to the south by the narrow shelf of the island of Crete. To the west it Copyfight 1997 by American Geophysical Union. Paper number 97JC /97/97JC-111 $9. the Cretan Sea and the Levantine Basin occur through the three eastern Cretan Arc Straits: the Rhodes Strait (17 km width, 35 m average sill depth); the Karpathos Strait (width 47 km, deepest sill depth in excess of 9 m); and the Kassos Strait (5 km width, deepest sill depth in excess of 1 m). Outside the straits the seabed plunges toward the deep basins of the Hellenic Trench to depths exceeding 4 m. Physiographically, the straits are characterized by a very high relief, showing large gradients of the seabed slope on either sides of the passages (Figure 1); the high relief of the area is in response to the complex tectonism it experienced, especially during the last 13 m.y. [Angelier et al., 1982]. The Cretan Arc Straits are associated (directly or indirectly) with a number of important circulation features of the eastern Mediterranean, including among others [Theocharis et al., 1993]: (1) the northwestern part of the cyclonic communicates with the Ionian Basin through the western Rhodes gyre located to the southeast of the eastern straits; Cretan Arc Straits. These comprise three different passages, (2) a branch of the Asia Minor Current (AMC) flowing close namely, the Elafonissos Strait (11 km width, average sill to the Asia Minor coast; (3) the Pelops anticyclonic and depth of 18 m), Kithira Strait (35 km width, average sill Cretan cyclonic gyres situated to the west and southwest of depth of 16 m) and Antikithira Strait (34 km width, deepest the western straits; (4) the Ierapetra anticyclone to the sill depth in excess of 8 m). The water exchanges between southeast of Crete; and (5) the cyclonic (generally) circulation pattern in the Cretan Sea. These features exhibit significant temporal variability [POEM Group, 1992], which influences their dimensions, strength, and duration. James Rennel Centre for Ocean Circulation, Southampton The main water masses associated with the Cretan Arc Oceanography Centre, UK. Straits include the Modified Atlantic Water (MAW) 2Department of Oceanography, Southampton Oceanography Centre, UK. 3Institute of Oceanography, National Centre for Marine Research, Athens, Greece. 25,5 characterized by comparatively low salinity [Malanotte- Rizzoli and Hecht, 1988], the saline Levantine Intermediate Water (LIW) [Ovchinnicov, 1984], the Eastern Mediterranean Deep Water of Adriatic origin (EMDW-1) [Pollack, 1951; Wust, 1961]; the Eastern Mediterranean Deep Water of Aegean origin (EMDW-2), and the Cretan Sea Dense Water (CDW) [Roether et al., 1996; Theocharis et al., 1997].

2 ß 25,6 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE Cyclades 37øN 36øN 35øN, ',---',.,. _ '. '--',, c> 9 ' / c, retan uasln '. - 1.,.---- = ½ ' 4.3ooo'. A Elafonissos Strait B Kithira Strait C Antikithira Strait D Kassos Strait E Karpathos Strait F Rhodes Strait 34øN onian Basin ß ;:; 3" Basin....;...::.,o 23øE 24øE 25øE 26øE 27øE 28øE 29øE Figure 1. Bathymetric chart of the region showing the location of the moorings denoted by triangles and the straits referenced in the text. The chart has been created on the basis of the General Bathymetric Chart of the Ocean (GEBCO) digital atlas. Contours are expressed in meters. Additionally, Transitional Mediterranean Waters (TMW) lying between LIW and EMDW-1 play an important role in the exchanges through the Cretan Arc Straits. Water masses originating at the deeper parts of the Cretan Sea were found to overflow through the straits and sink to The study aims in clarifying the vertical structure of currents and the extent to which current variability is coherent in all the straits. It also attempts to identify possible paths for the dispersion of the Cretan Dense Water to the eastern Mediterranean. depths of about 12 m in the Levantine Sea [Miller, 1963]. Nevertheless, their contribution to the creation of EMDW was considered to be small compared to the contribution of The Data water masses originating in the Adriatic Sea [Roether and Schlitzer, 1991]. A significant change in the circulation of Current Meter Data the region has been documented recently [Theocharis et al., 1992; Roether et al., 1996]. This change occurred between 1987 and 1995 and is basically induced by the increase of the deep water density (5(by about.2) in the Cretan Basin accompanied by intensification of the CDW outflow regime [Theocharis et al., 1997]. The transformation occurred at the deeper layers (> 5 m) changing their Levantine characteristics to a more saline (by about.1 psu) and cooler Fifteen Aanderaa RCM4 self-recording current meters, arranged in six moorings, were deployed at the Cretan Straits between April and September 1994 (for location, see Figures 1 and 3). Five moorings were located within the eastern straits which are characterized by larger water depths: one mooring was located between the islands of Simi and Rhodes (Rhodes Strait), two moorings were located between Rhodes and Karpathos (Karpathos Strait), with two between Kassos (by.4øc) water mass. The description and processes and Crete (Kassos Strait). The sixth mooring was deployed involved in this transformation will be dealt with elsewhere. near the northwestern edge of Crete at the Crete-Antikithira The understanding of the spatial and temporal variability of flow patterns and the identification of the paths followed by the deep Cretan water is clearly important for the Strait. A second mooring at the Antikithira Strait would have been useful, but instrument availability made this impossible. The deployment period together with the exact location and hydrology of the region and for monitoring possible changes the average water depth of each of the current meters are in deep water formations, as those evidenced by Roether et listed in Table 2. The depths at which the current meters al. [1996]. Moreover, chemical, biological, and sedimentolo- were deployed divide them in three layers. gical parameters are also defined by the water flows. Despite repeated measurements in the region, information related to the currents through the straits is fragmented and a clear The sampling interval of the instruments was set to 3 min and the mean current established for the sampling period. At the end of the 3 min period, instantaneous picture is not available yet (Table 1). Toward this objective, measurements of the current direction, pressure, salinity and an extensive European cooperative research programme (MAST/MTP-1 PELAGOS), covering numerous oceanoin situ temperature were also recorded. Following initial quality checks and the removal of graphic disciplines was carried out. This contribution "spikes" from the data sets, the current speed was averaged presents the results of the analysis of a comprehensive data to correspond to the time at which the direction set of current measurements collected over the sills of the measurements were taken. The time series of direction were Cretan Arc Straits, and its comparison with meteorological passed through a (.25,.5,.25) filter. Subsequently, hourly data from five stations in the Aegean (Figure 2). values were selected for analysis.

