JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, C11001, doi: /2006jc003775, 2007

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi: /2006jc003775, 2007 Circulation over the southeastern continental shelf and slope of the Mediterranean Sea: Direct current measurements, winds, and numerical model simulations Zvi Rosentraub 1 and Stephen Brenner 1,2 Received 22 June 2006; revised 9 May 2007; accepted 20 July 2007; published 3 November [1] Results are presented for the first time from an extensive, systematic campaign of direct current measurements that were conducted over the continental shelf and slope of the southeastern corner of the Mediterranean over a 10-year period. Concurrent cross-shelf hydrographic transects and coastal wind measurements were also conducted. The results are presented within the context of the general circulation of the Levantine Basin as simulated with a numerical model. The circulation over the shelf is northward throughout the year with strong seasonal variability, and maximum currents in winter and summer. In summer, the strong currents are confined to the upper layer with seaward intensification. Over the slope, a seasonally varying along-slope baroclinic jet also appears during summer and early winter. The shelf and slope current system is part of the general circulation of the Levantine basin composed of the bifurcating Atlantic Ionian Stream, the Mid-Mediterranean Jet, and the cyclonic basin-wide current that follows the coast. During winter storms, the strong southwesterly winds drive the northward flowing current and downwelling over the shelf. At this time the deeper level currents near the shelf break are also intensified. The alongshore synoptic currents are weakly coherent and strongly affected by the alongshore pressure gradient. In late winter the slope jet remains offshore and does not intrude over the narrow shelf. Both the measurements and the simulations confirm the strong seasonality and variability of the shelf and slope current system and hint at the potential for intense shelf-open sea exchanges in this region. Citation: Rosentraub, Z., and S. Brenner (2007), Circulation over the southeastern continental shelf and slope of the Mediterranean Sea: Direct current measurements, winds, and numerical model simulations, J. Geophys. Res., 112,, doi: /2006jc Introduction [2] The continental shelf opposite Israel lies at the southeastern corner of the Levantine basin of the Mediterranean Sea (Figure 1). Its bathymetry is relatively simple with depth contours aligned approximately parallel to the northsouth coastline. In the south, the coastline bends nearly at a right angle as it approaches the Egyptian Sinai coast. In contrast to the relatively wide shelf opposite the Sinai coast, the shelf opposite Israel is narrow (from 20 km in the south and narrowing to 10 km in the north) and therefore the deep sea is relatively close to shore. There are no major rivers that discharge along the coast although before the construction of the Aswan High Dam in 1965, the annual flood of the Nile River was an important source of water. Prior to the damming of the Nile these floodwaters could be traced as a tongue of relatively fresh water along the Israeli shelf in 1 National Institute of Oceanography, Israel Oceanographic and Limnological Research, Haifa, Israel. 2 Now at Department of Geography and Environment, Bar Ilan University, Ramat Gan, Israel. Copyright 2007 by the American Geophysical Union /07/2006JC early autumn during the natural flood [Hecht, 1964; Oren and Hornung, 1972]. Today, the discharge is less than one tenth of the original value [Gerges, 1976]. However, even at these reduced discharge levels, Nile outflow can still be seen in satellite images of the chlorophyll distribution of the southeastern Mediterranean [El-Sayed and van Dijken, 1995; van Dijken and Arrigo, 1996]. [3] The climate of the Levantine Basin is characterized by hot, dry summers with stable atmospheric conditions, cold, wet winters and relatively short transitional seasons in spring and autumn. During the summer, steady westerly to northwesterly winds are dominant over the basin, with a superimposed, well-developed coastal sea breeze. In contrast to summer, winter atmospheric conditions are unstable and variable, often characterized by cold air outbreaks from the north and local cyclogenetic activity [Ozsoy, 1981; Alpert and Reisin, 1986]. In fact, during the winter, the eastern Mediterranean appears on hemispheric charts as one of the three main midlatitude, cyclogenetic regions [Reiter, 1975]. The seasonal mean winds are mainly westerly, although migratory low-pressure systems moving eastward across the Mediterranean Sea [Alpert et al., 1990] force downwelling favorable, strong southerly to southwesterly winds along the Israeli coast. 1of21

2 Figure 1. Map of the study area showing the locations of the current meter and wind stations. The insert shows the location of the study area within the context of the eastern Mediterranean Sea as well as several prominent circulation features: 1, Atlantic Ionian Stream; 2, Mid-Mediterranean Jet; 3, Southern Levantine Current; 4, Mersa Matruh Gyre; 5, Asia Minor Current; and 6, Rhodes Gyre. [4] During the past two decades, there has been renewed interest in investigating the circulation in the Eastern Mediterranean Sea. In the early 1980s the circulation in the region south of Cyprus was studied in the Marine Climate (MC) program through a series of regular hydrographic cruises [Hecht et al., 1988]. These results clearly showed that a meandering jet and a highly energetic and variable mesoscale field are the dominant features of the circulation in this region [Robinson et al., 1987]. From 1985 to 1995 the highly successful Physical Oceanography of the Eastern Mediterranean (POEM) program provided the first set of quasisynoptic realizations of the circulation in the entire Eastern Mediterranean. The POEM results clearly showed that the basin wide circulation consists of dynamical features over a wide range of spatial and temporal scales including a basin wide thermohaline cell originating with deep water formation in the northern Adriatic [e.g., Robinson and Golnaraghi, 1994], the meandering Mid- Mediterranean Jet which connects various persistent and recurrent subbasin-scale gyres [The POEM Group, 1992], as well as the highly energetic, transient mesoscale eddy field [e.g., Robinson et al., 1987; Hecht et al., 1988; Brenner, 1993; Malanotte-Rizzoli et al., 1996]. Despite the abovementioned intensive open sea hydrographic surveys and the growing knowledge regarding the general circulation of the Levantine Basin, the circulation along the continental margin and specifically, along the narrow shelf and slope of the southeastern coast of the Mediterranean remained relatively unknown. On the basis of the trajectory of the Nile River flood waters [Hecht, 1964; Oren and Hornung, 1972], the 2of21

