Effects of a topographic gradient on coastal current dynamics

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jc002632, 2005 Effects of a topographic gradient on coastal current dynamics Antonio Cenedese, Céline Vigarie, and Emilia Visconti di Modrone Dipartimento Idraulica Trasporti e Strade, University of Rome La Sapienza, Rome, Italy Received 29 July 2004; revised 31 January 2005; accepted 10 February 2005; published 14 September [1] When a boundary current in a rotating environment encounters a significant change in depth, two typical behaviors are observed. The flow may bifurcate, with one branch moving away from the boundary following the topographic contours and the other branch following the boundary. Alternatively, the boundary current may rebound or retroflect from the topography, with the formation of a dipole that moves in the upstream direction. Which behavior is observed is determined, for the most part, by the geometry of the system and not by the initial direction of the boundary current. Bifurcation is typical in a right-handed geometry, i.e., in the case with the boundary on the right side looking from deep to shallow water, while retroflection is typical in a left-handed geometry. Laboratory experiments in a rotating tank using dye visualization and particle tracking verify this basic dichotomy but also reveal new aspects of the problem not examined in previous studies. In particular, it is demonstrated that in the enclosed basin, there is a return current flowing counter to the branch following the topographic contours. The experimental design allows comparison of the large time flow with the circulation in the Adriatic Sea. Citation: Cenedese, A., C. Vigarie, and E. Visconti di Modrone (2005), Effects of a topographic gradient on coastal current dynamics, J. Geophys. Res., 110,, doi: /2004jc Introduction [2] Coastal currents, also called boundary currents, are distinguished from the rest of the ocean circulation by their characteristic small width, high velocity, and strong temperature and salinity gradients. These currents typically exhibit strong fluctuations due to a variety of external forcing actions and inherent instabilities. Particularly interesting behavior is observed when such currents encounter significant variations in the bottom topography. In particular, we consider the case of a significant change in depth running transverse to the coast (an escarpment). When a boundary current encounters such a change in depth, it may bifurcate in two branches, one following the topographic contours away from the coast and the other continuing to flow along the coast. Many examples of this phenomenon can be mentioned: the Bering slope current in the Shelikof Strait, which lies between the Alaska Peninsula and the Kodiak Plateau; the Kuroshio current at the northeastern corner of Taiwan; the flow along the Iceland-Faeroe ridge; the Adriatic flow at the Jabuka Pit and the boundary current at the southeastern edge of Sicily. Changing sea conditions, irregularity of the coast and complex bottom topography makes the study of coastal current dynamics quite complicated. In order to gain some insight a number of analytical studies and numerical simulations, based on the shallow water and quasi-geostrophic models, and experimental studies have been performed. [3] Spitz and Nof [1990] studied the effect of a high step, running oblique to the coast on a boundary current Copyright 2005 by the American Geophysical Union /05/2004JC using an inviscid numerical f-plane model. This model was designed to reproduce the current bifurcation observed in the Shelikof Strait by focusing on two particular cases: the step-down case, in which the depth beyond the step (the deep region) is considered to be infinite and the step-up case, where the downstream depth is finite. The current was considered to have a linear profile and the approach was based on the nonlinear shallow water equations. The numerical results were compared to those found in laboratory experiments in a circular rotating tank with dye flow visualizations. For the stepdown case it was found that the current does not cross the step but flows offshore staying in the shallow region. In the step-up case, two different situations occur. For a small step (Figure 1) DH/(H DH) <Ro (where DH is the step size, H is the fluid height in the deep region, Ro is the Rossby number, Ro = V/fL, where V is the current velocity, L is the current width, and f is the Coriolis parameter, which will be considered positive (North Hemisphere) the current remains near the wall with no flow along the step, while for DH/(H DH) >Ro the current separates from the coast and flows offshore remaining in the deep region (without crossing the step). [4] In order to study the specific case of the Kuroshio current bifurcation, Stern and Austin [1995] developed another model to examine the influence of topography on current dynamics. They used a narrow slope instead of a sharp step. They produced a theory to predict current flow bifurcation and the velocity of the current following the topography. They carried out several experiments that instead showed a retroflection of the boundary current with the production of a dipole that moved offshore in the upstream direction. 1of11

