IP11a

Experimental study of the bottom boundary layer’s dynamics in the coastal zones

Evgeny A. Kontar

P.P. Shirshov Institute of Oceanology, Moscow, RUSSIA

The horizontal components of the flow velocity and the temperature of the bottom boundary layer (BBL) were studied 2.5 m above the bottom of the coastal zones of the Black and Mediterranean Seas and the Atlantic Ocean (in the vicinity of the Coastal Canary Upwelling Zone) over a 6-day period. Potok meters were incorporated into the self-recovering (pop-up) self-contained bottom stations. In the Black and Mediterranean Seas our study includes the discovery of bottom (benthic) storms in the coastal zone. These storms contain clear evidence of deposits, termed ‘turbidites’, which result from episodic down slope gravity transport of a dense suspension of water and sediment in a ‘turbidity current’. Our data thus suggests a strong unsteady mass transport in the coastal zone. The coastal zone BBL dynamics are complicated by a variety of physical phenomena that show up as bottom storms, sometimes correlating with temperature variations in the bottom water. This latter fact has not been previously observed. In the Atlantic Ocean our measurements indicated that the BBL in the area of the Coastal Canary Upwelling Zone has highly variable dynamics. The complex interaction of the counter-flowing strong geostrophic currents, with flow velocities of 22 and 26 cm/sec, that was identified in the BBL by our measurements inevitably give rise to energy-consuming processes, which may influence the development of the upwelling in the coastal zone. For example the recorded speeds of the bottom currents undoubtedly produce erosion of bottom sediments, increasing the turbidity of the water in the BBL.

The boundary region between the counter-flowing currents exhibits high levels of turbulence and a high degree of vortex generation, and these factors cause efficient transport of momentum and of substance in the coastal zone. The pronounced tidal phenomena in the measurement area increase the size of the zone in which the currents interact vigorously, and thus intensify eddy generation, which may follow the sea-floor relief to subsurface level or even emerge at the sea surface, forming the tongues and patches of relatively cold water typical of the upwelling.

IP11d

Guiana Current on the Shelf and Continental Slope off North Brazil

N.P. Bulgakov and S.N. Bulgakov

Marine Hydrophysical Institute, Ukrainian Academy of Sciences, Sevastopol, UKRAINE

At present, our knowledge of the Guiana current is very poor. Here the kinematic structure of this current is considered. Analysis is based on current meter observations made in cruise 41 of R/V ‘Academician Vernadsky’. Five current meter moorings were deployed off the north Brazilian coast in winter (February-March) of 1990. Buoys worked for twenty days. The obtained data enabled us to resolve the following questions: qualitative and quantitative characteristics of the vertical distribution of current velocity, its stability and variability, the influence of the baroclinic component of the horizontal pressure gradient on kinematic structure and to calculate water volume transport by the Guiana current.

The main results are as follows:

IP11e

Planetary Waves in Stratified Ocean of Variable Depth

G.M. Reznik, A.V. Bobrovich and T.B. Tsybaneva

PP Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia

Linear Rossby waves in a stratified ocean with a cylindrical bottom relief (the isobaths are straight parallel lines) are investigated. In the first part a two-layer model is considered. The case of a rough bottom relief (the wave scale is much greater than the bottom relief scale) is studied analytically by the method of multiple scales. A special numerical technique is developed to investigate the waves over a periodic bottom relief for arbitrary relationships between the wave and the bottom relief scales.

There are three types of modes in the two-layer case: barotropic, topographic, and baroclinic. The structure and frequencies of the modes depend substantially on the relative strength of the topography and beta-effect. For weak topography the modes are close to the ‘usual’ barotropic and baroclinic Rossby modes; the topographic mode degenerates in the limit of constant depth. The topographic and barotropic modes depend weakly on the stratification, and their frequencies increase monotonically with increasing height of the topography inhomogeneity. Both these modes become close to pure topographic modes for strong topography.

The dependence of the baroclinic mode on the topography is nontrivial. The order of magnitude of the frequency of this mode does not depend on the height of topography. At the same time the spatial structure of the mode depends strongly on the topography height H. For large H the motion in the mode is confined mainly to the upper layer and is very weak in the lower one. A similar concentration of mesoscale motion in an upper layer over an abrupt bottom topography was observed in the real ocean many times.

Another important physical effect is ‘screening’. It implies that for the horizontal bottom scale smaller than the internal Rossby scale the small-scale component of the wave is confined to the lower layer, whereas in the upper layer the scale of the motion is always greater than or of the order of the internal Rossby scale. Thus the stratification prevents the ingress of the motion with scale smaller than the internal Rossby scale into the main thermocline.

Similar effects occur also for the waves in a continuously stratified ocean over a cylindrical topography. The main dissimilarities from the two-layer case are the following: (i) there is a countable set of baroclinic modes in the continuously stratified model, (ii) for large H the motion in the baroclinic modes is confined to a region outside a thin near-bottom boundary layer, (iii) if the horizontal scale of topography is smaller than the internal Rossby scale then the small scale component of the wave is confined to the near-bottom boundary layer.

IP11f

Cascading of a Dense Water Plume Over a Shelf Break

G.I. Shapiro and A.E. Hill

P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, RUSSIA
School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, UK

Near bottom plumes of dense water are common features in the shelf/slope zone and have been reported for example in Spencer Gulf (South Australia), the Irish, Black and Arctic Seas. These structures can generate strong density-driven flows which have important implications for the dispersal/retention of contaminants, suspended sediments as well as marine biota. Previous modelling studies of dense plumes have been confined to simple bottom topography (eg: inclined or flat plane). Here dense plume evolution has been studied theoretically using a more general approach which incorporates arbitrary bathymetry and levels of friction corresponding to the shelf/slope zone.

We consider a meso-scale variability of the time scale which lies between several tidal periods and weeks and use the two-layer approximation. The lower layer consists of a water plume lying beneath a lighter fluid. The theory takes into account a pressure gradient caused by horizontal density inhomogenities, the Coriolis force, bottom and interfacial friction and non-linear effects. Turbulence in the bottom boundary layer and hence the level of friction near the shelf break is considered to be produced by energetic tidal motions rather than by residual currents. This allows us to use Ekman-like dynamics to resolve the vertical structure of the current. Analysis is not based on boundary-layer approximation and thus allows us to describe the cases of both thick and thin Ekman layers.

