DYNAMICS OF SEA ICE AND OCEAN IN POLAR SEAS
Location: Poynting Physics, S02 LT
Location of Posters: Bridge Poynting/Watson
Thursday 22 July AM
Presiding Chair: M.C. Gregg (University of Washington, USA)
HYDRAULIC CONTROL AND ENTRAINMENT IN STRAITS, SILL
FLOWS AND DENSITY CURRENTS
Introduction 0950
P14/W/10-A4 Invited 1000
REVIEW OF HYDRAULICS AND HYDRAULIC CONTROL IN THE OCEAN
Larry Pratt (Woods Hole Oceanographic Ins., Woods Hole, MA. 02543, USA. email: lpratt@whoi.edu)
A review of recent progress in understanding the hydraulics of straitand sill flows is given. Areas of interest include hydraulic control in the presence of continuous stratification, linking controlled flows to forcing and dissipation in upstream and downstream basins or marginal seas, hydraulic jumps and bores, rotation, and time-dependence. Also mentioned will be some longstanding problems whose importance is widely recognized but which have proved difficult to understand. These include hydraulic control in the presence of rapidly fluctuating currents, the presence of critical layers in 'hydraulically critical' flow, and the role of friction in rotating, hydraulically-driven flows. Finally, I will review two classes of hydraulic problems having well-developed theories but, at present, little or no observational support. These are coastal hydraulics and potential vorticity hydraulics.
P14/E/04-A4 Invited 1040
CIRCULATION IN STRAITS – A REVIEW OF RECENT OBSERVATIONAL STUDIES
Stephen P. MURRAY (Coastal Studies Institute, Louisiana State University,
Baton Rouge, Louisiana 70803, USA, email smurray@unix1.sncc.lsu.edu)
The important role of sea straits in controlling exchange between marginal seas and ocean basins and between adjacent ocean basins is well established, but in the last 20 years, modern instrumentation has provided a new depth of insight into the controlling processes. The Gibraltar Experiment (1985-86), for example, allowed a vast improvement in the determination of mass and salt transport between the Mediterranean and the Atlantic Ocean, leading to a significant revision in the Mediterranean evaporation rate. The Experiment also determined that as much as one-half of the exchange across the Gibraltar sill is controlled by tidal covariance (Bryden et al. 1994). Additionally, barotropic fluctuations caused by atmospheric pressure differences between the basins can cause transport fluctuation as much as 1 Sv--as large as the mean flow. A recent study in the Bab el Mandab Strait (linking the Red Sea and Indian Ocean), employing moored ADCPs, has documented the seasonal transport cycle of the two-layer regime that is present most of the year and the three-layer regime that prevails in summer. Highly energentic low frequency fluctuations in the transport are largely controlled by direct wind driving, producing both barotropic and baroclinic responses. A strong (0-0.8 Sv) seasonal modulation in the deep high density outflow is related to the annual cycle of the winds in the Arabian Monsoon but the controlling dynamics remain unclear. In addition to the above settlings forced by thermohaline, wind, andatmospheric pressure effects, other straits (e.g. in the Indonesian Archipelago) are controlled by inter-ocean basin sea level differences at the seasonal scale and coastal Kelvin waves at the intraseasonal line scale. These and other examples from the Bosphorus and Straits of Otranto, of Sicily and Hormuz, are discussed, compared, and summarized in terms of regional forcing mechanisms.
P14/W/14-A4 Invited 1140
ASPECTS OF STRAIT DYNAMICS STUDIED BY NUMERICAL LAYERED MODELS
Angelo RUBINO and Peter Brandt (both at the Institut für Meereskunde, Universität Hamburg, Troplowitzstr. 7, Hamburg, Germany, Email: rubino@ifm.uni-hamburg.de) Stefano Pierini (Dipartimento di Fisica, Università dell’Aquila, Via Vetoio, I-67010 Coppito (l’Aquila), Email: pierini@cds.unina.it)
Different multi-layer numerical models are employed for the study of three straits of the western Mediterranean Sea: the Strait of Gibraltar, the Strait of Sicily, and the Strait of Messina. The Strait of Gibraltar, that connects the Mediterranean Sea and the Atlantic Ocean, is investigated with the help of two different two-layer models. By using a one-dimensional, nonlinear, weakly non-hydrostatic model the generation and propagation of internal bores caused by the interaction of the barotropic tide with the Camarinal Sill and their disintegration into trains of internal solitary waves is simulated. The results are compared with different remote sensing data. By using a nonlinear, hydrostatic, two-dimensional boundary-fitted coordinate model, two-dimensional aspects of the strait dynamics induced by topography and earth rotation are simulated. In particular, it is shown that within the strait different areas in which the flow is supercritical exist, characterized by a complex temporal evolution. The Strait of Sicily, that connects the western and eastern Mediterranean basins is investigated with the help of a nonlinear, hydrostatic three-layer model. The layers represents waters of Atlantic origin, the Levantine intermediate water, and deep waters of the Mediterranean Sea. Quasi-stationary circulation patterns representing aspects of an idealized thermohaline circulation are produced by steady fluxes imposed at the open boundaries. The model is capable of capturing several features of the observed circulation in the area of the Strait of Sicily. The Strait of Messina, that connects the Tyrrhenian and Ionian seas, is investigated with the help two different models. The first model is a one-dimensional, two-layer coupled model. In the region of the strait sill the complex interaction of the lower layer with the topography induced by a strong barotropic tide is simulated by using a frontal model that, due to a numerical technique for the treatment of movable lateral boundaries, is capable of describing the dynamics of water layers having a localized extension. Outside the sill region the disintegration of the internal tidal bores produced in the sill region into trains of internal solitary waves is simulated by coupling to the generation model a nonlinear, weakly nonhydrostatic model. The results are compared with high-resolution in-situ and remote sensing data. The second model is a three-layer, nonlinear, weakly non-hydrostatic model. Using this model, the observed jet-like structure of the tidal flow outside the sill region is simulated.
