On pathways and residence time of saltwater plumes in the Arkona Sea

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110,, doi: /2004jc002848, 2005 On pathways and residence time of saltwater plumes in the Arkona Sea H. U. Lass, V. Mohrholz, and T. Seifert Institute of Baltic Sea Research Warnemünde, Rostock, Germany Received 16 December 2004; revised 17 June 2005; accepted 23 August 2005; published 29 November [1] Time series measurements of temperature, salinity, and current at the sills of the Arkona Basin have been performed during the winter seasons 1998/1999 and 1999/2000 together with ship-based measurements of stratification and currents covering the whole Arkona Basin. Observations were supported by model simulations of saltwater inflow into the Arkona Basin. The pathway of the saltwater plume passing the Drogden Sill stretches southward along the western rim of the Arkona Basin and turns into the deeper parts of the basin through a trench at the northern rim of Kriegers Flak discharging at the eastern tip of Kriegers Flak into the saltwater pool in the center of the Arkona Basin. The saltwater plume branches into a western filament, which passes the gap between Mön Island and Kriegers Flak if its mass flux exceeds the transport capacity of the trench. The western filament drops down to the bottom of the SW Arkona Basin, which causes mixing and eddy shedding. The salt water passing the Darss Sill flows onto a wedge-shaped submarine terrace stretching from the Darss Sill to Kap Arkona. This plume joins near Kap Arkona by dropping into the underlying western Drogden Sill plume associated with mixing and eddy shedding. The central saltwater pool of the Arkona Basin discharges geostrophically controlled through the Bornholm Gatt into the Bornholm Basin. An upper bound of the discharge time of the saltwater pool based on repeated temperature-salinity diagrams was estimated to be 3 months, while the residence time estimated by an analytical model was 1 month. Mixing of the saltwater pool with the overlying brackish surface water was observed. Citation: Lass, H. U., V. Mohrholz, and T. Seifert (2005), On pathways and residence time of saltwater plumes in the Arkona Sea, J. Geophys. Res., 110,, doi: /2004jc Introduction [2] The Baltic Sea is a large semienclosed sea located with its entire drainage basin in the humid temperate zone of the Northern Hemisphere. This implies a weak estuarine flow pattern driven by the freshwater surplus, which is largely maintained by the discharge of the rivers around the basin [Helsinki Commission, 1986; Jacobsen, 1980; Wyrtki, 1954b]. Shallow and narrow straits at the entrance limit the water exchange with the North Sea. The general stratification of the Baltic proper consists of brackish surface water which is separated by a permanent halocline in a depth of m from a dense bottom water pool in the basins. The salinity is decreasing from the entrance of the Baltic toward the Baltic Proper and the Bothnian Sea in both the brackish surface water in the range of 8 3 psu and in the dense bottom water pools in the range of 20 5 psu. [3] The process of saltwater inflow from the Kattegat via the Danish straits into the Arkona Sea, which is well investigated [see Kändler, 1951; Wyrtki, 1954a; Dickson, 1973; Börngen et al., 1990; Matthäus and Franck, 1992] is governed by processes with two separate timescales. The Copyright 2005 by the American Geophysical Union /05/2004JC exchange associated with the estuarine circulation is dominating on timescales longer than a few months. The outflow of the brackish surface water is driven by a small barotropic pressure gradient, whereas the inflow of the saline bottom water is driven by the permanent baroclinic pressure gradient established by the salinity difference between the surface waters of the Kattegat and the Arkona Sea. The water exchange on timescales shorter than a month is predominantly barotropic and is driven by the large sea level differences between the Kattegat and the Baltic Sea [e.g., Hela, 1944]. These sea level differences are established by the piling up of the water at the coasts of the North Sea and the Baltic Sea under the action of the wind [Lisitzin, 1974]. According to the seasonal cycle of the wind field barotropic inflow events are particularly pronounced in winter season [Matthäus and Franck, 1992]. [4] Barotropically driven inflow into the Baltic Sea advects thick layers of saline Kattegat waters through the Danish straits toward the Arkona Sea [Wattenberg, 1941]. A barotropic inflow event transporting a sufficient amount of Kattegat waters into the Baltic Sea which finally arrives in the Gotland basin is called a Major Inflow [see, e.g., Wyrtki, 1954a; Matthäus and Franck, 1992]. These events are important for the formation and ventilation of the deep water, for maintaining the haline stratification, and, subse- 1of24