Results provide information about along-isobath translation, evolution timescales and cross-slope fluxes. The inviscid core of the dense layer moves basically along-isobath whereas downslope fluxes are concentrated in the bottom and interfacial frictional layers. This results in a wider dispersion of water particles over the adjacent area than in case of flat bottom. At a developed stage the water plume propagates in the form of a near bottom tongue with thickness of the order of double the Ekman scale. Entrainment of water from the upper layer increases the propagation speed of the bottom plume. An alongslope current, depending on its direction and velocity, may assist or prevent downslope cascading. The theory is specifically applied to recent observations of a cold, saline plume which had cascaded over the shelf break onto the continental slope northwest of Scotland. It is shown that the observed spatial density structure of the water tongue would have formed in about two days.

IP11h

Cool Filaments and Coastal Counter Currents in the eastern North Atlantic

John Johnson and Ian Stevens

School of Mathematics, University of East Anglia, Norwich, UK

The final modelling results from the multidiscplinary project MORENA will be described. This project has been concerned with the circulation over the west Iberian continental shelf and the surrounding deep water. Observed and modelled features in this seasonal upwelling area include cool offshore filaments, shelf edge counter-currents, topographic effects due to capes and ridges, shelf waves and the Mediterranean outflow. Some speculative thoughts on the role of this region in climate modelling will be presented.

Prelininary results will be discussed from the new European project CANIGO which is investigating the eastern North Atlantic between the Canary Islands, the Azores and the Gibraltar Strait by in-situ observations, modelling and remote sensing. This project includes modelling of the upwelling region along the Northeast African coast, the interactions of the coastal current with the Canary Islands, the Mediterranean outflow and the formation of meddies in the coastal current along the southern Iberian shelf. A model of the Gulf of Cadiz is nested within a primitive equation North Atlantic numerical model and will examine the extent of the spreading of Mediterranean water and its influence on the circulation in the North East Atlantic.

IP11j

Hydrodynamic Aspects of River and Sea Waters Interaction in River Mouths

E.S. Povalishnikova and V.N. Mikhailov

Department of Hydrology, Faculty of Geography, Moscow University, Moscow, Russia

In the context of an active economic development of river mouth natural resources the study of the processes taking place in the zone of dynamic interaction and mixing of river and sea waters is of great importance.

The hydrological regime of the river flow influences greatly both the formation of the mixing zone in the river mouth and the temporal and spatial variations of its physical, chemical and biological characteristics as well. The largest part of the mixing zone represents the buoyant plume and occupies the nearshore zone of the river mouth. It also should be pointed out that besides the river flow a number of hydrometeorological (wind regime over the nearshore, seasonal variations of river and sea water temperature) and morphological (the type of the river mouth nearshore) factors influence the formation of the buoyant plume. The type of river mouth nearshore (open or semi-enclosed, deep or shallow) determines the water circulation patterns at the mouth, the type of water stratification (weak, partially and strong stratified) and the role of the particular governing factors in the formation of the hydrological and hydrochemical regime of the buoyant plume. Strong or partial water stratification is usually observed at deep nearshore (the Danube, Kuban, North Dvina river mouths, estuaries in the White Sea), while the weak stratification occurs predominantly at the shallow nearshore (the Volga and Don river mouths). Wind regime has the most significant influence on the buoyant plume dynamics at the open nearshore (the Volga, Danube and Kuban river mouths), whereas the wind effect over the semi-enclosed nearshore is limited. The estuarine circulation and tidal effect are the main and relatively stable mechanisms of spatial and temporal variability of the buoyant plume here. On the contrary, the wind regime over the nearshore is characterised by seasonal and considerable short-term variability and is the casual and hardly predicted factor. This is the reason that the dynamics of the buoyant plume at the open nearshore is the most difficult to study.

Using hydrodynamic laws the authors have considered the changes of velocity distribution, water discharge and salinity along the buoyant plume at the deep open nearshore under steady-state conditions and worked out the simple mathematical model of longitudinal salinity distribution in the buoyant plume. This model is based on the following equations: 1) of salt-water balance in the plume (S_s-S_x)/(S_s-S_r)=Q_o/Q_x, 2) corrected condition of conservation of linear momentum Q_o/Q_x=(V_x/V_o)^m; 3) exponential reduction of flow velocity in the buoyant plume V_x/V_o=j(^x/h_o, I, f_b f_l). Here Q_o, Q_x, V_o, V_x are river discharges and averaged over the cross-section flow velocities at a distance x = 0 and at a distance x from the coastline, S_s, S_r, S_x are salinity of sea and river waters and mixed water at a distance x from the coastline correspondingly, h_o is the depth at a distance x = 0, I is the water slope at the nearshore, f_b and f_l are the bottom and lateral friction coefficients, m is the empirical coefficient. Combined solution of the above mentioned equations gives the quantitative law of the longitudinal salinity distribution in the buoyant plume under the steady-state condition: S_x = S_s - (S_s-S_r) exp(-kmx/h_o). Here k is the united friction coefficient depending on f_b and f_l. In such a manner, the salinity in the plume increases proportionally to the growth of the plume discharge due to water entrainment. On the basis of the observed data and the subsequent computation for the Danube, Amudaraya, Jura, Kuban, Sulak and Terek river mouths it has been established that the flow velocity falls along the plume faster than the discharge increases, so the condition of the conservation of linear momentum needs a correction exponent m. It equals 0.6 for the Danube and Kuban river mouths. It was also shown that the lateral friction coefficient at the open nearshore is 10 times more than the bottom one and 100 times more than the friction coefficient at the sea and river waters interface. Value of the united friction coefficient k was obtained as 0.002-0.003. The authors have also found out that with the existence of the water slope at the shallow mouth nearshore (as at the Volga river mouths) the flow deceleration along the plume occurs slower than at the deep nearshore.

Under the low flow conditions or during storm surges and tides the mixing zone can penetrate into the rivers. The saltwater wedge is typical for deep and nontidal river mouths and estuaries. Conditions of saltwater intrusion differ greatly for river mouths in various climate zones. For example, the existence of a saltwater wedge in the nontidal Danube mouth is more frequent in the summer-autumn low flow period and induced by the winds of north and north-east directions. On the contrary, the saltwater intrusion into the Main Channel of the Yana delta occurs in the summer at low flow and in winter under the ice cover. The most up-to-date methods for prediction of the saltwater wedge length L_s, are the empirical and semi-empirical equations L_s=f(Q), L_s/h=j (Q/Q_cr), L_s/h=c (Fr_r) and the hydro-dynamic models (the advection-dispertion concept for the cases of well and partial mixing). Here Q_r is the critical water discharge for the beginning of saltwater intrusion, h is the channel depth, Fr_r is the densimetric Froude number.

ip11l

source sink flows over shelf and slope topography

P. Jacobs, P.A. Davies and G.J.F. Van Heijst

Centre for Water Research, University of Western Australia, Nedlands, AUSTRALIA
Department of Civil Engineering, University of Dundee, Dundee, Scotland, UNITED KINGDOM
Fluid Dynamics Laboratory, Eindhoven University of Technology, Eindhoven, NETHERLANDS

Results of analytical and experimental models are presented in which the role of various forms of bottom topography on continental shelf currents has been investigated.