2 Figure 1. Side view of the experimental configuration: DH is the step size, H is the fluid height in the deep region, H DH is the shelf depth, and W is the slope width. [5] In a quasi-geostrophic model, Carnevale et al. [1999] investigated the flow of a boundary current with exponential profile that intercepted an escarpment. Two basic configurations were considered: the right-handed geometry (Figure 2a) and the left-handed geometry (Figure 2b), according to the orientation of the topography with respect to the coastline (looking from deep to shallow fluid, if the coast is on the left (right) then we refer to a left-handed (right-handed) geometry). For each type of geometry, they classified the resulting flow behavior according to the value of the nondimensional ratio jh/wj (where h = f DH/H measures the effect of the topography and jwj = V/L is the maximum vorticity magnitude in the upstream flow) resulting in three possibilities: h f w ¼ DH H ¼ DH=H V Ro L 8 < < 1 ¼ 1 : : > 1 [6] In quasi-geostrophic theory, on which their theoretical and numerical models were based, the upstream direction of the flow along the coast is irrelevant to determining whether the flow will bifurcate or retroflect; only the handiness of geometry and the value of jh/wj determine this issue. The boundary conditions are considered to be open for the entire domain with the exception for the coastline where a free slip condition is considered. In this study numerical simulations have been carried out, varying the slope height (DH) and width (W) in order to see the influence of these two parameters on the current behavior. In the left-handed geometry they found the formation of a dipole near where the coastal current encounters the slope. This dipole leaves the coast, but does not follow the topographic contours. Instead, it propagates away from the coast moving upstream. The angle between the vortex trajectory and the coastline at depends primarily on the value of jh/wj. For the same value of jh/wj, with different slope width, the angle between the dipole and the coastline remains approximately constant, while for the same slope width but with different value of jh/wj, the angle varies. In contrast, for the righthanded geometry, if jh/wj < 1, part of the flow crosses the slope and goes downstream while the other part of the flow forms a dipole that propagates away from the coast along the step. A current trailing behind the dipole eventually results in a steady outflow of fluid along the contours of topography. For jh/wj = 1 and jh/wj > 1, there is no flow across the topography, but the whole current leaves the coast and flows along the topographic contours of the slope. This difference between flows in the right-handed and lefthanded geometries was also noted in the earlier studies of Allen [1988, 1996], Allen and Hsieh [1997], and Gill et al. [1986] where the problem was approached from the point of view of shallow water theory. [7] An experimental study verifying the theoretical and numerical results in Carnevale et al. [1999] has been carried out by Serravall [1998], in a rotating tank. The width and height of the step were kept constant, while the Coriolis parameter f and the water depth H were varied in order to reproduce the solutions found with the numerical simulation. The laboratory results confirmed the relationship between the angle at which the dipole leaves the coast and the value of jh/wj in the left-handed case, and showed a bifurcation of the current, for the right-handed case. Carnevale et al. [1999] assumed a semi-infinite domain, bounded only by the straight line coast. However, it is of interest to see how the flow over the step evolves also in an enclosed basin. For example, the Adriatic sea can conceptually be divided into at least two basins of different depths. Figure 2. (a) Right-handed and (b) left-handed configurations. The coastline is represented by a solid line, while the open boundaries are indicated by dashed lines. The slope is indicated by the shaded region. 2of11

