Observations on the circulation of the Saronikos Gulf: A Mediterranean embayment sea border of Athens, Greece

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1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2008jc005026, 2010 Observations on the circulation of the Saronikos Gulf: A Mediterranean embayment sea border of Athens, Greece Harilaos Kontoyiannis 1 Received 17 July 2008; revised 4 December 2009; accepted 31 December 2009; published 30 June [1] The Saronikos Gulf three dimensional flow structure is mapped by objective analysis in 12 acoustic Doppler current profiler surveys (4 seasonal surveys per period for the periods May 1998 May 1999, May 2000 May 2001, May 2003 May 2004) under various winds. Robust seasonal flows throughout the Gulf are basically induced by thermohaline effects (winter water subduction, different responses to atmospheric heating of shallow and deeper areas of the Gulf) and density contrasts with inflowing Aegean waters. Inner Gulf observations show that the seasonal flows are modified by the wind, while recurrent structures (cyclonic and anticyclonic) appear between the seasonal flows and the coast of Attica. In summer an anticyclonic and a cyclonic flow exists throughout the Gulf above and below the pycnocline, respectively, with a jet observed to meander within the Inner Gulf. In winter and early spring an anticyclonic flow prevails in the upper 100 m. In late spring early summer cyclonic and anticyclonic flow occurs in the upper ( 0 40 m) and deeper ( m) layers, respectively. The predominant northerly winds in summer and winter push the Inner Gulf eastward seasonal jet to the south and favor the formation of a recurrent cyclonic upwelling structure between this jet and Attica. Northwesterly, westerly, and southerly winds favor the northward meandering of the seasonal jet in the Inner Gulf and no recurrent structures appear in the limited space between this jet and Attica. In the southeast area of the Inner Gulf, the wind driven flow fluctuations at 30 m have time scales of days. Observed meandering period is 13 days; meandering characteristics are close to instability predictions. Time scales of flows due to recurrent structures are 8 days. Citation: Kontoyiannis, H. (2010), Observations on the circulation of the Saronikos Gulf: A Mediterranean embayment sea border of Athens, Greece, J. Geophys. Res., 115,, doi: /2008jc Introduction [2] The Saronikos Gulf draws the attention of marine science because it constitutes the natural marine gateway of the city of Athens and the Piraeus harbor and receives the treated wastes of 4 million people through a bottom source at a depth of 65 m in the northeast just south of the Psittalia island (Figure 1). The Gulf is a semienclosed embayment on the western coastline of the Aegean Sea in the eastern Mediterranean. It is bounded by the coast of Attika along its northern and eastern boundaries and the coast of Peloponisos along its western boundary, while it communicates with the Aegean Sea at its southern end. It is divided into a western and an eastern part. The so called western subbasin is to the west of the Aigina and Salamina islands, whereas the northern area of the eastern part, north of the 100 m depth contour, is called the Inner Gulf. The Inner Gulf is relatively flat, with a mean depth of 90 m, while the 1 Hellenic Center for Marine Research, Institute of Oceanography, Athens, Greece. Copyright 2010 by the American Geophysical Union /10/2008JC western subbasin is deeper, with an elongated north south trough having maximum depths of 230 m in the north and 450 m in the south. At the north of the Gulf lies the Elefsis Bay, with a depth range of 20 to 30 m; it connects to the Gulf via two narrow channels with sill depths of 10 m at the west and 30 m at the east. [3] The Gulf is subjected to a strong seasonal cycle of heating/cooling, with air temperatures ranging from 0 to 40 C. The predominant winds throughout the year blow from northern directions (Figure 2). Northerly winds called etesians or meltemia prevail consistently during summer, whereas in fall, winter, and spring, apart from the frequent northerly winds, westerly and southerly winds may also blow. No major river input exists except during heavy rainfall when the water runs rapidly into the sea through a couple of point sources in the northeast. [4] Circulation studies of the Saronikos Gulf have been of scientific and societal importance with respect to pollution management and environmental control, in view of the fact that since 1992 and up to the present (2009), the evolution of the dissolved oxygen concentration in the deep layers of the western subbasin has approached nearly anoxic conditions, with concentrations of <1 ml/l. The first systematic 1of23

2 Figure 1. (top) The eastern Mediterranean. The study area is within the bold rectangle. (bottom) The study area enlarged. Bathymetric contours are in meters as indicated at the upper right. C, current meter mooring; MS, meteorological station. 1, Psittalia Island; 2, Salamina Island; 3, Aigina Island, 4: Piraeus. scientific surveys in the Saronikos Gulf began in the early 1970s [Coachman et al., 1976] in the framework of the Saronikos Systems Project (SSP), which basically consisted of 21 hydrographic surveys in the period These surveys employed reversing thermometers and salinity samples and each one lasted 5 to 6 days on average. Consequently, the first circulation studies of the Saronikos Gulf were based on the SSP data pool and utilized either the spreading patterns of water characteristics [Hopkins, 1974, 1975] or the dynamic method, which was applied for all seasons including winter under homogenization conditions. Later, in 1977, 1983, 1984, and 1985, short term deployments of one or two simultaneous current meter moorings were realized at specific points, mostly near Psittalia [Papageorgiou, 1979; Papageorgiou and Balopoulos, 1984; Kardaras, 1984; Kaltsounidis and Lascaratos, 1989], while drogues were launched near Psittalia on other occasions and gave flow tracks for the upper 50 m that were at best 6 km long [Watson, 1986]. Most of the published work is in the form of technical reports in Greek. [5] Insufficient equipment or facilities compared to the present technology have resulted in certain drawbacks in 2of23

