Characteristics Of The Eastern Adriatic Current In The Coastal Area Between

Dubrovnik And BarNENAD LEDER1, GORDANA BEG PAKLAR2, BRANKA GRBEC2 & FRANO MATIĆ2

1 – Faculty of Maritime Studies, Ruđera Boškovića 37, 21000 Split, Croatia, nenad.leder@pfst.hr2 – Institute of Oceanography and Fisheries, Šetalište I. Meštrovića 63, 21000 Split, Croatia

ABSTRACTThe research of sea currents in the Adriatic has a long history. Numerous methods of current measurements as well as indirect methods of current determination were applied. A large number of numerical models were developed to explore variability of the Adriatic current system at various space and time scales.Direct long-term measurement of the sea currents in front of Dubrovnik in combination with the Princeton Ocean Model (POM) adapted for the whole Adriatic were used in this paper to describe a part of very stable Eastern Adriatic Current (EAC) in the coastal area between Dubrovnik and Bar where the coastline is generally oriented in the direction of NW-SE, without larger islands. It was found that at the current meter station with total depth of over 100 m off Dubrovnik, the current directions were dominantly between WNW and NW with very high stability factor, while the most intensive currents were recorded in WNW direction, parallel with the coastline. The maximum current flow near the surface was about 95 cm/s, while at the bottom it was weaker, about 40 cm/s. Intensity and stability of the East Adriatic Current flow in the area between Dubrovnik and Bar, as well as the existence of South Adriatic Cyclonic Gyre over the southern Adriatic Pit were confirmed also by the POM model results. Obtained results imply that all waste that reaches the sea in the coastal area southeast of Dubrovnik will likely pollute the whole area towards northwest.

KEYWORDS: currents, Eastern Adriatic Current, numerical model, Adriatic Sea.

I. INTRODUCTIONIn order to understand characteristics of the current field in a particular sea area, general outlines should be given about the main current generating forces. The main current generating forces are [1]: the force generated from horizontal differences in the sea density (gradient currents), tidal force causing tidal currents and wind drift force developed from the impact of tangential wind stress on the sea surface (drift currents).

Apart from generating forces, currents are also highly influenced by dimensions and topographic characteristics of the coast and the seabed of a particular basin.

To determine a current field at sea indirect and direct methods are used. Indirect method comprises distribution of some properties of sea water (for example temperature and salinity) to conclude on sea currents while direct method presents measurement of parameters needed for description of current field. Well known indirect method is determination of geostrophic currents from measured sea temperature and salinity, assuming that Coriolis force balances the horizontal pressure gradient [2].

For direct measurement of sea currents Lagrangian and Eulerian methods are usually applied. Lagrangian method includes spatial and temporal tracking of sea water particle (or tracers, drifters etc.) giving its trajectory. Eulerian methods include measurement of speed and direction at one position having as results stream lines at one point [3].

Indirect and direct methods were used to determine current field in the Adriatic paying more attention to the sea surface currents.

The first exploration of sea currents in the Adriatic was related to experience of sailors. Lorenz (1863) and Wolf and Luksch (1881) made a first set of „sea currents charts“ based on spatial distribution of measured temperatures and salinities. Wolf and Luksch (1887) edited a chart of sea surface water

circulation in the Adriatic Sea with set of cyclonic gyres and „recirculations“ at the levels of the Palagruža Sill and south of the Istrian Peninsula [4].

Direct measurement of sea currents in combination with indirect method of geostrophic approximation enabled making the „new“ charts of the Adriatic surface currents . One of these charts was made by Zore-Armanda in 1967 [5] (Fig. 1). After Zore-Armanda the sea currents along the east Adriatic coast have prevailing NW direction, stronger in winter than in summer. Along the west Adriatic coast prevailing currents are SE directed, stronger in summer than in winter. Apart from strong seasonal variability, cross-currents in the area of Jabuka Pit and South Adriatic Pit can be noticed in charts, and there is some indication of circular circulation in the Northern Adriatic basin. However, it should be emphasized that sea currents in inner coastal waters are not shown in charts.

By implementation of Lagrangian method in current measurements using drifters, until lately, only surface currents were measured, although recently drifters which can be programmed to measure through water column are developed. The mean Adriatic surface circulation based on all drifter data for the period from 1990 to 1999 was derived by Poulain in 2001 [6].

According to Poulain, Lagrangian measurements confirmed the classical basin-wide cyclonic pattern in the Adriatic Sea, with recirculation (cross-circulation) in the vicinity of Jabuka Pit and the southern Adriatic basin as well as in the northern Adriatic basin.

Resultant surface circulation in the Adriatic may be explained as modification of gradient currents under the influence of tides and prevailing winds [7] as well as under the influence of some other phenomena and processes like seiches and inertial oscillations [8] and [9].

Development of numerical models in oceanography enabled exploration of acting forces and their influence on the sea current fields. In the last 30 years a large number of numerical models were applied for the Adriatic [4].

Fig. 1. Sea surface currents in winter and in summer (from [5]).