3 TSIMPLIS ET AL.' CURRENT VARIABILITY OF THE STRAITS OF CRETE 25,7 < Z<O * I I I I

4 o 25,8 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE " '-:!!!!!!! ii ( O...:'.' ' ø ' ' ',' ::'""":'"'"...?' N " '"'"'"" '"" ': :-.-.'.:!i :::::... ',. ::'": ';,.;.. % ' * '" Rhodes / j XLXIIECL '.,.-, Figure 2. The location of the meteorological stations. Meteorological Data Methodology The formalism of rotary empirical orthogonal functions can be found in numerous publications [Klinck, 1985; Preisendorfer, 1988]. Consequently, only a short description of the method will be presented here. The eastward and the northward deviation from the mean, at current meter i and at time j is denoted by uy/and vy/ respectively. The data set can be written as an N x M matrix Z: --( - > + 4W ( - ( ) where i=1... M and j=l... N. The covariance matrix S can be formed as S = (1/N)ZZ*, where Z* denotes the complex conjugate transpose of Z. Matrix S is Hermitian, and therefore its eigenvalues are real. The eigenvalues 3,,, and the eigenvectors e,,satisfy Se,,= 3,,,e,,, and e,,.e,,* = 5,... Therefore the covariance matrix S is decomposed into a mutually orthogonal set of eigenfunctions. The eigenvectors can be considered as spatial patterns of velocity at the various current meters [Klinck, 1985]. The magnitude of each eigenvalue indicates the percentage of variance explained by the corresponding eigenvector. The data set of the individual time series can be transformed into a set of spatial patterns, which vary in time, by using M Afi=E Z. im½,rn (2) Atmospheric pressure data for the period under investigation from five meteorological stations were where y/ (j=i,n) are time series of the amplitudes of the kindly provided by the National Meteorological Service of ith component. The time series can be synthesized again as Greece (Figure 2). The time series of atmospheric pressure M were used to estimate pressure gradients between the Zfi.-E A.fm½im (3) m---1 stations which are proportional to the geostrophic winds. Both the atmospheric pressure time series and the Equation (3) can be used to reconstruct the current atmospheric pressure gradients were then compared to the meter records, including only those Empirical Orthogonal current meter data in the time and frequency domain to Function (EOF) modes that are considered significant. The extract any significant correlation. selection of the significant eigenvalues is based on the "bootstrap method" [Preisendorfer, 1988]. m--'l Data Analysis and Interpretation The present work concentrates on low-frequency current variations. The hourly time series were passed through a 6-point filter in order to remove high-frequency oscillations and then subsampled daily. The resulting time series had a common length of 127 days starting on April 14. B P2 q>" PI ' "' -3,..., Figure 3. Sketch of the mooring showing the bottom topography of each strait.

5 TSIMPLIS ET AL.' CURRENT VARIABILITY OF THE STRAITS OF CRETE 25,9 Table 2. Details of the Current Meter Moorings Mooring Strait Latitude Longitude Water Instrument STD of Start Day Record øn øe Depth, Depth, Instrument Length, m m Depth, m days P1S Rhodes 36o27.26 ' 27o52.39 ' 51 9/4/ P1B /4/ P2S Karpathos ' 27ø34.11 ' /4/ P2B /4/ P3S Karpathos ' ' /4/ P3M /4/ P3 B /4/ P4S Kassos ' ' /4/ P4M /4/ P4B /4/ P5S Kassos ' ' /4/ P5M /4/ P5B /4/ P6S Antikithira ' 23ø31.' /4/ P6B /4/ Water depth is the depth of the station as measured by echo-sounding' depth of current meter is the mean depth of the current meters in each mooring as estimated by the pressure sensors of the instruments. Mean Currents tion and will be discussed elsewhere, some of the results The current meters can be distinguished as representing are presented here in order to facilitate the discussion of three layers: The upper layer is described by six current the current meter measurements and to put the revealed meters deployed at depths between 54 and 7 m below surface, the bottom layer is described by six current meters circulation patterns into context. The three layers appear to have distinctly different deployed about 5 m above the sea bed, while the three behavior. The upper layer shows the largest current values remaining current meters representing the middle layer with currents oriented across the straits. The current meter were deployed at depths ranging from 255 to 335 m (Table at the Rhodes Strait (P1S) shows the largest mean value, 2). The time mean currents for the 127 days, for different corresponding to the inflow of Levantine water in the layers in the water column, are shown in Figure 4. The Cretan Sea. This current meter (P IS) is most probably results provide a spatial pattern, the departure from which located on a branch of the Asia Minor current that flows gives the variance of the current. Although the mean along the Asia minor coasts and has been observed in currents flow through the Cretan Arc passages, their orientation is not perpendicular to the cross sections of the previous studies of the Rhodes Strait [Theocharis et al., 1993]. The upper layer current meters located within straits but rather follows the complex bottom topography Karpathos Strait (P2S and P3S) exhibit currents opposite in (Figure 1). The mean currents together with the orientation direction; the current meter close to the island of Rhodes of the strait topography and the water massesurrounding (P2S) shows water moving out of the Cretan Sea, while the current meters are shown in Table 3. The water masses current meter P3S indicates water of similar hydrological were identified by the analysis of data collecteduring two characteristics (of Levantine origin) entering the basin at hydrographic cruises taken place just before and in the almost double the speed. This pattern of water movement middle of the current meter deployment period (March indicates the presence of a clockwise eddy over this area. and June 1994). Although the detailed analysis of the The presence of an eddy of an opposite orientation in the hydrological structure is beyond the scope of this contribu- area has been identified from hydrographic observations of Upper. Layer 4 ' ' 4. Mi,ddle Layer.. Be!to m Layer 4 1 cm/sec 2 '6/. P4 1 "-C 2 tp3 P5 P4 2 : 6..P1 lp2 pip3 i 1 Km '2øo ' i o ' io i '2' 4' 6-2, o 6 Figure 4. Vectors of mean currents for the three layers. '['he scales of the plots (in 14 m) show the real distance between the moorings and at the same time the current speed in centimeters per second and the current direction.