3 Table 1. Locations of Mooring and Coastal Wind Measurement Sites Sit Distance From Shore, km Latitude Location Longitude Nominal Bottom Isobaths Depth, m Orientation, T T N E T N E T N E T N E N N E H N E H N E H N E A N E A1P N E A N E A N E MW N E HW N E AW N E presence of Atlantic Water in the eastern Levantine basin [Oren, 1971], and the drift of floating coal debris [Golik, 1993], the circulation on the shelf was considered to be generally cyclonic, similar to the deep basin-wide circulation first presented by Nielsen [1912]. Some of the above mentioned features are shown schematically in Figure 1. Direct current measurements were rare and sporadic, and confined mostly to the inner shelf in water depth of less then 24 m [Rosen, 1980]. They were never incorporated or synthesized into an overall picture of the shelf circulation. [5] In order to close this gap in understanding the shelf/ slope circulation, from 1987 to 1996, the National Institute of Oceanography (NIO) conducted the first, long-term, program to investigate the circulation and the thermohaline structure over the continental shelf and slope of Israel. Within the framework of this program systematic observations of currents, temperature, salinity and bottom pressure at various subsurface mooring sites on the shelf and at one mooring site on the midslope were conducted (Figure 1 and Table 1). These were accompanied by hydrographic cross sections and wind measurements. This study, for the first time, provides extensive, systematic knowledge about the currents over the Israeli shelf, which had been lacking owing to the sporadic nature and limited number of measurements previously conducted. It also provides data about the circulation of the slope region, which is important owing to its possible impact upon the shelf, as well as for water exchange between the shelf and the open sea. The latter are especially important in view of the environmental problems concerning the semienclosed Mediterranean Sea. Recently Kunitsa et al. [2005] provided a preliminary statistical analysis and a simplified analytical wind-driven current model of a small subset of these data. They focused on the winter period over the inner and mid shelf, when the circulation is nearly barotropic. [6] In this paper we present and analyze the results of the full set of nine years of observations with an emphasis on the low-frequency variability ranging from the synoptic and up to seasonal and interannual timescales. We separate the current regime into the inner shelf, the mid to outer shelf, and the continental slope. The results of the direct current measurements are presented and discussed within the framework of their connection to the density structure, the windforcing, and the internal pressure gradients. Furthermore, in order to understand the current regime as part of the overall basin wide circulation, we compare the observations to the results obtained from a climatologically forced simulation of the Levantine Basin with an intermediate resolution (0.05 ) numerical model. 2. Measurements and Data Processing [7] Details of the mooring locations with the current meters and pressure gauges are given in Figure 1 and Table 1. During the initial period of measurements (from mid-1987), the moored current and pressure observations were conducted mainly over the continental shelf at the northern stations opposite Atlit. Near the end of the first year, the moorings were shifted southward and measurements carried out at various times until 1992 opposite Hadera, Netanya, and at the southern end of the Israeli shelf opposite Ashkelon. The shallowest current meter was generally fixed near the base of the mixed layer, while the two deepest current meters where positioned roughly 3 and 8 m above the seafloor, within the bottom boundary layer. After 1992, measurements continued at station A1 until 1994 and at H1 until Details regarding the periods of measurement and instrument depths on the various moorings are given in Figure 2. The mooring on the continental slope was installed in November 1993 at a bottom depth of approximately 500 meters opposite Hadera (station H5). Two months later an additional mooring was installed on the shelf break at a bottom depth of 120 meters (station H4). This station operated for half of a year. The measurements at station H5, however, lasted until the end of 1996, with cumulative measurement gaps of about one year. Station H5 had four current meters at nominal instrument depths of 50, 120, 300 and 480 m below the sea surface. [8] The current observations were conducted with Aanderaa vector measuring current meters (VMCMs) placed on taut subsurface moorings. These current meters recorded temperature and conductivity as well as instantaneous direction and averaged speed. On the mid and outer shelf moorings bottom pressure was measured with an Aanderaa Water Level Recorder attached to an acoustic release system, 1 m above the bottom weight or mounted on a stable platform fixed to the seafloor. Data were recorded at 10-min intervals and averaged into hourly bins. These data were low-pass filtered to suppress inertial, daily sea breeze, and tidal oscillations by means of a symmetrical cosine filter with a half-power point of 40 hours (0.6 cpd.). The resulting series was then decimated to 6-hourly values. In addition, a clockwise axis rotation transformation was applied to all of the current time series in order to express the velocity components in the local, along and across-isobaths coordinate system (Table 1). [9] Wind speed and direction were measured at three coastal stations. The locations were at Ashdod (station AW), Hadera (HW), and 10 km north of Hadera (MW) as shown in Figure 1. The majority of the data are, however, from the southern station, AW. In addition, some 4 months of wind 3of21

4 Figure 2. Measurement periods and depths of current meters (numbers in boxes) for the shallow and midshelf moorings. measurements (February to May 1989) were recorded at station ASW placed on oil drilling ship 16.5 km offshore from Ashdod, opposite station AW (location not shown). All of the raw wind data were treated in the same manner as the currents in order to obtain hourly and low-pass filtered time series. The 180-degree reversal of wind direction and rotational axis transformation were also performed (9.5 degrees for HW and MW, and 20 degrees for AW and ASW) in order to obtain along and across-shore wind velocity components. The hourly wind time series were used to obtain hourly (and later, low-pass filtered) along and across-shore wind stress time series using the conventional bulk aerodynamic formula with a constant drag coefficient of Results of the Field Measurements 3.1. Density Structure [10] In addition to the current measurements, throughout this study, a total of 17 hydrographic sections were conducted along lines perpendicular to the coast off Atlit, Hadera, Netanya and Ashkelon, using an Applied Microsystems STD or a Neil Brown CTD. The hydrographic sections during the summer season indicate the presence of strong stratification over the shelf and, in particular, over the slope (e.g., Figures 3a and 3b), reaching a density difference of 3 sigma-t units, in the upper 100 m. The seasonal pycnocline and the isopycnals typically tilt downward over the slope, intersecting the sea bottom at midshelf, thus leaving the inner shelf nearly mixed. Upwelling was found only on a few occasions during the last month of autumn when northeasterly winds prevail. During winter, the water over most of the shelf is homogenous as a result of winter deepening of the mixed layer due to surface cooling. In contrast to this, the water over the slope becomes well mixed and homogenous only at the end of the season (sigma-t difference in the upper 500 m of only 0.2 units). Until midwinter it may remain considerably stratified, with deeper isopycnals sloping downward, intersecting the bottom of the continental slope. The winter density sections for the northern sections (and occasionally for the southern sections) also exhibit the presence of a lens of lighter water 4of21