3 Figure 2. Stick diagrams of monthly wind averages in Saronikos Gulf (37.5 N, E) during obtained from National Centers for Environmental Prediction (NCEP) reanalysis data [Kalnay et al., 1996] (top) and from the National Meteorologic Service at station MS shown in Figure 1 at N, E (bottom). earlier circulation studies of the Saronikos Gulf, as follows. (1) Inferring the circulation via the distribution patterns of some hydrologic characteristics is an indirect method that basically provides indications but not observed flow structures with full three dimensional information and associated flow speeds. (2) The application of the dynamic method is very likely to provide misleading results (a) in narrow coastal regions of the Gulf such as the area around Psittalia, due to deviations from geostrophy by friction; (b) during winter, when horizontal density gradients and vertical current shear are very minimal because of the strong overall mixing in the upper layers; and (c) due to violation of synopticity by some of the SSP cruises, with average durations of 5 6 days, knowing that currents measured near Psittalia respond to changing winds in approximately 2 days [Kaltsounidis and Lascaratos, 1989]. (3) The results from one or two moorings, although valuable in other aspects, fall far short of providing information on flow structures and spatial variability. Previous studies have successfully identified the general presence of eddies and suggested the influence of wind. However, they could not reveal details of the circulation in a coherent way, not even basic aspects of it, such as the two layer structure during the stratification period or the role of Aegean Sea intrusions through the southern boundary. [6] Field observations of the current work started within the framework of the multidisciplinary environmental project, Monitoring of the Saronikos Gulf Ecosystem Affected by the Psittalia Sewage Outfall The initial objective of the physical oceanography was to detect the existing flow structures throughout the Gulf by means of direct and synoptic current measurements. More objectives, relating to the short term and seasonal variability of the flow as well as the dependence on forcing factors, were added later when the project was extended for the period , after the success of in tracing the treated sewage plume throughout the Gulf and its implications on the ecosystem. [7] Investigation of the circulation of the Saronikos Gulf in the present work is based on shipboard acoustic Doppler current profiler (ADCP) measurements on a grid of stations. Four seasonal surveys (August 1998, December 1998, February 1999, May 1999) were conducted in , with ADCP stations covering the northern part of the west subbasin, the area between Salamina and Aigina (Salamina Aigina passage), and the Inner Gulf. For all cruises, CTD (conductivity, temperature, depth) data were collected on a station grid with a coverage larger than that of the ADCP measurements. This data set allows us to relate, via geostrophy, the distributions of the hydrographic characteristics with the circulation structures observed directly in the ADCP data. We therefore show representative comparisons between horizontal flow structures derived from dynamic heights and horizontal flows derived from mapped ADCP data, to examine the reliability of the dynamic method and the conditions under which it can or cannot identify the general flow structures in such regions. Seasonal ADCP surveys were repeated in (May 2000, August 2000, December 2000, March 2001) and in (June 2003, September 2003, January 2004, March 2004), with dense station coverage in the Salamina Aigina passage and the Inner Gulf, and revealed even more detailed structures therein. An indicative picture of the short term (synoptic) variability of the three dimensional flow during winter and summer is also provided from three consecutive h surveys in August 1998 and February Finally, the flow response to the wind is examined at site C (37.82 N, E) in the southeastern area of the Inner Gulf (Figure 1) at a depth of 30 m from current and wind time series during the 3 month period November 2003 to January [8] After presenting a few technical aspects with respect to the data and the methodology used to treat them, we proceed with the results, which are comprised of (a) the seasonal structures of the general circulation in the study area, (b) their relation to the seasonal mass distributions and 3of23