The shelf currents are generated in a rotating cylindrical geometry by means of a source sink technique. A linear analytical model for a homogeneous fluid in this configuration predicts that the azimuthal (swirl) velocity above a flat bottom can be described by a 1/r profile, with r the radial coordinate, measured from the geometry centre. The velocity profile is altered if the bottom consists of a flat and a sloping part (modelling the continental shelf and slope). It is shown that a geometrical function has to be included to describe the azimuthal velocity profile above the sloping bottom. This function depends only on the slope angle a and differs only significantly from unity for large values of a (a > 30ƒ).

Descriptions of the (horizontal and vertical) boundary layer transports reveal that a combination of free Stewartson layers is generated above the shelf break to account for the azimuthal velocity shear between the two interior regions. The net vertical transport in these shear layers is again only important for large slope angle.

A laboratory model has been set up to verify the analytical description experimentally. A flow, driven by a source sink system, was created in a rotating cylindrical perspex tank containing a weakly linearly stratified fluid. The stratification was applied to disperse neutrally buoyant particles for flow visualisation, but was shown to be sufficiently weak to have negligible influence on the dynamics of the flow. Azimuthal velocity profiles were determined from particle streak photographs and with the video based Diglmage Particle Tracking software. Reference experiments have been carried out to measure the azimuthal velocity profiles above a flat bottom. Then, two models of a continental slope were inserted with slope angles of 25ƒ and 55ƒ, respectively. Comparison of the azimuthal velocity profiles of the 25ƒ with its equivalent reference case reveals no measurable difference, as expected from the analytical model. However, with the 55ƒ slope the difference between the interior regions above the slope and the flat bottom is significant and in quantitative agreement with the results of the analytical model.

IP11n

Effects of Mixing and of Bottom Topography on the Stability of the Mediterranean Outflow over the Iberian Shelf

Laurent M. Cherubin, David G. Dritschel, Yves G. Morel
and Xavier J Carton

SHOM/CMO, Brest, France
DAMTP, Univ. Cambridge, UK

The SEMANE'95 experiment at sea was conducted to evaluate the evolution of the Mediterranean outflow on the Iberian shelf, from the Straits of Gibraltar to the Tagus Plateau. Its hydrological and current meter data was processed and studied to assess the influence of bottom topography on flow stability, and the relations between this latter quantity and mixing.

To this purpose, temperature and salinity cross-sections and maps are analyzed, and potential vorticity distributions are computed. It is shown that:

A simple stratified quasi-geostrophic model is used to highlight these processes and to quantify their sensitivity to changes in the physical parameters (width and speed of the undercurrents, diffusivity, canyon geometry). It is shown that quasi-stationary meanders or eddies can be generated by submarine canyons on a thermocline-intensified coastal flow.

IP11O

SEA SURFACE ADVECTIVE VELOCITIES IN THE SOUTHWESTERN ATLANTIC DERIVED FROM AVHRR IMAGES

David Renato Ghisolfi and Carlos Alberto Eiras Garcia

The University of New South Wales, School of Mathematics, Sydney, AUSTRALIA
FundaÁ“o Universidade do Rio Grande, Departamento de FÌsica, Rio Grande (RS) BRAZIL

Sea surface advective velocity fields were derived in the region between 28-36ƒ S and 47-56ƒ W in the Southwestern Atlantic from a set of 11 pairs of AVHRR thermal images. The study was based on locating the maximum cross-correlation between windowed portions of sequential images (technique known as the MCC method). We considered six pairs of images obtained in winter of 1993, two pairs for spring of 1993 and three pairs for summer of 1994. The time lag between each pair of images varied form 12 to 24 hours. The derived velocity fields were then compared to those estimated independently from wind stress data, patterns of SST fields, and available in situ data of buoy drifts. In general, good agreement was found, specially for winter and spring when the Brazil-Malvinas Confluence determines sharp thermal fronts in the region covered by the images. The agreement was poorer in the summer. It is thought that this was due to both absence of thermal surface features and intradiurnal variability cycling of SST which can limit the use of MCC method within the 24 hours time lag.

Six pairs of sequential images are presented in this work and the results confirm that the remote sensing may provide an alternative way for estimating sea surface currents in the Southwestern Atlantic where the coverage of in situ data on upper layer dynamics is still poor.

IP11p

INTENSIVE MESOSCALE MOTION GENERATION BY BAROCLINIC OCEAN CURRENTS

D. Yu. Lysanov and G.M. Reznik

P.P.Shirshov Institute of Oceanology, RAS, Moscow, Russia

There are lots of publications on the open ocean mesoscale eddies but the main mechanism of their generation is not clear even now. The baroclinic instability of large scale ocean currents could play the role of such mechanism due to: 1) the "correct" space-time characteristics of growing disturbances, 2) the great excess of the mean current available potential energy over the total eddy energy. The problem is that in existing models of the nonlinear baroclinic instability the kinetic energy (KE) of developed perturbations (KEE) usually does not exceed the mean current kinetic energy (KEM) while in the real ocean KEE is much more (often by an order of magnitude) than KEM. The aim of this work is to examine the influence of different physical factors on the level of mature baroclinic disturbances. The main attention is paid to the mean current supercriticality level, external forcing nonstationarity and bottom topography.

Analysis is carried out in the framework of the model of quasigeostrophic two-layer zonal channel on the beta-plane. Instability characteristics of the initial flow were determined based on the Phillips’ model. Parameters were taken close to the real ocean parameters. The friction and dissipation were chosen to be small in order to minimize the eddy decay.