Eastern Adriatic Current (EAC) is a branch of the general Adriatic cyclonic circulation along the eastern part of the Adriatic Sea with dominant NW direction. It is well known that EAC varies seasonally, being strongest in winter and weakest in summer.

A very stable part of Eastern Adriatic Current in the coastal area between Dubrovnik and Bar will be described using long-term current measurements performed with Acoustic Doppler Current Profilers (ADCP’s). Current measurements were part of the scientific and research program – „The Adriatic Sea Monitoring Program“[10]. The measurements started in November 2007 and lasted until January 2009. Comparison between results of current measurements and numerical model will be presented too.

II. MATERIALS AND METHODSThe sea area off Dubrovnik, from an oceanographic point of view, is distinguished by a large depth gradient, which means that the sea depth rapidly increases with the departure off the coast (Fig. 2).

Currents were measured at station S20 (φ=42° 38,06' N, λ=18° 02,66' E, depth 105 m; WGS84) positioned at a distance of about 3 km off the coast (Fig. 2). In the winter ADCP was positioned at 105 m depth, while in the summer it was somewhat closer to the coast, at a depth of about 80 m (the station is therefore marked S20A). The measurements started on 28 November 2007 and lasted until 16 January 2009. Teledyne RD Instruments ADCP (Acoustic Doppler Current Profiler; Fig. 3) bottom mounted current meter was used with the vertical resolution of 2 m and sampling interval of 15 min.

Basic statistical analysis and spectral analysis of currents were applied [11]. Monthly and seasonal oscillations were described through the mean monthly current vectors and current roses.

Physical properties of the Adriatic for the period between 15 August 2007 and 15 November 2008 were calculated with Princeton Ocean Model (POM; [12]). POM is a three-dimensional nonlinear numerical model with complete hydro- and thermodynamics and turbulence closure submodel ‘Level 2 ½ [13]. The equations which capture the model physics are the traditional equations for conservation of mass, momentum, heat and salt coupled with the equation of state [14]. The equation of state is a modified UNESCO form.

Adriatic-scale POM model had horizontal resolution of 2.5 km and covers the Adriatic basin with 108x320 grid points. Along the vertical 22 unequally spaced sigma layers were defined with increased resolution in the surface and bottom boundary layer. POM model was controlled by atmospheric, hydrological and tidal forcing. Atmospheric forcing for the Adriatic model adaptation included wind stress [15], surface heat ([16];[17];[18]) and water (E-P) fluxes calculated on the basis of the surface fields from the mesoscale model ALADIN [19] and instantaneous sea surface temperatures obtained by POM. Model domain contains 41 river discharges parameterized according to [20]. Climatological discharges were scaled according to realistic values from Neretva and Po Rivers [21]. Initial temperature and salinity fields were obtained by bilinear interpolation of the summer climatological fields into the Adriatic model domain, while velocity field was initialized with the state of rest. At the POM southern open boundary 7 harmonic constituents - M2, S2, N2, K2, K1, O1, P1 - are applied for denivelation and transport, while radiation condition is applied for three-dimensional current field.

III. RESULTS AND DISCUSSIONTime series of vertical profiles of mean monthly current vectors at station S20 (S20A) in the period between December 2007 and January 2009 are shown in Fig. 4. Predominant circulation is WSW-WNW, throughout the water column, except in May and June 2008 when speeds were less than 3 cm/s. Most intensive circulation was recorded in March 2008, with average (maximum) speed about 20 cm/s (95 cm/s) in the surface layer and 7 cm/s (40 cm/s) in the bottom layer, with a very high stability factor: about 95% in the surface layer, 70% in the middle layer, and 40% in the bottom layer (Table I). The weakest currents were observed in May 2008 (Table II). It can be generally concluded that circulation at station S20 is more intensive during the cold season (December to April) than during the warm one (May to November).

Statistical parameters of circulation at station S20 for March and May are given in Tables I and II.

Fig. 2. Position of the ADCP station S20 near Dubrovnik.

Fig. 3. ADCP current meter system lowering into the sea prior to deployment at the sea bottom.

TABLE I. The statistics of the current flow at station S20 for March 2008. Vector and scalar means are given, together with the statistics of velocity magnitude (V) and the stability factor F.

Depth V- Dir V-avg V-std V-min V- max

F(m) (cm/s) (°) (cm/s) (cm/

s)(cm/s) (cm/s) (%)

6 22.2 298.2 23.4 15.0 2.1 70.7 95.210 21.1 293.0 22.6 14.8 0.5 69.3 93.3

20 17.1 294.4 18.7 12.0 1.3 57.9 91.130 13.3 296.7 14.7 10.0 0.6 45.2 90.5

40 10.1 298.6 12.1 8.1 0.3 35.9 83.350 7.9 300.4 10.4 6.8 0.6 34.4 75.5

60 6.1 299.2 9.0 5.7 0.5 33.4 67.270 5.0 295.9 8.0 5.1 0.2 29.9 62.8

80 4.1 297.4 7.6 4.7 0.5 26.8 53.890 3.1 298.2 7.2 4.3 0.4 22.8 42.4

100 2.6 281.5 6.5 3.7 0.2 19.1 39.1

TABLE II. The statistics of the current flow at station S20 for May 2008. Vector and scalar means are given, together with the statistics of velocity magnitude (V) and the stability factor F.