4 the underlying thermohaline mechanisms and/or Aegean intrusions, (c) the modifications of the general seasonal structures in the Inner Gulf that the wind can cause, (d) the short term, 6 day variability that the flow structures can exhibit in winter and summer, and (e) the site specific response to the wind of the flow at site C (37.82 N, E). In section 4 we comment on the role of the forcing factors and relate their influence on the flows to similar observations from other coastal regions. 2. Data and Methodology [9] ADCP measurements were collected with a 150 khz broadband instrument built by the RDI. At each station the ADCP was lowered to a depth of 4 m off the side of the ship. For all measurements 5 m bins and 32 pings in 45 s were used for each ensemble. Velocity estimates for each ping were transformed to Earth coordinates using internal compass and tilt sensors. Nearly all measurements were collected in bottom track mode, in which we averaged 15 ensembles at each station. In a few cases with bottom depths over 200 m in the western subbasin, when bottom tracking was not possible, we performed a longer ensemble average, depending on the ship s total drift and the error in reference velocity, which was kept at the order of 10%. During ADCP measurements in deep mode, care was taken to avoid possible GPS jumps. The near bottom velocity measurements within m over the bottom were usually noisy and unreliable owing to alias by the bottom reflection and were eliminated from the final ADCP velocity profiles [10] Raw ADCP measurements have not been detided. Tides in the entire Saronikos Gulf, with the exception of the narrow channels connecting Elefsis Bay with the Gulf, are very weak. On days of a full moon or new moon the 12 h current speed fluctuations have an amplitude of 3 cm/s, whereas they are indistinguishable in comparison to subtidal currents during the rest of the tidal month. This is confirmed further in section 3.4, in the unfiltered current meter time series shown in Figure 15. [11] The station snapshot data at specific depths are interpolated via objective analysis on a fine grid so as to produce flow maps. In this way we obtain the threedimensional structure of the flow based on direct current measurements. The approach followed for objective mapping of shipboard ADCP measurements is based on the objective analysis/optimal interpolation method presented by Bretherton et al. [1976] and adapted by Watts et al. [2001]. In this approach a stream function was assumed for the flow field, with a correlation of the form RðrÞ ¼½1 ðr=aþ 2 Š exp½ ðr=bþ 2 Š; where a =30.7kmandb = 4.6 km. This correlation function is rather sharp and results in near zero values for r ^ 12 km. Compared to a set of other relevant values of (a, b), it shows the best overall performance in producing mapped flows in agreement with the ADCP input currents for all cruises. Objective mapping errors are referred to as percentage of signal variance [Watts at al., 2001]. In our objective analysis input ADCP data there is a typical standard deviation of 3 4 cm/s. A mapping error near 70% implies a velocity error near 3 cm/s. For a sharp correlation function like the one we use, mapping errors decrease sharply toward areas where there is input data coverage. Nevertheless, in all the objective flow maps the input current vectors are superimposed on the mapped fields and their agreement confirms the quality of the mapping. [12] The National Meteorologic Service made available local wind data from a meteorologic station (MS) at (37.87 N, E) in the old Athens airport area (Figure 1) for the periods prior to and during all cruises. The wind time series for the period November 2003 to January 2004 were obtained from the meteorologic buoy of the Poseidon Project ( at the same site with the current meters. Monthly averaged wind data for investigation of seasonal wind patterns for the 24 month period January 1998 to December 1999 were obtained from National Centers for Environmental Prediction (NCEP) reanalysis [Kalnay et al., 1996] and from the National Meteorologic Service, but only for the first 17 months of the above period (Figure 2). The common parts of these records have mean speed deviations that are <1 m/s, but most importantly, the direction deviations are around 15 on average, which shows that the NCEP winds can be used at least qualitatively to successfully describe the seasonal wind changes over the study area. In addition, wind data during the surveys in and in the Salamina Aigina passage, obtained from the Poseidon Project (not shown), compare well with the corresponding wind data at site MS, showing that the wind data of the National Meteorologic Service are fairly representative of the wind conditions over the wider Saronikos Gulf. 3. Results 3.1. Seasonal Circulation Patterns [13] A seasonal pycnocline exists in Saronikos Gulf every year in the period from May to November [Kontoyiannis and Papadopoulos, 1999]. This pycnocline is fully developed in late August early September and extends between depths of 40 and 70 m. Afterward it erodes and in the period December April a typical picture is that the upper m is nearly or completely homogenized; that is, the Inner Gulf and the Aigina Salamina passage undergo vertical mixing to very near the sea bottom. For such stratification Figure 3. Objectively mapped currents (black vectors with arrows) in Saronikos Gulf at 20 m (left maps) and 60 m (middle maps) during (from top to bottom) August 1998, December 1998, February 1999, and May Short lines with a circle at the origin (vector sticks) indicate acoustic Doppler current profiler (ADCP) current measurements at the positions of the circles. Areas with an objective mapping error >70% are blank. Speed scale for currents is shown in the lower right of each map. Bold lines with arrows are characteristic gross streamlines (flow trajectories) used to ease visualization of flow structures. Right hand plots show wind time series obtained at site MS in Figure 1 prior to and during the ADCP surveys. The arrows on the time axis indicate the beginning and the end of the specific ADCP survey. 4of23