Numerical experiments show that in the absence of bottom topography at the quasi-equilibrium stage when KEE and KEM oscillate slightly near some constant mean levels KEE is less or exceeds slightly KEM (depending on the scale of the growing disturbance). At the same time there exists rather long initial period (hundreds of days) when KEE can be much more (dozens of times) than KEM. The inclusion of various physical factors does not change this tendency. The presence of a barotropic component in the initial current does not influence the baroclinic instability mechanism but reduces the relative amount of KEE with respect to KEM. External forcing temporal variability within 10 percents does not make a substantial effect, though it promotes the generation of disturbances. The influence of the bottom topography (both shape and height of the bottom relief) on the dynamics of the mean current-disturbance system is stronger. Meridional relief (with zonal isobaths) results in shorter periods of the eddy domination and less level of KEE. Relief in the form of a Gaussian mountain leads to a rapid generation of eddies but their amplitude and period of dominance do not become large in comparison with the case of constant depth.

IP11Q

A Numerical Study of a Density Current Descending Along Continental Slope

K. Tanaka, K. Akitomo and T. Awaji

Department of Geophyics, Graduate School of Science,
Kyoto University, Japan

Numerical experiments with a three-dimensional non-hydrostatic ocean model, having a continental shelf and slope, were carried out to investigate the descending processes of dense water, which are essential to the bottom water formation in polar oceans. Starting with a homogeneous condition at rest, dense water is supplied uniformly at the coastal end of the continental shelf. Two kinds of dynamical regimes are found in descending current system. The first is a descent of dense water in the bottom Ekman layer, where bottom friction breaks down the geostrophic balance between the downslope pressure gradient force and the upslope Coriolis force. This appears in the early stage of the experiments or in cases with small density flux. The descending volume transport is determined not only by the pressure gradient due to density deviation, but also by the surface pressure gradient, which is induced through the geostrophic adjustment to a barotropic flow. Using the pressure distribution based on the result of a OGCM, this Ekman transport can explain 32% of the Antarctic Bottom Water formation. Half of this (15%) is due to the effect of the wind driven barotropic flow around Antarctica. The other descent of dense water is caused by baroclinic instability on the slope, extending outside the bottom Ekman layer. This appears in the mature stage of the experiments or in the case with large density flux. The descending volume transport is about ten times larger than the Ekman transport.

Thus, this can be a complementary process for the bottom water formation in polar oceans if it occurs locally and temporarily.

IP11s

On the Transfer of Tracer by Eddies on the Beta-Plane

E.S. Benilov

Department of Mathematics, University of Tasmania, Launceston, AUSTRALIA

The problem of the evolution of a tracer "injected" in an eddy on the beta-plane is examined numerically using the pseudospectral method. The eddy is governed by the standard quasigeostrophic equation, and the tracer is governed by the transfer equation (with no diffusive term). At the initial moment of time, the streamfunction describing the eddy and the distribution of the tracer are both radially symmetric. It is demonstrated that the tracer rapidly loses radial symmetry and forms coherent spiral patterns; whereas the potential vorticity (which satisfies exactly the same transfer equations), as well as the streamfunction, remain almost perfectly symmetric! It turns out that this phenomenon has a simple theoretical explanation based on the "potential vorticity theorem".

Similar spiral patterns were observed in the cases where the tracer was initially concentrated in a "strip" or elliptic "spot". In the latter case, the tracer field looked very similar to the cloud patterns in tropical cyclones.

ip11t

Coastal Embayment Circulation and Cross-Shelf Exchange due to Atmospheric Cooling

X.H. Wang and G. Symonds

School of Geography and Oceanography, Australian Defence Force Academy, Canberra, Australia

A field experiment including current meter moorings, CTD surveys and weather monitoring was carried out in Jervis Bay, NSW, Australia, to investigate the response of a coastal embayment to atmospheric cooling during winter. The bay is small enough that a synoptic CTD survey of the bay could be achieved over a period of six hours but still large enough that Coriolis effects are important. During a cooling event vertical convection and surface wind stress combine to produce a well mixed water column. Continued cooling produces cold, dense water in the shallow regions of the bay and can be identified as a tongue of cold bottom water flowing out of the bay onto the adjacent shelf. The cold outflow produces a surface inflow to the bay of warmer shelf water resulting in a restratification of the water column within the bay. The response has been modelled using a three dimensional ocean model with a prescribed surface heat flux. Following a period of cooling the model produces a stronger anti-cyclonic gyre at the surface and a weaker cyclonic gyre nearer the bottom. As the cold bottom water flows out of the bay, the warm shelf water enters at the surface forming a barotropic, cyclonic gyre which, within a few days, occupies the entire bay. There is good qualitative agreement between the model and observations including the restratification following a cooling event and flow reversal at one of the current meter sites associated with the formation of the barotropic cyclonic circulation due to inflow from the shelf.

IP11u

GEOSTROPHIC CURRENTS ON THE CONTINENTAL SHELF IN the PRESENCE OF INTERNAL WAVES

A.E. Filonov and C.O. MonzÛn

UdG, CUCEI, Departamento de FÌsica. Apdo. Guadalajara, Jal., MŠXICO.

This paper discusses a method to calculate geostrophic currents when internal waves exist on the continental shelf. These waves cause important spatial and temporal variations in the density field.

Here an internal waves’ filtration method which is applied to research data is suggested. The method consists in smoothing the density field data obtained from an oceanographic station’s grid. It applies the smoothing process by a filter that is determined from the spatial correlation function of the density’s variability. An example of a fast survey of the density field of a micropolygon on the shelf of the west coast of Mexico is provided. Field measurements were done monthly during 1995 and 1996 with a CTD SBE-19. The CTD was provided with a special case and is towed at maximum speed by the research ship. An average of eighty samples were taken to a 200 m depth, thus covering a grid surface of 100x15 km on a daily basis. Results from the internal tides spatial and temporal structure on the shelf are discussed as well as the oscillations spectral compositions by means of its amplitude, frequency, wave number and direction. Suggestions about the construction of an optimum spatial grid on the shelf are presented in order to successfully filter the density fluctuations enhanced by internal waves.

ip11v

A Numerical Study on Vortex Dynamics

A. Rubino and K. Hessner

Institut f¸r Meereskunde, Universit”t Hamburg,
Hamburg, GERMANY

A nonlinear numerical model based on the reduced gravity, hydrostatic shallow water equations has been developed in order to simulate the dynamics of surface vortices with outcropping interface. Due to a technique for movable lateral boundaries, the model allows for the simulation of expansions and contractions of the surface area. As a model validation, the results obtained by the numerical model are compared with the results of an analytical model. These analytical solutions refer to a special class of frictionless circular vortices with quadratic depth and linear velocity profiles. It is shown that the spatial and temporal evolution of the vortex shape and the associated velocity fields are adequately described by the numerical model. Moreover, different numerical simulations are carried out by assuming different parametrizations for interface friction and entrainment. The results indicate that interface friction is much more efficient than entrainment in producing the vortex decay.