5 Figure 3. Vertical cross sections of density (sigma-t) across the shelf and slope region for summer along (a) the northern transect (b) the central transect, and for winter along (c) the northern transect and (d) the central transect. in the upper layer over the slope and the outer shelf, causing a weak front with the dense water over the shelf (Figures 3c and 3d). The appearance of shelf-like dense water close to the bottom over the shelf break and the upper slope is most likely related to the bottom friction driven seaward flow found to exist above the bottom at the outer shelf. Gravitational advection and downslope cascading of dense shelf water, induced by winter cooling and evaporation may possibly reinforce this transport Currents Over the Inner Shelf [11] The rotary power spectrum analysis of the current time series over the shelf shows quite pronounced daily rotational fluctuations during spring and summer due to the well developed sea breeze (which has a typical amplitude of 15 cm/s and accounts for roughly 40% of the current energy power spectrum) but no apparent peak for the semidiurnal tidal forcing (which has a typical amplitude of 2 cm/s). In winter a significant part of the kinetic energy of the current fluctuations is confined to the synoptic timescales of atmospheric forcing, ranging from 2 to 3 days to several weeks (Figure 4). In general the synoptic velocity fluctuations over the shelf were directed mainly along the local depth contours. This was especially true for the shallow stations A1 and H1 located at bottom depth of 26 m (current meters located at a depth of 18 m), which are characterized by unidirectional polarity of the two rotary spectrum components. [12] The statistics of the relatively long currents time series at these two stations provide some information concerning the interannual variability of the currents on the inner shelf. Throughout the years covered by the measurements, the dominant monthly mean current direction was approximately northward, following the bathymetry, as can be seen in Figure 5 where we show the multiyear monthly mean alongshore current (solid line) as well as the individual year means (various symbols) and their respective standard deviations ( error bars ). The currents were relatively strong during the summer and during the stormy winter months. They weakened during spring while during autumn they were close to zero and occasionally even directed southward during the early autumn. This is especially noticeable in September The largest multiyear, monthly mean velocity for the two stations occurred in February when it reached a value of 12 cm/s. This was due mainly to the very stormy weather during the winters of 5of21

6 Figure 4. Rotary power density spectra of the currents (left) for the spring and summer of 1990 at station N4 (bottom and instrument depths of 120/34 m, respectively) and (right) for the winter of 1993/ 1994 at station H1 (26/18 m, respectively). The 95% confidence intervals are also shown and 1994 when the monthly mean velocity at station A1 reached 20 cm/s. The multiyear, monthly mean velocity then decreased during spring, reaching its lowest value in May (3 cm/s), and then increased again during the summer, reaching a secondary maximum of 8 cm/s in July. [13] The individual monthly standard deviations of the alongshore velocity fluctuations generally exceed the monthly means, reflecting the frequent occurrence of southward current events in winter and during the transition seasons, even though the mean flow is northward. The standard deviations were largest during the winter with values ranging from 8 to 20 cm/s, and smallest during the months of September and October with values of only 2 to 7 cm/s. The summers were also characterized by relative low velocity fluctuations with standard deviations ranging from 4 to 7 cm/s. They are comparable to and, in July, even smaller than the monthly mean, thus indicating the steady and persistent northward flow during this season. [14] Comparing wind data at stations AW and MW reveals various mesoscale variations. The northern station, MW, is characterized by stronger synoptic wind fluctuations and is more affected by easterly or northeasterly winds during late autumn and winter, whereas sea breezes are more dominant at the southern station. Despite the mesoscale variations, the monthly mean alongshore wind stresses along the shelf are generally northward during winter owing to the winter storms. These are northward during summer as well owing, in part, to counterclockwise veering of the midbasin monthly wind direction to westerly or southwesterly wind when approaching the Israeli coast (Table 2). During the transition seasons, except for the months of November or March (which are sometime affected by an early or a long-lasting winter, respectively), the monthly winds are, however, northwesterly resulting in a southward wind stress component (Table 2). The monthly mean northward direction of the alongshore currents during winter and summer as well as the reduction in the northward monthly mean magnitude during the transition seasons fairly closely follows the monthly mean alongshore wind stresses. In addition, performance of a linear regression between the monthly alongshore wind stress at station AW and the monthly mean alongshore currents at the shallow stations A1 and H1, for about 20 months of current measurements, results in quite high positive correlation coefficients of 0.87 and 0.81, respectively. The residuals of these regressions are positive (Figure 6), hinting at the possible existence of an alongshore pressure gradient probably due to the affect of the whole basin circulation or wind action along the Sinai coast. It is also worthwhile to note Figure 5. Monthly means (symbols) and standard deviations (error bars) of the 40-hour low-pass filtered alongshore velocity component at the two shallow stations, H1 and A1 (right shifted) for the years Only months having at least 2 weeks of data available (in most case more than 25 days) were included. The solid line is the mean of the two stations. At both stations the bottom depth and instruments depth were 26 and 18 m, respectively. 6of21

