Final report February 1999

The present report is part of the Norwegian Ministry of the Environment's program "Transport and fate of contaminants in the northern seas" coordinated by the Norwegian Polar Institute in Tromsø and supported by the Norwegian Ministry of Foregin Affairs. The subject matter, physical oceanography, is coordinated by the Institute of Marine Research in Bergen. On 17 November 1998 there was a meeting at SINTEF to plan the laboratory simulations that would provide the modellers of the program with good benchmarks for developing and validating the hydrodynamic modules of various numerical models. We thank the participants of the meeting:

Paul Budgell, Institute of Marine Research (IMR)
Michael Iosjpe, Norwegian Radiation Protection Authority (NRPA)
Ole Anders Nøst, Norwegian Polar Institute (NP)

for their contributions in defining the project goals. We also extend our thanks to Ole Anders Nøst for his help in carrying out the laboratory work.

Trondheim, 28 February 1999
Thomas McClimans

SUMMARY

The results from a laboratory simulation of the ocean circulation in the eastern Barents and Kara seas have been prepared as a benchmark for the development and validation of numerical models of transport processes in the region. The circulation was forced by the monthly averaged inflows of 3 sources of sea water, 2 sources of brackish surface water and 3 river discharges for the period 1991-1995. Tide-like excitations to force residual tidal transports were produced by tilting the model. A fluorescent dye was discharged in the coastal current source in the Pechora Sea in March 1993 and monitored at selected sections until the end of 1995. It took about 2 years for the tracer to penetrate through the Kara Sea to the Central Plateau in the northeast. The flow of coastal water to the west and north of Novaya Zemlya reached the eastern slope of the St. Anna Trough within 7 months. The two routes appear to be independent of each other.

The model tilt in the present benchmark simulation was inadvertently large, giving larger tidal excitations and larger tidal induced circulation in some locations. A comparison of the present and earlier results with less tidal excitation leads to the conclusion that the two-way flow in the Kara Strait, including the origins of the Litke Current is enhanced by tidal action. Furthermore, the tides help propel water southward along the east coast of Novaya Zemlya, northeast along the delta slope and west-northwest in the region of the Ob and Yenisei plumes. The action also enhances the bifurcation (splitting) of the Yenisei plume with a narrow coastal current flowing eastward toward the Pyacina River mouth.

It is not known how much the transport times of the coastal current through the region depends on the tidal excitations. This can be studied numerically or simulated in the laboratory with weak tidal forcing. The general circulation pattern within the Kara Sea compares well with earlier model results, with less tidal action, and with composite, late summer field measurements from 1993-1995.

INTRODUCTION

A laboratory model of the Kara Sea was built to study the transport routes of contaminants in the northern region (Barents and Kara seas). Emphasis is placed on the possible discharge of radioactive contaminants from the Ob and Yenisei catchments and containers deposited in the Kara Sea. The first objective of the model simulations during late summer 1995, was to inform the research vessel R/V H.U. Sverdrup II of important locations to take field measurements of currents and hydrography, to assess the transport rates and routes in the Kara Sea. This was done under the auspices of the Arctic Nuclear Waste Assessment Program (ANWAP), administered by the Office of Naval Research (ONR) in Arlington, Virginia. A follow-on study and the reporting of the results were supported by the Norwegian Ministry of Foreign Affairs (McClimans et al., 1997). The geographical extent of the model, with the bottom topography, is shown in Figure 1 (Cherkis et al., 1990; USSR Ministry of Defense, 1990). The sources and sinks used in the model are noted on the figure.

Figure 1. The laboratory model with the sources, sinks, bathymetry and relevant names noted.

The results of the earlier simulations agreed well with data from cruises to the Kara Sea in 1993 – 1995 and to some available historical field data. It was therefore proposed that the model be used to establish a benchmark for numerical model development and validation within the framework of the Norwegian Department of the Environment's program "Transport and fate of contaminants in the northern seas" coordinated by the Norwegian Polar Institute. With well-documented boundary conditions, the hydrodynamic modules of numerical models can be compared to the independent results from the laboratory. In this respect, the physical laboratory results are an alternative reality to the full-scale physics in the ocean, with well-documented forcing conditions. These conditions include the inflows of ocean, coastal and river discharges and an ad-hoc tidal excitation.