2 quently, for the control of the ecological regime in the entire Baltic Sea. [5] The Kattegat waters are subject to a variety of mesoscale circulation processes while moving through the straits of the transition area [see Fennel et al., 1987; Schmidt et al., 1998]. These processes transform the inflowing Kattegat waters differently along their paths through the Great Belt and the Öresund. The inflowing water reaches the Drogden Sill faster and with a higher salinity than the Darss Sill according to Mattsson [1996], Fischer and Matthäus [1996], and Jakobsen and Lintrup [1996]. This is due to the shorter length and the smoother topography of the Öresund compared with the channel consisting of the Great Belt, the Fehmarn Belt, and the Mecklenburg Bight. [6] According to Kõuts and Omstedt [1993], strong mixing of the bottom water occurs in the Arkona Sea since the bottom water has an entrainment rate of 53% while passing this basin. The saline Kattegat water flowing into the Arkona Basin is spreading generally as gravity-forced dense bottom water cyclonically along the rim of the basin into the Arkona Basin [see Lass and Mohrholz, 2003], and is finally stored in more or less independent pools at the bottom of the Arkona Basin and subsequently in the Bornholm Basin [Krauss and Brügge, 1991; Liljebladh and Stigebrand, 1996]. [7] The kinematics and dynamics of dense bottom currents have been studied in the laboratory and in several sill regions of the world ocean, for example, in the overflow area between the Greenland Sea and the North Atlantic, the Strait of Gibraltar, etc. A review of the properties of gravity currents is given by Griffith [1986]. From these studies it is known that gravity, the Coriolis force, bottom friction, and the peculiarities of the bottom topography are important for the pathways of the dense bottom water plume and its mixing with ambient water. Lass and Mohrholz [2003] could show that differential advection [see van Aken, 1986] at the moving head of the plume is an important mixing process for propagating saltwater plumes in the Arkona Sea. Other mixing processes may be important as well depending on the particularities of the bottom topography along the path of the plume until it is absorbed into the pool. Moreover, the transformation of the bottom water may depend on the residence time in the pool of the Arkona Basin because of vertical entrainment of the bottom water and diapycnical mixing through the halocline. [8] The residence time and the pathways of salt water spreading from the sills into the Arkona Sea are not well known in detail yet in particular in the western Arkona Basin with its erratic topography. The work presented in this paper aims at elucidating the pathways of inflowing water and at estimating the residence time of the bottom water in the Arkona Sea with respect to the transformation of this water while passing this westernmost basin on its way to the basins of the Baltic proper. 2. Area of Investigation, Materials, and Methods [9] The Arkona Basin has a size of nearly 40 by 60 nautical miles with a maximum depth of 45 m; see Figure 1. It is connected with the Kattegat through the Great Belt and the Belt Sea in the west and through the Öresund in the northwest. The connections are rather narrow and shallow with sill depths of only 18 m at the Darss Sill and 8 m at the Drogden Sill, respectively. In the east the Rønne Bank separates the bottom layer of the Arkona Basin from the Bornholm Basin and depicts the eastern boundary. The Bornholm Gatt in the northeast connects the Arkona Basin and the Bornholm Basin as a channel without a pronounced sill, where the dense bottom water can leave the Arkona Sea toward the central Baltic. Kriegers Flak, a sea mount in the western part of the Arkona Basin, is another important topographic feature which may have influence on the pathways of inflowing bottom water. [10] During the winter seasons 1998/1999 and 1999/2000 time series at key positions in the Arkona Basin as well as near-synoptic basin-wide measurements of temperature, salinity, and current patterns were carried out in order to cover inflow events of saline Kattegat water and to reveal the pathways and pattern of saltwater plumes in the Arkona Basin. The online time series measurements at the Darss Sill and the Drogden Sill were used to monitor the inflow events at the sills and to trigger research cruises in order to collect data suitable to reconstruct the pathways and pattern of saline bottom water during or at least after an inflow event. The constraints with respect to the access to ship time allowed performing four cruises with R/V Professor Albrecht Penck in the Arkona Basin. [11] The dates of begin and end of each cruise are given in Table 1. It turned out that weak inflow events occurred in advance of the cruises 40/99/21 and 40/00/01 only Time Series [12] Seacat SBE16 measurements at four levels and current measurements with a bottom-mounted 600 khz RDI Broad Band ADCP were performed at the station Darss Sill together with surface meteorological measurements. TS measurements at four depths were also made at the Drogden Sill by Danish authorities. The access to TS measurements on both the Drogden Sill and the Darss Sill was online. This allowed a continuous monitoring of the actual state of saltwater inflow over the sills into the Arkona Basin. [13] The ADCP was configured to perform 300 contiguous pings every hour. The 300 profiles were vector averaged and stored in geographic coordinates in 1 m depth bins. The depth bins affected by the sidelobes of the instrument were removed from the measurements. [14] The sampling time of the SBE16 TS recorder was 10 min. Their data were checked by CTD profiles during the routine maintenance intervals of the instrument carrier at the Darss Sill Near-Synoptic Basin-Wide Measurements [15] Attempts were made within the logistic constraints to trigger field campaigns of near-synoptic hydrographic and current measurements in the Arkona Basin by the online observations of saltwater inflow events on the Darss Sill and the Drogden Sill in order to study the pathways of the dense bottom water. This resulted in basin-wide near-synoptic mappings of the hydrographic conditions taken during the cruises with R/V Professor Abrecht Penck according to Table 1. During these cruises only weak inflow events happened. [16] Along-track current measurements were performed by a vessel-mounted 300 khz ADCP of RDI interfaced with 2of24

3 Figure 1. Topography of the Arkona Basin and positions of the time series measurements of current, temperature, and salinity marked by triangles. At Drogden Sill, no current measurements were available. The dotted lines mark the hydrographic sections. the gyro compass of the ship. The ADCP was configured to measure in the bottom track mode. For current measurements an ADCP bin size of 2 m was chosen. The blank after transmit value was set to 2 m. Currents were recorded in Earth coordinates and vector averaged over 2 min. The comparison of current profiles measured on station and while the ship was sailing revealed that current profiles measured on sailing were disturbed in the upper 10 m of the water column. The thickness of the bottom layer disturbed by sidelobe effects of the ADCP was 14% of the depth. Hence currents of thin saltwater plumes could rarely be measured with the VMADCP. [17] Temperature and salinity profiles were measured using both a SeaBird 9/11 CTD lowered at stations and a SeaBird 9/11 CTD in an undulating Scanfish towed along selected tracks. The accuracy of pressure, temperature, and salinity were better than 1 dbar, 0.01 K, and 0.01 psu, respectively. A grid of hydrographical stations at transects depicted in Figure 1 covered the Arkona Basin, with distances of 2 3 nautical miles between the stations. Additionally, the surface salinity and temperature were measured by a SBE 21 thermosalinometer continuously. Standard meteorological parameters were measured continuously by an automatic ship weather station (ABWst V 2.4) implemented and serviced by the German Weather Service (DWD) Model Description: Setup of the SALPRO Simulations [18] Basin-wide field measurements performed in such a variable environment as the Arkona Sea are always contaminated by a mismatch of timescales and space scales because of the time it takes to cover the whole station net with the ship. Model simulations of the relevant processes may assist in overcoming these problems associated with ship-based field observations. Therefore numerical simulations were carried out with a three-dimensional Baltic Sea circulation model which is based on the modular ocean model (MOM-3) [Pacanowski and Griffies, 1999]. The Table 1. Cruises of R/V Professor Albrecht Penck During Which Hydrographic and Current Data Were Acquired in the Arkona Basin Cruise Number Start End 40/98/24 24 Nov Dec /99/04 15 Feb Feb /99/21 1 Nov Nov /00/01 3 Feb Feb of24