7 Table 2. Long-Term Monthly Wind Statistics at Coastal Stations AW and MW for Wind Magnitude and Direction, Standard Deviation of Along-Shore and Across-Shore Velocity Components (m/s), and Mean Along-Shore and Across-Shore Wind Stress (in dyne/cm 2 ) AW: MW: Vel. Std. Dev. Mean Stress Vel. Std. Dev. Mean Stress Mag Dir Along Across Along Across Mag Dir Along Across Along Across Jan Feb March April May June July Aug Sept Oct Nov Dec that no significant correlation was found between the monthly currents and the monthly across-shore wind stresses. [15] Although the alongshore current component at stations A1 and H1 were significantly correlated at synoptic timescales (R = 0.8 for 19 months), the correlation of the low-pass filtered alongshore velocities between those two stations varied considerably according to the months of the year. The latter correlations were generally weak and sometimes even insignificant as shown in Table 3. Furthermore, the correlation of the velocity does not appear to depend definitively on the correlation between the alongshore wind stresses at stations AW and MW. In particular, an increase in the coherence of wind stresses along the shelf does not necessarily result in an increase in coherence between the currents. The time-lagged correlations between the along shelf wind stress and the currents for the different months of the year shows they were generally significant except for the months of July September (Figure 7). The effects of the wind stress can explain from 30% to 80% of current variance. For correlations greater than 0.7 the currents lag behind the wind stress at AW by 6 to 18 hours. [16] As noted in the previous subsection, the monthly mean wind stresses as well as the synoptic variations during the different months of the year at the coastal wind stations are generally weak (Table 2). However, wind measurements conducted for a short period (February May 1989) on an oil drilling ship located near the shelf break some 18 km offshore from wind station AW indicate much stronger winds than measured at the coastal station (located 1.5 km inland). This can be clearly seen in Figure 8 from the stick diagram covering the entire 4-month period of concurrent onshore and offshore wind measurements. An empirical orthogonal decomposition of these data shows that the alongshore wind components are highly coherent, having an offshore amplitude twice that of the coastal station. Similar coherence and amplitude relationships were also found by a cross-spectral analysis at the synoptic timescale. [17] The simultaneous pressure measurements at the inner shelf stations H1 and A1 (100 km apart) during the years of , were used to obtain 40-hour low-pass filtered time series of pressure differences and pressure gradients and the corresponding standard deviations for several time periods, lasting from 2 weeks to 1 month. Owing to the lack of knowledge as to the absolute height of the pressure sensors and the exact difference of their depths below the sea surface, the absolute average pressure gradients are unknown. However, the standard deviations are still adequate to represent the pressure gradient fluctuations. The standard deviations of the pressure gradients, normalized by the density, were highest during winter, averaging cm/s 2, and lowest during summer, averaging cm/s 2. These were about 3 to 4 times larger then corresponding standard deviations of the along shelf component of wind-forcing at the inner shelf (obtained by dividing the wind stresses by density and water depth) or about the same magnitude, if the possible seaward increase of wind speed is taken into account. In either case, these findings demonstrate the significant role of the along-shelf pressure gradients as a driving force for the along shelf current, and may additionally explain the weak correlations Figure 6. Alongshore monthly mean wind stress at Ashdod (station AW) versus monthly mean alongshore velocities at the shallow current meter stations A1 and H1. The best fit lines for the two stations are also shown. 7of21

8 Table 3. Time-Lagged Correlation Between (40-Hour Filtered) Longshore Currents at Stations A1 and H1 (RC) and Between Longshore Wind Stress at Stations AW and MW a Month L RC lag RW lag L RC lag RW lag L RC lag RW lag Jan Feb March April May June July Aug Sept Oct Nov a Time lag is in hours; when positive: H1 lags A1. L is the period length (in days). which were found between the wind stress and the along shelf currents Currents Over the Mid and Outer Shelf [18] In Figure 9 we show the monthly mean alongshore velocity component for the two shelf break stations A4 and N4. Similar to the shallow stations, the monthly mean alongshore currents were generally directed northward throughout the year. Here the multiyear monthly mean velocities at the uppermost current meters (30 40 m below the surface) were weakest during the winter months of January February and strongest during the summer months of June July. This is in contrast to the shallow stations, which exhibited a bimodal seasonal structure with clear northward maxima in both summer and winter (Figure 5). The weaker monthly mean northward flow at the deeper stations, as compared to the shallow stations, is due to frequent episodes of southward flow near the shelf break, as for example, during February March 1989 (Figure 10). These months experienced more frequent easterly or northeasterly winds, which are capable of inducing southward currents. It is also possible that the southward currents seen, for example, in the upper layer at station A4 during 1 10 February 1989 (Figure 10) could be due to an off-shelf anticyclonic eddy, which would affect the outer shelf but only slightly affect the mid shelf. Another indication for offshelf eddy activity at station A4 was observed during the summer season causing a relatively long period (10 days) of southward currents during August 1990 (not shown). In the model simulations presented below transient, offshore eddies occasionally develop in this region. [19] As noted above, the strongest northward multiyear, monthly mean magnitudes occurred during the months of June and July when they reached values of 30 cm/s and 22 cm/s for station N4 and A4, respectively. The monthly mean velocities at the northern station (N4) remained higher than at A4 throughout the summer. In addition, the synoptic timescale currents at station A4 during summer were relatively more rotational, also indicating eddy activity. The dominant rotational component in the rotary spectra at the shelf break and over the slope was clockwise. Such eddy activity was probably also the cause for other episodes of southward currents. [20] During summer, both the monthly mean current and synoptic velocity fluctuations decreased with depth below the upper mixed layer. This decrease of the mean velocity is in agreement with the strong stratification and sloping isopycnals over the shelf, resulting in very low magnitudes of only a few cm/s close to the bottom at the outer shelf. Characteristic of this season is an offshore increase in the northward seasonal current within the upper water layer. During winter, however, there was no clear seaward increase in the northward monthly mean velocity in the upper layer (Table 4). Although strong currents were often found to exist over the outer shelf, the most energetic currents during winter storms were generally found at the shallow stations. This is probably due to the relative small mass of the water column. The seaward increase during summer, however, may be explained by combined effects of the seaward increase of the alongshore wind stress, the seaward thinning of the surface mixed layer, and the reduced bottom friction. An example for the summer season appears in the current time series off Atlit during June August 1988 (Figure 11) when the mean northward velocities increased seaward with values of 19, 30, and 41 cm/s for the Figure 7. Maximum monthly lag correlation between the 40-hour low-pass filtered alongshore wind stress at Ashdod and the corresponding alongshore currents at H1 and A1 (right shifted). The dotted line indicates the 95% significance level for 12 degrees of freedom. 8of21