ip11w

The Rhine Outflow Studied by the Analysis of ERS1/2 SAR Data and Numerical Simulations

K. Hessner, A. Rubino and W. Alpers

Institut f¸r Meereskunde, Universit”t Hamburg, Hamburg, Germany

Synthetic aperture radar (SAR) images acquired by the First and Second European Remote Sensing Satellite (ERS1/2) over coastal waters near estuaries often show sea surface signatures of river outflow fronts. In particular, the analysis of several SAR images showing the Rhine outflow region indicates that the outflow front is visible as a line of high radar backscatter. Location and form of the outflow front depend strongly on tidal phase and Rhine discharge. In order to simulate the dynamics of the Rhine plume in the outflow region, a two-layer, nonlinear numerical model based on the hydrostatic shallow water equation has been developed. Due to a numerical technique for moveable lateral boundaries, the model allows for the simulation of localized layers with an outcropping interface (front). The model is forced by imposing tidal and residual transport and river discharge at the open boundaries. The evolution of the Rhine plume as calculated by the numerical model is discussed with respect to tidal phase and Rhine discharge. Using a simple radar backscatter model relating the surface velocity convergence and shear to the relative radar backscatter, it is shown that the observed signatures of the Rhine outflow front can be explained by the variation of the surface velocity convergence and shear as calculated by the numerical model.

IP11x

Effect of Split Scales in a Diffusion Induced Boundary Current in a Continuously Stratified Liquid

Yu. D. Chashechkin, V.G. Baydulov and A.V. Kistovich

Institute for Problems in Mechanics, RAS, Moscow, RUSSIA

In a Schlieren image pattern of a density gradient distribution of a quiescent linearly stratified liquid one can see two accidents located near the upper and lower poles of submerged 2D and 3D bodies. The length of this density gradient accidents monotonicly grow with time. The reason of its formation is interruption of a molecular flux of a stratified component on the impermeable surfaces of the body. The linear theory of a transient boundary current on 1D, 2D and 3D obstacles is constructed in the Boussinesq approximation. Explicit solution for split flow induced on impermeable inclined plane is obtained. Asymptotic and numeric solutions are received for flows near a sphere, horizontal cylinder, 2D wedge-shaped cavity and inclined slot.

All solutions are monotonicly growing with time analytical functions. Given solutions are matched among themselves and with well-known analytical solutions. Applied solutions are analytical functions of all physical parameters of the problem (gravity acceleration, scale of buoyancy, local angle of surface inclination, kinetic coefficients).

IP11y

Nutrient Enrichment of Jervis Bay During the 1992/93 Algal Bloom

Mark T. Gibbs, Patrick Marchesiello and Jason H. Middleton

School of Mathematics, University of New South Wales, Sydney, AUSTRALIA

A massive algal bloom occurred in Jervis Bay, south-eastern Australia, during December and January 1992/93. The nutrient source for the bloom was thought to be deep nutrient rich slope water which was upwelled by the East Australian Current (EAC). The UNSW New South Wales regional model was used to simulate the synoptic EAC structure for the period immediately preceding the bloom. The simulations suggested that a cold core instability developed adjacent to the Jervis Bay shelf over the two week period preceding the bloom which forced cold slope water into the nearshore zone. Local winds were also upwelling favourably during this period. A regional model of the Jervis Bay shelf and slope was also developed to investigate the influence of the Jervis Bay protrusion on a strong alongshore EAC flow field.

IP11z

One Mechanism of Near-Bottom Dense Water Exchange Between Sea Basins

V. Zhmur, Yu Leonov and M. Yakubenko.

P.P.Shirshov Institute of Oceanology, Moscow, Russia

The behaviour of a thin near-bottom dense water layer on a steep continental slope is considered. The driving actions are: background ocean flows, Coriolis force (in a beta-plane approximation), gravity, and bottom friction. The problem is formulated in terms of Ekman boundary layer theory and the thickness of studied layer is no more then the order of some Ekman scales. Our approach covers both laminar and turbulent cases of dense water motion. The different regimes are investigated in theory and numerical simulations.

One of the most interesting regimes is the motion of a dense water tongue uphill. It may arise in areas of western boundary currents if the reduced gravity force is not extremely strong. In certain conditions it leads to upwelling of a dense water tongue to shelf zones and water exchange between deep sea basins through shallow water passages. Application of this theory to the problem of Indonesian throughflow is discussed.

The work is partially supported by RFBR grant 96-05-66316.

IP11aa

Slope currents on a zonal shelf: a numerical study using POM

John F. Middleton and Mauro Cirano

School of Mathematics, University of NSW, Sydney, AUSTRALIA

As a first step in understanding seasonal shelf circulation in the Great Australian Bight, a numerical study is made of the slope currents driven by a steady alongshelf windstress (0.1 Pa s), of finite fetch, (1000 km). The model domain extends 1000 km offshore and 4000 km to the east with a uniform shelf bathymetry. The Coriolis parameter is held constant for the zonal shelf. For the bathymetry and initial density field chosen the shelf slope is strongly stratified with a squared Burger number of 0.2. After 30 days, the westward upwelling favorable winds are found to drive a 30 cm/s coastal current to the west. Over the slope a weak 3 cm/s undercurrent evolves, but only at the western end of the wind forced region. In contrast, for a downwelling favorable wind stress, a relatively strong undercurrent (10 cm/s) is found to develop below the eastward coastal current and over the entire wind-forced region. At the eastern end of the forcing region, the undercurrent is "fed" by water drawn towards the coast with a mass transport that is about 1/3 of the surface Ekman transport. The difference between the current systems is in part attributed to the larger thermal wind shear that results from downwelling: the bottom boundary layers are much larger (40 m as against 4 m) and isopycnals are spread farther apart over the slope due to connective overturning. Slantwise connective instabilities are also found for the downwelling bottom boundary layer. These and other details of the circulation will be discussed.

IP11bb

Variations of Circulation and Water Exchange in Sagami Bay Induced by the Kuroshio Path

Yasuo Furushima, Takashige Sugimoto and Shingo Kimura

Japan Marine Science and Technology Center, Tokusuka City, Japan
Ocean Research Institute, University of Tokyo, Tokyo, Japan

Sagami Bay is located in the neighbourhood of Tokyo Bay and exposed to the Pacific Ocean. Onshore-offshore shift of the Kuroshio path influences the circulation and water exchange of the bay. The upper layer water of the bay is a combination of river influx, water from Tokyo bay and the offshore water.