5 Figure 3 5of23

6 conditions the internal Rossby deformation radius R i is 10 km in summer and 5 km in winter, according to R i = (g h) 0.5 /f, where g is the reduced gravity, h is the thermocline depth, and f is the Coriolis parameter. For investigation of circulation we utilize objective flow maps at 20 and at 60 m (Figure 3). In the stratified period (May November) the depth of 60 m is in or below the lower part of the seasonal pycnocline, whereas the depth of 20 m is above the pycnocline. Both depths are in the mixed part of the water column for the rest of the year (December to April). For each survey we also show the unfiltered 3 hourly wind data prior to and during the survey. The duration of each survey, defined by the respective arrows on the time axis of the wind time series in Figure 3, was approximately h, that is, 1.5 days at most. As mentioned in section 1, flow structures near Psitallia, which is a less energetic area in background flow relative to the rest of the study area, have been observed to respond to changing winds in 2 days [Kaltsounidis and Lascaratos, 1989]. Therefore, a duration of maximum 1.5 days for each of the surveys shown in Figure 3 satisfies the synopticity requirement. The general flows, as shown in Figure 3, are robust to high frequency wind changes such as sea/land breeze that occur in summer in the absence of the predominant etesians and do not supply enough energy to alter the general flow structure within a day or even less. [14] In the near surface layer (20 m) a general eastward flow from the western subbasin to the Inner Gulf exists in summer and winter (August 1998, December 1998, and February 1999). It follows a broad anticyclonic loop in the western subbasin, entering into the Inner Gulf through the central and southern area of the Salamina Aigina passage. The eastward flow in the Inner Gulf forms an anticyclonic meander, while smaller scale cyclonic recirculation structures of a recurrent nature exist upstream and downstream of this anticyclonic meander to the north or northeast of the eastward flow. The main flow exits to the south through the area between Aigina and Attika. It is likely that the nearsurface anticyclonic flow in the west subbasin, apart from providing a branch into the Inner Gulf, forms a closed loop in the west subbasin, as indicated by the flow field in February, with the additional ADCP station to the west of Aigina. In winter (December 1998 and February 1999) the flow field is highly barotropic; that is, the basic flow characteristics do not change with depth, as they are observed at both 20 and 60 m. In summer (August 1998), with the presence of a strong pycnocline, the general flow in deep layers (60 m) is reversed to the west, that is, from the Inner Gulf to the west subbasin. A meandering path is observed in the Inner Gulf, whereas a cyclonic path is followed in the west subbasin, with one branch of the flow shifting to the north at the southwest of Salamina along the local bottom topography (Figure 1). [15] In late spring (May 1999), during the first stages of the development of the seasonal pycnocline, the flow in the west subbasin and the flow in the Inner Gulf do not appear as one continuous structure spanning the two areas, as in the other periods (August 1998, December 1998, and February 1999). In the near surface layer (20 m) the general flow enters into the Inner Gulf through the area between Aigina and Attika. This flow proceeds to the north, providing mostly meandering branches of recirculation in the Inner Gulf and possibly some minor net flux to the west near the coast of Salamina. In the west subbasin the near surface (20 m) flow, after entering into the Salamina Aigina passage, shifts back to the west near the coast of Salamina and appears disconnected from the flow in the Inner Gulf. In deeper layers (60 m) the circulation of the Inner Gulf is reversed with respect to that in the surface, with an anticyclonic flow that exits to the south through the area to the east of Aigina, whereas in the west subbasin a flow branch is directed into the Salamina Aigina passage. The mapped flows in late spring (May 1999) are characterized by smaller scales and they are possibly in a transitional stage. Their investigation requires denser spatial sampling, like that during and used for the Inner Gulf and the Salamina Aigina passage. [16] At 20 m the typical flow speeds in the highly energetic core of the jets and away from the constriction of the Salamina Aigina passage are in general near cm/s throughout the year. At 60 m they decrease by a factor of 2. In the Salamina Aigina passage current speeds near 20 cm/s can typically occur near the surface intensified part of the flow [Zeri et al., 2009]. [17] In a first attempt to examine whether the seasonal variability in the flow structures is associated with the instantaneous or monthly averaged local winds, attention is focused initially on the instantaneous 3 hourly winds prior to and during the ADCP measurements in (Figure 3). They either were from northerly directions or were highly variable, with a minimal net effect on the flow field. Similarly, the monthly averaged winds during the month of each cruise were from northerly directions apart from February 1999, when there was a westerly wind on average (Figure 2). Therefore, the change in the general flow pattern in May 1999 in comparison to the flows in August 1998, December 1998, and February 1999 does not seem to be associated with the local wind forcing. The actual cause of the late spring (May 1999) circulation is revealed further in section Seasonal Mass Distributions [18] The seasonal distributions of the hydrologic characteristics in the study area are determined by the local thermohaline processes and by intrusions of Aegean waters through the open boundary at the south. The linking between mass/density distributions and observed flow structures through the dynamic height distributions and geostrophy provides information on the importance of the thermohaline processes and the Aegean intrusions in establishing flow structures. [19] The temperature, salinity, and density at 20 and at 60 m during summer are shown in the upper six maps in Figure 4. Knowing that in summer there is a two layer circulation, we consider dynamic height fields at 10 m with reference to 40 m for the upper layer (Figure 4, bottom left map) and at 50 m with reference to 65 m for the lower layer (Figure 4, bottom right map). Other choices of depths that would involve both layers in one dynamic height field are likely to produce obscure results with respect to the actual flow shown in Figure 3. At 20 m a tongue of relatively warmer water (contour of 26.2 C) originates from the shallow area of the Inner Gulf near the coast of Attika and spreads to the south and to the west into the central part of 6of23

7 Figure 4. Temperature, salinity, and density fields at 20 m (upper three left hand plots) and at 60 m (upper three right panels) during the survey in August Bottom panels are dynamic height (DH; m) fields at 10 m with respect to 40 m (bottom left) and at 50 m with respect to 65 m (bottom right) during the same survey. H and L in the dynamic height fields indicate areas of higher and lower dynamic height values or, equivalently, anticyclonic and cyclonic flow, respectively. In all maps crosses are positions of respective data points. The dashed arrow indicates the spreading route of the respective salinity feature. 7of23