9 Figure 8. Stick diagrams showing the wind vectors at the two southern stations AW and ASW (offshore) and at the northern station MW for the late winter and spring of uppermost current meters at stations T1, T2 and T3, respectively. The low-pass filtered alongshore velocity fluctuations were highly coherent within the water column and across the shelf, with a maximum of 82% of the variance explained by the first mode of the empirical orthogonal decomposition (using a time lag of 6 hours for T2 relative to T1, and T3 relative to T2). The amplitudes of the first mode were similar for the mid and outer shelf and both were larger than that of the inner shelf (Table 5). [21] During the autumn, the strong currents, initially confined to the upper mixed layer, spread over a deeper layer following the erosion of the seasonal thermocline. This nearly uniform layer of strong flow almost reaches the bottom of the outer shelf by early winter. During winter, both the monthly mean velocities and synoptic fluctuations are uniform within the water column. Data from station T3 show that, over the outer shelf, these currents can be very strong and uniform (Figure 12). The along-bathymetric synoptic fluctuations for this example, as well as for the entire winter season were found to be highly barotropic with more than 95% of the variance explained by the first vertical empirical orthogonal mode. Typical of the winter season is an increase in the northward current magnitude close to bottom for the shelf break stations (Table 4). This increase is consistent with the shoreward, upward slope of the isopycnals across the weak shelf-break front (Figures 3c and 3d). [22] Also during winter, when approaching the bottom, a counterclockwise rotation of current direction accompanies the strong northward near-bottom flow. This veering, caused by bottom friction, leads to a transition from a weak shoreward monthly mean flow component in the upper water layer, to a significant seaward flow component above the bottom, having a monthly mean magnitude of around 4 cm/s, 3 meters above the bottom at the mid and outer shelf. The monthly mean seaward velocity component at the inner shelf was around 2 cm/s, which is considerably less owing to the higher position of the current meter (8 meters above the bottom). A seaward flow component also exists during summer at the inner shelf but is absent at the shelf break sites (Table 4). [23] Considering the across shelf variations of the lowpass filtered, along-shelf velocity fluctuations, we found that the inner and mid shelf stations were highly correlated. However, the inner shelf and shelf break stations were only weakly correlated, probably owing to the latter being affected by the slope and open sea circulation and eddy field. Furthermore, in both winter and summer, only weak or insignificant along shelf correlations appear between the southern and northern shelf break stations, which are separated by some 100 km. The degree of correlation increases for the southern and northern inner shelf stations, which are still weakly but significantly correlated. The spring shows a much higher coherence of the alongshore currents over the entire shelf, including the shelf break stations. This can be seen, for example, in the current vector time series during March 1990 shown in Figure 13. Bottom Figure 9. As in Figure 5 except for the two shelf-break stations N4 and A4 (right shifted) for the years At both stations the bottom depth and instruments depth were 120 and m, respectively. 9of21

10 Figure 10. Stick diagrams showing the wind vectors and the corresponding current vectors at various stations (currents from several depth levels where available) for late January through March pressure measurements conducted during this weakly stratified season at stations A1 and A4 off Ashkelon, also show the along-bathymetric current component at midshelf is in geostrophic balance with the across-shelf pressure gradient (Figure 14). This figure also indicates a strong correlation between the along-shelf wind stress and the 12- to 24-hour lagged currents and pressure gradient. It is worth noting, however, that such geostrophic balance does not occur in summer owing to the effects of stratification Currents Over the Continental Slope [24] The currents over the continental slope at station H5 (bottom depth of 500 m) were predominantly northward in the lower part of the surface mixed layer and were found to be strongest during winter and again during summer (max- Table 4. Interannual Monthly Mean Velocity Magnitude and Direction for the Shallow Stations A1 and H1 and for the Uppermost (a) or Lowest (d) Current Meters at Stations N4 and A4 a A1 H1 N4-a N4-d A4-a A4-d Month V Dir V Dir V Dir V Dir V Dir V Dir a Magnitude is given in cm/s. Directions are relative to the local bottom contours, defined positive (counterclockwise) or negative (clockwise) to a contour s north-tending extension. These monthly means were obtained using all available data for the whole study period. 10 of 21

11 Figure 11. As in Figure 10 except for the summer of 1988, across the shelf off Atlit. imum hourly mean velocities of 90 and 60 cm/s, respectively). These currents decreased with depth below the pycnocline throughout most of the year (except late winter) in agreement with the horizontal density gradients existing over the slope. The seasonal cycle of the along-bathymetry monthly mean velocities m below the sea surface closely follow the seasonal variations over the inner shelf with a bimodal distribution as shown in Figure 15. The monthly mean velocities during summer were comparable to those in the upper layer over the outer shelf. The mean flow dominated the synoptic fluctuations, reaching their highest values of about 25 cm/s in July. For the example of the summer of 1994, as shown in Figure 16, the synoptic fluctuations were coherent throughout the water column. However, these fluctuations, as well as the mean velocity, decreased and were quite weak below the seasonal pycnocline. During the summers 1994 and 1995, the monthly mean current direction measured at 300 m was, in fact, directed southward with a weak magnitude of 1 to 4 cm/s. At 20 m above the bottom, however, the monthly mean velocity was directed northward during all three summers, having a magnitude of up to 3 cm/s (Table 6). At 50 m, the northward monthly mean velocities during the winter exhibited a strong interannual variability, being quite weak during the winter of 1995/1996 (less than 8 cm/s) but very strong during the winters 1993/1994 and 1994/1995 (exceeding 30 cm/s). The relatively high monthly mean velocities during the latter two winters dominated the synoptic fluctuations and extended at least to a depth of 120 m. At depths of 300 m or more, the monthly mean velocities Table 5. Statistics of the Low-Pass Filtered Alongshore Velocity Components off Atlit During Summer 1988 (15 June to 3 August) Station Instrument/Bottom Depth, m Velocity Standard Mean, cm/s Deviation, cm/s Principle Axis Direction, a deg EOF: Mode- 1 EigenValue, Eigen Vector (cm/s) 2 Variance Explained T1: 25/ (%) T2: 29/ T3: 29/ T3: 59/ a Principal axis directions are relative to the local isobath. 11 of 21