During non-large meander period of the Kuroshio path, the circulation in the inner part of the Sagami Bay is weakened about one half to one tenth of that in the large meander period. And, intrusion of the Tokyo Bay water into the inner part of Sagami Bay is enhanced during non-large meander period of the Kuroshio path and has greater influence on the water quality.

To clarify relationship of the fluctuation of Kuroshio frontal wave induced by the Kuroshio path and circulation in the inner Sagami Bay, we made direct current observations in the inner part of the bay using three sets of mooring systems for about four months from April 1996 to August 1996. Mooring systems were set at east side, the center and west side in the inner part of the bay mouth. Current meters were fixed at about 180 m and 300 m depth.

As a result, short time variation of circulation in the inner Sagami bay was influenced by frontal wave of Kuroshio Path and angle of inflow of Kuroshio branch current from west and east channels in the bay. The circulation patterns are classified into four types viz. Anticlockwise type, clockwise type, outflow type and inflow type. Among all those circulation patterns, the anticlockwise type and the outflow type were dominant. Power spectral peak of the currents obtained at each station show 3 peaks i.e., a few days, 7 to 10 days and about 20 days. 20 days period is similar to that of the frontal wave of the Kuroshio.

ip11cc

Filament formation on the southern Benguela shelf

Greville Nelson

Sea Fisheries Research Institute, Roggebaai, South AfricA

The southern part of the Benguela system in the vicinity of Cape Town is characterised by short but intense wind episodes in the summer months, resulting in the formation of an upwelling tongue with a typical lifetime of four to six days. The wind also generates coastal-trapped waves (CTWs) both locally and at the Cape Columbine upwelling site one degree of latitude to the north. The cross-shelf barotropic component of current associated with CTWs propagating poleward from Cape Columbine is large and can significantly affect the configuration of the tongue. Variations range from a thin filament structure, turned inwards towards the coast, to an extension of an offshore tongue into a filament reaching out beyond the shelf edge. The filament has a thickness of about 50 m.

In the latter case, an anti-cyclonic eddy to the north over the Cape Canyon with a diameter of some 50 km appears to ensure the retroflection of the filament water back to the shelf. The eddy has been identified both by ADCP measurements and a thermohaline depression at its centre, in particular by a depression of salinity down to 400 m. A possible reason for the existence of this eddy is the generation of anti-cyclonic vorticity as poleward moving water enters the deep canyon. Current meters have detected a net poleward flow in the canyon.

Upwelling in the Cape Town region does not appear to contribute to the development of a seasonal upwelling front, as occurs on the wider shelf further north, particularly as in the case of the vastly more extensive Luderitz and Walvis Bay upwelling cells. Of greater import is the configuration of the filament as an indicator of upper ocean current structures. The frequently observed shelf-edge jet, reaching down to 250 m, is known to be the mechanism by which pelagic eggs and larvae are moved rapidly from the western Agulhas Bank to nursery areas north of Cape Columbine. The filament interrupts the upper 50 m of this flow, but does not necessarily cause a loss of biotic material.

The Lagrangian trajectories in the upper 50 m are complex and are sensitive to the phase relationships of wind episodes at the two upwelling sites near Cape Town and Cape Columbine which jointly determine the tongue configuration. Is the level of complexity too great to be deterministic?

ip11dd

Efficiency of boundary mixing on the continental slope

Scott A. Condie

CSIRO Division of Marine Research, Hobart, AUSTRALIA

When a stratified current moves over the continental slope, downslope Ekman pumping can generate strong vertical mixing, which continues until the Ekman flux is shutdown by opposing buoyancy forces. This increases the potential energy of the density field at the expense of the kinetic energy of the interior geostrophic flow. Over longer timescales, kinetic energy is also lost to viscous dissipation through slow diffusion processes. The mixing efficiency (ratio of increased potential energy to lost kinetic energy) has been estimated both theoretically and experimentally. Scaling is used to demonstrate that the efficiency is inversely proportional to the Rossby number R₀, and independent of both bottom slope and buoyancy frequency. This is supported by laboratory experiments, which yield mixing efficiencies of (0.042±0.009)R₀^-1 over the duration of the spindown event.

IP11ee

Sub-inertial variability of the Brazil Current at 22ƒS

Jose A. Lima and Jason H. Middleton

School of Mathematics, University of New South Wales, Sydney, AUSTRALIA

Hydrographic transects and current measurements of the Brazil Current in the southwestern Atlantic Ocean at 22 ƒS are presented. The observations were made from an array of six mooring positions extending from the shelf break at the 200-m isobath to the middle slope at the 2000-m isobath over the period of one year. CTD profiles provide an insight into the thermohaline structure of the site during the period of time that the moorings were deployed. The Brazil Current in this region has a shallow structure extending less then 450 m deep with an average transport lees than 10 Sv southward. Its shallow structure is complemented by the Brazil counter-current which flows northward below 500 m carrying both South Atlantic Central Water (SACW) and Antarctic Intermediate Water (AAIW). The Victoria-Trindade ridge disturbs the flow and the Brazil Current jet is perturbed downstream by meanders and eddies. Current variability at the continental slope appears to be due to a combination of effects, such as coastal trapped waves, local winds, and intrinsic instabilities of the Brazil Current itself.

IP11ff

A regional model of shelf circulation near bass strait: a new upwelling mechanism

Simon R. Evans and John F. Middleton

School of Mathematics, University of New South Wales, Sydney, NSW, AUSTRALIA

A study is made of the low frequency, 3-dimensional circulation and upwelling for the western region of Bass Strait using oceanographic data and the Princeton Ocean Model. A novel radiation condition, which simulates the effect of an eastern shelf on Kelvin wave scattering, is successfully tested against analytic solutions for Coastal-Trapped Waves (CTW) scattering and generation. Using a realistic bathymetry for the Bass Strait the model was forced with 8 day period zonal winds within the strait and a model CTW paddle at the north-western boundary.

Results demonstrate the existence of a new mechanism for upwelling whereby the residual poleward flow over the steeply sloping Tasmanian shelf separates from a gyre that develops over the more gently sloping topography near the mouth of the strait. The resultant divergence of the velocity field leads to large vertical velocities, (26 m/day), and a plume of sub-thermocline water that is upwelled by 80 m and drawn 60 km towards the strait. While upwelling driven by the CTW paddle is less significant, data from the region show that the phase of the forcing mechanisms is such as to enhance upwelling over the shelf slope.