The transport routes of the various water masses are steered over detailed topography under the influence of the simulated rotation of the Earth, producing the so-called Coriolis effect.

OBJECTIVES

The main goal of the present work is to establish a good set of data from a well-controlled laboratory simulation of the circulation in the eastern Barents and Kara seas, to be used in several numerical models of the region. Within the framework of the program, there are 3 types of models to be considered:

• box model (NRPA) (Nielsen et al., 1998)
• process model (NP) (Song and Haidvogel, 1994)
• system model (IMR, DNMI) (Svendsen et al., 1996)

In addition to these models, IMR is developing a finite element model of the region to focus on the fine topographical features of key areas like the Kara Strait (Lynch and Werner, 1991).

The laboratory simulation was run for a 5-year period. The simulation is for coupled physics (stratified, rotating flow driven by advection and tides) with high topographical resolution. The results can be expanded by numerical simulations, using modules for wind, thermodynamics, chemistry and biology, as well as other values of inflow and tidal excitation.

The 5-year simulation included the actual runoff during the three years that Norwegian research vessels took measurements in the Kara Sea (1993 – 1995). The first two years of flow establishment (1991 – 1992) were given the climatological values of the earlier simulations, while the three actual years were steered according to monthly observations from the data base at GRDC (Global Runoff Data Center) in Koblenz, Germany (GRDC, 1996). To simplify the automatic steering, the temporal variability was made similar for all three rivers. These conditions are given in Chapter 3.

To document the 'fast track' flow of coastal water through the Kara Sea, a fluorescent tracer (Rhodamine) was discharged in the coastal current in the Pechora Sea, in May 1993, and monitored throughout the remainder of the simulation period.

It was originally proposed that, for this benchmark, the bottom topography in the Kara Strait should be smoothed to match the existing numerical models. However, IMR's finite element model can resolve the fine topography in the laboratory model and can show numerically whether or not the resolution of the bottom topography has a significant effect on the transports.

The tidal forcing was not changed from the earlier runs, since they showed good agreement with field data. However, it will be seen that there was an unexpected change in the calibration that led to amplified tidal action. This is an integral part of the present benchmark, as represented in the tidal current velocities in the Kara Strait (calibration).

MODEL THEORY, FORCING AND MEASUREMENTS

A laboratory model of the circulation in a stratified, rotating ocean requires similitude of buoyancy and Coriolis terms in the equation of motion, and proper forced boundary conditions (McClimans, 1990). The laboratory model boundaries of the Kara Sea, shown in Figure 1, are chosen to pass through regions that are either streamlines or regions in which the flux conditions are sufficiently easy to control. These conditions must be subjected to a validation. The 5 m diameter Kara Sea model simulates a geographical region of 1,500 km diameter, centered at 74° N, 67° E, thus, the ratio of horizontal lengths is

L_r = = 300,000 (1)

To avoid the effects of surface tension gradients in the laboratory, to secure that the dynamics of the fronts are dominated by buoyancy, it is necessary to exaggerate the vertical scaling in the model (McClimans and Sægrov, 1982). This can be done as long as the vertical accelerations in the model do not contribute to the pressure balance (the flow in the model must be hydrostatic). To model the bottom topography of this region in a 50 cm deep laboratory basin, at least to the 500 m isobath, the vertical ratio of length scales must be

H_r = = 1,000 (2)

This gives a distortion (L_r/H_r) of 300, which is typical for this type of model (McClimans and Nilsen, 1993). It satisfies the requirement for surface tension gradients for the Ob and Yenisei discharges, but the Pechora River discharge is so small that it is influenced significantly by an outward pull of the surface tension of the more saline water. However, it is expected that the river plume here is passive in the energetic coastal and tidal flows, and that it is not necessary to include surface tension gradients in the numerical calculations.