12 Figure 12. Stick diagrams showing the current profile at the midshelf station T3 for the first half of the winter 1987/1988. The wind vectors are also shown for comparison (top part of figure). decreased to only a few cm/s, but were still directed northward. In addition, for the available near bottom data for the winters of 1993/1994 and 1995/1996 we noticed an increase in the northward mean velocity toward the end of the mixing season reaching, for example in February 1994, a magnitude as high as 9 cm/s with a maximum hourly velocity of 30 cm/s. Within the upper layer (from the surface to a depth of at least 120 m) the monthly mean velocities during winter were generally uniform and the along-bathymetry synoptic fluctuations were coherent and barotropic. [25] In December 1993 (Figure 17a) rather strong northward currents were observed in the upper layer at station H5. This occurred despite the weak wind-forcing and in contrast to the weak currents measured at the same time at station H1 on the inner shelf (Figure 17a). However, January and February 1994 were relatively stormy months, resulting in quite strong currents over the slope, at the shelf break (station H4), as well as over the inner shelf (Figure 17b). From both the latter figure and an empirical orthogonal decomposition of the time series, the along-bathymetry synoptic fluctuations in the upper layer at station H5 were highly correlated with those at the shelf break. Despite this strong connection, the correlations of the slope and shelf break currents with the inner shelf station H1 were found to be much lower and weakly significant. For January and February 1996, when data were also available at station H1, no correlation was found between the slope and inner shelf. Such a low correlation is not surprising assuming the shallow stations are more affected by the local windforcing. Similarly, a cross-spectrum analysis shows no significant coherence between the along-bathymetry slope currents and the coastal wind stress components on synoptic timescales. This is due mainly to episodes of relative strong currents over the slope with only moderate or weak costal wind speeds, and furthers points to the potential influence of the open sea circulation. 4. Comparison With Numerical Simulations [26] While the in situ measurements presented above present an excellent record of the currents and their temporal variability, they are nevertheless limited to specific points and therefore provide only a very partial picture of the spatial variability. Furthermore, it is clear that the shelf and slope circulation is not an isolated domain and it is therefore important to understand this flow within the context of the overall basin wide circulation. During the past ten years, results have been published from various full Mediterranean models [e.g., Roussenov et al., 1995; Zavattareli and Mellor, 1995; Korres et al., 2000a, 2000b]. However, these models had a relatively low horizontal 12 of 21

13 Figure 13. As in Figure 10 except for the spring of resolution of about 25 km, which is insufficient to resolve the shelf circulation in the southeastern Mediterranean. More recently, as part of the Mediterranean Forecasting System Pilot Project (MFSPP), a higher resolution (1/8 ; 13 km), prototype, near real time, forecasting model was developed by Pinardi et al. [2003], however this is also too coarse to resolve the shelf circulation. Within the same project, Korres and Lascaratos [2003] developed a regional, Figure 14. Alongshore velocity at station A2 compared to the geostrophic velocity at midshelf computed from the cross-shelf pressure gradient between stations A4 and A1P for (left) mid-february through mid-may 1990 and (right) May through early June The two pressure gauge stations were 16 km apart. The geostrophic scale (40 cm/s) corresponds to a pressure difference of 5.3 hpa. For comparison we also show the alongshore wind stress at station AW (upper curves) and the alongshore velocity at station A4. 13 of 21

14 Figure 15. As in Figure 5 except for the midslope station H5 for the years eddy resolving model for the Levantine, Ionian, and Aegean basins with a resolution of 1/20 (5.5 km) which is potentially able to resolve the shelf circulation, but they used a minimum depth cutoff of 50 m thereby eliminating the zone that we refer to as the mid and inner shelf. Within MFSPP, Brenner [2003] developed a very high resolution (2 km) model of the southeastern corner of the Levantine basin. While this model focuses on the region that most interests us, the relatively small domain prevents us from being able to see the shelf circulation within the context of the entire Levantine basin. Therefore, here we have decided to apply a regional model similar to that of Korres and Lascaratos [2003] but with two main additions. First we extend the model to cover the entire shelf by specifying a cutoff depth of 5 m, and second we directly include atmospheric pressure forcing through the inverse barometer effect in addition to wind stress and heat fluxes Model Description [27] Following Brenner [2003] we have used the Princeton Ocean Model (POM) for our work. The model has been used extensively to simulate the coastal circulation in various regions around the world, including the Mediterranean Sea [e.g., Zavatarelli and Mellor, 1995; Korres and Lascaratos, 2003], and has been described elsewhere. A detailed description of the model equations and numerical formulation is given by Blumberg and Mellor [1987]. Briefly, POM is a three-dimensional, time-dependent, primitive equations model. It consists of prognostic equations for the two components of the horizontal momentum, potential temperature, salinity, and the free surface. It is completed by three diagnostic equations consisting of the hydrostatic equation, the equation of state, and an equation for the vertical velocity, which is derived from the mass continuity equation. In addition, POM contains an imbedded higher Figure 16. Stick diagrams showing the cross-shelf variations of currents along a transect off Hadera for midsummer For the midslope and shelf-break stations (H5 and H4, respectively) the vertical current profiles are also shown. For comparison, the wind vectors are included at the top of the figure. 14 of 21

