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Home , Aegean Sea, Chios, Crete, Island, Kythira, Lemnos

1898 JOURNAL OF PHYSICAL VOLUME 37

Aegean Surface Circulation from a Satellite-Tracked Drifter Array

DONALD B. OLSON,VASSILIKI H. KOURAFALOU,WILLIAM E. JOHNS,GEOFF SAMUELS, AND MILENA VENEZIANI Division of Meteorology and , Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

(Manuscript received 6 June 2005, in final form 14 July 2006)

ABSTRACT

A pilot experiment using an array of 45 drifters to explore the circulation in the north and central Aegean is described. The global positioning system drifters with holey-sock drogues provide positions every hour with data recovery through the Argos system. The drifters were launched in four separate deployments over a 1-yr period. The resulting trajectories confirm the existence of a around the rim of the basin consistent with a buoyancy plume created by the outflow of through the ( of Çanakkale in Turkish). The degree to which this is augmented by an Ekman response to the dominant northerly winds is not obvious in the dataset owing to mesoscale dynamics that obscure the existence of any westward Ekman flow. The mesoscale eddy field involves anticylonic eddies in the current around the rim of the basin consistent with eddies with low-salinity- cores. Cyclones are also seen, with the most prominent forming over deep in the basin topography. The array also documents the interaction of the currents with the through the and island groups. These inter- actions are complicated by the nature of the mesoscale flow and in some trajectories suggest a Bernouilli acceleration in straits; in others the flow through the island groups appears to be more diffusive and involves deceleration and eddy motions. The rapid sampling by the drifters reveals an extremely nonlinear submeso- scale eddy field in the basin with length scales less than 4 km and Rossby numbers of order 1. A better understanding of the dynamics of these features is of importance for understanding the circulation of the basin.

1. Introduction biogeochemistry of the , where the Aegean forms a transition between the eutrophic Black Sea and the The with its complex , highly highly oligotrophic Mediterranean (Siokou-Frangou et irregular coastline, and combination of semi-isolated al. 2002). deep basins presents a number of challenges in our un- A specific aspect of the Aegean Sea dynamics is the derstanding of both mean and time-dependent circula- complicated topography (Fig. 1) that consists of a tion in marginal . It shares many aspects with other unique island archipelago of over 2000 , numer- marginal seas—such as the Adriatic, the , the ous gulfs, embayments, and straits. The northern Ae- Indonesian, and the South Seas—in that it com- gean contains extended shelf areas east and west of the bines wind-driven and thermohaline-driven flows and . South of this peninsula is a deep complex mesoscale eddy fields. The combination of this trench with depths reaching 1500 m (the forcing and the Aegean Sea’s role as a connection be- Trough). This is divided into three subbasins: the Spo- tween the Black Sea and the greater rades, Athos, and Basins. The central Aegean leads to a setting with important implications for the is characterized by extended deep areas such as the dynamics of the latter and the World as a whole Basin and the areas surrounding the Ba- (Balopoulos et al. 1999). The details of the physics of sin. These are bounded by the elongated Evoia Island the Aegean Sea are also crucial for understanding the in the west, the arc of the Cyclades Islands in the south, and the Minor and the neighboring Chios and Lesvos Islands to the east. The boundary between Corresponding author address: Dr. Donald B. Olson, Division of Meteorology and Physical Oceanography, RSMAS, 4600 Rick- the northern and central basin is a rather artificial one, enbacker , Miami, FL 33149. generally taken to lie between the Sporades and Lesvos E-mail: [email protected] Islands. The southern Aegean is well defined by the

DOI: 10.1175/JPO3028.1

© 2007 American Meteorological Society

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FIG. 1. Bottom topography of the Aegean Sea (map adapted from Karageorgis 1995) and locations mentioned in text. Island abbreviations are TH: Thassos, SM: Samothraki, IM: Imroz (Gökçeada in Turkish), LEM: Lemnos, TN: (Bozcaada in Turkish), LES: Lesvos, SK: Skyros, CH: Chios, IK: , MY: , RH: , KA: , and KY: .

Cyclades and the wide strait south of the passages between and Kythira, Crete and Karpa- Chios Basin. The southern basin is distinguished by thos, and Karpathos and Rhodes. The topography in substantially different water masses as compared to the the different subbasins has a pronounced impact on the central Aegean (Zervakis et al. 2000). This part of the prevailing flows, as is clearly evident in the drifter data. Aegean is bounded by the island of Crete and commu- The Aegean circulation is driven by a near-surface nicates with the through the thermohaline circulation involving the low-salinity out-