4 Figure 2. Model topography (m) and geographic coverage of the IOW Baltic Sea model. Thin lines indicate the stretched grid (each mesh corresponds to grid cells). The grid resolution varies from 1 nautical mile in the central region to 9 nautical miles. The land contour indicates the dry model points. adaptation of the model to the Baltic Sea required a module for the treatment of open boundaries. A modified version of the algorithms given by Mutzke [1996] was used. A radiation condition is applied to signals coming from inside the model domain. Temperature and salinity are relaxed to climatological profiles at the open boundary. The along boundary shape of the free surface elevation is extrapolated from the model interior, but the average elevation keeps track with prescribed data taken from gauge records. The model is driven by prescribed atmospheric fields like wind, air pressure, and air temperature. The coupling of atmosphere and ocean is done by a boundary layer module applying parameterizations for heat fluxes and wind stress taken from literature [Schmidt et al., 1997]. The brackish waters of the Baltic Sea result from an estuarine circulation of saline deep water inflows and strong input of approximately 480 km 3 of fresh water per year coming from rivers and precipitation. [19] Since the investigations were focused on the spreading of saline water through the Belt Sea, a model resolution of approximately 2 km was applied within the area to eastern longitude and to northern latitude. Such a high resolution is necessary to reproduce the essential dynamical features like eddies, coastal jets, and coastal upwelling within this area of the Baltic Sea, as shown by Fennel et al. [1987] and Schmidt et al. [1998]. [20] The full model covers the whole Baltic Sea and the North Sea with a coarsening model grid, indicated in Figure 2 by grid lines for grid cells. The highresolution domain comprises cells with a spacing of 2 0 in longitude, and 1 0 in latitude. In the outer parts the grid spacing is increased by a factor of 9 leading to a total of grid cells. Thus the open boundary is located far from the area of investigation at 4 W. The other margins of the North Sea are closed. [21] The model bathymetry was derived from the digitized Baltic Sea data set compiled by Seifert et al. [2001] and the DYNOCS data for the North Sea compiled by Weiergang and Joensson [1996]. The averaged water depths were mapped onto 60 model layers: 30 upper layers of 3 m thickness, 2 transition layers of 3.75 m and 5.25 m, and 28 layers of 6 m down to a maximum water depth of 267 m. Thus the deep basins of the Baltic Sea are completely covered. Only the small region of the Landsort Deep and greater water depths in the Skagerrak and the northern North Sea are neglected. The upper 30 model layers allow an adequate resolution of the shallow Belt Sea and of the mixed surface water in the Baltic Sea down to the halocline. The critical thresholds for the inflow of saline bottom water are resolved by at least 3 active model layers at Drogden Sill, and 7 layers at Darss Sill. Despite the high grid resolution, the model bathymetry had to be corrected in 4of24

5 [24] Subscale processes not resolved by the model grid were parameterized by the following MOM options: The scheme of Smagorinsky [1993], which evaluates horizontal mixing coefficients from the local current shear, was applied with a scale parameter of ko = 3 [see Pacanowski and Griffies, 1999]. The ratio between momentum exchange and tracer mixing (Prandtl number) was set to 10. The vertical mixing was described by constant coefficients of 5 cm 2 /s for momentum and 0.5 cm 2 /s for temperature and salinity. A quadratic law with a drag coefficient of was used for the bottom friction. [25] The high model resolution requires integration time steps of 120 s for the baroclinic velocity components and the tracer equations and 15 s for the free sea level and the barotropic transports. The model behavior was analyzed on the basis of snapshots of the hydrographic fields within the Belt Sea, written every 3 hours of simulation time, and daily averages covering the full model domain. Moreover, the spreading of neutral density test particles moved by the actual velocity components was saved on 3 hour time slices. Figure 3. Initial model distribution of salinity shown for the (top) whole model region and (bottom) region of interest. the Danish Straits. The narrow entrance regions of the Little Belt and the Sound were widened up to 2 2 adjacent active grid cells, and the minimum cross section water depth was set to at least 24 m. [22] The initial model fields were derived from idealized temperature and salinity profiles corresponding to a typical stratification during the winter season. Figure 3 shows the locations where the initial profiles were prescribed. The data were distributed to the enclosed model domain by successively filling adjacent empty grid cells. Thus the initial stratification is characterized by brackish waters of salinity 8 to 10 within the southeastern part of the Belt Sea and high salinities beyond 20 up to the Danish Straits. Strong initial gradients occur across the Fehmarn Belt at 11 E and the Drogden Sill at N. The Baltic Sea and the North Sea were filled with the climatological data for January given by Janssen et al. [1999]. In order to avoid strong baroclinic currents at the model start from rest, the initial sea surface elevation was adjusted to the mean local salinity. Setting zero level as reference for salinity 35, the sea level was raised by 1 cm to compensate approximately a decrease of 1 in salinity. [23] Because of the great buffer volume of the North Sea the influence of the open boundary on the investigation area is considered to be negligible during the simulation period. Therefore temperature and salinity at the open boundary were relaxed to the initial profiles, and the sea surface was nudged to the reference level zero. 3. Results 3.1. Observed Salinity Distributions in the Arkona Basin [26] The time series measurements at the Darss Sill (this part is not shown here) revealed an inflow of saline bottom water between late October and mid November The dense bottom water passing the Darss Sill had a maximum salinity of about 17 psu and a temperature of about 10 C. Subsequently an outflow of brackish surface water followed until early December During this time the surface water cooled down to about 6 C. Hence the hydrographical cruise carried out end of November 1998 could cover the distribution of saline water which was flown into the Arkona Basin roughly 3 weeks in advance of this cruise. The TS diagram of all profiles taken during the cruise is shown in Figure 4. The water mass of the dense bottom water pool is characterized by a salinity of 21 psu with a temperature of 9.5 C, which is obviously a mix of dense bottom water flown over the Drogden Sill and the Darss Sill into the Arkona Sea early November After the inflow event the bottom water obviously was mixed with the slowly cooling surface water resulting in a TS diagram bending toward the brackish surface water mass of about 5 C observed during the cruise in early December [27] The upper layer of the dense bottom water pool, characterized by the 11 psu isohaline, is about 40 m deep in the northern and central part of the Arkona Basin and bends up to about 30 m depth at the southern rim of the basin; see Figure 5. The current observations suggest that mesoscale eddies exist in the whole basin which are strongest in the southwestern part. The bottom water of the pool is flowing out through the Bornholm Gatt as a weak geostrophically adjusted bottom current from the Arkona Basin toward the Bornholm Basin; see Figure 6. [28] A weak inflow event occurred at the sills during 4 to 12 February No active inflow was observed during the cruise 40/99/04. The pool of the dense bottom water consisted of a few water masses differing in temperature (see Figure 7), indicating that they are remainders of inflow events observed at the Darss Sill in December 1998, 5of24