6 25,1 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE Table 3. Summary of Mean Current, Direction, and Maximum Speed of the Low Passed Current Meter Data Current Mean Direction, Maximum Topography Water Mass Meter Speed, Speed, Orientation, March 1994 cm s - deg cm s - deg June 1994 P1S LSW P 1B LIW P2S LSW P2B TMW P3S LSW P3M TMW P3B TMW/CDW P4S LIW P4M TMW P4B CDW P5S LIW P5M LIW/TMW P5B TMW/CDW P6S MAW P6B CDW SWLO TMW CDW SWLO TMW CDW MAW/LIW CDW Directions are measured clockwise from north. The orientation of the parallel slope topograp hy estimated from GEBCO [IOC et at., 1994] is also shown. The last two columns provide the results of hydrographic analysis. The acronyms are MAW, Modified Atlantic Water; LSW, Levantine Surface Water; SWLO, Surface Water of Levantine origin; LIW, Levantine Intermediate Water; TMW, Transitional Mediterranean Water; and CDW, Cretan Dense Water. the summer season in a previous year [Theocharis et al., 1993]. The current meter records within Kassos Strait (P4S and P5S) suggest that water flows out of the Cretan Sea. The outflowing water masses were observed to have hydrological characteristics indicating Levantine Sea origin; this suggests recirculation of these water masses in this area of the Cretan Straits. The current meter within Antikithira Strait (P6S) shows mean flow toward the Cretan Sea in a northeasterly direction; the hydrological character of the inflowing water masses suggests that they are of Modified Atlantic Water origin. In summary, during the observational period, water enters the Cretan Sea through the northern part of the eastern straits (Rhodes Strait) and the western strait (Antikithira Strait). There is a recirculation of waters just to the southwest of Rhodes (Karpathos Strait), and water outflows through Kassos Strait. The midwater current meters (P3M, P4M, and P5M), located between 25 and 335 m, recorded currents of much lower velocities showing a slow flow of water into the Cretan Sea These velocities are real, as they retain their characteristics even when all the values below instrumentation threshold were replaced by zero instead of half the threshold value as was the standard practice. The inflowing water is characterized by low temperature and salinity, having similar character to that of the transitional water mass (Transitional Mediterranean Water) found between the Levantine Intermediate Water (LIW) and the Eastern Mediterranean Deep Water (EMDW-1). The water flow in the bottom layer (5 m above the sea-bed) shows the most dramatic and unexpected features. The current meters at Rhodes and Karpathos Straits (P1B, P2B, P3B) reveal only very low velocities, generally in a northern direction. In contrast, the two current meters in Kassos Strait (P4B and P5B) show strong outflow currents (mean values of 13 and 31 cm s - respectively) which are almost perpendicular in direction. The bottom current meter at the Antikithira Strait reveals mean flow of 19 cm s - at 13 ø. The bottom current meter at this strait was located below sill depth in the Ionian Basin [Intergovernmental Oceanographic Commission (IOC) et al., 1994], and therefore it is difficult to associate this along slope current with inflow or outflow of water. The energetic water masses in Kassos and Antikithira Straits have CDW characteristics, suggesting significant outflow of CDW into the eastern Mediterranean. The bottom currents are not, generally, perpendicular to the cross section of the straits. The most probable mechanism affecting the orientation of the bottom currents is topography; therefore the relative orientation with the upper layer currents is not meaningful in terms of flow dynamics. Nevertheless, it may be noted that the rotation of two of the energetic bottom currents (P4B, P5B) relative those in the upper layer is almost 9 ø, while at the Antikithira Strait the angle of rotation is slightly larger (15ø). Low-Frequency Variability The principal axis for each of the (mean removed) current time series has been determined, and then the current vector was rotated in reference with the orientation of the relevant strait. Furthermore, as most of the variance is associated with the current meters located within the upper layer, each time series has been standardized by reference to its standard deviation so as to extract most of the coherent variability. The results of the bootstrap method indicated that only the three first eigenvalues can be distinguished from a

7 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE 25,11 random process, with the percentage of the variance explained by each of them being 2%, 14%, and 1%, respectively. When the time series are reconstructed, mode by mode, starting from the largest eigenvalue (and for the first three significant modes), a residual variance can be defined as the sum of the squared differences between the reconstructed and the original time series. The resulting values for the standardized and the original case are shown in Table 4. In the standardized case, the first three EOFs explain at least 15% of the variance for all current meters; the lowest is for the upper layer current meter in the western strait (P6S). Notably, even for the low variance in the middle and bottom currents, a significant part of the variability is explained by the three EOFs. The first EOF explains 1% of the variance, in all but P4M and P1B. These records are both very low in variance, and it may be suspected that the analyzed signals are below the measurement capabilities of the instrumentation. Nevertheless, P3B, which exhibits the lowest variance of all current meters, has around 15% of its variance explained by the first EOF mode; this observation reinforces confidence in the analysis undertaken. Indeed, the second and the third mode explain 53% of P4M and 27 % of P1B. In the case of nonstandardized data, the first three modes explain approximately 7% of the total variance, Table 4. Analysis of the Variance Ratio Explained by the EOFs Ratio of Residual Variance After Fitting EOFs Mooring Variance, Standardized Not standardized cm 2 s Upper Layer P 1S P2S P3S P4S P5S P6S Middle Layer P3M P4M P5M Bottom Layer P1B P2B P3B P4B P5B P6B The second column is the original variance. The third to fifth columns are the residual variance, after subtracting the reconstructed velocity at each current