The most significant current energies at site S20 (Fig. 5) are recorded at long periods. Within subsurface layer (depth of 6 m) current energies are ten times bigger than in near bottom layer (depth of 100 m). Significant spectral maxima are pronounced at diurnal tidal period (larger near the surface) and at semidiurnal tidal period (relatively larger energies near bottom).

Current rose in the surface layer shows that dominant current directions were between WNW and NW, while the most intensive currents were recorded in WNW direction, parallel to the coastline. In the bottom layer the currents were directed towards W and WNW, but also to the E direction, being a result of the tidal forcing (Figs. 6 and 7). Mean current vectors also show that the direction of the resultant vector throughout the most of the water column was directed towards WNW, due to the entering Adriatic NW current – East Adriatic Current (EAC) which embrace the whole water column in that area.

It can be concluded that current flow at S20 is stronger and with lower variability in the surface layer in comparison to the flow in the bottom layer. The recorded current flow is a part of the general East Adriatic Current which in front of Dubrovnik comprises the northern section of the South Adriatic cyclonic gyre.

Intensity and stability of the East Adriatic Current in the area between Dubrovnik and Bar is obtained by POM model too. Averaged model current fields for March 2008 (the most intensive circulation) and May 2008 (the weakest circulation) are shown in Fig. 8. It should be pointed out that the coastal area between Dubrovnik and Bar is generally oriented in the direction of NW-SE, without larger islands, which from the topographic and bathymetric point of view represents a significant condition for the intensification of the East Adriatic Current coming from the area of Otranto. The existence of the cyclonic gyre in the southern Adriatic is also evident in the modelled monthly average currents (Fig. 8). The presence of South Adriatic Cyclonic Gyre was obtained in almost all modelled monthly currents between December 2007 and January 2009 (not shown).

Depth V- Dir V-avg V-std V-min V- max

F(m) (cm/s) (°) (cm/s) (cm/

s)(cm/s) (cm/s) (%)

6 1.4 232.1 9.6 7.6 0.5 38.5 14.510 1.3 203.9 10.1 7.4 0.1 37.8 13.020 1.2 144.7 9.0 5.9 0.4 31.0 13.8

30 2.5 110.1 6.8 4.7 0.4 23.3 36.640 1.9 95.9 5.2 3.2 0.0 16.6 36.7

50 0.8 83.8 4.3 2.2 0.4 12.0 18.460 0.6 358.6 4.2 2.0 0.3 10.6 14.1

70 0.9 343.5 4.2 2.3 0.2 11.5 21.380 0.8 342.6 4.5 2.6 0.5 12.8 17.0

90 0.5 355.7 4.6 2.7 0.2 15.0 11.0100 0.5 308.9 4.0 1.9 0.1 9.5 12.9

Fig. 4. Time series of vertical profiles of monthly mean current vectors at station S20 from December 2007 till January 2009 (from [10]).

Fig. 5. Total current spectrum in subsurface and near-bottom layer at the station S20 for time interval from 28 November 2007 to 20 July 2008.

Fig. 6. The distribution of current speed and direction at station S20 at depths of 6 and 100 m for 16 main directions (from 28 November 2007 to 20 July 2008; from [10]).

Fig. 7. The distribution of current speed and direction at station S20 at depths of 6 and 74 m for 16 main directions (from 21 July 2008 to 23 January 2009; from [10]).

Fig. 8. Mean monthly current fields for March 2008 (up) and May 2018 (down) at the depth of 5 m obtained by POM model.

IV. CONCLUSION

Characteristics of the Eastern Adriatic Current in the coastal area between Dubrovnik and Bar were explored on the basis of direct current measurement at the single station in front of Dubrovnik and numerical model simulations.

At the deep current meter station S20 with total depth exceeding 100 m off Dubrovnik, the predominant current directions were between WNW and NW with very high stability factor in the cold part of the year, while the most intensive currents were recorded in WNW direction, parallel with the coastline. The resultant vector almost throughout the water column was in WNW direction, which suggests that the inflowing Eastern Adriatic Current affects even the deepest layers of the sea. The maximum current flow near the surface was about 95 cm/s, while at the bottom it was weaker, about 40 cm/s. The most intensive circulation was in March 2008 while the weakest was in May 2008.

Results of spectral analyses of long-term measured currents off Dubrovnik have confirmed that East Adriatic Current can be explained as dominantly gradient current under the influence of tides and winds.

Intensity and stability of the East Adriatic Current flow in the area between Dubrovnik and Bar, as well as the existence of South Adriatic Cyclonic Gyre were confirmed by the POM model simulations.

It is important to emphasize that described current measurements and numerical model results indicate very important ecological consequence for the coastal area between Otranto Strait and Dubrovnik. The obtained results indicate that all waste (e.g. oil, communal waste or any other pollution) that reaches the sea in the coastal area southeast of Dubrovnik will likely pollute the whole area towards northwest.

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