P14/E/02-A4 Invited 1210
ENTRAINMENT BY OVERFLOWS AND DENSITY CURRENTS, A REVIEW
JAMES F. PRICE, Woods Hole Oceano. Inst., Woods Hole, MA, 02556 USA (jprice@whoi.edu)
Simulation of past or future climate will require ocean general circulation models (OGCMs) having comprehensive physics, and including a realistic treatment of the most important marginal sea overflows. This presents a considerable challenge since the scales and dynamics inherent to marginal sea overflows are different from that of the general circulation over most of the open ocean. Overflows are density-driven bottom currents that have small vertical scales, being typically about 100-200 m thick, and currents that are O(1 m s$^{-1}$). As they Descend into the ocean they may mix intensely with overlying oceanic water, typically doubling their initial transport and diluting their tracer properties. Because of these distinctive scales and dynamics, realistic marginal sea-ocean exchange and deep water production do not arise spontaneously in large-scale OGCMs, even those with the greatest resolution. Consequently, it will be necessary to parameterize marginal sea processes in OGCMs. The aim of this review will be to describe what an ideal parameterization should do, and to consider some candidates.
Thursday 22 July PM
Presiding Chair: E. OZSOY (Institute of Marine Science, Middle East Technical University, Turkey)
P14/W/08-A4 1400
HYDRAULIC CONTROL IN THREE-LAYER EXCHANGE FLOWS AND APPLICATION TO THE BAB AL MANDAB
David SMEED and Gregory Lane-Serff, Southampton Oceanography Centre, University of Southampton, Southampton. SO14 3ZH UK.
There is a strong seasonal variation of the flow in the strait of Bab al Mandab. In winter dense salty water leaves the Red Sea and fresher surface water enters the Red Sea from the Gulf of Aden. In the summer the outflow of dense salty water Is much reduced, the direction of flow in the surface layer is reversed and a sub-surface layer of cool fresh water enters the Red Sea between the two outflowing layers. A model of hydraulic control in three-layer exchange flows has been developed. The model predicts the observed flow regimes and supports the hypothesis that upwelling, dependent upon the monsoon, in the Gulf of Aden is primarily responsible for the seasonal variation. The model has applications to other strait and sill flows. The problem has been examined further in a series of laboratory experiments. Measurements of the exchange fluxes and interface heights agree well with the model predictions.
P14/W/05-A4 1420
THE CONTINUOUS DYNAMICAL MODES OF THE BAB AL MANDAB AND THEIR HYDRAULIC INTERPRETATION
Heather E. DEESE (Woods Hole Oceanographic Ins., Woods Hole, MA, 02543, USA, email: hdeese@mit.edu); Larry J. Pratt (Woods Hole Oceanographic Ins., Woods Hole, MA, 02543, USA, email: lpratt@whoi.edu); Steve Murray (Coastal Studies Institute, Louisana State University, Baton Rouge, LA, USA, email: smurray@lsuvax.sncc.edu); William Johns (RSMAS, University of Miami, Miami, FL, USA, email:johns@ibis.rsmas.miami.edu)
The continuous dynamical modes of the exchange flow in the Bab al Mandab are computed in an attempt to assess the hydraulic character of the flow at the sill. Hydrographic and direct velocity measurements taken from April to November of 1996 using moored CTDs and a bottom-mounted ADCP are first used to construct monthly mean density and velocity profiles. Next, an extended version of the Taylor-Goldstein equation is solved for the phase speeds and vertical structures of the first and second internal long wave modes. The 'extended' equation takes cross-strait topography into consideration. The sill flow is found to be subcritical during each month, but it hovers near a critical state with respect to the second internal mode during the non-summer months. A series of calculations using idealized velocity and buoyancy frequency profiles are presented to illustrate the effects of the topography on the vertical modes.
Wave speeds are also calculated for a variety of instantaneous conditions at the extremes of the tidal cycle. The results indicate that the tides often push the flow into a critical or supercritical regime with respect to the second mode and, somewhat less frequently, with respect to the first mode. We can only speculate on the connection between classical hydraulic control and the observed intermittent critical and supercritical conditions. A series of calculations using idealized velocity and buoyancy frequency profiles are also presented to illustratethe effects of the topography of the vertical modes.
P14/E/06-A4 1440
INVESTIGATING THE FEEDBACK BETWEEN STRAIT EXCHANGE AND WATER FORMATION USING A HYDRAULICALLY CONTROLLED BOX MODEL OF THE MEDITERRANEAN
Stephan MATTHIESEN and Keith Haines (Dept. Meteorology, Univ. Edinburgh, James Clerk Maxwell Building, Edinburgh EH9 3JZ, UK, email: stephan@met.ed.ac.uk and kh@met.ed.ac.uk)
We have built a 2 layer model consisting of a hydraulically controlled strait coupled to a 2 box model of the Mediterranean. This is the simplest system that can respond to changes in the strait flow. Unlike most previous models which have assumed maximal exchange at all times, our model also allows the exchange to become submaximal. A wide range of strait cross sections can be accommodated in the model. For the maximal regime, the model includes a hydraulic jump towards the Mediterranean with the possibility of mixing.
