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1898 JOURNAL OF PHYSICAL OCEANOGRAPHY 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 Physical Oceanography, 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 Sea 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 current around the rim of the basin consistent with a buoyancy plume created by the outflow of Black Sea waters through the Dardanelles (Strait 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-water cores. Cyclones are also seen, with the most prominent forming over deep regions in the basin topography. The array also documents the interaction of the currents with the straits through the Sporades and Cyclades 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 region, where the Aegean forms a transition between the eutrophic Black Sea and the The Aegean Sea with its complex archipelago, 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 seas. It shares many aspects with other unique island archipelago of over 2000 islands, numer- marginal seas—such as the Adriatic, the Caribbean, the ous gulfs, embayments, and straits. The northern Ae- Indonesian, and the South China Seas—in that it com- gean contains extended shelf areas east and west of the bines wind-driven and thermohaline-driven flows and Chalkidiki Peninsula. South of this peninsula is a deep complex mesoscale eddy fields. The combination of this trench with depths reaching 1500 m (the North Aegean 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 Mediterranean Sea rades, Athos, and Lemnos 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 Ocean as a whole Skyros Basin and the areas surrounding the Chios 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 Asia Minor coast 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 Causeway, 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 Unauthenticated | Downloaded 10/02/21 02:28 AM UTC JPO3028 JULY 2007 O L S O N E T A L . 1899 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: Tenedos (Bozcaada in Turkish), LES: Lesvos, SK: Skyros, CH: Chios, IK: Ikaria, MY: Mykonos, RH: Rhodes, KA: Karpathos, and KY: Kythira. Cyclades Island arc and the wide strait south of the passages between Crete 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 eastern Mediterranean through the thermohaline circulation involving the low-salinity out- Unauthenticated | Downloaded 10/02/21 02:28 AM UTC 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 Gulf (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 flat 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. (etesian, 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.