Due to the highly sub-critical flow in the ocean (much less than tidal wave speeds) it is sufficient that the simulation of the physics in the model obey the densimetric Froude and Rossby model laws. The ratios of buoyancy, inertial and rotational forces in a vertically distorted geometry are simulated correctly, provided that the flow is approximately hydrostatic in the laboratory, and that viscous and surface tension effects are small (McClimans, 1990). The most significant frictional effect is wind shear, which is not simulated here. This is relegated to numerical simulations.

Within the above framework, it is possible to tailor the buoyancy (or density) ratio

g'_r = (3)

where g' is the so-called reduced gravity = g (r - r _o)/r _o, g is the acceleration of gravity, r _o is a reference density and r is the density which varies in space and time.

The densimetric Froude number is

F = (4)

where u is the flow velocity. For similitude, the ratio

F_r = = 1 = (5)

where u_r = L_r/t_r and t_r is the time ratio t_r = t_nature/t_lab.

By choosing a convenient density scaling, we can tailor the time so that a day in nature is an integral number of seconds in model time to facilitate the control of the monthly variable inflows. Here, the choice is g' = 1.74, giving a time scale t_r = 7,200, for which a day is modeled in 12 s, and each second in model time simulates 2 hours in nature.

The Froude similitude above is compatible with the Rossby similitude

Ro_r = (6)

where f is the so-called Coriolis parameter (= 2 x Earth rotation rate x sine of latitude). Thus, f_r = t_r^-1. With this scaling, the volume flux scaling is

Q_r = L_r²H_r/t_r = 300,000^2× 1,000/7,200 = 1.25× 10¹⁰ (7)

A source flux of 1 Sverdrup (1 Sv = 10⁶ m³/s) in the ocean requires a model source flux of 80 cm³/s or 4.8 l/min.

The model is constructed in a UTM 67 projection, which is exact along the 67° E longitude and has a maximum deviation of about 2 km at the eastern and western boundaries. Due to the rotation of the model, to simulate the geophysical flows, the water surface describes a paraboloid of revolution. This datum is taken into account in forming the bottom topography from sea charts (Cherkis et al., 1990; USSR Ministry of Defense, 1990). To accommodate the sources and sink conditions in the circular model domain, changes were made in the northeast (Voronin Trough) and the southwest (western Pechora). The actual model topography used is included in the benchmark (see APPENDIX) as a file on a discette. The geographical coordinates of the isobaths are given to obtain the best horizontal resolution on slopes.

The rotating basin provides a constant f. In nature, f varies with ± 2.7% over the model domain, but only ± 1% from its average value over the flat delta. Thus, the variations in depth dominate over the differences in the Coriolis parameter when it comes to potential vorticity dynamics (Pedlosky, 1979). Key isobaths are noted in Fig. 1.

A crucial problem for simulating flows is obtaining accurate forcing data. In the present model, the main forcing is the density (salinity) and volume fluxes of 8 sources and the division of the downstream (outflow) into 3 paths. The monthly values of the forcing data, given in Table 1, are applied for each year. These are based on the forcings used in the Barents Sea model study and the climatology of (Pavlov et al, 1993). The forcing conditions for land sources (river discharges) are given in Table 2 for a 5-year sequence (GRDC, 1996).

In addition to the advectively forced flows, the model is slightly tilted to simulate a tidal excitation of flow through the Kara Strait. This is a sloshing that we refer to as 'tidal forcing' in the model. It produces a diurnal period (pendulum day). As noted earlier, the calibration was larger than intended. In view of the earlier results and recently published tidal current data (Utne, 1995; Johannessen et al., 1998) the excitation should rather have been reduced.

Table 1. Monthly flux boundary conditions in Sverdrups (Sv = 10⁶ m³/s).

* The source symbols are as follows: MC = Murman Current, CBC = Central Bank Current, CC = coastal current and PW = Polar Water. The underflow of Atlantic Water to the east of Franz Josef Land is held at a constant 1 Sv, for lack of better data.