The model is used to investigate the feedback between the water formation processes in the basin and the strait dynamics and the relevant timescales. We discuss the implications of the fact that the adjustment time for salinity changes is of the order of decades, while the adjustment time for water budget changes are of the order of years.
This study also addresses the stability of the maximal and submaximal regimes for different situations. In most cases, the water formation rate inside the basin determines whether the strait flow is maximal or submaximal. When mixing in the hydraulic jump is included, the system can exhibit stable multiple equilibria for the same net evaporation over the basin, where one state is submaximal with weak mixing, and the other maximal with strong mixing. Comparisons with GCM simulations are also shown.
P14/W/11-A4 1500
LARGE AMPLITUDE INTERNAL WAVES IN THE STRAIT OF GIBRALTAR
Harry L. BRYDEN and Brian A. King (both at Southampton Oceanography Centre, Southampton, SO14 3ZH UK, email h.bryden@soc.soton.ac.uk or b.king@soc.soton.ac.uk)
Large amplitude internal waves are generated close to the time of high tide at or near the sill in the Strait of Gibraltar, particularly during strong Spring tides. We examined the subsurface characteristics of these large amplitude waves using new technology including calibrated, three-frequency backscatter profiles from an EK500 and accurate velocity profiles from shipboard ADCP measurements combined with three-dimensional differential GPS navigation as well as using a slowly-towed CTD package to measure the hydrographic properties of the interface between inflowing Atlantic water and outflowing Mediterranean water. In the early morning hours of 4 successive days over Easter weekend 1998, 100 m amplitude waves propagated eastward past the nearly stationary ship at locations east of the sill. Maxima in the vertical profiles of backscatter, particularly at 38 kHz, provided dramatic, real-time portraits of the waves. The maxima in backscatter, however, appear to be a feature of the Mediterranean layer and not a feature of the interface as the maximum lies 20 m below the 38-isohaline; furthermore, the backscatter maximum appears to evanesce before the complete wave train has passed so it is an unreliable indicator of the waves. More robust signatures of the waves are measured by the shipboard ADCP: upward and downward vertical velocities as large as 50 cm s-1 are an outstanding feature of the waves and the depth of the maxima in the vertical shear of the eastward current from the ADCP tracks the lower part of the interfacial region (salinity of about 37.7) for much longer periods than does the backscatter.
On 14 April the large amplitude waves are observed at two alongstrait locations separated by 15 km and 2 hours implying an eastward propagation speed of about 2 m s-1. As the waves propagate over 15 km, their amplitude, maximum vertical velocity, and maximum steepness decrease by about 30% from 120 m to 80 m, from 50 to 35 cm s-1, and from 0.7 to 0.5. As a measure of nonlinearity, horizontal advection accounts for about half of the local time changes in interface depth while vertical velocity accounts for the remainder.
P14/W/12-A4 1520
GROWTH MECHANISM OF TOPOGRAPHIC INTERNAL WAVES GENERATED BY AN OSCILLATORY FLOW
Tomohiro NAKAMURA and Toshiyuki Awaji (Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan, Email: nakamura@kugi.kyoto-u.ac.jp)
A new amplification mechanism for topographic internal waves generated by tidal flow is presented to reveal the unknown growth processes of the internal waves in the Kuril Straits (about 47N in the North Pacific) reported by Nakamura et al. (1998). Accordong to their investigation, the nature of the wave properties is determined by the nondimensional parameter (k U0)/sigma_f, where k is the curvature of topoguraphy, U0 is the flow speed amplitude, and sigma_f is the frequency of tidal flow, through which the topographic internal waves can be classified into the three wave types; (1)unsteady lee waves (k U0/sigma_f >> 1), (2)mixed tidal lee waves (k U0/sigma_f \sim 1), and (3)internal tides (k U0/sigma_f << 1). Notably, their numerical model showed that the growth processes of the first two waves were quite different from those predicted by conventional internal wave theories, thus suggesting the presence of another criterion for wave amplification. For this issue, our theoretical investigation based on the ray tracing of individual waves generated at various time reveals the following interesting facts. Unsteady lee waves are always amplified when the maximum frequency is sufficiently smaller than the buoyancy frequency (i.e., sigma_f << k U0 << N), because their phase speeds and amplitudes are equal and proportionate to the tidal flow speed at their generation, respectively. Fast mixed tidal lee waves are also effectively amplified as well as unsteady lee waves, when the rotation effect is significant (i.e., f \sim sigma_f \sim k U0 << N). Accordingly, amplification of unsteady lee waves and fast mixed tidal lee waves can occur even if the conditions indicated by previous theories (i.e., the critical slope and the critical Froude number) are not satisfied. Since our theoretical model covers generation and amplification processes of topographic internal waves in a broader parameter range than before, it may contribute to a better understanding of the Boundary Mixing processes.
P14/E/10-A4 1540
FLOWS IN THE SILL REGION OF THE CLYDE SEA.