meter from the original data. In this case the data are standardized over the STD of each current meter. Columns six to eight represent the same ratios when the analysis is performed on the original data without standardization. with almost all of this variance being concentrated within the upper current meters in the eastern straits. The upper layer current meter in the Antikithira Strait and the middepth and the bottom current meters have less than 15% of their variance explained by the three first EOFs. The first mode is defined by all the upper layer current meters east of Crete, with larger weights associated to those located to the north. The second mode is determined mainly by P5S, while the third mode is determined again by P1S, P2S, P3S, and P4S. As the nonstandardized approach does not appear to convey any significant amount of information for the middle and bottom layers of the water column, we will concentrate on the standardized EOF analysis, focusing on the coherent structures throughouthe whole of the water column. The spatial distribution of the three statistically significant EOF modes are shown in Figure 5. The eigenvectors were derived by normalising their amplitudes. As this procedure is not sufficient to determine a unique set of eigenvectors [Preisendorfer, 1988], it was also demanded that the largest component is real; this corresponds to eastward current. Consequently, the shown orientation of the vectors is somewhat arbitrary. The vectors could be rotated as groups to any suitable direction, provided that the corresponding EOFs are also rotated. The resulting spatial structures are complex, indicating that even the coherent part of the variability is obscured by the different orientation of the straits and probably is contaminated by noise. Nevertheless, some interesting general observations can be made. For example, the middle layer results always show a pair of the current meters being nearly parallel with the third current meter being about 15 ø apart. The vectors from the upper layer current meters in Karpathos Strait have opposite directions for all the three modes. The vectors in Kassos Strait (P4S and P5S) for the first two modes are about 14 ø apart, while the third mode is not influenced significantly from the northern current meter (P4S) recordings. The relative orientation of the vectors in the Anikithira Strait indicates a similar direction for modes one and three, whereas the second mode reverses and also diminishes in size. In the bottom layer, the vectors associated with the energetic current meter in the western part of Kassos Strait (P5B) have directions similar to those related to the current meter in Rhodes Strait (P1B). In contrast, the vectors for the first two modes (at P5B) show direction opposite to the vectors at P6B and similar direction for the third mode. The temporal variability of the first three EOFs (Figure 6) shows generally different patterns The first EOF is associated with a large inflow of water which lasted almost 15 days (until the end of April) and to four other events (between days 2 and 35, 5 and 6, 8 and 95, and 15 and 115, respectively). The last two events are nearly opposite in phase to the first event, while the second and third events are also opposite in phase to each other. The second EOF indicates large shifts of the phase over its most energetic part (between days 2 and 4 and 9 and 12). The third EOF is highly variable, although there are some periods in which it shows steady directions; this pattern suggests that there is more than noise in its signal. The event which occurred between days 5 and 6, in the

8 , 25,12 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE 4 Eigenv. ector 1, Upper 4 Eigenv,ecto r 1.Middle 4 Eigenv,ector 1,Bottom 2 P6 134 IP1.,...,,. P 2 2 P3 2 P2, 1-2 i i loo Km ' 2'o ' ' 6-2 '1o'4'o'6o o 4'o'6o 4 Ei(:jenv, ector 2. Upper 4 Eigenvector 2 Middle Eigenvector 2 Bottom 2 P1 P6,,., _2 2 2 "'2 P1 P5-2 i I 1 Km ' io ' 6-2 ' 2' 4' ' 6 2' ' 4' ' 6 4 Eicjenv. ector 3. Upper 4 Eigenv.ector 3.Middle 4 Eigenv,ector 3.Bottom 2 2 P3 2 P2_ 1 P1-2 I1 oo KIm -2-2,. 4''6 '1'4''6 i'4' 6 Figure 5. The spatial characteristics of the three significant EOF modes. The absolute length of the vectors is arbitrary, but the relative lengths and directions are meaningful. beginning of June, although not very energetic, is evident in all the three EOFs as a period where the phase is nearly constant. The period after day 1 is almost steady for EOFs one and two, while it appears to be variable for the third EOF. The power spectra of the two components of each EOF are shown in Figure 7. The length of the time series does not allow for high resolution. All three significant EOFs describe energy at low frequencies. The first EOF peaks at about 5 days in accordance with the time scales of the five events described above. A peak of lower energy occurs for the first EOF at 3.7 days. The power spectrum of the second EOF is equally energetic to the spectrum of the first EOF for the u component and for periods of up to 5 days. It then becomes more energetic than the first EOF for periods between 2.9 and 5 days. As this EOF explains some variance for all current meter records with the exception of those located in the Antikithira Strait, the spectrum analysis indicates that the days variability has a part which is coherent in the eastern straits only. The v component of the second EOF shows a peak at around 4 days but is an order of magnitude less than the peak in the first EOF. The second EOF does not exhibit any other significant peaks. The third EOF has a rapidly falling spectrum of the u component, with a peak at about 5 days. The v component is more energetic than the second EOF at low frequencies and peaks at around 25 days. The peak at 5 days is clearly demonstrated and is shown to be more energetic than EOFs 1 and 2 at the same frequency. It is interesting to note that atmospheric pressure oscillations of similar periodicity (4-5 days) have been previously detected in the Mediterranean [Gupta and Singh, 1977]. Nevertheless we were unable to detect a similar signal in the atmospheric pressure data we used. Therefore the 5- day peak in the current meter data is probably produced by larger-scale atmospheric pressure variations. The coherent part of the variability has been described as expressed through the standardised time series. Nevertheless, correlation between a few current meters may still exist; such correlation may not be included in the EOFs if it is associated with small variability. In order to investigate this possibility, as well as to extract any