There are 3 outflows: an overflow weir accommodating most of the surface water in the region where the Persey Current flows west, a diversion of 0.03 Sv of the surface outflow to the northeast, toward Vil'kitsky Strait, and a deeper outflow under a broad skirt at 110 m depth in the north, which accommodates about 1/3 of the total outflow.

The instrumentation of the model is divided into in situ and remote sensing. Monthly in situ samples of salinity and fluorescent dye are withdrawn at a total of 34 locations to monitor the stratification on the delta and the spread of plume water. The sampling stations are noted with red dots in Figure 2. The geographical locations and depths of all measurement points are given in Table 3.

Figure 2. Locations of salinity and tracer sampling (red dots), and FOV of cameras (rectangles).

Table 2. Monthly river discharges used in the laboratory for the period 1991 – 1995.

The remote sensing is accomplished with 3 video cameras and a 35 mm Nikon F6 with a wide lens to view the entire region of interest. A motor drive gave series of 7 pictures at 4.1 s intervals (a total time span of two days). The fields of view (FOV) of the cameras are shown in Figure 2. To achieve a good mapping and quantification of the flow, surface buoys and dye clouds are used at critical times and locations. Latitudes and longitudes are marked on the bottom for reference.

In the present simulation, the transport times for the coastal water flow through the Kara Sea is measured by following the distribution of an equivalent of half a million tons of the fluorescent Rhodamine dye in the coastal current source in the Pechora Sea in May 1993. The locations and depths of the sampling points are given in Table 3 (see also Figure 2).

Table 3. Locations and depths of salinity and tracer samplers used in the benchmark simulation.

* Central Plateau

LABORATORY BENCHMARKS

In this section, the data from the laboratory simulation are presented in a form that can be used for comparisons with different numerical models. We call these the benchmarks. The core results and the relevant boundary conditions are published in an accompanying diskette (see APPENDIX A).

Tidal residuals

The model is tilted to excite tidally induced motions. Such a system was studied by Hsueh et al. (1973). The purpose of inducing tidal-like oscillations is to produce a realistic transport due to tidal pumping in the Kara Strait. The degree to which the model velocities match the actual tidal speeds in the Kara Strait is a measure of the 'calibration'. Five selected particle paths in the Kara Strait region are shown in Figure 3, starting with asterisks. The dots mark 1 s intervals (= 2 h in nature) during the first two days. An arrow after the number 2 points to the location of the particle after 5 days of travel (noted with the number 5). The computed maximum speeds are almost 3 m/s. This greatly exceeds the measured speeds according to available data (Utne, 1975; Johannessen et al, 1998), and is over twice the value of the earlier runs. It may be associated with an unexpected movement in the laboratory building due to nearby excavation activities.

The 'tide' induces a two-way flow in the Kara Strait, as before, with the origins of the Litke Current near Novaya Zemlya. There is, however, a more expanded exchange region near the Kara Strait. The modeled 'tidal' residuals in the Kara Sea are shown in Figure 4. A region of highly distorted tidal motions has been identified along the slope to the northwest of Dikson (shaded region). This may have some consequences for the local mixing and the local propulsion of water along the slope. Also, to the west of the Sverdrup Island there are strong jets toward the northwest and west. These features and their comparison with earlier simulations (McClimans et al., 1997) will be discussed later. Conclusions on the role of tidal forcing in the active regions can be made by comparing the two different excitations. It would be quite valuable to simulate the transports with very weak tidal excitation, both for the understanding of the role of tides, and to provide a 'cleaner' benchmark for numerical studies.

Figure 3. Pure 'tidal' induced motions in the Kara Strait.

Figure 4. Map of 'tidal' residuals without source flows.

Tracer discharged in the coastal current in the Pechora Sea

The equivalent of half a million tons of fluorescent Rhodamine dye was discharged in the coastal current in the Pechora Sea in May 1993. The observations from the Pechora Sea are listed in Table 4 (see Table 3 for locations). Time histories of the concentrations in the remaining sections are shown in Figure 5.