3 Figure 3. Schema of the experimental apparatus. The exchange of fluid between these basins is to some degree accomplished by a boundary current. The effect of having an enclosed basin in this case will modify the results obtained for a semi-infinite domain. This effect was not the object of Serravall [1998] and is not discussed in regard to her experiments. The aim of this paper is to illustrate new qualitative laboratory results found with the dye flow visualizations and quantitative results obtained with the particle tracking method. In the next section the experimental apparatus and the main parameters will be described. In the third section results will be given and compared to the numerical simulations given by Carnevale et al. [1999]. The fourth section presents a comparison between experimental results and the Adriatic circulation. In the final part, the conclusions will be drawn about the role of the enclosing boundaries. 2. Experimental Apparatus [8] The experiments were performed in a tank ( cm (Figure 3)) rotating counterclockwise in order to simulate the current dynamics in the Northern Hemisphere. The deep water region (41 84 cm) and a shallow water region (43 84 cm) are connected with a linear slope (4 cm wide with a depth variation of 6 cm) making an angle of 90 with the tank wall. [9] The tank was illuminated from above with two neon lamps. A CCD video camera (25 frames/s) was used to visualize and record the flow. Following Stern and Whitehead [1990], the boundary current is produced by Figure 4. Schema of the jet, with nozzles positioned on a width of 3 cm at 5, 10, and 15 cm of height. 3of11

4 Table 1. Experimental Conditions a Geometry H, cm f, s 1 X 0 V,cms 1 Ro = V/fL jh/wj RH 1 b RH 2 b RH RH 4 b RH 5 b RH RH 7 b RH RH LH 1 b LH 2 b LH LH 4 b LH 5 b LH a RH is the right-handed geometry, and LH is the left-handed geometry. H is the fluid height in the deep region, f is the Coriolis parameter, X 0 is the distance between the source of the jet and the slope, V is the current velocity, and jh/wj is the ratio between the topographic amplitude and the maximum vorticity magnitude. b Cases in which particle tracking has been applied. allowing water to flow at a constant rate from a reservoir into a perforated vertical tube on the sidewall of the tank (Figure 4). The holes in the tube were located at depths of 5 cm, 10 cm, and 15 cm which aided the establishment of a vertically uniform jet flowing along the tank wall. This same jet has been used for both right-handed and lefthanded geometry. The hydraulic circuit is composed of a volumetric pump, with a low flow rates, which is controlled by voltage generator and which takes the dyed water from a reservoir. In the experiments the tank was filled with three different water heights (20 cm, 27 cm and 13 cm) in different experiments. The period of preliminary rotation of about 30 min allowed the fluid to come into solid body rotation with the tank before each experiment. The experiments began with the release of water (dyed with methylene blue) from the reservoir. Then constant flow rate from the reservoir continually was maintained throughout the rest of the experiment. [10] The flow velocity has been determined by evaluating the ratio between the distance covered by the flow and the time necessary to cover it; the velocity was regulated by varying the generated voltage and the distance between the jet and the slope, which varied between 15 and 20 cm to avoid the jet separation that occurs for higher distances due to viscous boundary layer effects. The resulting current width (L) is not the width (3 cm) of the source. The flow coming from the source is pushed against the tank wall by the Coriolis force, resulting in a current width closer to 1 cm. In the experiments the slope height (DH), the current width (L 1 cm) and the slope width (W) have been kept constant, while the Coriolis parameter (f), the fluid height in the deep region (H) and the current velocity (V) have been varied such that the range of Rossby numbers is 0.2 < Ro < 1.4, representing values comparable to those found in the Adriatic Sea. 3. Results [11] Initially the experiments were carried out with the dye flow method in order to visualize the current evolution qualitatively. Subsequently, the particle tracking technique was applied to quantify some of the results. Given errors in the measurements, the transition case (jh/wj = 1) was not easy to determine and observe, and, thus, results are given for cases with (jh/wj > 1 and jh/wj < 1. Table 1 gives a list of the experimental conditions for each run Right-Handed Geometry [12] In the numerical simulations of quasi-geostrophic flow in Carnevale et al. [1999] and Serravall [1998], the flow along the boundary of the rotating fluid could be taken in either direction, that is the boundary current could have the boundary to its right or left. Thus fro a given geometry and topography, the current could encounter either a step-up or a step-down depending on the direction of the boundary flow. However, with our laboratory design, it was only possible to create boundary currents flowing in the direction with the tank wall on the right. That is in the right handed geometry (looking from shallow to deep the coast is on the right) the only experiment possible with this set up, in which the flow always has the wall to its right, is the stepdown case. In the left handed geometry, the only possibility is the step-up case. This is because the Coriolis effect forces the flow to bend to the right as it exits the source resulting