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Fig 1 live 4/C 1900 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37 flow from the Black Sea and the additional input of around the Aegean. The questions involved in the wind freshwater from rivers flowing into the Aegean. These driving of the Aegean circulation are the relative domi- are countered by a flux of more saline waters from the nance of the topographically controlled along-axis eastern Mediterranean that balance the Aegean salinity (north–south) winds in forcing upwelling and down- budget. The river runoff is very important in certain welling flows along the eastern and western sides of the northern Aegean subbasins such as the Thermaikos Aegean versus the contribution of wind stress curl over (Kontoyiannis et al. 2003; Kourafalou et al. 2004) subbasin interiors (Bakun and Agostini 2001). The and contributes to the overall salt budget (Poulos et al. dominant winds from the north are expected to drive 1997). However, the outflow of low-salinity waters from upwelling along the Asia Minor coast and a southward the Dardanelles is the most important lateral buoyancy geostrophic current opposing the inflow of eastern forcing, exceeding that of all rivers combined (Koura- Mediterranean waters. Downwelling on the western falou and Barbopoulos 2003). The Dardanelles outflow side of the basin also leads to a southward geostrophic consists of generally cooler waters with higher nutrient tendency, which enhances the buoyancy-driven current content than the oligotrophic Aegean (Siokou-Frangou system. Here these currents are quantified using a et al. 2002). This allows the outflow to be detected in drifter array deployed over a little more than a calendar sea surface temperature (SST) and in ocean color im- year. This paper will proceed with a description of the agery (Zodiatis et al. 1996; Jönnson 2003). The re- array, and then a charting of the currents across the sulting water mass is referred to as Black Sea Water Aegean, which is followed by a statistical analysis of the (BSW) from its source. The BSW is responsible for the buoyant rim current, the gyres in the central basins, and lower salinities in the northern Aegean (Zodiatis and the inflow from the eastern Mediterranean. The discus- Balopoulos 1993; Zodiatis 1994; Zervakis et al. 2000; sion is concluded by an analysis of the mesoscale eddy Kourafalou and Barbopoulos 2003). These studies dis- fields across the Aegean as seen in the drifter array. cuss frontal areas in the northern Aegean related to the spreading of BSW above the saltier modified Levantine 2. The drifter array Intermediate Water (LIW) whose source is in the The drifters deployed in this experiment were stan- southern Aegean. The studies indicate seasonal pat- dard World Ocean Circulation Experiment (WOCE) terns in the BSW pathways. In particular, the climato- surface drifters with the exception that the first deploy- logically forced numerical simulation in Kourafalou ments involved 10-m rather than 15-m drogues. To pro- and Barbopoulos (2003) shows the summer and early vide finescale resolution of the eddy field all of the autumn seasons as favorable periods for BSW spread- drifters were equipped with global positioning system ing along the northern shelf areas exiting southward (GPS) devices that take positions hourly. An estimate along the western coastline. Seasonal stratification fa- of the velocity uncertainty was of order 2 cm sϪ1 based cilitates the offshore spreading of near-surface buoyant on a noise floor in the velocity spectra at periods waters. A buoyancy-driven, northward pathway devel- between the 6 h and the 2-h Nyquist frequency. There ops, while wind-driven westward rapid advection is are some areas around the Greek islands with their high also observed, due to the prevailing strong northerlies topography where there are a few dropouts in the GPS. (, meaning annual winds). The latter was shown These sections have been removed from the datasets both in the above modeling studies, but also through prior to calculation of the velocity fields. (The reader observations of the northwest Aegean in Kontoyiannis can judge the effect of these filters by considering tra- et al. (2003). Jönnson (2003) used a 5-yr time series jectories and velocities in Fig. 5, where position jumps (1998–2002) of Sea-Viewing Wide Field-of-View Sen- and a fisherman’s recovery of a unit are included in the sor (SeaWiFS) ocean color images to show that the trajectory plots.) Velocities charted here are 40-h low- Dardanelles plume has a pronounced westward exten- passed (40 HLP) data except where otherwise noted. sion with initial pathways branching around Lemnos An alternative would be to let the turbulence filter the Island. Zodiatis et al. (1996) attribute the bifurcation data and just subsample at scales longer than the auto- pathways to wind forcing with the summer Etesian correlation time scale. This works because the trajecto- winds (northerly direction) enhancing the branch south ries are an integral of the velocity field the drifter of Lemnos. samples (Monin and Yaglom 1971). The velocities were To summarize, the Aegean is forced by buoyancy also calculated by just subsampling the trajectories at input from BSW coming through the Dardanelles as 40-h intervals. While these are not shown, they were augmented by riverine inputs and modified by the ef- also computed and the differences were found to be fects of winds over the region. The overall tendency of minimal (Յ0.001 m sϪ1 in velocity and Ͻ2 km in posi- the buoyancy forcing is to lead a cyclonic circulation tion).

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FIG. 2. Chart of the Aegean with all of the drifter trajectories superimposed; these are all 45 units deployed in 2002 and 2003. (a) Trajectories are based on hourly data, and (b) velocity vectors are 40-h low-passed (40 HLP) estimates. The shading under the drifters denotes the bottom topography as shown by the grayscale at the bottom of the figure.

The deployments were accomplished by the Hellenic subbasin-scale gyres closely linked to the complex to- National Center for Marine Research (HCMR) with pography and an intense southward flow along the help from the University of . The deployments western boundary (east coast of the Evoia Island). took place during four separate cruises from the We describe in detail below the circulation in various HCMR R/V Aegaeo (March, June, and September 2002 of the Aegean and the most prominent fea- and February 2003). The drifter trajectories for the tures emerging from the drifter analysis. overall experiment and from the separate deployments are shown in Figs. 2 and 3. The initial deployments a. The Dardanelles plume region focused on the northern Aegean and the Dardenelles Several drifters were placed in the periphery of the outfall plume. Later deployments filled in some of the Lemnos (Fig. 5a) and they all exhibited strong eastern Mediterranean inflow region. The average life offshore displacement with anticyclonic tendency. span of the 45 drifters was about 5.5 months; eight drift- Drifters that were deployed south of the Lemnos pla- ers had short duration of 1–2 months and eight drifters teau first move rapidly offshore, then experience either lasted over 8 months. We recovered and redeployed a sharp anticyclonic turn northward or continue in a nine stranded drifters. The dataset with a full metadata westward jetlike pattern. The latter is consistent with list have been submitted to the National Oceanic and offshore advection of near-surface low-salinity waters Atmospheric Administration. under the etesian upwelling-favorable winds that im- pact the eastern Aegean during summer and autumn. 3. The general circulation Branching of the buoyant waters around and between From a subjective evaluation of the total array (Fig. the islands of Lemnos and Tenedos also occurs. As seen 2) a new schematic of the circulation in the Aegean is in Fig. 3, westward displacement of drifters in the Dar- constructed (Fig. 4). The overall circulation revealed by danelles plume region was strongest during the Septem- the drifters is similar to other schematics previously ber 2002 deployment. This could be either due to an constructed for the Aegean Sea (Georgopoulos 2002; enhanced westward Ekman drift associated with the Nittis and Perivoliotis 2002), but with more details. A strong northerly winds in autumn or a response to sea- basinwide cyclonic circulation is revealed with several sonally varying discharge from the Dardanelles. Al-