Rik MIDGLEY & John Simpson (School of Ocean Sciences, University of Wales, Bangor, Gwyned LL57 2DG, U.K. email oss088@sos.bangor.ac.uk)
Observations of the vertical structure and currents in the Clyde Sea over a seasonal cycle identified the sill as a region of major uncertatinty. At times when the cross-sill density difference was maximum, the rate of exchange was below average, indicating that other forces control exchange. Renewal was generally episodic, indicative of wind induced exchange, and in the summer, when horizontal gradients were minimal, persistently high rates of exchange were observed.
New observations in the sill region in 1995 were used to construct a dynamical schematic of the structure. In March, a rapid cross-sill exchange (flows ~10 cm-1), which appeared to be essentially estuarine, and influenced by rotation, was observed. In contrast, in September cross-sill flows were observed to be relatively weak and spatially complex. A 3-dimensional model was used to confirm that changes in wind direction could substantially increase or diminish exchange by enhancing or blocking the estuarine circulation, and the summer time retreat of the front at the mouth of the Clyde, due to low freshwater inflow, could enhance exchange by increasing the availability of saline water over the sill.
P14/E/03-A4 1620
MODELING OF MEDITERRANEAN OUTFLOW IN THE BLACK SEA. DEPENDENCE ON TOPOGRAPHY.
Elissaveta PENEVA and Emil Stanev ( both at Dept. of Meteorology and Geophysics, University of Sofia, Sofia 1126, Bulgaria, emails: elfa@phys.uni-sofia.bg,stanev@phys.uni-sofia.bg)
Numerical simulations of Mediterranean outflow in the Black Sea using reduced gravity model with horizontal resolution of 600 m have been carried out. The Mediterranean water spreads in the neighbouring Black Sea as gravity plume accelerated by the the large density contrasts and topography slopes. This process is of major significance for the deep and intermediate water mass formation in the Black Sea. A number of numerical experiments have been run in attempt to better understand the topographic control of the current. In the refference experiment we use realistic topography and the model is tuned to adequately simulate the observed characteristics of gravity currents. Idealistic topography H=H(y), consisting of Bosphorus exit, shelf, continental slope and abyssal plain, is constructed in such a way that the geometric parameters of topography elements (slope and length) are comparable to the real ones. In order to investigate the impact of roughness on the gravity current simulations are carried out with idealistic but rough topography with roughness characteristics similar to observed ones. Additional experiments with deviating element in north-western direction after Bosphorus exit were run too in order to study the effects of Bosphorus underwater extention. The results show that the entrainment rate reaches its maximal values over the continental slope due to the acceleration of the plume. The roughness affects both entrainment rates and dispersive properties of the model. It was also found that the presence of Bosphorus underwater channel in north-western direction is quite significant for the dynamics of gravity currents.
P14/E/05-A4 1640
FLOW AND MIXING IN THE BOSPHORUS
Michael GREGG (University of Washington Seattle, WA 98105 USA); Emin Ozsoy (Middle East Technical University Erdemli, Turkey)
Current meter moorings and high-resolution surveys during late summer of 1994 in the Bosphorus revealed important details of the exchange flow between Mediterranean and Black Sea waters. We conducted the surveys with a narrowband ADCP, a high-frequency acoustic backscatter system, and a microstructure profiler carrying a CTD. The flow was quasi-steady and mixed most vigorously south of the principal contraction in the strait. The mixing was demonstrated by trains of billows,density overturns, high dissipation rates, and rapid thickening of the interface between the surface and bottom layers. Nevertheless, composite Froude numbers were much less than critical through the Contraction and may have been critical only several kilometers south of the Contraction, when the surface layer became too thin and shallow for us observe its speed.
P14/W/16-A4 1700
THE BOSPHORUS STRAIT MIXING AND THE DENSE WATER OUTFLOW ON THE BLACK SEA SHELF: EXPERIMENTS AND MODELLING
Emin OZSOY (Institute of Marine Sciences, Middle East Technical University, P.K. 28 Erdemli - Icel 33731 Turkey, Email: ozsoy@ims.metu.edu.tr), Daniela Di Iorio (NATO SACLANT Undersea Research Centre Fine Scale Acoustics and Oceanography Group, Viale S. Bartolomeo 400, 19138 La Spezia (SP), Italy, Email: daniela@saclantc.nato.int), Michael Gregg (College of Ocean and Fishery Sciences, 522 Henderson Hall, University of Washington, 1013 NE 40th Street Seattle Washington 98105-6698, Email: gregg@apl.washington.edu), Sciences, 522 Henderson Hall, University of Washington, 1013 NE 40th Street Seattle Washington 98105-6698, Email: gregg@apl.washington.edu), Jan Backhaus (Institute for Oceanography, University of Hamburg, Troplowitzstr. 7, D-22529, Hamburg, Germany Email: backhaus@dkrz.de).
The exchange and mixing of waters from the Black Sea and the Mediterranean Sea occurs throughout the Turkish Straits, and intensively in the Strait of Bosphorus. Entrainment and mixing processes are studied through the Bosphorus, and on the Black Sea continental shelf / slope, based on the historical data base and the high-resolution intensive data sets obtained in 1994 and 1996. The changes in water properties along the Strait and in the outflow regions are characterized by distinct entrainment / mixing regimes, such as near the southern end of the Bosphorus, within the northern reach of the Strait and across the northern sill and the shelf region. A reduced gravity model of the dense bottom current is employed to study the flow across the northern shelf and to the continental slope. The flow is shown to be very sensitive to chosen parameters, to the environmental conditions in the Black Sea, and especially to the accurate representation of the bottom topography. High resolution, improved topographical data constructed from adcp and swath echosounder measurements are used to improve model predictions. Measurements of hydrographic and current features are used for verification of the results.