in a current that flows with the boundary on the right. According to quasi-geostrophic theory, the direction of the flow is not relevant to determining if the flow will bifurcate or retroflect, only the handedness of the geometry matters. Unfortunately, this symmetry of the quasi-geostrophic equations cannot be tested with these experiments because the direction of the boundary current cannot be reversed. [13] If jh/wj > 1 with H = 20 cm (Figure 5, experiment RH 1), the current turns to follow the topography when it encounters the slope. The flow over the slope repeatedly forms anticyclonic structures, in the zone over the slope and near the wall. On the shallow side of the slope, a current flowing counter to the offshore slope current is formed, shearing the offshore flow. This countercurrent joins a Figure 5. Experimental result for the right-handed geometry (experiment RH 1; jh/wj = 1.5; H = 20 cm). The bold arrows show the main current direction, while the thin arrows show the countercurrent direction. 4of11

5 Figure 6. Velocity and vorticity fields in the right-handed geometry (experiment RH 4; jh/wj = 1.5; H = 27 cm). The shallow region is in the upper part of the image, while the deep one is in the lower part; the slope is at y = 33. narrow boundary current that flows along the wall in the shallow part of the tank. The countercurrent and narrow boundary current do not occur in the quasi-geostrophic predictions for jh/wj > 1. The presence of these currents in the shallow water basins is difficult to understand because the conservation of potential vorticity implies that, for jh/wj > 1, none of the positive relative vorticity in the upstream current can survive crossing the step. There appears to be no source for positive vorticity in the shallow part of the tank, but the observed circulation in the shallow part of the tank is cyclonic requiring positive relative vorticity. This cyclonic circulation was also observed by Spitz and Nof [1990]. Neither the quasi-geostrophic prediction of Carnevale et al. [1999] nor the shallow water theory of Spitz and Nof [1990], which are both based on semiinfinite geometries, offer an explanation for the formation of this circulation. A full investigation this effect will probably require an analysis of the flow in the entire enclosed basin. If the water height is increased to H = 27 cm, the current has the same qualitative behavior, but with a lower velocity than in the previous case with H = 20 cm. [14] Particle tracking was used to measure the current bifurcation and the development of coherent structures over the slope. For H = 27 cm, the velocity fields obtained show that the maximum velocity in the jet direction is decreasing as the run proceeds, while the maximum velocity along the slope remains constant. The images of the velocity fields show the presence of a countercurrent on the shallow water side of the slope, confirming the result found with the dye flow visualization. On the other hand, the vorticity fields (Figure 6) clearly point out the development of the negative vorticity structures in the flow over the slope; in fact the images show an initial, predominantly positive, vorticity in the outflow from the current generator, but as soon as the current encounters the slope and detaches from the wall, the positive vorticity decreases substantially, and the negative vorticity structures become evident. The anticyclonic structures have a maximum vorticity that tends to remain constant. [15] The solutions in Carnevale et al. [1999] do not reveal either the countercurrent or the anticyclonic structures observed in these experimental results. Perhaps the countercurrent is a result of the enclosed geometry and the development of coherent flow structures may be the result of the presence of horizontal shear between the topographic current and the countercurrent. Figure 7. Experimental result for the right-handed geometry (experiment RH 7; jh/wj = 1.5; H = 13 cm). The bold arrows show the main current direction, while the thin arrows show the countercurrent direction. 5of11

6 Figure 8. Experimental result for the right-handed geometry (experiment RH 2; jh/wj = 0.3; H = 20 cm). Compare the numerical simulation for jh/wj = 0.5 [Carnevale et al., 1999]. [16] For H = 13 cm (experiment RH 7 (Figure 7)), the flow did not bifurcate. Rather, the entire flow followed the slope in agreement with the numerical simulations for this case. Again there was the formation of a countercurrent on the shallow side of the slope and the formation of coherent structures over the slope, but this time the coherent structures were predominantly cyclonic (Figure 7). [17] When jh/wj < 1 for H = 20 cm and H = 27 cm, the current does not bifurcate as it encounters the slope, but instead it enters the shallow basin and meanders and expands. The fluctuations in the flow direction seem to begin where the flow encounters the slope (Figure 8). The quasi-geostrophic numerical simulations