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FIG. 3. The trajectories (hourly data) arising from each of the deployments in (a) March, (b) June, and (c) September 2002 and (d) February 2003. Since many drifters were still in operation at the time of the next deployment, the actual array is more dense than any single deployment (see Fig. 2). Dots indicate deployment sites here and in the other figures. though accurate estimates of the Dardanelles inflow are certain limitations on spatial and temporal coverage currently not possible, climatological estimates of the must be undertaken with caution. upper-layer discharge from the Dardanelles suggest an annual mean rate of approximately 40 000 m3 sϪ1 with b. The northern Aegean a maximum in late spring (ϳ55 000 m3 sϪ1) and a mini- Ϫ 1) NORTHEASTERN AEGEAN AND CHALKIDIKI mum in fall (ϳ30 000 m3 s 1;Og˘uz and Sur 1989; PENINSULA Bes¸iktepe et al. 1993, 1994; Tug˘rul et al. 2002; Bes¸ik- tepe 2003). This would suggest a maximum cyclonic The interaction of the Dardanelles-induced coastal flow around the basin in summer and a minimum in flow with the complex topography in the northern Ae- winter if one assumes an adjustment time for the plume gean produces a set of anticyclonic gyres. The shallow of several months. However, inferring seasonal patterns Lemnos plateau is in the immediate vicinity of the from a little more than a year’s worth of data with buoyancy-driven anticyclone associated with the Dar-

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FIG. 4. A schematic of the general circulation of the Aegean as gleaned from the drifter trajectories. The currents are superimposed over the bottom topography (map adapted from Karageorgis 1995). danelles plume. The anticyclonic circulation encom- shelf continues throughout the northern portion of the passes the Lemnos and Gökçeada (Imroz in Greek) basin. The northeastern anticyclone extends across the Islands on the northern portion of the Lemnos plateau shelf and encircles the Samothraki Island. A smaller and extends across the 200-m isobath onto the coast anticyclone is found across the shelf west of Thassos. In (Fig. 5). This cross-topography nature of the trajecto- between, the flow makes a large cyclonic arc over the ries implies that the flow is surface intensified and iso- deep topography in the North Aegean Trough. The lated from topographic influences. mean flow consists of a drift around these gyres with a The tendency for unconstrained exchange across the net displacement in the trajectories to the west. To the

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but is resolved consistently in all the deployments and is interpreted as an eddy of outflow water caught in the Aegean’s complex topography. To the north, several trajectories suggest inflow of buoyant plume waters to the Chalkidiki embayments with the inflows concen- trated on the right-hand . As seen in Fig. 5, the drifters enter the Singitikos with the coast to their right and then proceed up the narrow basin. There they form eddy systems that at their largest scale fill the bay. Exits from the bay are all on the southwest corner. The nature of these flows suggests that the large-scale in- duction of fluid into the narrow bay is part of the over- all adjustment process to the Dardanelles plume. The possible role of winds in introducing water into the em- bayments must also be considered. The dominant northerly winds should weaken inflows on the eastern sides of bays and strengthen the outflows to the west. However, this is the opposite of what is seen in the drifter trajectories where the inflowing velocities to the east exceed the outflowing currents.

2) NORTHWESTERN AEGEAN (THERMAIKOS GULF AND SPORADES BASIN) As the Dardanelles plume approaches the western side of the northern basin, it bifurcates along the 200-m

FIG. 5. Enlargement of the trajectories in the northeastern Aegean isobath (Fig. 5). A portion of the flow turns southward showing the gyres in the interior, the bifurcation along the Athos while the other turns northward into the Thermaikos peninsula, and the flow into the anticyclonic gyres in the central Gulf. This bifurcation is very similar to the situation to Aegean. Note the entrainment of drifters northward into Singiti- the east (near the Athos Basin) where the flow onto the Bay and the smaller-scale eddies filling the width of the bay. shallow topography is very different and less coherent (a) Trajectories (hourly data), and (b) velocity vectors (40 HLP). than the southward flows along the 200-m isobath. It is the strong stratification tied to the low-salinity water of east there are large slow loops onto the shelf. As the Black Sea origin that allows free communication with flow approaches the side of the Athos peninsula at the shallow regions. The portion of the current that 40.4°N, it bifurcates. This point is approximately along stays outside the 200-m isobath, however, retains higher the 200-m shelf break in the region. Again, drifters energy. Again, there are coherent gyres in the gulf and freely move northward into the shallow shelf areas on another rim current that is intensified by riverine input the north. The character of these trajectories and those within the gulf (Kourafalou 2001). that sharply turn to the south along the topography are The flow to the south again splits with some of the very different. The flows onto the northern shelf are drifters proceeding through the straits between the slow and very variable with respect to the drifters avail- westernmost Sporades Islands, while others are en- able. The flow to the south around Athos involves a trained into a gyre over the deep Sporades Basin. This narrow set of trajectories, all at high velocity. In fact feature is less persistent than the anticyclone to the east these are some of the highest seen in the entire experi- and has been previously observed to generally occupy ment with speeds over 0.30 m sϪ1. the upper 400 m of the water column and to change This flow into the deep Athos Basin south of the rotation during different hydrographic surveys (Dur- complicated three-pronged Chalkidiki Peninsula sets rieu de Madron et al. 1992; Kontoyiannis et al. 2003). In up the highest energy system in the entire experiment. the drifter data, the Sporades eddy is cyclonic most of This is a strong anticyclone centered on the mouth of the time (see Figs. 6 and 15) and anticyclonic during the Singitikos Bay (Figs. 5 and 15). To the south over the autumn of 2003; see Fig. 6. Analysis of drifter tracts in deepest part of the Athos Basin there is an anticyclonic the Sporades basin suggests that September 2003 had gyre. The anticyclonic gyre is highly time dependent, the most persistent anticyclonic flow in this area. We infer

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FIG. 6. Enlargement of the region including the passage through the Sporades and the area of Ther- maikos Gulf. There is a bifurcation of the flow north of the Sporades along the 200-m isobath with some drifters making the passage through the Sporades straits or becoming entrained into the gyre that fills the western deep Sporades Basin. Results from (top) the first three deployments and (bottom) the last deployment. (a), (c) Trajectories (hourly data), and (b), (d) velocity vectors (40 HLP). that low-salinity waters originating from the Darda- Kontoyiannis et al. (2003) found evidence of a strong nelles had filled the Sporades Basin during this period. salinity front, due to intrusion of waters of Black Sea Seasonal surveys in the north Aegean, described in origin, that extended to the Sporades Basin and was Lykousis et al. (2002) and Zervakis and Georgopoulos associated with an anticyclonic Sporades eddy. We sug- (2003), make this a very likely occurrence in the month gest that the presence (or absence) of lenses of BSW of September. Also based on seasonal surveys in the over the Sporades Basin is the likely cause of changes in Thermaikos Gulf, September was the only month that the eddy rotation. This agrees with the model results