9 ...._ TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE 25, EOF 1 v v =o EOF 2 6 [... 1 o '... I.11 '_, Irl.I, _ '... t/1... I f 4 o EOF 3 18o 9.fl cl -9 o o 8O 1 12 Figure 6. The temporal variability of the three significant EOFs. Directions correspond to outward flow (9ø). 13 I' / Bandwidth.35(cycles/day) 12 correlated signal from all three statistically significant EOFs, lagged cross-correlation analysis was used. Cross Correlation of the Time Series 11 1 L,,,-,,r-..., ' j.' The cross-correlation coefficient (C o ) and the veering angle (i-j) [Fofonoff and Hendry., 1985] has been expanded to include lagged correlation. If u i and vi are the eastward and northward components, respectively, of the current velocity after the means are removed, then FREQUENCY (cycles/day) I... I... I... I... : T 9 Degrees of Freedom ' / Bandwidth.35(cycles/day). (4) 12 _, O/o ß. where a positive angle implies a counterclockwise rotation 11 of the signal j, with respect to i. The level of significance is calculated on the basis of the procedure described by,,.-. Fofonoff and Hendry [1985]. The autocorrelation function 1 o of the data from all but one current meters became [... EOF-1,' 'x... insignificant after 7 days. This indicates approximately 18 degrees of freedom for the full time series [Bryden and 1-1 f... Pillsbury, 1977] Equation (4) was applied initially to the three EOFs. FREQUENCY (cycles/day) The first and the second EOFs are statistically correlated Figure 7. The power spectrum of the first EOF modes. zero lag for the.5 level of significance. There is a 55

10 25,14 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE Table 5. Significant Cross-Correlation Coefficients at the Residual Time Series of the Current Meters Current Meters Lag Type P1B-P5M 1,2 P2B-P4M 1,2 P2S-P4S 1,2,3,12 P3S-P5S 1 P4B-P5M 1 P4M-P4S 1 P2S-P3M 2,3 outer inner inner inner inner inner inner part of their variance. A similar cross-correlation analysis of the original time series did not show any lag at which data from the current meters within the Antikithira Strait were significantly correlated to the records the eastern straits. As the coherent part of the circulation at the two connections of the Aegean Sea with the adjacent Ionian and Levantine basins is less than 2% of the current variance, the above correlation refers to only a small part of the variability. This estimation corresponds only to the variability in the currents rather than the mean flow which is expected to be connected in a more consistent way. Comparison of Current Meter Data difference indicating anticlockwise rotation. No other to Meteorological Forcing significant correlation, at any lag of up to 8 days was apparent in the data sets. The sea level in the Aegean Sea is inversely (although Equation (4) is applied then to the residual time series nonisostaticaly) correlated to atmospheric pressure to search for any possible correlatio not absorbed by the EOF analysis. The resulting significant correlations are listed in Table 5. Apart from the correlations between 4M- 4S all the other correlations refer signals between different changes, with the major component of the sea level variability being an in-phase response of the basin as a whole [Lascaratos and Gacic, 199; Tsimplis and Vlahakis, 1994; Tsimplis, 1995]. Such sea level changes moorings. Likewise, they refer mostly to the shorter period necessarily lead to changes of the flow in the Cretan Arc of the frequency variation (1-3 days) which is not absorbed Straits. When the atmospheric pressure decreases over the by the EOFs. These correlations do not account for Aegean, the sea level increases and a net inflow through significant parts of the variability; however, they indicate the straits occurs modifying the currents accordingly. In that one or two of the smaller EOFs may include some information. Information in the Antikithira Strait is provided by a single mooring (P6). Therefore it is useful to present its cross correlation to the current meter records at the eastern straits at zero lag. This analysis has been undertaken on the reconstructed time series of the three significant EOFs order to examine the relation between atmospheric pressure and flow through the straits, atmospheric pressure data, as well as geostrophic winds (represented through pressure gradients) were compared with the current meter data. The first two EOFs of the atmospheric pressure time series "explain" 83% and 13% of the variance respectively (Figures 8d and 8e). The first mode represents an in-phase (Table 6). Within the upper layer, all cross correlations oscillation of the Aegean, almost equally weighted on all have shown to be statistically significant with the observa- the time series, while the second mode represented an out tions from moorings P1S and P5S being the best corre- of phase oscillation between Alexandroupolis in the North lated. Clockwise rotation relative to the P1S and P2S and and Rhodes in the south Aegean, with very small weighing anticlockwise relative to the remaining upper layer current for the other three stations. meters are shown to occur. The cross correlation of P6S, Comparison between the first EOF of the atmospheric with the bottom and middle layer, shows anticlockwise pressure (Figure 8d) and the EOFs of the current meters rotation; the higher correlation is with P3M. The bottom (Figure 6) shows good correlation during the first period of layer at the Antikithira Strait (P6B) is better correlated the measurements. The cross correlation between the first with the data from the bottom current meters at moorings EOF of the current meters and the two EOFs of the atmo- P2B and P4B, showing anticlockwise rotation. Surprising- spheric pressure were -.41 and.2, respectively. Howly, the highest correlation exists with the upper layer ever, cross-spectrum analysis between the time series did current meter (P1S) which is the farthest away. Neverthe- not provide statistically significant coherence, although the less, both P6S and P6B contribute least to the three significant EOFs, and therefore these results refer to a small coherence was generally higher at the longer periods (> 3 days). Table 6. Results of Cross-Correlation Analysis Between the Current Meters at the East and West Straits for the Reconstructed Time Series P1S P2S P3S P4S P5S P6S P3M P4M P5M P1B P2B P3B P4B P5B Cross-Correlation Coefficient P6S P6B Veering Angle, deg P6S P6B