Table 4. Observations of the concentration of Rhodamine (ppb) in the Pechora Sea.

Figure 5. Time histories of the fluorescence (Rh, ppb) in the measured sections: a) SW Kara Sea, b) Dikson section, surface, c) Dikson section, bottom, d) Yenisei, at Dikson, e) Central Plateau, and f) St. Anna Trough slope.

It can be noted from these data that there is a rapid flushing of coastal water in the source area in the Pechora Sea, and that the tracer front reaches the St. Anna region before it penetrates to the Dikson section. This shows that the major part of the coastal current turns north in the Pechora Current and flows NE along the west and north coasts of Novaya Zemlya. The penetration through the complicated topography in the southwestern Kara Sea takes much longer.

The results from the Central Plateau show that this is an independent route connected to the throughflow in the Kara Sea. The core of coastal water at the eastern slope of the St. Anna Trough implies that this is the cold water mass identified by Quadfasel et al (1993) on the basis of temperature alone. The very low water temperatures must have been isolated from summer heating and mixing with the warmer, modified Atlantic Water below. Newer analyses of water masses in the north confirm this picture (Ivanov and Korablev, 1998; Schauer et al., 1998). The Rhodamine concentrations imply a core of coastal water at 20 – 60 m depths. It takes almost 2 years for the tracer to penetrate through the Kara Sea to the Central Plateau in the northeast. The transit time for the flow of coastal water to the west and north of Novaya Zemlya, to the eastern slope of the St. Anna Trough is only 7 months.

Salinity fields

The salinity was measured at the same locations as the tracer (see locations in Figure 2 and Table 3). The local measurements in the Pechora Sea are listed in Table 5. Figure 6 shows the time histories of salinities corresponding to Figure 5. The most obvious signal is the seasonal variability of the surface layer to the north of the river mouths (5 m section northwest of Dikson). Also, the interannual differences with the various discharges (Table 2) are apparent. In 1994 there is a more intense vertical penetration of the brackish layer at Dikson than in 1995. In the deeper layers off the delta, there is an apparent slow trend to lower salinities, but not enough to make a difference in the dynamics of the ocean currents. The salinity in the submerged sink (at 110 m depth) was close to 34.8 throughout 1993-1995.

The general level of the salinities in the Dikson section is in accord with the historical Russian data (Morison, 1997).

Table 5. Observations of salinity (psu) in the Pechora Sea (see Table 3 for locations).

Figure 6. Time histories of the salinity in the measured sections: a) SW Kara Sea, b) Dikson section, surface, c) Dikson section, bottom, d) Yenisei, at Dikson, e) Central Plateau, and f) St. Anna Trough slope.

Current fields and particle paths

With the above caveat on the amplified tidal excitations, benchmarks for the circulation in the Kara Sea are derived from the motion of surface particles. Eulerian fields for a spring (March 1995) and a fall (August 1995) situation are shown in figures 7 and 8, respectively. It should be pointed out that these data are snapshots of a multi-scale time variability, and must be used with caution.

To show the transport routes at the larger space and time scales, selected particle paths were measured at 5-day intervals during the fall of 1994, starting 1 August (Figure 9). Some of the tracks extend over several months (noted by number). The particles reveal some regions of 'fast track' currents and regions of long residence times, associated with the topography.

River plumes

There is a special variability in the river plumes that is captured both by particle trajectories and dye clouds. Figure 9 shows dye cloud fronts at 5 and 10 days during the major discharge in June 1994. Corresponding field measurements were taken during September – October of that year (Johnson et al., 1997). The bifurcation (dividing) of the Yenisei plume has been observed both in the field data and the earlier simulations with weaker tidal excitation. It appears to be greatly exaggerated in the present benchmark simulation. The transport time of Ob water to the outer Dikson section is about 3 months, compared to 4½ months in the earlier simulations.

Figure 7. Surface currents for March 1995.

Figure 8. Surface currents for August 1995.