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FIG. 7. Details of the flow through the Sporades showing the trajectories (hourly data) for drifters passing through the western most passages in (a) the first, (b) second, and (c) fourth deployments. Only one drifter approaches the area in the third deployment, which is not shown. Passage through the straits in (a) involves slowing of the drifters and eddy motions along the islands. In (b) the drifters accelerate as they approach the straight and reach maximum velocities in the center of the strait. in Kourafalou and Barbopoulos (2003), where the Dar- wide. This is much smaller than the scales of flow north danelles plume does not connect with the Sporades of the Sporades, but larger than the radius of deforma- ϭ Ј 1/2 Basin during the winter season. Furthermore, the tion. The radius of deformation, Rd (g h) /f,isap- drifter data show that, when the circulation over the proximately 6 km in winter and 12 km in summer based Sporades Basin is anticyclonic, there is no evidence of on regional hydrographic estimates of the reduced flow through the straits in the western part of the North gravity gЈ and the pycnocline depth h (Kourafalou Sporades (Figs. 6c,d). Instead drifters skirt the Spo- 2001). rades to the east and then proceed to join a flow along The trajectories through the Sporades Island arc the Evoia coast to the south. Although the drifter array (Fig. 7) vary considerably and suggest three different is perhaps too sparse to catch a continued flow into the modes of interbasin exchange: 1) diffusive exchange Thermaikos Gulf during winter, the data suggest that without acceleration; 2) Bernouilli acceleration as a jet the presence of an anticyclone blocks at least some the passes through the passages; and 3) blockage of the flow from the east, routing it southward to the east of pathway in the later deployments and flow into the the Sporades instead. Evoia coastline to the east of the Sporades. The flow reorganizes itself south of the Sporades and forms a c. The central Aegean (Evoia coastal jet and boundary current along the Evoia coast (Fig. 8). All of Chios gyre) the trajectories along this coast suggest intensified flow The North Sporades Islands represent a possible - with a rather narrow scale along the coast. rier to the rim current and can change the organization The Evoia coastal flow consists of a fairly intense of the overall circulation (see discussion below). Pas- boundary current that widens to the south. Many drift- sages through the islands are between 17 and 29 km ers enter this flow after passing through the Sporades,

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FIG. 8. The flow along the Evoia coast into the southern Aegean. Note that the eddies are filling the embayments along the coast and the tendency for the width of the current as defined by the trajectories to widen to the south. (a) Trajectories (hourly data), and (b) velocity vectors (40 HLP). particularly when a cyclonic eddy is present in the Spo- further interesting feature of the flow off Evoia is the rades Basin. The drifters join the flow along Evoia, in anticyclones locked to the topography of the embay- winter 2003, from the interior. The transit time along ments in the coast. the Evoia coast is around 18 days (15–22 days). The South of Evoia, as the solid coastline breaks into a current as defined by the envelope of the trajectories is chain of islands that make up the eastern Cyclades Is- around 10 km wide at 38.8°N. It narrows rounding the , two flow pathways occur (Fig. 9). The drifters on at 24.2°E and then broadens and slows to the the westward side of the flow pass quickly through the south (Ͼ15 km at 38.4°N) where the flow comes in straits between the islands and across the Cyclades pla- contact with the gaps in the Cyclades Island arc. A teau. Those on the outer side of the broadening current

FIG. 9. The details of the Chios gyre in the deep basin that defines the south central Aegean and the flow westward onto the Cyclades plateau. Smaller loops in the trajectories along the edge of the gyre are

M2 . This gyre is consistent there throughout the experiment and is cyclonic. (a) Trajectories (hourly data), and (b) velocity vectors (40 HLP).

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FIG. 10. The Cyclades plateau and the circulation in the south- ern Aegean. All of the units entering onto the plateau through the northern passages have come down the Evoia coast. Only one drifter enters the plateau from the deployments to the south (close to the Island ϳ36.9°N, 24.8°E) and is promptly swept back eastward again. (a) Trajectories (hourly data), and (b) ve- locity vectors (40 HLP). are entrained into a large cyclonic gyre that is locked to the deep basin between the Cyclades arc and Chios Island to the east. This cyclonic gyre—which we refer to FIG. 11. Mesoscale field over the deep topography in the central as the Chios gyre—dominates the flow in the east- Aegean and along the eastern Aegean. Here drifters enter from both central Aegean. This gyre is present in all four of the the north and the south. The gyres are not as persistent as the Chios gyre to the south or the anticyclone in the north central Aegean. deployments. Although a cyclone in this location has (a) Trajectories (hourly data), and (b) velocity vectors (40 HLP). been mentioned in a limited number of studies (as in Georgopoulos 2002; Nittis and Perivoliotis 2002), the southern Aegean through the strait south of the Chios drifter data provide for the first time observa- gyre, between Mykonos and Ikaria (ϳ37.5°N), the fast- tional evidence of a permanent cyclonic feature. The est one going through the western side of the strait. The Chios gyre fills the southern entrance to the central outfall of the Dardenelles plume seems to leave the Aegean and entrains drifters that do not leak through Aegean by leaking southwestward through the Cycla- the Cyclades to enter the . An extension of des to form another coherent boundary flow along the the Evoia coastal current therefore forms the western edge of the Saronikos Gulf (Fig. 10). side of this gyre while the Mediterranean inflow forms Most of the trajectories that exit the Chios gyre do so the eastern side. A number of drifters remain in the to the north where they join more transient, smaller Chios gyre for many revolutions (Fig. 9). cyclones in the deep basins that make up the Aegean’s The flow along Evoia therefore does not lead to a central part. In the later deployments, which concen- direct outflow from the Aegean. Rather, it participates trated more on the southern Aegean, the incoming in a leakage of fluid to the southwest through the Cy- drifters are entrained into the area between the Chios clades as well as addition of low-salinity waters to the and Lesvos Islands along the Asia Minor coast (Fig. Chios gyre. Only two trajectories actually pass into the 11). These trajectories suggest closure of the Aegean