6 Figure 4. Temperature-salinity (TS) diagram of all stations during the cruise 40/98/24 from 24 November to 3 December January 1999, and February 1999 before the cruise 40/99/04 started. No traces of the saline bottom water entering the Arkona Basin during the inflow event of October November 1998 (compare Figure 4) were found about 3 months later in February These 3 months are an upper bound of the residence time of the bottom water in the Arkona basin. The halocline was observed in about 35 m depth during the cruise 40/99/04 (Figure 8), indicating that most of the recently entered dense bottom water had flown into the Bornholm Basin already. The strongest currents were observed north of Rügen. [29] The hydrographical cruises in November 1999 and in early February 2000 were well positioned in time in order to study the pathway of inflowing saline bottom water plumes; see Figure 9. The continuous observations at the sills revealed a sequence of inflow events starting in the first half of October 1999 which transported saline water with more than 20 psu over the Drogden Sill and more than 17 psu with a temperature of about 13 C over the Darss Sill. This inflow over the Darss Sill was delayed and weaker compared with the inflow over the Drogden Sill. The inflow activity ceased for about 10 days until saline water of more than 20 psu with a temperature of slightly more than 10 C spilled again over the sills from end of October until the first days of November 1999 when the cruise 40/99/21 started. It is interesting to note that this inflow occurred earlier over the Darss Sill than the corresponding inflow over the Drogden Sill. Obviously, the saline water west of the Darss Sill retreated only a small distance toward west during the intermediate outflow. Thereby the saline water was in a favorable initial position enabling it to reach the Darss Sill in a short time after the start of the following inflow event. [30] The TS diagram of all stations covered during the cruise in the first half of November 1999, shown in Figure 10, indicates bottom water originating from the inflow event occurring at the end of October that was mixed with bottom water entering the Arkona Basin about 3 weeks earlier. The surface water of the Arkona Basin was cooled to a minimum temperature of 8 C during the cruise. [31] The Arkona Basin was covered 3 times with CTD observations. This enabled the observation of the spreading of saline bottom water that recently entered the basin. The 6of24

7 Figure 5. Depths distribution of the 11 psu salinity surface during the cruise 40/98/24 from 24 November to 3 December The velocity vectors depicted as sticks are interpolated on the depth of the 11 psu surface from vessel-mounted ADCP measurements. This layer was chosen since it represents the upper layer of the bottom water pool which was in the majority of cases outside the range of the ADCP depth cells affected by sidelobe effects. Figure 6. Cross section of salinity through the Bornholm Gatt during the cruise 40/98/24 from 24 November to 3 December of24

8 Figure 7. TS diagram of all stations during the cruise 40/99/04 from 15 to 19 February depth distribution of the 11 psu surface during the three legs, shown in Figure 11, reveals that the saline water entering over the Drogden Sill is flowing southward along the western rim of the Arkona Basin and confluences with the water flowing over the Darss Sill. The joint saltwater plume continues to flow cyclonically from the southwestern corner of the Arkona Basin along the southern rim of the basin toward about 14 E longitude in the eastern part of the basin where the deepest part of the old saltwater pool is located. A cyclonic eddy was obviously shed northward off Arkona into the center of the basin. This eddy was already geostrophically adjusted. The injection of a second eddy from Arkona just started during the first leg. [32] The second leg was performed 1 week later. The distribution of the depth of the 11 psu surface (Figure 11) indicates that the inflow over the Drogden Sill relaxed but that over the Darss Sill it was still in progress. The bottom water advanced along the southern rim of the Arkona Basin toward the Bornholm Gatt and filled the old bottom water pool in the northeastern part of the basin. The eddy in the central Arkona Basin moved slightly northward, and a second eddy appeared in the northeastern part of the basin. The eddy activity at Arkona, associated with strong current patterns, intensified during the second leg. [33] The third leg was performed with a towed CTD about 3 days after the second leg. The general distribution of the 11 psu layer (Figure 11) did not change very much compared to the second leg. The eddies, however, were well exposed during this leg. Two geostrophically adjusted cyclonic eddies were found at the northern rim of the basin. The eddy observed off Arkona during the second leg was displaced toward the Darss Sill. [34] Details of the spreading dense bottom water plumes in the western part of the Arkona Basin after entering across the sills can better be revealed by suitably located vertical sections of salinity. The sections crossing Kriegers Flak in north-south as well as west-east direction (see Figure 12) reveal that the dense bottom current south of the Drogden Sill splits into two branches, one flowing southward along the western rim of the Arkona Basin toward the Darss Sill and the other branch flowing clockwise around Kriegers Flak. While the flows along the northern edge of the Kriegers Flak and through the gap between Mön and Kriegers Flak seem to be dominantly geostrophically bal- 8of24