To examine the circulation and upwelling under realistic conditions, the model is forced by observed winds and the CTW paddle modulated using low-pass filtered sea-level data. The results reproduce some of the 30 cm/s and 20 cm variability apparent in current and sea-level data obtained for the region. Moreover, the combined wind and CTW forcing is predicted to lead to upwelling rates of 36 m/day and the growth of an upwelled plume of sub-thermocline water which is displaced 120 m in strait.

IP11GG

The internal circulation produced by multiple localised regions of deep convection

Alan Wong and Ross W. Griffiths

Research School of Earth Sciences, The Australian National University, Canberra, AUSTRALIA

We investigate the circulation driven by two or more regions of deep convection in the oceans by simply treating the convecting regions as isolated turbulent plumes, each of which carries a buoyancy flux from the ocean surface to its spreading depth in the interior. In this idealised description, we begin with the non-rotating case. The theories for a single buoyant plume with turbulent entrainment from all levels above its spreading depth and for the so-called "filling box" mechanism by which a plume establishes a density stratification are extended to cover multiple sources of buoyancy. The source with the largest buoyancy flux produces the "bottom waters" of the basin. Its outflow supplies water to the whole basin through upwelling while at the same time sets up a circulation pattern consisting of a number of horizontal layers of water moving in alternating directions. Each of the sources with smaller buoyancy fluxes produces water that spreads at an intermediate depth. Numerical solutions and analytic approximations for large times show that this outflow depth is dependent on the 2/3-power of the ratio of the buoyancy flux from this source to that from the strongest source. The outflow from a plume, as it spreads laterally, is influenced by the circulation pattern generated by the dominant source. This circulation can enhance the spreading of the outflow into other regions of the basin at some depths or constrain spreading to areas near the plume's position at other depths. The results for this simple case form a basis for understanding the behaviour as the system is modified to include multiple basins and the effects of rotation, and may have implications for the transport of water properties and tracers in the oceans.

IP11hh

A RELOCATABLE ATMOSPHERIC - OCEAN MODELLING SYSTEM

Graeme D. Hubbert and Ruth Preller

Global Environmental Modelling Systems, AUSTRALIA
Naval Research Laboratory, Stennis Space Center, USA

Over the next three years Global Environmental Modelling Systems is developing a fully relocatable atmospheric - ocean modelling system in conjunction with the Naval Research Laboratory at the Stennis Space Center. The system, when completed, will incorporate a graphic user interface, a mouse driven grid generator, a mesoscale atmospheric model, an ocean thermodynamics analysis and an ocean model driven by winds, tides and thermodynamics. The system will draw upon global files of climatological and terrain type entities such as topography and bathymetry, tides, albedo, soil moistures, roughness length etc. with embedded high resolution regional data sets. On completion, this relocatable system will be used for basic research and real time forecasting. Coupling between the atmospheric and ocean mesoscale models will initially be only dynamical but full thermodynamical coupling will be investigated as part of the research program enabled on completion of the relocatable system.

The first version of the system was installed in October 1996. This version contained a mouse driven grid generator, a relocatable mesoscale atmospheric model and a barotropic three dimensional ocean model. Global climatological and terrain type files have been established but only a few embedded regions have been developed. This early version of the system is now being tested, with particular emphasis on verifying the relocatable mesoscale atmospheric model. The key aspects of the mesoscale model will be described and results of verifications for a number of locations around the world, including Australia and the USA will be presented.

The next upgrade, to be installed early in 1997, involves adaptation of the Princeton Ocean Model to make it fully relocatable and the inclusion of several improvements such as stable, relocatable open boundaries and tidal forcing. Details of these improvements will be given and results of testing against observations in the Yellow Sea will be presented.

IP11kk

The tectonic cause of the transformation the oceans streams

B. Kapochkin, V. Nagrebetsky and N. Kucherunko

Institute of Hydrometeorology, Odessa, UKRAINE.

In report consider the results the measurement of the seas streams on the shelf of the Black Sea on autumn 1990. The measurements were do by the 4 automatic bottom station. At the same time the stream were observe with the help of the drifters. In May-July 1990 this region was studying from Satellite. The tectonic and geological survey of this region we made studied on may 1990.

We establish the dependence between the direction of the tectonic fault and the change of the direct of the sea stream. We discover that the current change the direct near the sea fault. In the report prepared the explanation of the influence the oceans faults to oceans streams. Change the direct of the stream influence to the velocity of the stream. The change the velocity of the oceans current influence to condition of the transport particles.

IP11ll

The effects of the East Australian Current on near shore processes

Peter R. Oke and Jason H. Middleton

Mathematics Oceanography Laboratory, University of
New South Wales, Sydney, Australia

The internal, dynamical balances of nearshore processes in the presence of a strong, steady western boundary current on a narrow shelf will be discussed by considering the preliminary results of an experiment in October of 1996.

Observational data was collected from a shore normal array of current meter and thermistor string moorings on the continental shelf off the far north coast of New South Wales, north of Byron Bay. This is a region of strong East Australian Current activity over a very narrow continental shelf.

The relative importance of the East Australian Current and the wind in driving the nearshore processes, such as upwelling fronts, and surface mixed layer deepening will also be discussed.

IP11mm

Advection in coastal upwelling regions: The equatorward surface jet and the poleward undercurrent

Robert L. Smith, J.A. Barth, A. Huyer, P.M. Kosro
and S.D. Pierce

College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, USA

Equatorward surface jets and subsurface poleward undercurrents are ubiquitous features of coastal upwelling regions. Their alongshore continuity has been a matter of much speculation but few observations. Two studies have provided direct evidence of their transport and continuity off the North American west coast.

One study focused on the coastal upwelling front and equatorward coastal jet: Observations in May and August 1995 with CTDs on SeaSoar, ADCP, and satellite-tracked drifters, showed a persistent coastal upwelling front and a jet that transported coastal upwelling water across the continental margin as it evolved from a relatively small jet (0.5 Sv) over the shelf north of 43ƒN to a strong oceanic jet (3 Sv) south of 42ƒN. Drifters showed continuity of the equatorward jet over hundreds of kilometers as it separated from the shelf and meandered equatorward.

Another study, part of a NOAA/NMFS whiting survey in July and August 1995, made ADCP sections across the shelfbreak from 34ƒN to 51ƒN at about 20 km alongshore spacing. A strong equatorward surface jet was observed only north of 40ƒN, suggesting that the main equatorward jet was seaward of these sections (<100 km wide) by 40ƒN. A poleward undercurrent with width > 10 km, and thickness > 100 m at 200 m, was observed in 86 of the 110 sections; the mean poleward velocity was 20 cm/s. The data suggest continuity of the undercurrent over the entire latitudinal range, with a slight poleward weakening and deepening. Evidence of its continuity in time was provided by current meters at 38.5ƒN (August 1992 to June 1994) arrayed from the inner slope (410 m depth) to the abyssal plane (3650 m). The mean poleward flow was maximum over the inner slope (11 cm/s at 175 m) and decreased offshore with an e-folding width of 24 km.