Unauthenticated | Downloaded 10/02/21 02:28 AM UTC JULY 2007 O L S O N E T A L . 1909 circulation via northward flow in the eastern basin. The ern coastlines, consistent with the expected adjustment central portion of the Aegean is also the site of a vig- of a buoyant flow in the . This orous mesoscale eddy field. As shown in more detail westward tendency ends with a flow southward in a below, the mesoscale is dominated by a population of narrow Evoia jet along the western coast of . anticyclones over the raised topography north of the This flow has not been resolved in data surveys, but is Chios gyre, and in the northern portions of Fig. 11. A evident in numerical results with high-resolution wind persistent cyclone is also found over the Skyros Basin, fields (Kourafalou and Tsiaras 2007). The prevailing in the lower left of Fig. 11. basinwide northerly winds could be a factor in main- taining this flow, although local wind effects are also d. The southern Aegean possible. The presence of the Evoia capes enhances the As the western boundary flow leaves the Cyclades, it cross- velocity component, allowing the widening feeds into the and then the Sea of Crete, of the alongshore flow to the south of the capes. Addi- which is part of the Mediterranean proper (Fig. 2). This tional topographic constraints are present downstream, flow mirrors the situation north of the Sporades with given the complex nature of the Aegean topography. the incoming diffuse flow forming anticyclonic gyres and Along this route the flow navigates a wide range of an intensified flow along the western side of the basin. coastal embayments and must pass through several is- This last boundary flow along southern Greece also ex- land groups. hibits some strong cyclonic and anticyclonic eddies. In The deployments here only provide one realization this regard it is similar to the flow to the north. The flow of the seasonal circulation in the Aegean (Fig. 3). in the Sea of Crete is not well resolved in the drifter Within the dataset the buoyancy-driven flow induced at array and its general circulation will not be discussed. the Dardanelles is consistent, but changes its route to The southern Aegean is the area that connects the the western Greek coast as the circulation between the basin with the eastern Mediterranean general circula- Thermaikos Gulf and the Sporades Basin changes from tion. The influx of modified Leventine Intermediate anticyclonic—consistent with a gyre of low-density Water (high in temperature and salinity) along the east- fluid—to a cyclone, as found over other deep basins ern Aegean and at depths of about 100 m is a known (Fig. 4). Under the latter condition drifters move south- feature of the Aegean Sea circulation (Theoharis et al. ward to the east of the North Sporades, but they are still 1993). The near-surface circulation depicted by the entrained into the coastal flow off Evoia. The general drifters also includes a northward drift along the east- circulation in the northern Aegean during the drifter ern Aegean, which is particularly evident north of the experiment did not appear to be significantly affected Cyclades arc (the wide strait between the Mykonos and by the wind stress curl. Analysis of the wind field during Ikaria Islands) and is generally considered the bound- the drifter experiment period confirms that strong ary between the central and the southern Aegean. A northerlies dominate the Aegean wind regime, consis- number of the later deployment drifters exhibit north- tent with climatology. The magnitude of the northerly ward flow at the east of the cyclonic Chios gyre pro- winds has a tendency to decrease from east to west, ceeding northward up the eastern side of the central which would suggest that flow due to the wind curl Aegean Basin. would be anticyclonic, that is, opposite to the observed prevailing trajectories. 4. Discussion At the boundary between the central basins and the The overall circulation of the Aegean revealed by the northern Aegean many drifters in the later deploy- drifters consists of a cyclonic flow around the rim, ments are entrained into proximity of the flows de- driven by the Dardanelles buoyancy input. The eastern scribed in Fig. 4. None of these drifters enter the side of the Aegean Sea is dominated by an inflow of Thermaikos Gulf and the area north of the western Mediterranean Water that completes the cyclonic cir- Sporades. Several do return to the south via the east- culation. The inflow is not as organized as the outflow ernmost Sporades channels and enter the outer por- along the eastern coast of Evoia Island. This may be tions of the Evoia flow. During a portion of this period tied to the prevailing northerly winds that would en- the area north of the western Sporades is dominated by hance the Evoia flow but oppose the inflow from the a cyclonic flow (Figs. 6a,b), which may block their en- Mediterranean. The surface Ekman flow to the west try. Alternatively it is possible that they are on the may also augment the offshore spreading of the Dar- outer portion of the east–west circulation in the far danelles plume. Drifters also show anticyclonic motion northern basin and are sweeping out a pathway that is off the mouth of the Dardanelles. These waters then semipermanent. An exception is a drifter from the last move in a cyclonic pattern along the northern and east- deployment that reached the Sporades Basin during

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from 0.5 to 1 day and yields the independent sample statistics shown in Fig. 12. The best sampling both in terms of seasonal coverage and total number of inde- pendent data occurs in the deep Sporades Basin. Por- tions of the eastern Aegean have good overall data coverage, but suffer from poor seasonal coverage. The poorest coverage is in the southern Aegean. This area will not be a major focus of the following discussions. The mean and eddy velocities, based on the number of independent samples, are shown in Fig. 13. These statistics provide another view of the flow described above. It is interesting to note that poorly sampled ar- eas tend to have higher mean flows and variations. This is inherent in Lagrangian sampling in that drifters do not stay in high flow regimes for long. The plot also shows a decrease in both mean and eddy velocities to the north of the Sporades passages and an intensifying current along the Evoia. The Chios gyre shows up well in the mean flow and has a decrease in eddy activity toward its center. There is also a decrease in both mean and eddy flows as drifters approach the island barrier that separates the Chios basin from the southern Ae-