9 Figure 8. Depths distribution of the 11 psu salinity surface during the cruise 40/99/04 from 15 to 19 February The velocity vectors depicted as sticks are interpolated on the depth of the 11 psu surface from vessel-mounted ADCP measurements (see Figure 5 for the reason for selecting this level). anced, the flows at the eastern and the southern edge of Kriegers Flak seem to be governed by more complicated dynamical processes. The southward flow through the gap between Mön and Kriegers Flak seems not to confluence with the saltwater plume crossing the Darss Sill since it decreases into the bottom layer of the SW corner of the Arkona Basin while the Darss Sill discharges the saline water on a submarine terrace built by the topography in about 25 m depth (see Figure 13). The second branch of dense bottom water flowing from Drogden Sill along the northern edge of Kriegers Flak obviously fills the bottom water pool in the central Arkona Basin, but a part of this water may follow the bottom contours of Kriegers Flak and fill the bottom water pool in the southwestern corner of the Arkona Basin as well (Figure 13). [35] Between the cruises 40/99/21 and 40/00/01 several inflow events occurred; see Figure 9. In December 1999, salt water of about 17 psu passed the Darss Sill with a temperature of about 5 C. A second inflow event occurred at the beginning of January 2000 carrying water of about 15 psu and 3 C over the Darss Sill. A third inflow of saline water started at the end of January The overflow over the Darss Sill was delayed and weaker compared with those over the Drogden Sill during these events. The TS diagram obtained from the CTD measurements taken during the cruise 40/00/01 covering the western Arkona Basin (Figure 14) reveals two bottom water masses: the warmer and less saline one that entered the basin in December 1999 and a colder somewhat more saline bottom water passing the sills in January No trace of bottom water was found at least in the western Arkona Basin in February 2000 from the inflow event which started end of October The two bottom waters were mixed with the brackish surface water and with each other. The horizontal distribution of the 11 psu salinity surface, shown in Figure 15, again revealed a cyclonic spreading of the dense bottom water plume from the sills along the rim of the basin. The vertical sections of salinity near the Darss Sill (Figure 16) reveal that the saline bottom water passes the Darss Sill geostrophically adjusted and spreads eastward on the wedge-shaped submarine terrace from the Darss Sill toward Arkona. The plume of the bottom water from the Drogden Sill again was observed to branch out at Kriegers Flak in two filaments: one spreading southward between Moen Island and Kriegers Flak and a second filament surrounding Kriegers Flak clockwise along the northern edge discharging at least into the central bottom water pool; see Figure Modeled Spreading of Dense Bottom Water in the Arkona Basin [36] The observations of the spreading of dense bottom water plumes in the Arkona Basin suffer from several types of sampling deficiencies. First of all, research vessels are barely available when a randomly distributed intermittent inflow event occurs. However, when a cruise takes place, in order to obtain quasi synoptic observations a trade-off is required between the station distance, which should be of the order of the baroclinic Rossby radius, and the time it takes to cover the whole station net, which should not be longer than the duration of an inflow event. Since observations are not always quasi-synoptic under real conditions, modeling of the inflow process is a supplementing tool to study the relevant processes. [37] The barotropic model experiment aimed to simulate a succession of inflows and outflows which are not influenced by the local wind in the Belt Sea and the Arkona Basin. Therefore an initial step in the sea surface elevation was prescribed in the Danish Straits at N, and 55 N, respectively. This was achieved by subtracting a constant offset of 30 cm from the adjusted sea level within the Belt 9of24

10 Figure 9. Time series measurements of salinity at the (top) Drogden Sill and at (middle) Darss Sill and vertical integrated salt transport due to inflowing and outflowing current at the (bottom) Darss Sill. The bars mark the time ranges of hydrographic cruises. Sea and the Baltic Sea. This is an idealized setup of a typical preinflow situation: The Belt Sea is filled with brackish surface water (see Figure 3), and the mean Kattegat sea level is elevated by 30 cm. Moreover, the air pressure on the Baltic Sea was varied with a linear zonal profile between E and 30 E. The air pressure was considered as an artificial barotropic forcing with a background value of 1000 hpa. [38] The model experiment comprises four periods. The simulation starts at 1 January of an arbitrary year from the rest. Within the first period of 30 days an inflow of saline water is driven by the sea level difference between North Sea and Baltic Sea, and the air pressure gradient amounts to a pressure difference of 65 hpa across the Baltic. The following outflow in the second period is driven by an overenhanced rise of air pressure up to 1195 hpa and runs over 20 days from 31 January to 20 February. The following two periods repeat the succession of the inflow and the outflow by the same forcing. However, the model starts at the later periods with a different initial distribution of 10 of 24