did not show this meandering or the expansion of the flow in the shallow water region. [18] The velocity fields, obtained with the particle tacking for H = 27 cm, also showed the presence of a second flow along the slope, a result which the dye flow method did not reveal. This secondary flow involves the entrainment of water from the deep region. The images of the vorticity fields (Figure 9) show that the main flow over the topography is composed of two vortices, a negative one near the tank s wall and a positive one. The countercurrent is not evident because it is very weak in this case. As the runs proceeds, the negative vortex expands, while moving downstream, dominating over the positive vortex. The negative Figure 9. Velocity and vorticity fields in the right-handed geometry (experiment RH 5; jh/wj = 0.6; H = 27 cm). The shallow region is in the upper part of the image, while the deep one is in the lower part; the slope is at y = 18. 6of11

7 Figure 10. Experimental result for the right-handed geometry (experiment RH 8; jh/wj = 0.4; H = 13 cm). The bold arrows show the main current direction, while the thin arrows show the countercurrent direction. maximum vorticity magnitude remains approximately constant during this process. [19] If H = 13 cm a completely different current behavior is observed (Figure 10), in fact in the transition zone between the shallow water and the deep water regions, a clear bifurcation of the flow is observed. The current along the slope is sheared by a countercurrent while in the part of the flow which follows the coast a large cyclonic structure develops and leaves the coast. [20] For the transition case jh/wj 1, with H =20orH = 27, the flow (Figure 11) seems to have a intermediate behavior between the two previous cases; in fact, one part of the flow follows the topographic step, while the other part expands over the slope. For H = 13 cm the current bifurcates as in case(jh/wj < 1) and develops a big cyclonic structure over the slope Left-Handed Geometry [21] For the left-handed geometry, in the step down case (the only possibility in our experiment), only the results obtained with H = 20 cm and H = 27 cm will be given because for the lowest water level the current seems to ignore the slope and flows entirely along the wall with no regard to the value of jh/wj. Many experiments have been performed with increasing water depth in order to find the critical value of H for which the flow behavior is determined by jh/wj, and the critical value was found to be about 18 cm. For both water depths (H = 20 cm and H = 27 cm) the current behavior is very similar with the exception that, with the increasing water level, the current velocity decreases. [22] In the left-handed geometry, for both water depths, the presence of the slope makes the current retroflect into the shallow zone with the formation of a dipole. For the case jh/wj < 1, the correspondence between the numerical experiments and the laboratory results is very good as shown in Figure 12. This kind of dipole formation was observed in the experiments of Stern and Austin [1995] who used a lefthanded geometry. Before the dipole detaches from the wall, the maximum vorticity is obtained (Figure 13), while as it proceeds in the shallow area the vorticity decrease. The velocity in the jet direction, along the wall toward the topography, increases during the dipole propagation while the velocity in the slope direction moving away from the coast, remains constant. In the dye flow results it is possible to see that the dipole detaches from the slope at a distance of 5 cm from the wall, while in the numerical simulation the dipole leaves the slope much closer to the wall. It is also interesting to note that far from the wall, there is no outward flow over the slop, but rather the flow over the slope is inflow toward the boundary that supports the jet. [23] In the transition case jh/wj = 1, the current initially follows the slope (Figure 14) but shortly afterward leaves it and propagates into the shallow area; the position where the current leaves the slope seems to depend on jh/wj, but this result could not be clearly highlighted since the transition Figure 11. Experimental result for the right-handed geometry (experiment RH3; jh/wj 1; H = 20 cm). The bold arrows show the main current direction, while the thin arrows show the countercurrent direction. Compare with numerical simulation for jh/wj = 1[Carnevale et al., 1999]. 7of11