FIG. 12. Number of independent velocity estimates based on the gean. Again, this reflects the loss of momentum tied to integral Lagrangian autocorrelation time scale (Figueroa and Ol- flow through the island passages. son 1989; Bauer et al. 1998). This time scale ranges from 0.5 to 1 The basin eddy kinetic energy distributions are day in the dataset. The estimates of the mean circulation and eddy shown in Fig. 14. The highest energy levels are in the kinetic energy in Figs. 13 and 14 are based on independent esti- center of the northern Aegean over the Athos Basin. mates obtained by breaking the dataset up into segments. The graphic here shows the number of such independent estimates The energy maximum extends to the west over the Spo- that go into each 0.2° ϫ 0.2° bin. rades Basin. The flow along the Evoia peninsula also has elevated eddy energies. Although the sampling in September 2003 (7 months after a position near the the southern Aegean is poor, there appears to be an Lemnos plateau in the Dardanelles plume) and domi- extension of the higher eddy kinetic energy there and nated the flow field with strong and persistent anticy- into the Sea of Crete. In contrast the eastern basin has clonic motion. As discussed earlier, data and modeling lower kinetic energies except in local areas such as the support westward advection of waters originating at passage between the islands of Chios, Lesvos, and the Dardanelles during the late summer/early autumn sea- Asia Minor coast where the 200-m isobath turns son, due to the preceding high inflow conditions, the abruptly to the east (Figs. 1, 14). prevailing strong northerlies, and the stratification of the water column that allows the winds to act on the b. Subbasin-scale gyres and flows near surface buoyant waters. This has implications on Before exploring the various subbasin-scale gyres the water properties in the northwestern Aegean and that dominate the dataset, it is worthwhile to consider on the rotation of the Sporades eddy. the dynamics behind their formation. In the Aegean case there are two types of dynamics that can lead to a. Flow statistics and basin energetics the gyres. The first is the strength of the Dardenelles A means of judging the representation of the flows in outflow. A buoyant discharge released from a river or the Aegean that the drifter array provides is to consider strait will produce a plume with the coastline to its right the independent velocity estimates by 0.2° ϫ 0.2° bins in the Northern Hemisphere (see Chao and Boicourt (Fig. 12). Here velocity estimates are assumed to be 1986; Chao 1988; Garvine 1987; Münchow and Garvine independent if they are separated by times longer than 1993; Kourafalou et al. 1996). Temporal adjustments to the Lagrangian integral time scale (decorrelation time these flows would occur through edge or Kelvin waves scale) computed from the drifter velocities (Monin and (Garvine 1995; Nof and Pichevin 2001) or through Yaglom 1971; Figueroa and Olson 1989; Bauer et al. mesoscale mixing. In a simple linear two-layer system, 1998). This Lagrangian time scale for the array varies the width of the plume flow is limited to the scale of the

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Fig 12 live 4/C JULY 2007 O L S O N E T A L . 1911

FIG. 13. The mean velocity field with variance ellipses for the Aegean drifter array. Color code divides the estimate into areas with greater than 10 estimates (red) and those with less than 10 but greater than 5. Statistics have been calculated in 0.2° ϫ 0.2° bins. baroclinic Rossby radius of deformation and its maxi- conclusion then is that the outflow is perhaps subcritical mum speed is limited to the downcoast Kelvin wave for most of the year and supercritical during maximum speed. This gives an upper bound for the plume trans- outflow conditions. The conclusion is that at least in port of order gЈh2/(2f ). In the Dardanelles case, with a some months the outflow exceeds the ability of the lin- reduced gravity of approximately 0.05 m sϪ2 (Bes¸iktepe ear process to adjust to it, and one expects a nonlinear et al. 1993, 1994; Bes¸iktepe 2003), a thickness of 10–20 adjustment (Nof and Pichevin 2001). This can consist of m (Og˘uz and Sur 1989; Kourafalou and Barbopoulos enough mixing to modify the parameters in the adjust- 2003) yields an estimate of the critical transport of ment, that is, the reduced gravity and pycnocline depth ϳ50 000 m3 sϪ1. This compares to a mean Dardanelles h, or it will involve the formation of eddies (anticy- outflow of ϳ40 000 m3 sϪ1 with an annual range from clones) near the outfall that fill the flow around the ϳ30 000 to ϳ55 000 m3 sϪ1 (Tug˘rul et al. 2002). The northern and western side of the Aegean.

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Fig 13 live 4/C 1912 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37

FIG. 14. The distribution of eddy kinetic energy (EKE) in the Aegean based on the drifters. Bins with less than five independent estimates are dropped; contours are at 100 cm2 sϪ2 intervals.

Representative examples of these anticyclones are active scale changes as eddies split apart on topography those found south of Singitikos Bay (39.9°N, 24.3 °E). and then coalesce again to fill bays and basins (Nof One drifter makes three loops around one of these in 1988a,b; Simmons and Nof 2000). March 2002. Decomposition of the eddy shows swirl The tendency for the flow to produce gyres that fill velocities of Ϫ0.5 m sϪ1 (negative for anticyclones) at a the available space can be explained as a case of red radius of approximately 15 km. This suggests a fairly cascade in the mesoscale turbulence field (McWilliams nonlinear feature with a Rossby number of ␷/(fr) ϭ 1984; Griffiths and Hopfinger 1987; Melander et al. Ϫ0.33. The trajectories suggest that the eddy is moving 1987, 1988). Cascading to larger eddy size occurs as a little west of south at a translation speed of 0.07 m sϪ1. eddies merge together. The rough topography, how- This is higher than expected due to ␤ drift (Nof 1981), ever, also acts to drain fluid out of eddies and split them which is a fraction of the long Rossby wave speed, |c| Ͻ up (Nof 1988a; Simmons and Nof 2000). The balance 0.002 m sϪ1, suggesting that advection of the eddy field between these two tendencies is crucial for understand- is dominant. Interestingly the drifter leaves the eddy to ing the organization of the flow around the Aegean. the north and is entrained into Singitikos Bay where it Disruption of larger scales occurs as the buoyancy circulates around the embayment in a sense consistent plume and its associated large-scale eddies are forced with a buoyancy plume. Some drifters show anticy- through the narrow straits that appear along the bound- clonic eddies within the bay itself. The dynamics of ary. This is followed by an increase in eddy scale as the eddy interactions with coastlines and islands involves flow leaves the proximity of the topography. The pic-