11 Figure 10. TS diagram of all stations during the cruise 40/99/21 from 1 to 14 November salinity, which is taken from the last snapshots of period 2 and which resembles a nearly realistic stratification for an Arkona Sea after a relatively short outflow period. [39] The modeled saltwater plume stretches 5 days after the onset of the sea level elevation from the Drogden Sill toward Moen Island. The plume splits in two branches at the northwestern corner of Kriegers Flak; see Figure 18. One branch flows anticlockwise around this bank and descends at the eastern flank of the Darss Sill filling the southwestern part of the Arkona Basin. The branch flowing clockwise around Kriegers Flak moves along the northern rim and detaches at the southeastern corner of the bank into the center of the Arkona Basin filling the saltwater pool of the Arkona Basin. This branch is prevented from encircling the bank by the anticlockwise flowing branch which already filled the southwestern part of the Arkona Basin before the clockwise branch arrived there. The position of the confluence of the two branches of the saltwater plume encircling Kriegers Flak is controlled by friction which determines together with gravity and the Coriolis force the angle of descent along the slope of the basin. The core of the northern plume has both higher salinity and higher eastward velocity compared with the southern plume; see Figure 19 top. [40] The modeled saltwater plume starting from the Fehmarn Belt fills the Lübeck Bight during the first 5 days of model time and propagates thereafter toward the Darss Sill where it arrives at about day 10 of the model time. The saltwater plume crossing the Darss Sill flows on a wedgeshaped submarine terrace on top of the anticlockwise branch of the plume coming from the Drogden Sill; see day 15 of model time. The plume continues to propagate along the southern rim of the Arkona Basin (day 15 of Figure 18), and finally it arrives the Bornholm Gatt at model day 20 (Figure 18). The plume passing the northern coast of Rügen is displaced east of Rügen from the bottom slope of the southern Arkona Basin toward the center of the Basin and turns back toward the slope in the Bornholm Gatt. [41] Baroclinic jets with core velocities of about 40 cm/s transport water with salinities in excess of 15 psu along the trench at the northern rim of Kriegers Flak, see Figure 19. The trench is filled with saline water up to the upper level of Kriegers Flak. Both salinity and eastward velocity decrease in the plume with time after day 5 into the beginning of 11 of 24

12 Figure 11. Depths distribution of the 11 psu salinity surface during leg 1 (2 6 November), leg 2 (7 11 November), and leg 3 (10 13 November) of the cruise 40/99/21 in November The velocity vectors depicted as sticks are taken from the 15 m depth level of the vessel-mounted ADCP measurements since the current was rather barotropic in the eddies except in a thin bottom boundary layer. 12 of 24

13 Figure 12. Cross sections of salinity passing Kriegers Flak during the cruise 40/99/21 from 1 to 14 November of 24

14 Figure 13. Cross sections of salinity from Kriegers Flak to the Darss Sill during the cruise 40/99/21 from 1 to 14 November of 24

15 Figure 14. TS diagram of all stations during the cruise 40/00/01 from 3 to 6 February Figure 15. Depths distribution of the 11 psu salinity surface during the cruise 40/00/01 from 3 to 6 February The velocity vectors depicted as sticks are interpolated on the depth of the 11 psu surface from vessel-mounted ADCP measurements (see Figure 5 for the reason for selecting this level). 15 of 24

16 Figure 16. Cross section of salinity (top) over the west-to-east-aligned trench on the southern part of the Darss Sill and (bottom) between Kriegers Flak and the southern rim of the Darss Sill during the cruise 40/00/01 from 3 to 6 February of 24

17 Figure 17. Cross section of salinity (top) from Drogden Sill toward Kriegers Flak and (bottom) from Moen over the Kriegers Flak during the cruise 40/00/01 from 3 to 6 February of 24

18 Figure 18. Depth of the salinity equal to 11 psu surface and velocities at this surface of the modeled sea level driven inflow event during different days of phase 1. The depth of the isosurface is color-coded where it is between the sea surface and the bottom. inflow. Two cores of salinity and eastward velocity are found in the trench south of Kriegers Flak. The lower one belongs to the plume from the Öresund encircling Kriegers Flak anticlockwise, while the upper one, appearing after day 10, belongs to the plume passing the Darss Sill and propagating eastward on the submarine terrace at about 25 m depth. Both saline plumes merge close to the bottom of the Basin at Kap Arkona, where the terrace disappears (see Figure 20 top). The plume detached east of Kap Arkona into the center of the basin while losing both salinity and momentum (Figure 20 bottom). [42] The integrated salt transports were calculated through several sections stretching from Kriegers Flak northward, westward, and southward as well as through a section stretching northward from Kap Arkona, and they are shown in Figure 21. The salt transport through the section north of Kriegers Flak starts growing 1 day after the beginning of the inflow over the Dogden Sill until it reaches a maximum at day 5 and is decaying thereafter until day 7 when it obtains an equilibrium value. The salt transport through the section west of Kriegers Flak accelerates after the transport through the northern section reached its maximum. This growing stopped at day 8 and the transport decreases slowly thereafter. Salt transport through the northern Rügen section is also growing as through the Moen section but its maximum is delayed by 1 day compared with the Moen section. The salt transport through the southern Rügen section is affected by the inflow from the Öresund until day 5 of the model time, and thereafter the salt transport is dominated by the inflow over the Darss Sill with a time delay of a few days. The salt transport through the section north of Kap Arkona is determined by the flow from the Öresund until day 9 and becomes affected by the inflow over the Darss Sill at day 14 of model time. [43] The salinity differences are decreasing steadily after the arrival of the plume front at the sections which are affected predominantly by inflow from the Öresund, while the density differences were inclined to be constant after the passage of the front at those sections dominated by inflow from the Darss Sill. [44] The inflow event in phase 3, which starts with a stratification established in the Arkona Basin after a 20 day lasting outflow event during phase 2, depicts a quite similar pattern of the saltwater plumes propagation into the Arkona Basin. However, the time shift between the fronts passing 18 of 24