Figure 9. Particle tracks: The asterisks show the start position, 1 August 1994. A dot is shown for each 5 days and the numbered circles show the transport time in months. The dashed lines to the north of the Ob and Yenisei show 5-day displacements of color fronts in the estuaries in June.

APPLICATIONS TO NUMERICAL MODELS

To be useful as benchmarks for the development and validation of hydrodynamic modules of numerical models, the data presented in Section 4 are published digitally on a diskette. A list of the benchmarks is given in APPENDIX A. This set-up is based on the experience of the benchmark study in the Skagerrak (McClimans et al., 1999). The strategy in the Skagerrak work was to model, numerically, the exact topography and sources and sinks of the laboratory model. However, the overcritical flows in the downstream sinks had to be treated in a different manner in the computer. In the present benchmark, the boundary conditions in the north, with a deep outflow and stacked sources may present a challenge to the modeller.

The tidal variability was filtered out in the Skagerrak benchmark, but in the Kara Sea, it appears to give a significant propulsion. Thus, to treat the 'tidal' excitation in this special laboratory setting, a procedure like Hsueh et al. (1973) is recommended.

The laboratory results can also be applied to regional models that have boundaries within the geographical extent of the laboratory model. In this case, the forcing boundary conditions must be obtained from the simulated velocity fields, in stead of the controlled boundaries of the model basin. There are, however, only a limited number of data derived from the videotapes of the simulations, and a more detailed analysis of the particle motions is needed to use this approach.

As long as the observed velocity fields are not used to calibrate a given model, they are data that can be used for validating the model, on par with field data. The advantage with the present results is that the boundary conditions are well-defined, and do not contain random meteorologic forcing. Although the meteorologically derived forcings may be reliable, there are few data to use to determine the inflows of sea, coastal and polar water masses. Judging from the similarities of the present and the earlier model results with field observations, these are important boundary conditions.

COMPARISONS WITH FIELD DATA

The scope of the project does not include the comparison of the laboratory simulations with field data. However, some references have been made to relevant data sets, and earlier comparisons were made in McClimans et al. (1997). In the following, some comments will be made that are relevant for the comparison of the present benchmarks to field data.

Actual tides vs laboratory tides

The excitation in the laboratory is a sloshing which is intended to reproduce the tidal currents at a chosen location in the model domain (the Kara Strait). This causes 'tidal' motions in other places which have no correspondence to the tides that are excited from the open sea to the north (e.g. Harms and Karcher, 1999). The exaggerated tidal forcing in the present benchmark must also be taken into account.

Comparisons with current maps

The maps in this report are based on residual currents. The hourly currents were shown only for the 'calibration' in Figure 3. The highly energetic excitations in Figure 4 cannot be expected to agree with field data (Nøst, 1995; Johnson et al, 1997, among others), since they are produced by an exaggerated 'tidal' forcing. To account for this result, the 'pure tide' circulation in Figure 4, in principle, can be subtracted from the results in figures 7 and 8. However, it should be recalled that the results are snapshots of a current with variability over longer time scales than 2 days. Thus, the results of Figure 9 should be taken into account. To illustrate that the simulated currents are qualitatively realistic on the delta, the results of a composite of late summer current measurements for 1993-1995 are shown in Figure 10 (Johnson et al., 1998). However, as mentioned earlier, the circulation is faster in this benchmark study. It should be noted that the results in the northwest and west, in Figure 10, are observations from 1996.

In the present benchmark there is a narrow, southward flow near the east coast of Novaya Zemlya (not shown) that is stronger than in the earlier simulations (McClimans et al., 1997), and which is not captured in the field data.

Evaluating the effects of wind and thermodynamics

There is no wind or thermodynamic forcing (freezing/melting) in the present benchmark. Again, comparisons of the laboratory results with field data can be used, in principle, to determine the role of winds and/or thermodynamics on the circulation. There are, however, few data from the winter, which lasts most of the year in this region. Further, due to the exaggerated tides, care must be made in the choice of data sets and locations to be compared. All of these effects can be effectively studied in the suite of numerical models discussed earlier, or their future developments.