P14/W/15-A4 1720
A COMPARISON OF LARGE EDDY SIMULATIONS WITH OBSERVATIONS FROM THE CENTRAL CONTRACTION IN THE STRAITS OF BOSPHORUS
HARVEY SEIM, Skidaway Institute of Oceanography, Savannah, Ga 31411, USA, email:seim@skio.peachnet.edu Kraig Winters, Applied Physics Laboratory, University of Washington, Seattle, WA, USA and Centre for Water Research, University of Western Australia, Nedlands, Australia, email: winters@cwr.uwa.edu.au
Large eddy simulations of an internal hydraulic control at a contraction are found to support both maximal and submaximal exchange flows. The model results are compared with observations collected from intensive shipboard sampling by Gregg and Oszoy of the central contraction in the Straits of Bosphorus. A two layer decomposition of both the model results and the observations facilitates comparison with two-layer inviscid hydrostatic theory. A three layer decomposition, in which the upper and lower layer are well-mixed and the third layer comprises the interface between them, highlights the extent and location of mixing between layers. The comparison suggests the flow in the contraction is submaximal exchange, and that this can occur without flooding other hydraulic controls along the Strait, but that this configuration still generates copious mixed fluid.
Thursday 22 July PM
Presiding Chairs: M.C. Gregg (University of Washington, USA),
E. OZSOY (Institute of Marine Science, Middle East Technical University, Turkey)
P14/E/07-A4 Poster 1400-01
ROLE OF DENSITY CURRENTS IN STRAIT DYNAMICS AT COCHIN INLET- SOUTHWEST COAST OF INDIA.
K. J. AJITH and A.N. Balchand (Department of Physical Oceanography, Cochin University of Science and Technology, Fine Arts Avenue, Cochin 682 016, INDIA. email: oceans @md3.vsnl.net.in)
The shores of the tropical regions are embedded with tidal inlets that are grouped under straits as the dynamics of these inlets mimic flow patterns similar to that which occur in larger straits. As these inlets serve as openings of estuaries in many instances, the dynamics of two layer exchange present a vivid analytical situation brought about by the circulation of coastal waters vs. inland runoff. Also the physical configuration of the inlet controls the amount of exchange between the protected embayment and the coastal sea. The southwest coast of India, often a region of many geomorphologic features is no exception, where Cochin inlet (9o 58'N, 76o14'E), provides an insight on the strait dynamics. This paper elucidates information on the inflow and outflow transport rates by density currents due to the change in sigma-t with appreciable salinity differences. A monthly sigma-t analysis at Cochin inlet authenticates the variation in density currents with seasonal changes in mixing. The tidal conditions and freshwater influence is also considered while analysing the strait dynamic features of this tropical tidal inlet. The study also covers aspects on the influence of land sea exchange of material and the role of inlet hydraulics in the protection of the embayment.within.
P14/E/01-A4 Poster 1400-02
A 3-LAYER HYDRAULICALLY CONTROLLED BOX MODEL OF THE RED SEA AND BAB AL MANDAB STRAITS
David COBBY (ESSC, Univ. Reading, Harry Pitt Building, 3 Early Gate, Reading RG6 6AL, UK, email: dmc@mail.nerc-essc.ac.uk) Keith Haines and Stephan Matthiesen (Dept. of Meteorology, Univ. Edinburgh, James Clerk Maxwell Building, Edinburgh EH9 3JZ, UK, email: kh@met.ed.ac.uk and stephan@met.ed.ac.uk)
We have built a 3 layer model for the Red Sea in which the inflow and outflow of the layers is controlled hydraulically. The Bab al Mandab Strait connecting the Red Sea to the Gulf of Aden exhibits a 2 layer anti-estuarine exchange in winter which becomes a 3 layer flow in summer under the influence of summer Monsoon winds which raise an intermediate water mass up in the Aden Gulf, which then penetrates into the Red sea. The main control for the hydraulic flows is taken at a sill and narrows which are assumed coincident. Other controls and associated hydraulic jumps are detected by their influence on the flow rates.
Our model simulates the full seasonal cycle of flow as the Gulf of Aden intermediate water level is raised and lowered outside the straits. The layers inside the Red Sea exchange water through mixing and E-P water conversions and maintain both mass and salt budgets. The results are compared to observations from both current meter in the strait, layer thicknesses and sea level changes. The impact of the summer season 3-layer flow on the average properties of the waters inside, and emerging from, the Red Sea is discussed.
P14/W/01-A4 Poster 1400-03
CONTROL OF CONVECTIVELY DRIVEN EXCHANGE FLOW IN A SILL-ENCLOSED SEA
TIMOTHY D. FINNIGAN and Gregory N. Ivey (both at Department of Environmental Engineering, Centre for Water Research, University of Western Australia, Nedlands, WA 6907, Australia email: finnigan@cwr.uwa.edu.au)
Flux of buoyancy through the surface of a semi-enclosed or marginal sea, due to cooling or evaporative processes, generally results in a circulation within the sea and an exchange of fluid with the adjoining water body (ocean). The Red and Mediterranean Seas are well known examples of systems exhibiting this type of circulation. We describe laboratory experiments of the flow in an idealized semi-enclosed sea, with a configuration similar to the natural examples, where the sea is uniform in width and separated from an ocean by a shallow sill. Steady withdrawal of buoyancy by cooling at the sea surface results in turbulent convection and an associated lateral buoyancy gradient across the sill. A mean flow is thus driven into the sea near the surface becoming progressively more dense as it traverses the length of the sea before turning 180 degrees and flowing back out of the sea at depth. This circulation persists above a deep stagnant layer trapped behind the sill.