15 Table 6. Vertical Distribution of Monthly Mean and Standard Deviation of the Along-Bathymetric Velocity Component (cm/s) at Station H5 During Winter and Summer a Winter 1993/ / Dec Jan Feb Jan Feb Dec Jan Feb Dec Nominal Depth, m v std. v std. v std. v std. v std. v std. v std. v std. v std Summer June July Aug June July Aug June July Aug Nominal Depth, m v std. v std. v std. v std. v std. v std. v std. v std. v std a Nominal bottom depth is 500 m. order turbulence closure scheme to simulate the vertical mixing [Mellor and Yamada, 1982]. [28] The equations are solved using finite differencing in both space and time. In the horizontal it uses the Arakawa C scheme while in the vertical it uses a terrain following sigma coordinate [Phillips, 1957]. The time integration is done with a split explicit scheme in which the barotropic and baroclinic modes are integrated separately with a leapfrog scheme but with different time steps Model Setup and Forcing [29] The domain of the model covers the entire region east of 24 E and south of 37 N and includes the entire Levantine basin, the eastern part of the Ionian Sea, and the Figure 17. As in Figure 16 except for (a) the first half and (b) the second half of the winter of 1993/ of 21

16 Figure 18a. Monthly mean simulated currents from the last 2 years of the model runs for locations corresponding to the two shallow current meter stations (see Figure 5). southern part of the Aegean Sea. The horizontal grid spacing is 0.05 (5.5 km) and the water column is divided into 24 unequal thickness sigma layers. The model bathymetry was bilinearly interpolated from the US Navy 1-min (1/ 60 ) gridded data set and the minimum depth in the model was set to 5 m. [30] The model is forced at the surface (wind stress, heat flux, atmospheric pressure, and salinity flux) and at the lateral (western and part of the northern) open boundaries. For the wind stress we use the monthly mean values computed by Korres and Lascaratos [2003] on the basis of the ECMWF reanalysis data. Similarly, the heat flux components (solar plus total cooling) were the adjusted monthly means taken from the final year of the simulation of Korres and Lascaratos [2003]. Atmospheric pressure was computed as monthly means also from the ECMWF reanalysis. The spatial resolution of these surface-forcing fields was 1 1. The salinity flux was computed on the basis of the difference between the latent heat flux (evaporation) from Korres and Lascaratos [2003] and the monthly mean climatological precipitation from Jaeger [1976]. [31] At open boundaries, lateral boundary conditions for the total velocity, the barotropic velocity (or free surface), temperature, and salinity were spatially interpolated from the eighth year of the coarse grid climatological simulation of the entire Mediterranean run by Pinardi et al. [2003]. After interpolation, the normal velocity component was adjusted to preserve the total mass flux from the coarse grid across the open boundary. At inflow points, temperature and salinity were taken from the same coarse grid simulation while at outflow points they are advected from the first interior grid point. All of the coarse grid values were available as 10-day means and were therefore linearly interpolated in time to the model time step. Finally, the initial conditions were taken as 10 January from the coarse grid model. Our model was integrated for six years Model Results [32] In Figure 18a we show the annual cycle of the monthly mean along shelf currents, averaged at the two model grid locations (three dimensional) that are closest to the locations of the two shallow current meters, H1 and A1 (see Figure 5). Similarly in Figure 18b we show the simulated currents averaged at the grid locations closest to the two shelf break current meters, N4 and A4 (see Figure 9). In both plots we show the model results averaged for years 5 and 6 for the simulations without atmospheric pressure forcing (dashed line) and including atmospheric pressure (solid line). For comparison we also show the observed monthly mean currents (from Figures 5 and 9, respectively). [33] At the shallow stations, the model simulated the typical current speed of 10 cm/s or less as well as the predominant northward direction throughout the year. The simulation with atmospheric pressure forcing reproduced the seasonal cycle of the speed with a summer and an early winter maxima, a spring minimum, and a weak fall minimum, although there is a 1-month shift in the timing of the simulated summer maximum. Brenner [2003] noted a similar problem in a higher-resolution shelf model where the temporal shifts between the simulated and observed extremes were even larger. Here this latter difficulty has been significantly improved with the simulated minimum correctly appearing in September, although the speed is still too high. Furthermore by comparing the simulations with and without the atmospheric pressure forcing, it is clear that the surface pressure gradient is especially important in late summer and autumn. [34] At the shelf break stations, the improvement in the timing of the seasonal cycle of the alongshore flow in the present simulations as compared to that of Brenner [2003] is even more noticeable. Here the observations show weak currents throughout the spring followed by a very pronounced summer maximum in July/August, a local minimum in September, followed by a weak secondary maximum in November. Both simulations (with and without Figure 18b. As in Figure 18a except for the two shelfbreak stations (see Figure 9). 16 of 21

17 Figure 19a. Monthly mean simulated free surface height and 30 m velocity from the final year of the simulation including the atmospheric pressure forcing for the months of January April. atmospheric pressure) reproduce the weak spring currents followed by a summer maximum, although there is a delay of about 1 month. Nevertheless this is much better than Brenner s [2003] work, where the delay in the timing of the summer maximum was 2 months. Furthermore, the October minimum and November maximum are indeed reproduced in the simulation with atmospheric pressure forcing, in contrast to Brenner s [2003] work where they were completely missed. [35] Nevertheless, one weakness of the model is that it significantly overestimates the current speed at the inner and mid shelf stations in the summer and in the autumn. Several factors may account for this. First, the wind stress we used is based on monthly mean climatological values so that synoptic and higher-frequency fluctuations are missing. Therefore the turbulent kinetic energy imparted to the sea is insufficient to generate a deep enough mixed layer in these seasons, and therefore the near surface currents are too strong and confined to a relatively thin layer. Another possible reason is that the model resolution of the continental shelf is still too coarse so that the simulated shelf break jet is located too close to the coast. While there is room for further improvement in the simulations, given the various limitations the model performs reasonably well, at least qualitatively. [36] As noted above, one of the major motivations for comparing the measurements to the model simulations at this stage is to be able to understand the wider context of the observations and to see how the coastal circulation interacts and interfaces with the basin wide circulation. In Figure 19 we show the monthly mean currents at 30 m from year 6 of the simulation with atmospheric pressure forcing for the months of January April (Figure 19a), May August (Figure 19b), and September December (Figure 19c). In all cases, the free surface height field shows a very clear gradient from higher values in the southern, and especially the southeastern part of the basin, to lower values in the north and northwestern region. This gradient is maintained by the combined affects of the mean, large-scale wind field, which has a very strong westerly to northwesterly component throughout the year, and the resulting surface layer Ekman transport directed toward the coast of Sinai. [37] Another consistent feature that appears in all months is the Atlantic Ionian Stream, AIS, [Robinson and Golnaraghi, 1994; Malanotte-Rizzoli et al., 1999], which appears as a jet entering from the western open boundary along the North African coast. The eastward extent of this jet clearly fluctuates according to the seasons. In general it remains coherent as a well-defined coastal jet until 27 E 28 E, thus defining the eastern part of the Southern Levantine Current. There it separates from the coast and begins to form anticyclonic meanders, some of which can grow quite large while others pinch off to form warm core eddies. This meandering current continues flowing eastward through the center of the basin as the Mid-Mediterranean Jet, which forms a clear separation between the persistent cyclonic circulation in the northern 17 of 21