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Fig 14 live 4/C JULY 2007 O L S O N E T A L . 1913 ture is one of a complex turbulent flow forced by the by the freshwater plume and the Taylor tendency over Dardanelles buoyancy source, the along-axis prevailing the deep topography. This conclusion is in agreement winds, and the complex geometry. with the observations in Kontoyiannis et al. (2003) and A second origin of gyres involves forcing over topog- numerical simulations by Kourafalou and Barbopoulos raphy. Theory and observations suggest that the circu- (2003). lation above should be anticyclonic The edges of the Chios gyre are marked by entrain- (Bretherton and Haidvogel 1976; Vallis and Maltrud ment of the outer edge of the Evoia flow and the in- 1993). Conversely, the persistent cyclonic gyres over a troduction of Mediterranean waters on the eastern side deep topographic basin, like the more ephemeral cy- of the gyre. The latter is not well sampled except in the clonic gyre in the Sporades Basin in winter 2003, can be last deployment. Other features obvious on the edges rationalized as a Taylor column response over the deep of the gyre are small-scale loops in the trajectories that basins such as those associated with faults in the Ae- have frequencies consistent with the M2 semidiurnal gean. The tendency to form cyclones over deep basins lunar . This is one of the few regions in the entire stems from a consideration of the potential vorticity, basin where tides are evident in the drifter data, con- (f ϩ ␨)/H, where ␨ is the relative vorticity and H is the sistent with the overall weak tidal forcing in the Ae- basin depth. As advection forces a column over the gean. deep areas, H increases leading to an increase in ␨ to The Chios gyre is connected to the northern portions conserve potential vorticity (see Holloway 1978). Simi- of the Aegean through a broad inflow of Mediterra- larly the flow forced by random mesoscale inputs over nean water along its eastern flank and a more chaotic a deep basin will be cyclonic. linkage to cyclonic features that occupy the deep basins The Aegean has a number of deep subbasins and in the central region between Skyros and Lesvos in the (Fig. 1). The subbasins in the Aegean arise east–west and Lemnos to the north (Fig. 11). The meso- from a series of faults. There are three deep basins in scale field in this region is weaker and more variable the northern Aegean (Lemnos, Athos, and Sporades in with a consistent appearance of cyclones over the deep the North Aegean Trough) that capture cyclonic flow, topography, but with a great deal of variability. The as expected, if not occasionally overwhelmed by the gyre scales are smaller than that of the Chios gyre. The high pressure, anticyclonic tendency induced by the inflow from the Mediterranean leads to eddies in the Dardanelles plume. The central Aegean has two deep interisland passages to the east toward the Turkish areas. The southern of these (Chios Basin) is a large coast. These circulations are not as well resolved as the deep basin directly under the Chios gyre. The northern freshwater outflow to the north and west. one (Skyros Basin), also has cyclonic flow over it. There As important as the two gyre-producing mechanisms, are several smaller topographic depressions throughout 1) formation of high pressure eddies in the buoyant the Aegean. The drifters show cyclonic flows over de- plume and 2) rectified flows over topography, are the pressions south of Lesvos Island and north of Ikaria boundary conditions that limit them. This includes the Island (Fig. 3). nature of the coastlines that surround the Aegean, with The tendency for cyclonic currents around topo- its roughness in terms of capes and , and the in- graphic depressions is consistent with Holloway’s ternal boundaries induced by the island arcs. (1992) Neptune effect. These can be induced by ran- c. Properties of the eddy field dom forcing over the topographic depressions either by the wind, tides, or the mesoscale eddy field itself (Griffa A picture of the eddy field including both transient et al. 1996). The primitive equation simulations in and stationary features can be obtained by using an Griffa et al. (1996) provide an estimate of the spinup ellipse fitting routine (Brown et al. 1983). Here only time of the cyclonic motion. In the model simulation trajectories that have at least 270° rotations are consid- spinup occurred in approximately four mesoscale eddy ered. The results are shown in Figs. 15a,b. The first ϭ turnover times, Te L/Vrms, where L is the scale of the panel of the figure shows the eddy mean axis lengths as gyre and Vrms is the root mean squared velocity for the given by the average of the major and minor semiaxes mesoscale eddies. Substitution of the Chios gyre scale from the fits. Each eddy is plotted at the estimated and the velocity estimates suggest a spinup time of the center location. The average swirl velocity around the cyclonic flow over the deep basin on the order of sev- eddies is shown in Fig. 15b. The eddies are fairly evenly eral days. The Chios basin, separated from the high distributed across the sampled area: however, there is a pressure tendency induced by the Dardanelles waters, string of anticyclones around the edge of the basin ex- maintains this cyclonic flow. Apparently the system tending from the northeast basin along the north, down north of the Sporades alternates between domination the Evoia coast, then through the Cyclades, on along

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FIG. 15. The distribution, (left) size and (right) strength, of eddies in the Aegean Sea based on fitting drifter trajectories to ellipses (see Brown et al. 1983). Cyclonic eddies are shown as blue circles and anticyclones are red. The position of the features is determined by the fit and represents the center estimate. The size is the average diameter (equivalent circle) representing the average of the major and minor . the edge of the Saronikos Gulf, and into the Myrtoan in velocities for the cyclones (0.2 m sϪ1) exceeds that of and Cretan Seas. There are also two bands of weaker the anticyclones (Ϫ0.13 m sϪ1) as is expected from the anticyclones in the central basin centered at 39.4° and gradient current balance (Olson 1991) that demands 38.3°N. In general, the cyclonic eddy field is shifted into lower velocities for anticyclones with the ampli- the interior of the basin as compared to the anticy- tude pressure gradient. This arises owing to the positive clones. In particular, in the northern portion of the ba- definite centripetal term (␷ 2/r) in the gradient balance sin cyclones are found to the south of the inferred axis (␷ 2/r ϩ f ␷ ϭ gЈdh/dr), where gЈ is reduced gravity and of the cyclonic mean flow around the basin. This buoy- h is pycnocline depth. This balance depends on the non- ant rim current is wrapped up into anticyclonic eddies. linearity of the eddy as compared to the geostrophic There is a particularly persistent set of cyclones just balance, that is, the balance between the Coriolis and north of the Sporades island arc. In the winter 2002/03, the pressure gradient (Fig. 17). The last figure reveals during the reversal of the gyre circulation north of these the degree of nonlinearity in the field of eddies in the islands from cyclonic to anticyclonic, there is a persis- Aegean. Most of the eddies fall within a range of tent set of smaller-scale cyclonic eddies that are first Rossby numbers between 0.2 to 1.0 with a similar dis- advected counterclockwise around the cyclonic gyre tribution for both cyclones and anticyclones. and then clockwise around the subsequent intensifying larger-scale anticylonic circulation. This forms the ring 5. Conclusions of cyclones around the edge of the deep basin north of the Sporades (Fig. 15a). In the Chios gyre there is also The circulation in the Aegean is dominated by an a population of cyclonic eddies that are advected intensified rim current system that is consistent with the around the larger-scale circulation. buoyancy introduced from the Dardanelles outflow and The distribution of eddy sizes and velocities is shown along axis wind forcing from the north. Within the as histograms in Fig. 16. The mean radius of eddies dataset it is difficult to differentiate the influence of displays peaks at less than the radius of deformation, buoyancy from that of the wind. Flows over the north- consistent with trajectories that are within eddy cores eastern portion of the basin are largely consistent with for the most part. The anticyclonic eddies show a ten- a westward Ekman drift of less than 0.03 m sϪ1 (Fig. dency for some larger features with higher speeds. 13). The flow around the edge of the Aegean, however, These are representative of the strong transient anticy- starts with acceleration around a cyclone to the west of clones south of Singitikos Bay and elsewhere in the the Lemnos plateau. The flow along the northern rim of buoyancy-driven rim flow as discussed above. The peak the Aegean Sea is more consistent with a buoyancy