8 Figure 12. Comparison between the left-handed numerical simulation for jh/wj = 0.5 [Carnevale et al., 1999] and the left-handed experimental result (experiment LH 2; jh/wj = 0.4; H = 20 cm). The coastline and the bottom boundary are represented by a solid line, while the open boundaries are indicated by dashed lines. case is difficult to reproduce. Also in this case the result is very close to the numerical simulation; in fact, it is possible to see that the dipole leaves the slope at a larger distance than in the previous case as predicted by the numerical simulations. [24] For jh/wj > 1, together with the dipole formation there is also an outflow along the slope and a countercurrent as in the right-handed geometry (jh/wj < 1). The countercurrent appears to result from entrainment of slope water on the deep side of the slope. This current opposes the outward flow coming from the boundary with the jet. The numerical simulations, for the semi-infinite domain, predicted that as jh/wj increased, the flow would leave the slope further away from the boundary, creating an outward flow at least for some distance from the boundary. Such an outward flow was also found in our laboratory experiments with the lefthanded geometry for cases with jh/wj > 1. However, instead of the relatively smooth outward flow found in the numerical simulations, we observed repeated dipole formation near the coast with associated strong fluctuations in the current flowing outward along the topography. An instantaneous view illustrating the multiple dipole formation and the complicated nature of the flow along the topography is shown in Figure 14. Even when the experiments were continued for a very long period, strong fluctuations continued to occur. Another example of this is shown in Figure 15 along with an example of the type of flow found in the quasi-geostrophic numerical simulations for large jh/wj > 1 in the left-handed geometry. The vorticity fields clearly show that the flow consists of positive vorticity in the shallow part of the transition zone (between the wall and the slope) and negative vorticity over the slope (Figure 16); both vortices subsequently propagate in the shallow water region forming a dipole. In this case the countercurrent is also clearly visible in the velocity field but is not so evident in the vorticity field due to its low vorticity compared to that of the dipole. 4. Discussion of the Adriatic Circulation [25] In order to examine the long-term effect of the closed boundaries, several long runs have been carried out. These experiments can be considered model flows for the Adriatic Sea. To some degree the Adriatic, a semienclosed sea, can be considered to be formed of three basins. The South Adriatic Pit reaching depths over 1300 m forms the southernmost basin. The intermediate basin is a combination of the relatively flat Palagruza sill, north of the Gargano Figure 13. Vorticity fields in the left-handed geometry (experiment LH 5; jh/wj = 0.3; H = 27 cm). The deep region is in the lower part of the image, while the shallow one is in the upper part; the slope is at y = 25. 8of11

9 Figure 14. Comparison between the left-handed numerical simulation for jh/wj = 1[Carnevale et al., 1999] and the left-handed experimental result (experiment LH 3; jh/wj = 0.9; H = 20 cm). peninsula, with a depth of about 150 m, and the Jabuka pit with depths that reach somewhat more than 200 m. On the northwestern side of the pit, there is a strong topographic gradient forming a boundary with the relatively shallow northernmost of the three basins in which there is only a gradual variation as the depth decreases from about 80 m as one moves to the northern end of the Adriatic. In our experiments, the gradient between our two basins can be taken as a simple model of the topographic gradient separating the southernmost basin, the Adriatic pit, from the intermediate basin formed by the Palagruza sill and Jabuka pit. Alternatively, we could consider it a model of the transition from the intermediate basin to the northern most basin because the topographic gradient on the northwestern side of the Jabuka pit is strong relative to the topographic gradient in either the northern basin or that on the southeastern side of the Jabuka pit. Mediterranean Seawater entering the Strait of Otranto follows the eastern boundary of the Adriatic. When this current encounters the strong topographic gradient dividing the southernmost basin from the intermediate basin, it bifurcates with one branch flowing along the coast and the other flowing along the Figure 15. Comparison between the left-handed numerical simulation for jh/wj = 2[Carnevale et al., 1999] and the left-handed experimental result (experiment LH 1; jh/wj = 1.5; H = 20 cm). 9of11

10 Figure 16. Vorticity fields in the left-handed geometry (experiment LH 1; jh/wj = 1.5; H = 20 cm). topographic contours toward the Italian coast (Figure 17). This current bifurcation generates two cyclonic circulations, one in the shallow northern area and one in the deep southern area, a phenomenon which is almost always present in spite of seasonal changes. A similar bifurcation also occurs where the topographic gradient of the northwestern side of the Jabuka pit encounters the coast of Croatia. This also produces a current flowing across the Figure 17. Adriatic circulation. 10 of 11