The net effect of the surface buoyancy flux is reflected in the density of the lower outflowing layer at the sill. The two-layer exchange at the sill is hydraulically controlled and thus coupled with the surface buoyancy forcing through the density signature in the lower layer. The hydraulic control, the mean flow, and the mixing are linked in a complicated non-linear feedback loop. We present observations of the mean and turbulent fields, obtained through particle tracking methods, and interpret the effect of mixing on the control condition and the outflow density and volume flux. In addition, the long-term erosion of the deep stagnant layer is described and related to the geometric scales of the sea and the buoyancy forcing.
P14/L/01 Poster 1400-04
EVOLUTION OF SALINATION OF THE BLACK SEA DURING HOLOCENE
Mehmet Karaca, Eurasia Inst. of Earth Sciences, ITU Maslak 80626 Istanbul Turkey,
Achim Wirth and Michael Ghil, Dept. of Atmospheric Sciences and Inst. of Geophysics and Planetary
Physics, UCLA Los Angeles, CA 90095 USA
The evolution of the Black Sea's salinity after the opening of the Bosporus about 7500 years ago is investigated using a simple two-box model. The model consists of watermass and salt conservation equations, allows for changes in thermocline depth. The paleoceanographic box model is forced by the present-day Mediterranean inflow and outflow, and atmospheric forcings. Analytic solutions for the evolution of the box volumes are given. Model salinities reach 90% of their present -day values in both boxes about 2,500 years after the opening of the Bosporus. The evolution of salinities is shown to be almost independent of the evolution of the box volumes and the results are shown to agree with the existing paleoceanographic proxy records.
P14/W/03-A4 Poster 1400-05
SILLS AND INTERNAL BORES
Gregory LANE-SERFF (School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, Southampton, SO14 3ZH, UK, email gfl@soton.ac.uk)
Flows through straits and over sills are often strongly modulated by the tide. Basing calculations on mean conditions and treating the flow as steady may significantly underestimate the mixing that takes place in straits. A common feature of tidally modulated strait flows is the generation of internal bores. It is important to determine how much of the mixing takes place during transient events (such as the passage of an internal bore and its associated internal waves) compared with the mixing that occurs at other times. A striking example of a tidally generated internal bore is that in the Strait of Gibraltar, where the interface between the outflowing Mediterranean water and the inflowing Atlantic water can oscillate by 100 m or more in the vertical over periods of only a few minutes as the bore passes. Oceanographic measurements of the internal bore in the Strait were recovered during a cruise in 1998 led by Dr H L Bryden, using acoustic backscatter, ADCP and an oscillating CTD package. The simultaneous use of these different types of observations allows a comprehensive description of the internal bore to be made.The flow in a two-layer system with a combination of sill and internal bore is also analysed mathematically using shallow water equations and hydraulic control theory. The theory gives predictions for the dependence of bore speeds and layer depths on the initial stratification and the net flow through the strait. The same system is modelled in the laboratory using tanks of saline solution of different density. The laboratory experiments are recorded on a video system and processed to give quantitative information about the flow. The laboratory experiments also give information about the character of the bore, whether it is abrupt or marked by a series of internal waves (undular bore). For the undular bores, the experiments give the amplitudes and wavelengths of the waves. The results from all these methods of investigating unsteady flow through straits are compared, and we outline directions for future research.
P14/W/04-A4 Poster 1400-06
QUASI-STATIONARY TWO-LAYER EXCHANGE FLOWS THROUGH LONG STRAITS
Vladimir MADERICH (Institute of Mathematical Machine and System Problems NASU, Glushkova pr. 42, Kiev 252187, The Ukraine. Email vlad@maderich.pp.kiev.ua)
Quasi-stationary water exchange through the narrow strait with two-layer flow structure and net barotropic transport was investigated both experimentally and theoretically. The experimental facility consisted of the tank that was divided on two basins, connected by a shallow and narrow rectangular channel. One basin was heated at the bottom. Another basin was cooled at the top. The results of measurements were given for the short, intermediate and long straits in dependence on the magnitude of the net barotropic transport. The measured depths of interface and volumetric flow rate in the laboratory analog agree reasonably well with computed by a laminar hydraulic model that based on the principle of maximal exchange. The influence of the sills placed inside of the long strait, on the exchange was studied. The results showed strong influence of the position of sill on the exchange in the long straits. These results have been extended on the sea straits with predominated turbulent bottom and interfacial friction. Finally, the response of the chain of the seas connected by the long straits to the seasonal variations of the freshwater budget was investigated for the case Black Sea - Bosphorus - Marmara Sea - Dardanelles - Mediterranean Sea. The one-and-a-half-dimensional sea models supplemented the strait models. The computations showed a complicated response of exchange flows and level in adjacent seas on the seasonal forcing.