11 ._ TSIMPLIS ET AL.' CURRENT VARIABILITY OF THE STRAITS OF CRETE 25,15 4 /, ' ' ' Upper EOF-i (36%)' 2O O 12 4 F M,ddle EOF-1 (41%) [ (b) /j ' 2 I Bottom EOF-1 (33%) ' which is clearly connected to atmospheric pressure variability appears in the EOF of all the layers. A secondary event in the atmospheric pressure between days 5 and 6 is also evident in the current meter data, 44 particularly in the bottom layer. Nevertheless, as the EOFs (c) 1 were derived from standardized values this observation does not mean that the energy involved is higher in the bottom layer. The second EOF of atmospheric pressure, which describes the pressure difference between Alexandroupolis and Rhodes is significantly correlated with the first EOF of the bottom and middle layer current meters for the period starting after day 2 of the measurements. The cross-correlation coefficients of EOF 1 of the upper, middle, and bottom layers with EOF 1 of atmospheric pressure were -.29, -.16, and -.39, respectively, while the correlation coefficients for EOF 2 of atmospheric pressure were.8,.17, and.33. No,_ 4 significant cross correlation between EOFs 2 and 3 from O 2 the upper layer with the atmospheric pressure EOFs was found. Nevertheless, EOFs 2 and 3 of the middle layer - -2 exhibited significant correlation to EOF 2 of atmospheric a pressure. In order to identify the particular current meters which. o o are affected by the major atmospheric pressure event (days 1-15) the variability around the mean of the cross-strait -5 (e) component of each current meter (Figure 9) was compared with the atmospheric pressure data. The cross-strait component of the current velocity was taken to be along the axis which explained most of the current variability. 18 The atmospheric pressurevent that occurreduring the first period of the measurements appears in current meters with the exception of P5S and the two bottom current o meters. Cross-correlation coefficients between EOF-1 of -18 atmospheric pressure and the cross-strait components of the current speed are significant for all current meters with the exception of P1B, Middle During the first 15 days of the current measurements an atmospheric pressure low passed over the Aegean (Figure o._ Q -9 8d). This must have resulted in a sea level response which -18 (g) must have caused an increased influx of water through the straits. Indeed, at three of the upper layer current meters (P2S, P4S, and less at P6S) an inflow has been recorded. On the other hand, current meters P1S, P3S, and P5S record outflow during the relevant period. Therefore the response is not homogeneous over the area. In addition, it Bøttøm t (h)! can be argued that similaresponse to this meteorological event is evidenthroughout the water column at moorings, P2, P3 (not P3B), P4, and P5, while moorings P6 and Pl Figure 8. The EOF analysis for independent layers. The show clearly different behavior. In understanding how the, amplitude and phase of first EOF of (a) and (f) the upper layer, expected, increased net inflow can be consistent with the (b) and (g) middle layer, (c) and (h) bottom layer; (d) and (e) the first and second EOFs of the atmospheric pressure times series. observed outflow at three of the current meters, it should be noted that these are measurements at a particular point, while oceanic features may be shifting in space. Thus it can be speculated that the increased inflow at P2S is The next question to arise is whether the coherence accompanied with a shift of the Asia Minor Current closer during the first period is derived from a particular layer or to the Asia Minor coast; this may result in a reduced, particular current meters. The EOFs for each layer were relative to the mean, inflow at P1S. Moreover, an thereforestimated (Figure 8). Only the first EOF of each anticlockwise circulation at Karpathos Strait, i.e., inflow layer were found to show variability similar to that of the through its eastern part (P2S) and an outflow through its atmospheric pressure during the first period of the western part (P3S) developeduring this event; during the measurements. It is worth noting that this first event, remainder of the recording period (when the

12 25,16 TSIMPLIS ET AL.' CURRENT VARIABILITY OF THE STRAITS OF CRETE 5 2 ' o -2o ' 2!3 o ß B ' ,I ii I,l,I, I Figure 9. The cross-strait component of current variability. Positive values indicate inflow of water at all current meters relative to the mean current. i meteorological events were milder) the water circulation in pheric pressure and the N-S component of geostrophic the strait was reversed, resulting in the development of a wind or, by the pressure titne series at Rhodes and Kithira. consistent clockwise eddy. The outflow recorded at P5S Cross-strait components of P5S, P5M, P5B, and P6B also cannot be easily explained. To complicate the matter even had more than 1% of their variance explained. further, P5S is the only current meter significantly correlated with the EOF 1 of the atmospheric pressure Conclusions during the whole period (Table 7). However, the currents at this station seem to be best correlated to EOF 2 of Current meter data from 15 instruments deployed in the atmospheric pressure during April. High cross-correlation Cretan Arc Straits were used to estimate the mean flow coefficients were derived for all the current meters for the and study the low-frequency variability. Three layers with first 2 days of observations (during April). distinctly different characteristics are identified. The upper The pressure gradient between Alexandroupolis and layer (5 m below the sea surface) exhibits, in general, the Iraklion was used to estimate the E-W compon,snt of the strongest currents which reached values higher than 34 cm geostrophic wind. Similarly, the atmospheric pressure s - in all current meters. The waters of this layer corregradient between Rhodes and Kithira was used to estimate spond to surface waters of Levantine origin carried in the