18 Figure 19b. As in Figure 19a except for the months of May August. basin and the variable circulation in the southern basin. In March the eastward penetration of this jet is limited where as in the other months shown the jet reaches to the far eastern end of the basin where it turns northward along the coasts of Lebanon and Syria. In summer, autumn, and the first half of the winter, as the AIS separates from the coast it also bifurcates and forms a secondary coastal current, which flows eastward along the coasts of Egypt and Sinai and then turns northward along the coast of Israel. In late winter and early spring this coastal current is very weak or completely absent. [38] The variability of this coastal jet explains much of the signal that is recorded in the observed current meter data. In late winter and spring this current tends to weaken (see Figure 19a). In the March there is only a hint of a very weak current off the coast of Sinai. In August (Figure 19b) and November (Figure 19c) this current is well defined and reaches the area of Haifa where it separates from the coast and turns westward to the open sea to forms a large, offshore, cyclonic eddy. A similar feature was found in the higher-resolution simulations of Brenner [2003]. In October (Figure 19c) this current is somewhat weaker than in August and November and appears to dissipate just north of Haifa. This latter intermonthly variability explains the late summer/autumn interruption in the monthly mean current as seen in Figures 5, 9, and Discussion and Conclusions [39] This study summarizes the results of a program to systematically study the circulation on the Mediterranean shelf and slope off Israel on the basis of direct current measurements conducted between 1987 and 1996 and subsequent comparison to climatologically forced model simulations. The results showed that monthly mean currents on the shelf throughout the year were directed mainly northward, following the bathymetry. The along-isobathic monthly mean velocity variations on the shelf throughout the year were highly correlated with the monthly mean alongshore wind stresses. They were strongest during the summer and during the stormy winter months, weaker during spring, and were close to zero and occasionally even directed southward, during the first two months of autumn. The alongshore, monthly mean and synoptic fluctuations were vertically coherent and uniform during winter with a slight increase in the monthly mean velocity close to bottom at the shelf break. During summer the strong currents were confined to the upper layer with magnitude increasing away from the coast. The currents weakened with depth in a manner consistent with an across-shelf, sloping pycnocline and, in contrast to winter, they were very weak close to bottom over the outer shelf. The winters were characterized by downwelling, accompanied by a seaward Ekman drift in the bottom boundary layer across the entire shelf. At the shelf break sites the seaward flow was also associated with a stronger northward current in the vicinity of the bottom as compared to the upper water column. This is quite characteristic of cascading shelf water due to winter buoyancy losses [Davies and Xing, 2000; Shapiro et al., 2003; Ivanov et al., 2004] and is further substantiated by the density cross sections over the shelf/slope during midwinter (Figures 3c 18 of 21

19 Figure 19c. As in Figure 19a except for the months of September December. and 3d). The possibility of downwelling in the presence of cascading and a resulting enhancement of the seaward transport is, indeed, a direction for further research. [40] Over the slope, at a bottom depth of 500 m off Hadera, the currents measured in the upper layers exhibit a seasonal cycle similar to that of the monthly mean alongisobathic velocity variations found over the shelf, but with much stronger magnitudes during the summer and during the stormy winter months. The strong vertical sheer in the current speed during summer together with the sloping pycnocline, lead to a weak, and occasionally a reversed flow at middepths of the water column. During winter the strong currents were confined to the uppermost 200 meters, in agreement with the deeper, sloping permanent pycnocline. [41] In order to present an overall view of the shelf/slope circulation, in Figure 20 we show the monthly mean alongshore currents from all stations (i.e., the curves from Figures 5, 9, and 15). We also include a plot of the monthly mean alongshore wind stress component averaged for station AW and MW. Evident is the seasonal similarity between the along shelf and along slope currents as well as the good correlation with the alongshore wind stress. The seasonal similarity between the shelf and slope circulations may be attributed, however, to the seasonality in the borderscale wind regime over the Levantine Basin. In both summer and winter, but not in the transitions seasons, the wind stress has a strong eastward component [Roussenov et al., 1995] which supports the overall basinwide, cyclonic circulation. The stable and persistent atmospheric conditions during summer, as opposed to the higher variability and unstable conditions of winter [Demirov and Pinardi, 2002] are reflected in a more persistent along slope jet during the summer as compared to winter. On the basis of the model simulations, it is clear that the northward currents during summer off the coast of Israel are part of a cyclonic boundary current formed by the continuation of the eastward flow of the Southern Levantine Current along the shelf and slope of Egypt and Sinai. According to the model this boundary current prevails until midwinter in parallel with the Mid-Mediterranean Jet, thereby resembling the general circulation scheme described by Pinardi et al. [2005]. However, the current measurements off the Israeli coast do not support the presence of such boundary current during spring and autumn. Although the model properly simulates the interruption of the alone slope jet during spring, it fails to do so for autumn. As we noted in the previous section this is probably due to the use of monthly the mean wind stress which produces too little vertical mixing thereby resulting in near surface currents that are too strong, and the relatively coarse horizontal resolution over the slope and shelf. [42] We also found it helpful to compare the measurements to hindcast fields of the full Mediterranean forecast/ analysis system of MFSTEP. The spatial resolution is a bit coarser than our model, but it includes weekly assimilation of atmospheric and oceanic data. Even though this covers different years than our measurements, it is nevertheless 19 of 21