8 the west subbasin. The northern area of the west subbasin is occupied by relatively colder water (contour of 25.6 C). A tongue of this water spreads to the east through the area near the southern coast of Salamina. The water mass of lower temperature in the northern west subbasin is likely to be originating from coastal upwelling. The east west orientation of the coast, the predominant northerly etesian winds during summer, and the steep bottom slope near 23.2 E (Figures 1 and 4) are all factors favoring coastal upwelling in this particular area. In the summer months the bottom layers of Elefsis Bay are occupied by cold water that is formed during winter. It is unlikely that in the summer months the Elefsis bottom water near m has any substantial outflow to the west subbasin through the shallow west channel, with a sill depth of 10 m. In the upper 40 m the relatively warmer water of the Inner Gulf and the relatively colder water of the northern area in the west subbasin develop an extended anticyclone of higher (H) dynamic heights and a smaller scale cyclone in the Salamina Aigina passage of lower (L) dynamic heights, respectively. These account for most of the observed structure in the directly measured currents of the upper layer (depth of 20 m) (Figures 3 and 4). In the northernmost part of the Inner Gulf, the geostrophic flow representation by the dynamic height field in the upper 40 m cannot account for the actual flow field (Figures 3 and 4). It is likely that the frictional layers and the topographic steering of the Salamina and Attika coasts, which approach each other in the northernmost part of the Inner Gulf, are inducing nongeostrophic effects. [20] In the deep layers two water blobs of lower temperature that appear in the temperature distribution at 60 m (contour of 15.8 C) give rise to two cyclonic centers, one in the west subbasin and the other in the Inner Gulf, whereas a smaller scale tongue of warmer temperature (contour of 16.2 C) south of Salamina develops a weak anticyclone. The cold blob in the western basin seems to be isolated, but its salinity values, which are near 38.6, indicate that it is likely to be originating from the area near the south boundary of the Gulf to the southeast of Aigina, where salinities near 38.6 also occur. The cold blob in the Inner Gulf is apparently originating from the same area since it is directly associated with a lower salinity tongue ( ) that spreads north in the central part of the Inner Gulf as shown by the dashed arrow in Figure 4. The combination of these three circulation elements can basically account for the observed flow at 60 m (Figure 3) apart from the anticyclonic branch to the east of Aigina, which appears in the ADCP flow field only at the particular depth of 60 m and is not robust in the entire layer between 50 and 65 m. [21] In early winter (December 1998) a west east density section in the upper 100 m shows that the vertical mixing has reached almost 60 m (Figure 5). The distributions of temperature and density at 20 m show that a lower temperature, lower salinity water mass spreads north from the southern open boundary and affects mostly the Inner Gulf. The low salinity values, in the range of , indicate that this mass contains water quantities of Black Sea origin. Qualitative similar distributions (not shown) occur at 60 m, with Black Sea water salinities around The combined effect of both temperature and salinity at 20 m results in a density distribution with a gradient from the lowest densities in the southwest to the highest densities in the northeast. [22] At 75 m a water mass of lower salinity ( ) is again observed in the south near the open boundary with the Aegean. This mass is spread north and affects mostly the west subbasin; its salinity values ( ) are much higher than the corresponding values ( ) at 20 and 60 m and cannot be directly associated with Black Sea origin because it is also characterized by warmer waters in comparison to the waters in the inner parts of the Gulf. This lower salinity, higher temperature water mass that spreads into the west subbasin establishes a lower density tongue and an anticyclonic (H) dynamic height area therein, shown in the dynamic height field at 65 m with reference at 78 m. [23] The particular dynamic height distribution at 65 m with reference at 78 m, below the homogenized part of the water column, captures the basic features of the actual circulation in December 1998 (Figures 3 and 5), whereas the dynamic height field at 10 m with reference to 60 m, that is, for a pair of levels within the homogenized part of the water column, cannot account for the observed flow in December (Figures 3 and 5). From the raw ADCP measurements we find that the appropriately averaged vertical current shear for the 65 to 75 m layer is higher by a factor of 2 than the corresponding shear for the 15 to 65 m layer. The application of the dynamic height method cannot resolve the existing weak shear in the nearly homogenized part of the water column or, in other cases, there may be active mixing and the flow field is nongeostrophic. [24] A picture qualitatively similar to that of December 1998 occurs in February 1999 (Figure 6). In late winter (February 1999) the temperature distributions at 20 and at 75 m show that colder water masses originate in the Inner Gulf, due to the more effective response to atmospheric cooling that this shallower area has in comparison to the deeper western subbasin. At 20 m lower temperatures in the east are associated with higher salinities originating from the southeast at the open boundary with the Aegean Sea. At a depth of 75 m, below the homogenized part of the water column, lower temperatures in the Inner Gulf and the west subbasin are now associated with lower salinities. The local minima in density at 75 m (contours of in the west and 28.0 in the east) are determined by local minima in salinity, which are not associated with any water quantities appearing near the open boundary at the south and are possibly subducted winter waters with freshwater contributions from rainfall. It is likely that atmospheric precipitation plays a large role in the late winter decreases in salinity because, on the one hand, the precipitation effect and the subduction mechanism are strengthened as winter progresses and, on the other hand, the Black Sea water inflow has a maximum in the northern Aegean around summer to autumn [Besiktepe et al., 1993], with a peak influence in the southern Aegean around late autumn to early winter. In late winter the Black Sea water influence in the south Aegean is expected to be diminishing. The local density minima result in anticyclonic (H) dynamic height distributions in these deeper layers (dynamic height at 65 m relative to 78 m; not shown) that successfully capture the general circulation structure of February 1999 shown in Figure 3. [25] In late spring (May 1999), when the seasonal warming has already started but the thermocline is not fully 8of23

9 Figure 5. Top row: Density section (right hand plot) from the surface to 100 m along the line indicated in the left hand plot from station S6 to station S11 during the survey in December Second, third, and fourth rows: Temperature, salinity, and density fields at 20 m (left hand plots) and at 75 m (righthand plots) during the same survey. Bottom row: Dynamic height (DH) fields at 10 m with respect to 60 m (bottom left) and at 65 m with respect to 78 m (bottom right) during the same survey. H and L as defined for Figure 4. 9of23

10 Figure 6. As the upper eight panels in Figure 5, but for the survey in February developed yet as in summer (August 1998), a saline tongue of Aegean water at 20 m spreads north northwest along the coast of Attica and toward the Salamina Aigina passage, whereas an isolated core of warm water is observed in the west subbasin at 37.8 N (Figure 7). These features develop an extended area of higher densities at 20 m along the saline tongue and a lower density core in the west subbasin. In the deeper layers (60 m) a steep temperature and density gradient exists along the east west direction between the Inner Gulf and the west subbasin. The temperature distribution clearly depicts the different response to atmospheric warming between the shallow eastern area of the Gulf, particularly the area along the Attica coast, and the deeper western subbasin. The dynamic height distributions for the upper and the deeper layers (not shown) cannot account for the observed flow in this transitional period of late spring. The analysis of the flows in late spring/early summer is continued in section 4, after further observational evidence from the surveys of May 2000 and June 2003 is reported in the next section (3.3) Highly Resolved Structures in the Inner Gulf; Wind Influence [26] Seasonal flow patterns in the Inner Gulf are shown in Figures 8 11 from the surveys of and , in which a dense grid of ADCP stations was em- 10 of 23