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Fig 15 live 4/C JULY 2007 O L S O N E T A L . 1915

FIG. 16. Histograms of (a) the mean radius of the semiminor and semimajor axes and (b) the mean velocities from the eddy fits in the Aegean. In the plots the anticyclonic features are negative using the usual convention for velocities in cylindrical coordinates. current than one driven by the wind. This conclusion gion is in areas downstream of passages around the extends to the nature of flows within the embayments complex topography and transition of the flow either where the inflow velocities are higher than the outflows around abrupt capes, as in the northern kinetic energy on the western sides of the bays. A wind-driven flow maximum (Fig. 14), or in the reorganization of the flow would be more symmetric. It is also hard to definitely account for the source of the Evoia jet, which is con- sistent with a buoyancy-driven model, but can also be augmented by an interior response to the wind forcing. This problem can be addressed by consideration of the boundary layer scaling (Pedlosky 1987). For interior flows of U Յ 0.05 m sϪ1 an inertial boundary layer scal- ing as ␦ ϭ (U/␤)1/2 is on the order of 50 km wide. The observations suggest a narrower current. A similar cal- culation for the frictional layer thickness using the Lagrangian diffusivity estimate based on the array of ␬ ϳ1.3 to 3.9 (ϫ 103 m2 sϪ1) yields [␦ ϳ (␬/␤)1/3] a scale of 50 km. Again the scale of the boundary current in the dataset is less than this. It is not clear that the open basin eddy diffusivity is the correct diffusivity to use. Spall (2003) has suggested that the circulation around semienclosed basins is very sensitive to the turbulent diffusion in proximity to the boundary. The measure- ments suggest a forced coastal flow rather than an in- terior driven response. This flow is nonlinear with mean currents along the coast of Evoia having a Rossby num- ber ϳ0.2. The eddy field is well measured by the drifter array. It is also moderately nonlinear with a Rossby number Ͼ0.2 for most of the eddy population. There is a larger FIG. 17. Inertial radius ␷/f (km) plotted vs mean radius (km) for range of anticyclonic eddy scales (Fig. 17). A fairly the Aegean eddy field. Lines denote the Rossby numbers ␷/fr for large proportion of eddies have Rossby numbers ap- points falling on them. The shallowest slopes are for Rossby num- proaching unity. The maximum eddy energy in the re- ber 0.2, the intermediate: 0.5, and the steepest: 1.0.

Unauthenticated | Downloaded 10/02/21 02:28 AM UTC 1916 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37 in the southern Aegean. The eddy field is characterized eddy field is closely linked to the bottom topography ϭ by scales that are well less than the Rhine’s scale LR and the geography of islands and coastlines and that (uЈ/␤)1/2, where uЈ is the root-mean-square eddy ve- their interaction is a crucial aspect of understanding locity (Rhines 1975; Spall 2003), estimated from the marginal seas. Further understanding of these features ϭ array statistics to be approximately LR 100 km. Since should be an observational and theoretical priority. essentially all eddies (Fig. 15) are under this scale, the conclusion is that the basin is governed by mesoscale Acknowledgments. This work was funded by the Of- eddy mergers and scale increases rather than western fice of Naval Research under Grants N000140110134 propagation of Rossby waves (Rhines 1975). The tur- and N000140310439. The effort could not have been a bulent eddy field, instead, seems to modulate a cyclonic success without the support of our Greek collaborators. flow around the edge of the basin(s). This tendency is Haris Kontoyiannis and Vassilis Zervakis from the Hel- replaced by the production of cyclones over the deep lenic Center for Marine Research in Athens were en- basins such as the series of basins across the northern gaged in the drifter shipment and related permits and in Aegean and the Chios region. These features seem to the drifter deployments. Together with Sarantis Sofi- be persistent in the dataset, although the cyclone in the anos from the University of Athens, they provided sug- basin north of the Sporades appears to be covered over, gestions on the overall experiment planning. We thank or replaced by, an anticyclone associated with Dar- the captain and the crew of the HCMR R/V Aegaeo for danelles waters during a portion of the period. incorporating the drifter deployments in their opera- The pilot drifter array produces a quasi-synoptic look tions. We thank Yiannis Krestenitis from the Univer- at the Aegean circulation and its associated eddy field sity of and the Greek Coastguard authori- over an annual period. The hourly GPS sampling pro- ties for help with drifter recovery. Wind data for the vides detailed statistics on the mesoscale and subme- drifter experiment period were provided by Tassos soscale eddy structures. The drifter array provided ad- Papadopoulos of the HCMR operational equate coverage of the northern and central Aegean, forecast system and were further analyzed by Kostas but only a suggestion of the nature of the flow in the Tsiaras of HCMR. Professor Emin Özsoy and an southern Aegean and Cretan Seas. anonymous reviewer added significantly to the quality The existing array presents several intriguing conclu- of the manuscript. sions concerning the nature of the Aegean circulation. The first is the lack of evidence of the expected west- REFERENCES ward Ekman drift in the basin suggested from the pre- Bakun, A., and V. N. Agostini, 2001: Seasonal patterns of wind- vailing northerly winds (Bakun and Agostini 2001). induced upwelling/downwelling in the Mediterranean Sea. J. Converting the mass transport in Bakun and Agostini Mar. Sci., 65, 243–257. (2001) to westward velocities using a 30-m Ekman Balopoulos, E. T., and Coauthors, 1999: Major advances in the depth suggests an expected Ekman flow of 0.03 m sϪ1. oceanography of the southern Aegean Sea–Creatan Straits system (eastern Mediterranean). 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