P14/E/08-A4 Poster 1400-07
TRANSFORMATION OF ATLANTIC WATERS IN THE BARENTS SEA AND THEIR FLOW TO THE EURASIAN BASIN THROUGH ST. ANNA TROUGH
URSULA SCHAUER (Alfred-Wegener-Institut fuer Polar- und Meeresforschung, D-27515 Bremerhaven, Germany, email:uschauer@awi-bremerhaven.de Bert Rudels (Finnish Institute of Marine Research, FIN-00931 Helsinki, Finland) Harald Loeng (Institute of Marine Research, N-5024 Bergen-Nordnes, Norway)Robin Muench (Earth & Space Research, Seattle, WA 98102-3699 USA) Jim Swift (UCSD Scripps Institution of Oceanography, La Jolla, CA 92093-0214, USA)
The ventilation of the Arctic Ocean basins is largely associated with the input of water from the Barents and Kara Seas. Using hydrographic observations, made in the Barents Sea and along the shelf edge of the Kara Sea between 1991 and 1996, and time series of current, temperature and salinity from moorings in the eastern Barents Sea, we discuss the flow and the modification of the water masses between the western Barents Sea and the St. Anna Trough. The inflow of warm, saline Atlantic-derived water from the Norwegian Sea and the low salinity Norwegian Coastal Current feed a permanent eastward flow. An additional input of fresh water is melted ice which is advected from the central Arctic Ocean and the Kara Sea.These three water masses are modified through cooling and the freezing/melting cycle. Two distinct modes are formed, which leave the Barents Sea eastward and descend down the St. Anna Trough. The lighter mode is winter water of low salinity, thus confined to the upper layers, which leaves the Barents Sea seasonally at temperatures close to freezing point. The largest and most dense contribution consists in more saline but only moderately cold bottom water which is formed in the polynya west of Novaya Zemlya through highly brine-enriched water. Subsequent lateral mixing with warmer Atlantic water feeds a bottom-intensified flow throughout the year. At present conditions, both modes form a low salinity input of about 2 Sv to intermediate depths of the Arctic Ocean.
P14/W/09-A4 Poster 1400-08
VARIATIONS IN THE MAIN KUROSHIO PATH SOUTH OF JAPAN
Yoshihiko SEKINE (Institute of Oceanography, Faculty of Bioresource, Mie University, 1515 Kamihamachou, Tsu Mie 514-8507 Japan.)
Characteristics of the variation in the Kuroshio path south of Japan is studied by use of the observational distance of the main Kuroshio path from Japanese Coast over the period of 1975 - 1993. It is pointed out that a large meander formed in 1975 has a larger distance from the Japanese coast from Murotomisaki ( Muroto Pen.) to Daiohzaki ( Daioh Pen.), while its distance from coast in east of Omaezaki (Omae Pen.) is small. In contrast to this, large meanders formed after 1980 show an opposite tendency to have short distances from coast from Murotomisaki to Shionomisaki (Shiono Pen.), while they have larger distance from eastern area of Omaezaki. It is also resulted from the correlation analysis of the distances of the main Kuroshio path from Japanese coast that the path pattern of the large meander path formed in 1975 has a tendency to shift westward, while that formed after 1981 has a tendency to shift eastward. It is shown that the flow pattern of the large meander path is different before and after 1980. It is also resulted that there exists one month ( two months) time lag between sea level difference across Nishinoomote - Naze that represents the surface volume transport of the Kuroshio and distance of the main Kuroshio axis off Toimisaki (Murotomisaki), which indicates that an increase in current velocity of the Kuroshio occurs before formation of the small meander of the Kuroshio and its eastward shift. Some other results that give important suggestion on the Kuroshio path dynamics are also detected.
P14/W/06-A4 Poster 1400-09
MODELLING STUDIES ON ASPECTS OF THE DYNAMICS OF THE STRAIT OF SICILY
S. PIERINI(Dipartimento di Fisica, Università dell'Aquila, Via Vetoio, I-67010 Coppito (l'Aquila) Italy, Email: pierini@cds.unina.it) A. Rubino (Institut für Meereskunde, Universität Hamburg, Troplowitzstrasse 7, D-22529 Hamburg, Germany, Email: rubino@ifm.uni-hamburg.de)
In order to describe aspects of the baroclinic dynamics of the Strait of Sicily a high resolution multi-layer model has been implemented in a central Mediterranean region including the Tyrrhenian and the Ionian Sea. Three layers have been considered representing waters of Atlantic origin (MAW), the Levantine Intermediate Water (LIW) and deep waters of the Mediterranean. Quasi-stationary circulation schemes representing the effect of an idealized Mediterranean thermohaline circulation are produced by steady fluxes imposed remotely along the open boundaries. In order to follow the evolution of the oceanic response, a norm in the space of the flow patterns in each layer is defined. Thus, by studying the temporal evolution of the "distance" between our numerical solution and the initial motionless state we identified an initial adjustment time of 1-2 months followed by a very slow evolution of the system in which dissipating mechanisms appear to play a major role. We therefore selected our quasi-stationary states just after the adjustment phase. Such states can be interpreted as possibledynamic scenarios of the seasonal variability. In the simulations an inflow of MAW and an outflow of LIW through the Strait of Sardinia, an outflow of MAW and an inflow of LIW through the Ionian boundary and an outflow of MAW through the Corsica channel are imposed, resulting in a vanishing total net transport in each layer within the basin. For realistic values of the imposed transports the model captures some of the main features of the observed circulation in the area of the Strait of Sicily, such as: (a) the separation of the Algerian current intotwo branches (one directed toward the Tyrrhenian and the other toward the strait) of relative intensity in agreement with observations; (b) a secondary bifurcation of MAW within the strait giving rise to a southward moving current which follows the Tunisian continental slope and to a current that, after flowing south-eastward along the southern Sicilian coast, travels northward as a coastally trapped current; (c) a bifurcation of LIW at the strait level leading to a main current directed toward the Strait of Sardinia and to a weaker current crossing the strait and bending eastward along the northern Sicilian coasts. Variations of the boundary fluxes in the MAW and LIW layers have also allowed to determine that the horizontal distribution of the MAW flow across the Strait of Sardinia does not exert a relevant influence on the circulation in the zone of interest, and that the bifurcation of the Algerian current strongly depends on the LIW transport, i.e. the Tyrrhenian branch of the MAW decreases as the LIW transport decreases.