11 Figure 7. As the upper six panels in Figure 4, but for the survey in May ployed for this area. A comparison is made of these detailed flow structures with the relevant seasonal observations in and some interpretations are presented that associate their variability with the wind within a specific season August September [27] In August 2000 and September 2003 the general flow above the thermocline (20 m) is from west to east (Figure 8) as in August 1998 (Figure 3). A deep subthermocline reversal of the general flow is observed only in September 2003, with a deep pattern that resembles that in August In August 2000, however, the west to east flow at the Salamina Aigina passage is persistent throughout the water column, and a signature of a subthermocline flow reversal appears only at 70 m under the core of the surface cyclone at N, 23.6 E, where the flow reverses to become anticyclonic. In the surveys of August 1998 and September 2003 the predominant winds in the 2 day period prior to the respective cruise were blowing from the west northwest, whereas the corresponding winds in August 2000 were persistently shifted from directions farther to the north, basically from the north and north northwest, and were almost twice as strong as in August 1998 and September [28] It appears that winds from purely northern or northnorthwestern directions suppress the anticyclonic northward meandering of the eastward flow through the Inner Gulf and favor the formation of cyclonic flow to the north of the eastward flow. Winds from the northwest, west, and south, as shown in the following, have an opposite effect favoring the penetration and anticyclonic meandering of the eastward flow into the Inner Gulf and allowing for less space for cyclonic flows upstream and downstream of the anticyclonic meander December January [29] The flow structures observed in December 2000 and January 2004 (Figure 9) resemble those observed in December In December 2000 an anticyclonic meander is formed at the western part of the Inner Gulf after the flow entrance through the Salamina Aigina passage. The winds prior to the cruise were weak, blowing from western, southwestern, and northwestern directions. During January 2004 the anticyclonic meander has a larger lateral extent than in December 2000, particularly in the deep flow field at 11 of 23

12 Figure 8. As Figure 3, but for the surveys in August 2000 and September 2003 and the depths of 20 m and 70 m. H/L indicate areas of anticyclonic/cyclonic flow. 70 m. It is likely that the deeper part of the flow at 70 m, with the larger anticyclonic meander, reflects the earlier wind conditions around 4 January, with northwesterly winds reaching 15 m/s. In the upper part of the flow at 20 m, however, the anticyclonic meander is reduced in horizontal dimensions, giving more space to the cyclonic structure to the northeast of it, possibly under the influence of the later wind conditions, around 6 and 7 January, when strong winds ( 10 m/s) were blowing from the north northwest on average March [30] The flow in March 2001 has a distinct similarity to the flow observed in February 1999 (Figures 3 and 10). A deep anticyclonic loop of the eastward jet in the Inner Gulf coexists with two cyclones (L), one upstream and another downstream of this loop, while the basic shape is preserved Figure 9. As Figure 8, but for the surveys in December 2000 and January of 23

13 Figure 10. As Figure 8, but for the surveys in March 2001 and March with depth. The wind has been very weak, with speed of <3 m/s, possibly insignificant as a forcing factor. [31] In March 2004, however, the anticyclonic loop of the flow that enters into the Inner Gulf at 20 m is observed for the first time to be displaced toward the western part of the Inner Gulf. In our set of observations presented for the periods , and , the survey of March 2004 was the only one in which strong ( 15 m/s) and persistent winds from southern directions happened to be blowing on 8 and 9 March 2004, 3 to 2 days prior to the cruise (Figure 10). The southerly winds were immediately followed, on 10 March, by westerly northwesterly winds and, during the cruise on 11 March, by northeasterly winds, all of much weaker speed ( 7 m/s). It appears that the westward displacement of the anticyclonic meander at 20 m near Salamina and its deep penetration to the close prox- Figure 11. As Figure 8, but for the surveys in May 2000 and June of 23

14 Figure 12. Wind time series in August 1998, with arrowheads on the time axis indicating the duration of three consecutive surveys (top plot), and objectively mapped flows as in Figure 3 for the surveys on (upper maps), (middle maps), and (lower maps) August 1998 at depths of 20 m (left hand maps) and 60 m (right hand maps). Parallel vertical lines, solid or dashed, indicate half of a meander wavelength (l/2) during (solid) and displacement (DS) of meander crest from to (dashed). imity of Psittalia were either purely driven or significantly affected by the strong southerly winds. A cyclonic structure was observed westward of the deep anticyclonic loop at 20 m. [32] The difference in the flow structure in March 2004 at 70 m versus the one at 20 m is the northward extension of the cyclone, which, at 70 m, separates two different anticyclonic structures of smaller scale. With the limited snapshot observations of March 2004 we can only speculate about a possible mechanism responsible for the northward extension of the cyclonic structure in the deeper layers. If the flow field at 20 m, compared to the flow field at 70 m, has characteristics of a closer response to wind conditions during 8 10 March, then the flow at 70 m is likely to be still affected by the wind conditions prior to 8 March. Winds on 7 March 2004 were from the east and east northeast at a speed of 6 m/s. 14 of 23