the N-S component of the geostrophic wind. The N-S wind Aegean by a branch of the Asia Minor current and trapped component was found to be associated both with the atmo- in a gyre at the Karpathos Straits. Levantine Intermediate spheric low during the first period of measurement and the Water (LIW) recirculates through Kassos Strait, whereas manifestation of it at the current meters. Modified Atlantic Water (MAW) moves into the Aegean at In order to investigate the ability of the available the upper layer through the Antikithira Strait. The records meteorological time series (EOFs of atmospheric pressure, from the current meters at the middle layer revealed mean geostrophic wind estimates, and station pressures) to current speeds of less than 2 cm s -l and maximum speeds describe current variability, a linear regression was run for of less than 14 cm s -. These slow currents are associated each current meter cross-strait component. The with the Transitional Mediterranean Water (TMW), the independent variables were (1) EOF 1 and EOF 2 of water mass found between the LIW, and the Eastern atmospheric pressure; (2) two station pressures' or (3) a Mediterranean Deep Water (EMDW-1). combination of an EOF of atmospheric pressure and a The bottom current meters (deployed 5 m above the wind component. The variance "explained" by the multiple sea bed) showed nearly zero mean flow for the Rhodes regression was, in most cases, below 1% for most of the and Karpathos Straits, representing LIW and TMW water current meters. Nevertheless, 38% of the variance of P4S masses, respectively. In contrast, the two bottom current could be explained by the use of either EOF 1 of atmos- meters in the Kassos Strait showed strong mean outflow of

13 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE 25,17 Table 7. Monthly Cross-Correlation Coefficients Between the EOFs of Atmospheric Pressure and the Cross-Strait Component of the Currents EOF 1 EOF 2 April May June July August April May June July August P1S -.52 P2S.53 P3S -.47 P4S.85 P5S.27 P6S.31 P3M -.4 P4M.77 P5M -.43 P1B -.11 P2B -.58 P3B.31 P4B.84 P5B -.69 P6B Cretan Dense Water (CDW) toward the Levantine Basin (mean flows of 13.1 and 31.3 cm s 4 for P4B and P5B, respectively). The daily maximum speeds observed were 21 and 39 cm s 4. These findings indicate that the outflow of water observed by Roether et al. [1996] using hydrographic data takes place through the Kassos Strait in the form of a strong and steady outflow. According to Roether et al [1996], the western straits also contribute to the outflow of CDW into the eastern Mediterranean (Ionian Basin). Indeed, the current meter in Antikithira Strait (P6B) recorded a strong flow of water with CDW hydrological characteristics. However, as this observation site is located at the outer side of the strait and below sill level, it must be determined whether the observed strong currents are only due to the sinking of CDW following its exit from the strait or also to a strong recirculation along the complex topography of the slope toward the Ionian Basin. Nevertheless, if these strong currents were mainly due to the recirculation along the slope would have not, probably, shown good correlation with some of the current meters in the eastern straits. layers below 6 m. The cross-sectional area of the straits was based on GEBCO digital topography [IOC et al., 1994]. The results show an outflow of.36 Sv through the Kassos Strait;.2 Sv outflow through the western part of the strait and.16 Sv through the eastern part of the strait. It is worth noting that due to the sill topography, these two branches of the flow may not be completely independent, as there is a relatively deep channel connecting the locations of the two moorings (P4 an P5). The Antikithira Straits contribute.16 Sv to the outflow of CDW. Thus the total CDW outflow is estimated to be about.5 Sv with a range of.3 Sv. This outflow of CDW represents an annual flow of 16.4 x 13 km 3 which accounts for 34% of the total volume of the Cretan Sea and implies a renewal period for the Cretan Sea waters of about 3 years. Although this rate of creation and outflow of CDW is large, it represents only half the minimum value stated by Roether et al. [1996] with a renewal period for the Cretan Sea waters of just 1.5 years. Estimates of the fluxes for all layers based on the hydrographic data and ADCP data will be reported elsewhere (H. Kontoyiannis et al., personal communication, 1996). The high fluxes of CDW (.5 Sv) evidenced during the The results of the mean values of the bottom layer current meters when compared to earlier current meter measurements (Table 1) indicate that significant changes in present study indicate that major changes in the the thermohaline circulation of the Cretan Sea occurred thermohaline circulation have taken place in the Cretan between 1987 and In order to quantify these Sea and, in particular, in the patterns of creation of its changes, an estimation of the water mass exchanges deep water masses [Georgopoulos et al., 1989; Theocharis through the Cretan Arc Straits is necessary. However, the estimation of these exchanges on the basis of current meter measurements alone involves assumptions on the structure of the current field and the spatial distribution of each water mass in the straits. This distribution, even if determined during a particular hydrographic survey is unlikely to be steady during the whole period of current measurements. Therefore we attempted to estimate only the outflow of the CDW, which was found to be nearly steady during the period of measurements. The estimation is based on the assumption of steady and uniform flow across the straits and assuming the CDW occupies the et al., 1997]. They also suggesthat the CDW has become recently a major contributor water mass in the creation of the EDMW, more important than the deep water masses from the Adriatic Sea which accounts for.3 Sv [Roether and Schlitzer, 1991 ]. Seasonal variability of the flow cannot be examined on the basis of observations lasting only few months. Nevertheless, it is interesting to consider the relative variability of monthly mean values of the currents. In this respect, the currents in the upper layer exhibit the largest variability while the bottom currents at the most energetic locations are remarkably steady (Table 8).