19 the Drogden Sill and the Darss Sill was significantly shorter compared with phase An Analytical Model of the Bottom Water Residence Time in the Arkona Basin [45] The residence time of water in the saltwater pool of the Arkona Sea may be important for an estimation of the mixing with the ambient water by entrainment, diapycnical mixing, and upwelling. Successive bottom water mass observations in the Arkona Basin revealed an upper limit of about 3 months of the bottom water residence time. An attempt is made to estimate the residence time of the bottom water by an analytical model. We can assume the interface between saline bottom water and brackish surface water in the Arkona Basin to be located well below the Darss Sill and Drogden Sill; we further assume the Arkona Basin to be a closed basin filled with a two-layer fluid which is connected by a gap in the wall, the Bornholm Strait, with the Bornholm Basin. The upper layer of the Bornholm Basin may have the same density as that of the Arkona Basin but having a thickness which Figure 19. Meridional sections of salinity cutting Kriegers Flak during the modeled sea level driven inflow phase 1 after (top) 5, (middle) 10, and (bottom) 15 days. Salinity is rendered by contour lines, and the zonal component of velocity (cm/s) is rendered by a color code. Figure 20. Meridional sections of salinity cutting (top) Arkona and (bottom) Adler Grund during the modeled sea level driven inflow phase 1. Salinity is rendered by contour lines, and the zonal component of velocity (cm/s) is rendered by color code. 19 of 24

20 Figure 21. Salt transport (solid line) through the sections (a) Sealand, (b) Darss, (c) Moen, (d) Falster, (e) northern Rügen, (f) southern Rügen, and (g) Arkona and (h) overview. The salinity difference between bottom and surface layer (dashed line) of each section is added to the corresponding panel as kg/m**3. 20 of 24

21 is larger than the depth of the Arkona Basin. Then a horizontal pressure gradient directed from the Arkona Basin into the Bornholm Basin is maintained as long as a bottom water pool exists in the Arkona Basin. Gill [1982] has described how geostrophic adjustment is controlled by propagating Kelvin waves in a straight channel of a rectangular cross section. Liljebladh and Stigebrand [1996] showed that this kind of dynamics may control the outflow of the bottom water pool of the Arkona Basin via Bornholm Gatt into the Bornholm Basin. Therefore we assume the water exchange through the gap to be controlled by internal Kelvin wave dynamics. The net outflow through the gap may be zero since the outflow of the bottom layer is compensated by the inflow in the upper layer. The volume change of the lower layer in the closed basin is balanced by the outflow of this layer through the gap. The characteristic time for the change of the interface depth may be larger than the travel time of the Kelvin wave around the closed basin. Then the dynamics of the basin is quasi-steady and governed by the volume flow per unit length of the two layers and, using the notation given in Appendix A, the outflow U 0 between the Kelvin wave fronts is described by U 0 ðyþ ¼ c 2 h 0 y o e R 2 In order to obtain the total baroclinic transport T we have to integrate over the whole width L of the gap T ¼ Z L 0 U 0 h ðyþdy ¼ c 2 h 0 0 R 2 e y R 2 i L h i ¼ c 2h 0 0 R 2 1 e L R2 0 We assume that the Kelvin wave exited at the northern coast of the Bornholm Strait by lowering of the interface is totally scattered along its path around the Arkona Basin such that the depth of the interface at the southern rim of the Bornholm Strait is determined by the depth of the interface in the interior of the Arkona Basin. Assuming that the saline bottom layer is rather thin, i.e., h H, and setting h 0 = h 0 we obtain approximately, using the relation for the Rossby radius, Tt ðþ¼ eg h f 1 e L R 2 i h 02 ðþ t The maximum transport capacity of the strait is given according to (3) by the density difference between both layers, the maximum thickness the lower layer can obtain, and the width of the channel in relation to the internal Rossby radius. The mass balance of the enclosed basin ¼ Tt ðþ where F is the area of the interface in the basin between the upper and the lower layer. We assume the gap to be wide compared with the internal Rossby radius and neglect the dependence of R 2 on h 0. Inserting (3) into (4) results in a Bernoulli differential equation of the following þ eg h i ff 1 L e R2 h 02 ðþ¼0 t ð1þ ð2þ ð3þ ð4þ ð5þ With the initial condition h 0 (t = 0) = h 0 0 we obtain for the time dependence of the interface h 0 ðþ¼ t 1 þ egh0 0 ff h 0 0 h i 1 L e R2 t The gravity is g = 9.81 m/s 2, the inertial frequency at 54 latitude is f = s 1. A typical density for the upper layer of the Arkona Basin is r = kg/m 3, and for the bottom layer it is r 0 = kg/m 3, which results in e = The water depth is H = 45 m, the thickness of the upper layer is h = 35 m, and the thickness of the bottom layer is h 0 = 10 m. Then we obtain for the group velocity of the baroclinic Kelvin wave C 2 = 0.62 m/s and for the Rossby radius R 2 = 5.1 km. The characteristic width of the Bornholm strait is about 20 km. The radius of the Arkona Basin is approximately R = 55 km. Then it takes the baroclinic Kelvin wave about 4 days to circumnavigate the Arkona Basin. [46] The time to reduce the volume of the salt pool in the Arkona Basin by half compared with the initial volume is t 1 2 ð6þ pr 2 f ¼ h i ð7þ geh 0 L R 0 1 e 2 amounting to 31 days taking the numbers given above. 4. Summary [47] Continuous time series observations of salinity at both the Darss Sill and the Drogden Sill have been used to monitor the saltwater inflow into the Arkona Basin during the winter seasons and On the basis of these observations, field measurements were performed to estimate residence time of the saltwater pool and to study the pathways of the saltwater plumes propagating from the sills into the Arkona Basin. These measurements comprised CTD profiles and current profiles measured by vessel-mounted ADCP on a grid covering the whole Arkona Basin. Observations were supported by simulation of inflow of saline water into the Arkona Basin by a numerical circulation model. An attempt was made to estimate the residence time of the saltwater pool by an analytical twolayer model. [48] The propagation of salt water in the Arkona Basin is governed by gravity, the Coriolis force, and friction and mixing. This results in pathways of the saltwater plumes encircling the Arkona Basin cyclonically along its rim and which are slowly deepening with increasing distance from the sill. As a result of this mechanism the pathway is strongly affected by topography. The pathway of the saltwater plume passing the Drogden Sill stretches southward along the western rim of the Arkona Basin and bends eastward discharging into the trench at the northern rim of Kriegers Flak. Evidence was found that a part of this filament discharges bottom water at the eastern tip of Kriegers Flak into the bottom water pool in the central part of the Arkona Basin. Reaching the maximum transport capacity of this trench after 5 days, a western branch of the Öresund plume splits off and passes the gap between Mön Island and Kriegers Flak proceeding along the eastern flank of the Darss Sill at depth levels well below the sill 21 of 24