15 Figure 13. As Figure 12 but for the wind time series and the three consecutive surveys in February 1999 (5 6, 6 7, and 8 9 February 1999). Dashed line during 6 and 7 February indicates that the ADCP survey in the Inner Gulf occurred on 7 February during strong northwesterly wind. [33] Flow structures similar to those at 70 m in March 2004 were observed in the upper 20 m in December 2000 and January 2004, with all of these periods representing winter conditions. In these two cases of the flow structures in the upper 20 m, northeastern wind vectors appear during 18 and 19 December 2000 and on 6 and 7 January 2004 (Figure 9), that is, just before and during the respective two cruises, and thus they can have a more direct impact on the surface field. The results suggest that northeastern and eastern wind components, apart from limiting the anticyclonic meandering, pushing it to the southwest of the Inner Gulf and giving more space to a cyclonic circulation 15 of 23

16 structure within the Inner Gulf, also favor the formation of an anticyclonic branch with water that appears to be pinching off the northeastern side of the cyclonic structure May June [34] The flows at 20 m during May 2000 and June 2003 (Figure 11) provide additional evidence of the flow progression to the west in the layers above the thermocline during late spring and early summer that was discussed with respect to Figure 3 for May In May 2000 a broad cyclonic branch dominates throughout the Inner Gulf, transporting water to the west subbasin, while the winds have been variable and mainly weak, not exceeding 6 m/s. In June 2003, with winds mostly from the northeast, reaching 10 m/s, the corresponding cyclonic branch is displaced to the west southwest and is reduced in size, thus giving more space to a well developed anticyclonic structure to the east northeast of the Inner Gulf. The general westward flow observed at 20 m in May 2000 and June 2003 is reversed into a general eastward flow below the thermocline at 70 m (Figure 11) Short Term (Synoptic) Variability of Circulation Structures [35] The evolution of the flow structures in the Saronikos Gulf at 20 and at 60 m for a 6 day period is shown in Figures 12 and 13 for summer (August 1998) and winter (February 1999), respectively. Observations on this shortterm variability are based on respective ADCP data obtained during three consecutive 30 to 36 h surveys in each period. [36] In August 1998 at 20 m we observe the formation and growth of an anticyclonic meander of the general eastward flow at E in the Inner Gulf from 24 and 25 August to 26 and 27 August and its subsequent decay and eastward translation during 28 and 29 August. The southerly winds observed in the wind time series on the 25th may have favored the northward penetration of this meander on 26 and 27 August, while the northerly winds on the 26th and the 27th justify its suppression on the 28th and 29th according to the observations and the mechanism discussed in the previous section. Simultaneously with the evolution of this meander, the cyclonic eddy seen between Aigina and Salamina on the 24 and 25 August displaced westward by the anticyclonic meander on 26 and 27 August, and consequently the eastern branch of the anticyclone in the west subbasin at E is displaced westward, resulting in an intensification of the speed and a purely southward flow therein. It appears that the shrinking and intensification of the anticyclonic loop in the western subbasin during the second survey (26 and 27 August) are accompanied by a deepening of this flow structure. Therefore the corresponding flow at 60 m in the second survey resembles that at 20 m, whereas in the first and third surveys the flow at 20 m was opposite to the flow at 60 m. [37] In the flow structure at 60 m some synoptic variability is observed in the cyclonic structure of the Inner Gulf that provides the westward flow into the western subbasin. From the first to the second survey the center of the cyclone is displaced southward, thus providing southward outflow from the Inner Gulf to the east of Aigina. In the third survey this cyclonic structure appears as an open loop without any recirculation in the Inner Gulf. [38] In February 1999 (Figure 13) the standard grid of ADCP stations was employed for the first and third surveys, similarly to all surveys in August 1998, while for the second survey, on the 6th and 7th, we used a dense grid of 55 stations to search for the existence of any smaller scale flow features that are not captured by the standard grid. More detail was revealed regarding the anticyclone in the west sun basin and new information became available with respect to the cross sectional structure of the existing jets, which are not presented in this work. It was also shown that the standard grid adequately captures the basic meandering features. [39] On 5 and 6 February the anticyclone in the western subbasin is providing a branch of flow into the Inner Gulf that forms a deep anticyclonic meander extending to the east and has been possibly favored by the northwesterly and westerly winds of 3 and 4 February. The same basic features are observed in the first survey at both 20 and 60 m. Toward the middle of the 6th, when the first survey of the Inner Gulf was completed, the wind shifted from southern directions and the second survey of the west subbasin started. During the 7th, when the wind shifted from the north and northnorthwest, the area between Salamina and Aigina and the Inner Gulf was surveyed. The immediate impact of the wind on the near surface flow field is shown at the depth of 20 m, where the flow in the Inner Gulf and in the area between Salamina and Aigina has a strong southeastward component, whereas the flow connection between the Inner Gulf and the west subbasin is disrupted. In this second survey and at the depth of 60 m, the anticyclone in the west subbasin still provides a flow branch into the Inner Gulf, whereas the anticyclonic meander observed in the first survey recedes from the east and progresses slightly to the north and cyclonic northward flows are developed to the east of it. Finally, in the third survey the near surface flow connection between the western subbasin and the Inner Gulf reappears at 20 m, whereas two anticyclonic meanders of the eastward jet are observed with a cyclone between them that preserves its structure down to 60 m. At this depth (60 m) a northward extension and lateral shrinking of the anticyclonic meander at E appears, which is observed on 9 February. Southerly winds favoring the northward extension of anticyclonic meanders in the Inner Gulf have prevailed during the second half of 8 February Site Specific Response of Near Surface Flow to the Wind [40] In fall of 2003 two surface buoys with flashlights on top were launched in the Saronikos Gulf to monitor the meteorologic and wave conditions in the area of the sailing competition during the Olympic Games of 2004 in Athens. That provided the opportunity for nearby deployments of underwater buoys of current meters that would be protected from sea traffic and trawlers owing to the flashlights attached to the surface buoys. It turned out that, apart from trawling nets, all fishing gear was cast near the surface buoys so as to be protected from trawling nets and, finally, tangled the current meter releasers. One time series of currents was the outcome of three deployments near these otherwise protected meteowave buoys. [41] Figure 14 shows the unfiltered time series of local wind and current at 30 m at site C over a bottom depth of 16 of 23