14 25,18 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE

15 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE 25,19 The analysis of the variability of the currents (relative to the mean) revealed that 44% of the standardized variance can be explained by the first three significant EOFs. This coherent part of the variability is related to the events longer than 2 days as well as to events with periods around 5 days. The spatial characteristics of the EOFs are obscured by the effect of topography on the flows. The two current meters located within the upper layer at Karpathos Strait have characteristics of rotation both in their mean values and their variability indicating, probably, the presence of a clockwise eddy in that area. However, the current meters at the bottom of these two mooring do not show similar mean current characteristics, thus indicating that the eddy is probably constrained at the top layer. The comparison of the current meter data to the atmospheric pressure data revealed a complicated response of the currents to the forcing which is far from being either spatially uniform or barotropic. Nevertheless, the strongest meteorological event of the period, associated with a drop of atmospheric pressure, produced measurable effects and significant cross-correlation coefficients between the current meter data and the meteorological forcing at most of the current meters. A tentative picture describing the response of the current field when the atmospheric pressure is lowered over the Aegean has been produced. This was the only atmospheric event that could be clearly detected in the current meter records. Measurements during the winter period would probably include a larger number of strong meteorological activity which would allow for a better understanding of the response of the system. The surface stations at Kassos Straits have a significant part of their variance (38% and 16%, respectively) explained by a linear regression on EOF 1 of the atmospheric pressure and the N-S wind component derived as the pressure gradient between Kithira and Rhodes. This indicates that their response is not only due to the overall change of pressure over the Aegean but also due to differences in atmospheric pressure forcing between the Levantine and Ionian basins. The cross correlation between the atmospheric pressure and the cross-strait component of the current variability is significant in Kassos Strait for periods longer than 2 days. Finally, the strong currents recorded in the bottom layers in Antikithira and Kassos Straits also have sedimentological implications. Strong bottom flows may have been one of the factors responsible for the thin sedimentary cover [Wong and Zarudzki, 1969; Stanley and Perissoratis, 1977] and low Holocene accumulation rates [Rohling et al., 1993] observed over the sills of the Cretan Arc. Moreover, sedimentary structures associated with strong bottom currents have been also identified during previous investigations the area [Vittori et al., 1981] suggesting that significant flow/sediment interactions have been frequent in the area. If this is the case, then the recent changes in the circulation and outflow of CDW may form a part of a larger cycle in the circulation patterns of the deep waters of the eastern Mediterranean. Acknowledgments. Our colleagues in the National Centre for Marine Research (Athens, Greece) and the crew of its R/V Aegaio are thanked for their effort in collecting and supplying data to the authors. We would also like to thank the National Meteorological Service of Greece for providing the meteorological data, Harry Bryden for reviewing this paper prior to submission and Marion Raaymakers for producing the cameraready manuscript. The first author was supported through EC contract HCM ERB45PL93172 and the second author was supported through EC contract CHRX-CT The collection of the data was undertaken in the framework of the Mediterranean Targeted Project (MTP) - PELAGOS Project. We acknowledge the support from the European Commission's Marine Science and Technology (MAST) Programme under contract MAS2-CT93-59 References Accerboni, E., and G. Grancini, Measures hydrologiques en Mediterranee Oreientale (Septembre 1968), Boll. Geofis. Teorica Appl., 14(53-54), 3-24, Angelier, J., N. Lyberis, X. Le Pichon, E. Barrier, and P. Huchon, The tectonic development of the Hellenic Arc and the sea of Crete: A synthesis, Tectonophysics, 86, , Bruce, J.G., and H. 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16 25,2 TSIMPLIS ET AL.: CURRENT VARIABILITY OF THE STRAITS OF CRETE renewal on the basis of chlorofluoromethane and tritium data, Dyn. Atmos. Oceans, 15, , Roether, W., B.B. Manca, B. Klein, D. Bregant, D. Georgopoulos, V. Beitzel, V. Kovacevic, and A. Luchetta, Recent changes in Eastern Mediterranean deep waters, Science, 271, , Rohling, E.J., F.J. Jorissen, C. Vergnaud Grazzini, and W.J. Zachariasse, Northern Levantine and Adriatic Quaternary planctonic foraminifera; Reconstruction of palaeoenvironmental gradients, Mar. Micropalaeontol., 21, , Stanley, D.J., and C. Perissoratis, Aegean sea ridge barrier and basin sedimentation patterns, Mar. Geol., 24, 97-17, Theocharis, A., D. Georgopoulos, P. Karagevrekis, A. Iona, L. Perivoliotis, and L. Charalambidis, Aegean influence in the deep layers of the eastern Ionian Sea (October 1991), Rapp. Comm. lnt. Mer Medit., 33, 235, Theocharis, A., D. Georgopoulos, A. Lascaratos, and K. Nittis, Water masses and circulation in the central region of the Eastern Mediterranean: Eastern Ionian, South Aegean and north west Levantine, , Deep Sea Res. II, 4(6), , Theocharis, A., M. Gacic, and H. Kontoyiannis, Physical and dynamical processes in the coastal and shelf areas of the Mediterranean, in The Sea, Wiley-Interscience, New York, in press, Tsimplis, M.N., The response of sea level to atmospheric forcing in the Mediterranean, J. Coastal Res., 11(4), , Tsimplis, M.N., and G.N. Vlahakis, Meteorological forcing and sea level variability in the Aegean Sea, J. Geophys. Res., 99, , Vittori, J., H. Got, P. Le Quellec, J. Mascle, and L. Mirabile, Emplacement of the recent sedimentary cover and processes of deposition of the Matapan Trench margin (Hellenic Arc), Mar. Geol., 41, , Wong, H.K., and E.F.K. Zarudzki, Thickness of unconsolidated sediments in the eastern Mediterranean Sea, Geol. Soc. Am. Bull., 8, , Wust, G., On the vertical circulation of the Mediterranean Sea., J. Geophys. Res., 66, , Zodiatis, G., On the seasonal variability of the water masses circulation in the NW Levantine Basin-Cretan Sea and flows through the eastern Cretan Arc Straits, Ann. Geophys., 1, 12-14, Zodiatis, G., Circulation of the Cretan Sea water masses (eastern Mediterranean Sea), Oceanol. Acta, 16, , M.B. Collins and A.F. Velegrakis, Department of Oceanography, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, United Kingdom. ( soc.soton.ac.uk; A. Theocharis, Institute of Oceanography, National Centre for Marine Research, Elliniko 1664, Athens, Greece. M.N. Tsimplis, James Rennell Division for Ocean Circulation, Southampton Oceanography Centre, European Way, Southampton, SO 14 3ZH, United Kingdom. ( (Received June 14, 1996; revised December 17, 1996; accepted March 11, 1997.)