22 depth. The sinking of this plume to the bottom of the southwestern Arkona Basin may be associated with mixing of the plume with the ambient water and with eddy formation by conservation of potential vorticity. This transformed plume continues to flow at the bottom of the southwestern Arkona Basin toward Kap Arkona. A part of the northern filament may follow the bottom contours of Kriegers Flak anticyclonally and joins with the western filament in the southwest corner of the basin. The salt water passing the Darss Sill through the southerly gap flows onto a wedge-shaped submarine terrace located in about 25 m depth, which stretches from the Darss Sill as far as Kap Arkona. That means that the Darss Sill plume flows atop of the western Drogden Sill plume along this range. Both plumes are constituents of the coastal jet regime, which is frequently observed off the northern coasts of Hiddensee and Rügen [Lass and Talpsepp, 1993]. However, both plumes join near Kap Arkona since the submarine terrace ends there. The confluence of these plumes is associated with strong mixing by overturning and mesoscale cyclonic eddy activity because of the conservation of potential vorticity. Well-developed geostrophic eddies have been observed to shed from Kap Arkona into the interior of the Arkona Basin and to propagate through the basin. [49] The pathway of the joint saltwater plume east of Kap Arkona could not be evaluated in detail. However, indications have been found that at least a part of the plume propagates northeastward along the southern rim of the Arkona Basin. Model simulations suggest a northward meandering of the saltwater plume into the center of the basin east of Kap Arkona, which finally swings back toward the southern slope in the Bornholm Gatt. Eastward of Kap Arkona a well-developed saltwater pool was observed most of the time in the center of the basin [see also Liljebladh and Stigebrand, 1996]. This indicates that at least east of this cape the saltwater plume feeds the central pool either directly or by shedding of mesoscale eddies from the slope of the basin. The details of the location where the plume feeds the saltwater pool may depend on the intensity and the duration of the saltwater inflow over the sills as well as on the intensity of friction and diffusion processes. [50] The central saltwater pool of the Arkona Basin discharges through the Bornholm Gatt into the Bornholm Basin. An upper bound of the discharge time of the saltwater pool, derived from repeated TS diagrams, was estimated to be 3 months. An analytical two-layer model of the bottom water outflow through the Bornholm Strait provided a volume half time of the bottom water of 1 month. During the storage time of a newly built saltwater pool, mixing of the salt water with the overlying brackish surface water was observed. However, details of the mixing mechanism could not be evaluated. Possible candidates for mixing both water masses are diapycnic mixing through the halocline, entrainment, and upwelling of saline bottom water near the coast by Ekman offshore transport. Czanady [1982]. Any disturbance of the system may elevate the sea surface by h and the interface between both layers by h 0, respectively. [52] We define the density parameter e ¼ r0 r r 0 ¼ O 10 3 ða1þ We consider a Cartesian coordinate system where x is the along channel axis, y is directed across the channel, and z is directed upward. The corresponding components of the velocity vector are u, v, and w. The volume transport in the upper layer per unit length along the channel U and across the channel V are given by ðu; VÞ ¼ Z h hþh 0 and correspondingly in the bottom layer by ðu 0 ; V 0 Þ ¼ Z hþh 0 H ðu; vþdz ða2þ ðu 0 ; v 0 Þdz ða3þ Calculating the volume transport of both layers from the set of linear equations of motion results in a coupled set of equations for both layers. This set can be decoupled by introducing eigenvalues b and eigenfunctions for both the barotropic and the baroclinic modes. [53] The equation for the eigenvalues is b 2 b 1 þ þ h0 e h0 h h ¼ 0 ða4þ for small e the two roots of the eigenvalue equation can be expanded resulting in for the barotropic mode and b 1 ¼ 1 þ h0 h0 e h h þ O e2 b 2 ¼ e h0 h þ O e2 ða5þ ða6þ for the baroclinic mode. Neglecting forcing and assuming that there is no transversal motion in the channel, i.e., V 0 = 0, we consider Kelvin wave dynamics and obtain for the governing ¼ eg hh0 ða7þ Appendix A A1. Two-Layer Model of the Arkona Basin Outflow [51] To describe the dynamics of a two-layered ocean with the density r in the upper layer of thickness h and r 0 in the lower layer of thickness h 0, respectively, we follow fu 0 ¼ eg hh0 ða8þ ða9þ 22 of 24