P14/W/07-A4 Poster 1400-10
A THREE-LAYER MODEL FOR THE STUDY OF TIDALLY INDUCED DENSITY CURRENTS
Rainer Weigle, Peter BRANDT, and Angelo Rubino (Institut für Meereskunde, Universität Hamburg, Troplowitzstr. 7, D-22529 Hamburg, Germany, Email: brandt@ifm.uni-hamburg.de)
The dynamics of tidally-induced internal waves is investigated by using a numerical three-layer model based on the weakly nonlinear, weakly non-hydrostatic Boussinesq equations. Numerical solutions of the three-layer Boussinesq equations are compared with analytical solutions of different equations describing internal solitary waves in a two-layer system. In contrast to the two-layer models, the three-layer model is capable of describing the evolution of sub-surface jets. As a result of the three-layer model these jets develop undulations that finally disintegrate into internal solitary waves. Several characteristics of these waves, like e.g., wave amplitude and distance between the first two solitary waves of a wave train are studied as a function of the initial jet velocity and the stratification. Results of the numerical simulations are compared with results of high resolution in-situ measurements carried out north and south of the Strait of Messina, in the European Mediterranean Sea. Implications of the presented study for a possible inversion of sea surface manifestations of oceanic internal waves into characteristics of the interior ocean are discussed.
P14/E/09-A4 Poster 1400-11
MODELLING THE RIFT-VALLEY FLOW OF THE MID-ATLANTIC RIDGE
A.M. THURNHERR, K.J. Richards, and M.M. Lam (School of Ocean and Earth Sciences, Southampton Oceanography Centre, Southampton SO14 3ZH, UK)
The hydrography and flow field of the AMAR segments of the Mid-Atlantic Ridge (35 Deg 35' N - 36 Deg 45' N) were investigated by analysing CTD data from two hydrographic surveys, a set of LADCP profiles, and data from an array of moored current meters deployed for one year. The bathymetry of the rift valley can be approximated as a sequence of deep basins separated by shallow sills. Hydrographic sections across these sills show potential density contours consistent with unidirectional, hydraulically controlled, along-valley flow. This is supported by the current meter observations, which show unidirectional flow of order 0.1 Sv along the valley throughout the one year deployment. Based on the T/S characteristics of the rift-valley water as well as direct density observations, the flow is most likely fed by an inflow sill on the eastern valley wall of the southernmost basin of the segment.
Based on these observations a simple basin/sill model for the along-valley flow is constructed. Topics to be studied using this model include: the timescales of the flow; the nature and variability of its forcing; the most likely destiny of the rift-valley water; and the sensitivity of the flow to rift-valley processes, such as the localized hydrothermal density flux which occurs just upstream of one of the sills studied.
P14/W/02-A4 Poster 1400-12
LATERAL CIRCULATION DRIVEN BY AN OVERFLOW
Jody M. KLYMAK and Michael C. Gregg (both at APL/U. of Washington, 1013 NE 40th St., Seattle, WA 98105; 206-543-1300; e-mail: jklymak@apl.washington.edu)
The importance of interfacial stress and entrainment on the dynamics of an overflow is generally recognized. Interfacial stress slows the overflow, while mixing dilutes it and makes it lighter. However, the importance of stress and mixing on the water above an overflow is not usually discussed.
Recent observations in Knight Inlet (B.C.) show that an overflow can drive strong recirculations. We believe that shear instabilities along the top of the overflow accelerate the overlaying water. This acceleration is local because it only occurs over the overflow, which creates a cross-channel divergence in the overlaying water. A matching pair of recirculations transport water across the channel to fill this divergence. These recirculations strongly affect the flow in Knight Inlet. For instance, they draw mixing ingredients into the flow laterally, they bias volume budgets, and they confuse determinations of the level of no-motion which has traditionally been used to define the vertical extent of an overflow.
We hypothesize that these recirculations are a general phenomena in supercritical flows. First, supercritical flows are often susceptible to shear instabilities which exchange momentum and fluid between the overflow and the overlaying water. In addition, supercritical flows are too fast to allow pressure signals to propagate back upstream. This means that the local acceleration in the lee of the sill cannot change the flow upstream of the sill crest, and the increased flow can only be supplied by circulation in the lee of the sill. Since the major overflows are almost all supercritical at some point, we expect that they all drive lateral recirculations.