11 multiple basins to examine the role of topography in establishing the circulation patterns. Figure 18. Methylene blue visualization of the whole tank on a right-handed geometry and for jh/wj = 1.5 (>1). The shallow water region is on the upper part of the image, and the deep water region is on the lower part. Adriatic toward Italy (Figure 17). For these two bifurcations, estimated values of jh/wj in the range 1 3 would be reasonable, and this overlaps the range used in our laboratory experiments. The topographic gradients DH/W, however, are very small (about 0.05 for the northwestern side of the Adriatic Pit and about for the northwestern side of the Jabuka Pit) compared to those used in the laboratory experiments ( ). The long experimental runs were performed in the right-handed geometry with jh/wj 1.5 and water height H = 20 cm and H = 27 cm with a slope of 1.5. Furthermore, stratification effects in the Adriatic will tend to lessen the topographic influence on the currents in the upper part of the fluid column. Nevertheless, the resulting circulations were very similar to that shown for the Adriatic in Figure 18 in that there is cyclonic flow in both basins. In the region of the topography, there is clearly the offshore flow from the bifurcation of the boundary current, but also evident is the return flow or countercurrent running in the opposite direction along the topographic contours. This countercurrent was already evident in the short-term runs that were discussed above. The horizontal shear between the offshore current and the countercurrent appears responsible for the unstable nature of the flow along the topographic contours and the resultant production of eddies in this current. [26] In the Adriatic, the flow is sometimes composed of three cyclonic regions, the third being in the very shallow northern part of the sea where the depths are order 50 m. This suggests a further laboratory experiment with three basins. Also we can imagine more general experiments with 5. Conclusions [27] The results obtained with experimental method were not always similar to the numerical simulations. This discrepancy is in part due to the different boundary conditions and the different boundary current profile. As explained in the introduction, the numerical model considers open boundary conditions for the entire domain with the exception of the coastline, while in the tank the boundary conditions are closed. Another difference is that in the laboratory the boundaries are all no-slip boundaries, while in the numerical experiments, the boundary was free-slip. Because of the no-slip condition in the tank, it is impossible to create a boundary current of vorticity of a single sign like that used in the numerical simulations. The presence of both signs of the vorticity in the boundary current complicates the behavior and the analysis considerably. The no-slip boundary condition and the change of sign of vorticity in the boundary current in the laboratory can introduce instabilities and strong fluctuations not present in the cited numerical simulations. Another important difference is that in the cited quasi-geostrophic simulations, it is assumed that the topographic variations are very small. Here, however, we had variations in depth of up to fifty percent of the fluid depth. Finite depth effects could account for some of the differences between the simulations and the laboratory experiments. [28] Acknowledgments. This work was supported by the Italian Ministry of University (MIUR). The authors thank George Carnevale for his useful suggestions and revision of the final version of the manuscript. References Allen, S. E. (1988), Rossby adjustment over a slope, Ph.D. thesis, Univ. of Cambridge, U. K. Allen, S. E. (1996), Rossby adjustment over a slope in a homogeneous fluid, Phys. Oceans, 26, Allen, S. E., and W. W. Hsieh (1997), How does the El Niño generated coastal current propagate past the Mendocino escarpment?, J. Geophys. Res., 102, 24,977 24,985. Carnevale, G. F., S. G. Llewellyn Smith, F. Crisciani, R. Purini, and R. Serravall (1999), Bifurcation of a coastal current at an escarpment, J. Phys. Oceanogr., 29, Gill, A. E., M. K. Davey, E. R. Johnson, and P. F. Linden (1986), Rossby adjustment over a step, J. Mar. Res., 44, Serravall, R. (1998), Dinamica di una corrente costiera in presenza di un gradiente topografico, Ph.D. thesis, Univ. à degli Studi di Roma La Sapienza, Italy. Spitz, Y. H., and D. Nof (1990), Separation of boundary currents due to a bottom topography, Deep Sea Res., 38, Stern, M. E., and J. Austin (1995), Entrainment of shelf water by a bifurcating continental boundary current, J. Phys. Oceanogr., 25, Stern, M. E., and J. A. Whitehead (1990), Separation of a boundary jet in a rotating fluid, J. Fluid Mech., 217, A. Cenedese, C. Vigarie, and E. Visconti di Modrone, Dipartimento Idraulica Trasporti e Strade, University of Rome La Sapienza, Via Eudossiana 18, I Rome, Italy. (antonio.cenedese@uniroma1.it; c.vigarie@saibos-kizomba.com; e.visconti@siif-energies.fr) 11 of 11