Figure 10. Near surface circulation vectors composited from ADCP and moored current meters from data obtained between 1993-1996. There were no data from 1996 in the delta region. Chlorophyll-a as a fresh water tracer is mapped from the 1995 expedition only within the Kara Sea (Johnson et al., 1998).

Topographic steering

There is no doubt that topography is the most important boundary condition for the proper simulation of ocean currents. This is due to potential vorticity dynamics at these large scales. Even random forcing over the topography can produce the general features like the 'fast-track' slope currents (Holloway, 1987). Thus, the first entry in the benchmark is the modeled topography. The interaction of tidal forcing with topography is also a well-known driving mechanism for the slope currents (Huthnance, 1981). Due to the importance of bottom slopes in steering the flow, the topography is stored as geographical coordinates of isobaths. This gives higher spatial resolution where it is needed.

The objective of the tidal calibration is to produce the tidal currents and tidal residuals in the Kara Strait, although other locations can be used as reference. The exaggerated tidal forcing in the present simulation has led to significant current propulsions in the shallow waters: more than intended. This 'tare' is then a part of the benchmark. In this respect, it should also be noted that the geographical distribution of the excitation in the laboratory model is not the same as that of tidal forcing from the open ocean in the north. Thus, the 'tare' will show some regions of relatively large tidal velocities that have no counterpart in nature.

Interannual variations of river discharge

The actual time history of river discharges to the Kara Sea during 1993 – 1995 (Table 3) were used to give a better basis of comparing the laboratory results to field measurements. As a benchmark, it also gives a well-defined boundary forcing, like the monthly average (seasonal) inflows from the ocean sources (Table 2). Both of these are part of the benchmark. The results of the different river discharges can be seen in the surface salinity distribution of Figure 6b and the vertical distribution at Dikson (Figure 6d). The general character of the hydrography agrees with the historical Russian data (Morison, 1997).

Tracer in the coastal current

The discharge of Rhodamine in the coastal current source in May 1993 was followed through the Kara Sea to the end of 1995. The routes around Novaya Zemlya, on the shelf, into the river plumes and out to the Voronin Trough were documented in Figure 5. Tables of the observations of salinity and Rhodamine concentrations are included in the benchmark. The 'fast track' in the system is around the outside of Novaya Zemlya. Here the source water takes about 7 months to reach the eastern slope of the St. Anna Trough. Within the Kara Sea, the 'fast track' along the tortuous slope topography takes 17 months to the Dikson section, 21 months to the Yenisei estuary and almost 2 years to the Central Plateau.

Particle trajectories

From the particle trajectories, Eulerian current fields are constructed for the spring and fall of 1995, as typical periods for transport scenarios. The data shown in figures 7 and 8 are digitized in geographic coordinates and included in the benchmark for use in numerical model studies. For comparison purposes, the 'tare' mentioned above should be considered, taking into account that these results are 2-day snapshots of a system with variability at larger time scales.

Differences with earlier laboratory data

Due to the unintentional exaggeration of the tidal excitation, there are several results that are different from the earlier laboratory simulations (McClimans et al., 1997), mostly local, but a few that may have large-scale transport consequences. The most notable are:

• a more significant bifurcation of the Yenisei plume,
• a stronger plume jet toward the northwest, with a partial purging of Ob plume water,
• a significant mixing region at the slope break to the northwest of Dikson,
• a stronger southern current near the east coast of Novaya Zemlya, and
• a more active outflow in the Litke Current.

By comparing these results with the previous simulations, it is possible to draw some conclusions on the effects of tidal excitation. The through-flows seem to be less affected by this action. In effect, the excitation redistributes the water masses within each domain, but does not seem to add significantly to the main advectively forced transports. This contention can be tested using numerical simulations of the flow with different levels of sloshing.

In view of the large local effects of the exaggerated tidal forcing, it would be appropriate to simulate the same conditions with near-zero tides in the laboratory before the model is dismantled. The results would be reported on a diskette as a second benchmark, without the effects of the sloshing.

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List of benchmark files

A diskette with these benchmarks can be sent by request.