17 Figure 14. Time series of surface wind (m/s) and current (cm/s) at 30 m at site C (Figure 1) during the period from November 2003 to January V/U velocity components are north south/east west, respectively. Dotted, solid, and dashed lines indicate respective periods as discussed in the text. 17 of 23

18 Figure 15. Square rooted variance ellipses and mean velocities for the surface wind and the current at 30 m, both at site C (Figure 1), for the period from 3 November 2003 to 28 January Mean wind and current velocity are 1.35 m/s and 0.45 cm/s at 170 and 180 T, respectively. Maximum and minimum square rooted variance of wind are 3.65 and 1.80 m/s at 160 and 70 T, respectively. Maximum and minimum square rooted variance of current are 6.50 and 4.25 cm/s at 175 and 85 T, respectively. 70 m (Figure 1). The original recordings are every 3 h for the wind and every 0.5 h for the current. Figure 15 shows the mean values and the variance ellipses of both wind and current data. The wind is predominantly from northern directions, with a few (four or five) events with southerly southeasterly winds. The east west (U) component of the wind is substantially lower than its north south (V) component in both variance and mean value (Figures 14 and 15). The current is nearly equally distributed between northern and southern directions, with a mean value of 0.46 cm/s at 178. Its U component has a smaller mean and variance than the V component, but in comparison to the wind, its U component makes a greater contribution to the total variance; that is, the current s variance ellipse is not as elongated as the wind s (Figure 15). Nevertheless, for both wind and current, it is their north south components that account for most of the variability in the forcing and the possible response, respectively. We examine the wind and current time series, at first qualitatively in the time domain, for periods of increased cross correlation during particular events. [42] In the vector time series of wind and current (Figure 14) there are periods during which: [43] (a) The wind and the current are qualitative correlated, with their V components fluctuating nearly in phase, with at most a 1 day lag, when (i) a generally northerly/ southerly wind is associated correspondingly with a generally southern/northern flow, that is, from 2 to 11 November and from 16 to 28 January, with both periods indicated by a thin solid line, or (ii) a northerly wind is initially associated with a southern flow, but then a northern flow is developed in the presence of the northerly wind, that is, the period from 1 to 9 January, indicated by a dotted line. In the beginning, near zero values of V wind correlate with near zero values of V current ( 1 2 January), and then, when the flow switches to the north, near zero values of V wind correlate with local positive maxima in V current ( 4 5 and 8 January). The relevant flow structure at the current meter site around 8 January is shown in Figure 9 to be associated with a recurrent smaller scale cyclone that appears between the semipermanent eastward jet and the coast of Attica. [44] (b) The wind and the current are again correlated but with a time lag in their V components of 3 4 days, that is, the period from 3 to 16 December in the wind with the period from 4to 20 December in the current, indicated by a dashed line in the time series, when a northerly wind is generally associated with a southern flow and current peaks in V components lag by 3 4 days with respect to V wind peaks. [45] (c) A northern flow event appears in the absence of a significant wind forcing, that is, from 15 to 23 November, indicated by a thick solid line. This northern flow may be associated with the generation of a recurrent cyclonic gyre to the north northeast of the general anticyclonic flow that prevails in early winter in the Inner Gulf as shown also in Figure 9. [46] In the frequency domain Figure 16 shows the spectra of all wind and current components and the coherences between the V wind (north south component) and both current components. Apart from the notable spectral peak in the V wind at 10 days, other spectral peaks also appear at 4, 2.5, and 1.5 days. Corresponding current spectral peaks appear at 3.5, 2.5, and 1.5 days, common for both current components. A close wind current coupling, with statistically significant coherence, exists around days. These are apparently the time scales of the correlated wind and current signals, that is, the wind driven flow fluctuations, shown in Figure 14. Statistically significant coherence also exists at higher frequencies near 1.5 days, 1 day, and 16 h, indicative of higher frequency coupling such as the Ekman response and inertial oscillations. 4. Discussion [47] In this study we have employed snapshots of directly observed three dimensional flow fields in a confined coastal domain, that of Saronikos Gulf, and have determined the general circulation on a seasonal basis as well as modifications of the general seasonal flows in the northeastern part of the domain, that is, the Inner Gulf, that appear to be linked with different wind patterns. [48] The purely summer (August 1998) flow that is anticyclonic above the thermocline and cyclonic below it, is associated with a high dynamic-height structure and a low one respectively. The high above the thermocline is due to a warm water mass originating in the shallows of the Inner Gulf near the Attica coast that has a more effective response to the atmospheric heating than the rest of the study area. The wind can have no effect in the generation of this warmer water mass, and in that respect this evidence points toward a thermohaline generated flow, that is, due to the generation and distribution of water masses with different densities, independently of internal adjustment to geostrophic equilibrium induced by other causes. Similarly, in the deeper 18 of 23