IP11nn

The East Australian Current and wind-driven upwellings off central eastern Australia

George Cresswell and Jan Peterson

CSIRO Division of Marine Research, Hobart, AUSTRALIA

Observations with a research vessel, moored instruments, satellite radiometers, satellite tracked drifters, tide gauges and meteorological instruments were used in a study of the currents of central eastern Australia in late 1989. The East Australian Current (EAC) was a major influence and ran southward just beyond the shelf edge with its core at 50-100 m depth. In this core the speed steadily increased from 0.9 to 1. 6 ms^-1 between 27ƒ S and 29ƒ S. The nearshore edge of the EAC often reached in to the inner shelf and sometimes the coast. The inshore edge of the EAC commonly developed instabilities downstream of its separation from the shelf. Each commenced as a small meander to the west from which a warm plume reached back and around a growing cyclonic eddy in its wake. The instabilities formed roughly every 5 days, were carried southward at 0.3-0.5 m s^-1, and were separated by about 120 km. Individual satellite images showed several of them in various stages of growth.

A current meter at 90 m depth at the 200 m isobath showed that the EAC was certainly not steady, having steps lasting 1-2 months, plus variability at periods of several days. The current during the September 1989 to March 1990 ranged from 1.5 m s^-1 southward to 0.5 m s^-1 northward. The current decreased with depth so that at 130 m and 190 m depth on this mooring it was about 1/2 and 1/5 respectively of the 90 m value. Moorings across the shelf showed a progressive transition from domination by the EAC at the 200 m isobath to domination by local wind forcing. The current variability at the innermost mooring (at the 44 m isobath 7 km from shore) had a dominant period of ~1 week with amplitude ~0.25 m s^-1 and a subordinate period of 2-3 days with amplitude ~0.05 m s^-1. After late October, when summer stratification presumably overcame winter mixing, two effects could be observed: Downwelling-favourable winds (southerlies) raised coastal sea level and on the inner shelf they drove a northward current with an offshore component near the bottom. Temperature increased near the bottom. The opposite occurred with upwelling-favourable winds (northerlies). Growing and collapsing upwelling plumes could be seen in satellite images.

IP11oo

ocean circulation under the Amery Ice Shelf, East Antarctica: A new perspective

M.J.M. Williams, J. Determann, R. Gerdes and K. Grosfeld

Institute for Antarctic and Southern Ocean Studies and Antarctic Cooperative Research Centre, University of Tasmania, Hobart, AUSTRALIA
Alfred-Wegener-Institut fur Polar- und Meeresforschung, Bremerhaven, Germany

Ocean circulation under the Amery Ice Shelf in East Antarctica, is primarily driven by heat transfer between the ice shelf and the underlying ocean. The ice shelf thickness ranges from about 300 m at the ice front to 900 m at the grounding line, while the underlying water column is up to 700 m thick. The thickness of ice shelves presents a major obstacle to in situ observations, making numerical modelling the standard tool for studying these unusual parts of the ocean.

We have developed a three-dimensional ocean model adapted to this domain, allowing a more comprehensive picture of the circulation than in previous studies. We find that the predominant area of melting is in the southernmost section of the ocean cavity, where the ice shelf draft is deepest. The transition from this cavity to the northern part of the domain involves a steep decrease in ice shelf thickness as well as a rise in the bathymetry of about 200 m over a meridional distance of 40 km. Very cold Ice Shelf Water (ISW) that was formed by admixture of melt water in the deepest parts of the cavity accumulates south of this obstacle. Once past the obstacle the ISW flows into a large bathymetrically driven gyre, which dominates the circulation in the northern portion of the domain. Within the gyre the in situ freezing point at the ice ocean interface rises as the ice shelf draft shallows, allowing refreezing of the now supercooled ISW to occur. The patterns of marine ice formation from this refreezing are consistent with observations and offer an explanation to the long standing puzzle surrounding the thick layer of marine origin ice on the base of the Amery Ice Shelf.

ip11pp

On the Effects of Coastline Irregularities on Eastern Boundary Current Systems

Mary L. Batteen

Department of Oceanography, Naval Postgraduate School, Monterey, USA

A high resolution numerical model is used to study the effect of coastline irregularities in the wind-driven California Current system (CCS) and the Portugal Current system (PCS), and in the thermally driven Leeuwin Current system (LCS). In the CCS and PCS, irregularities in the coastline geometry are shown to be important for anchoring upwelling and filaments, as well as for enhancing the growth of meanders and eddies. Cyclonic eddies tend to form in the coastal indentations in the vicinity of capes, while anticyclonic eddies tend to form in the coastal indentations between capes. In the CCS, the region of Cape Blanco is identified as the location where the coastal, equatorward flow off Oregon leaves the coast to develop a meandering jet off California, while in the PCS, the coastal equatorward flow leaves the coast near Cabo de Sao Vicente, where the Portugal Current changes direction.

In the LCS, maximum current velocities occur at Cape Leeuwin, where the Leeuwin Current changes direction, while offshoots, anticyclonic meanders and eddies occur at preferred locations: in the vicinity of coastal indentations, and at Cape Leeuwin. Cold, cyclonic meanders and eddies form from the limbs of the warm, anticyclonic eddies, and propagate westward with the anticyclonic eddies as eddy pairs.

IP11qq

On the West Florida Continental Shelf Circulation

Huijun Yang, Robert H. Weisberg and Z. Li

Department of Marine Science, University of South Florida,
St. Petersburg, USA

The west Florida continental shelf region is unique among the US continental shelves because of its broad width, gentle slope and the influence by the Loop Current. A three-dimensional, primitive equation, numerical ocean model is being used in conjunction with an in-situ measurement program to investigate the circulation over the shelf. We have implemented two versions of the Princeton Ocean Model with rectilinear and curvlinear coordinate. Numerical experiments with monthly climatological wind stress forcing as well as synoptical wind forcing have been conducted. First, from the monthly climatological wind forcing experiments there emerge two basic patterns, a winter and a summer pattern. The results show an offshore cross-shelf surface transport in winter and an onshore cross-shelf surface transport in summer. Accordingly there is an upwelling in winter and a downwelling in summer near the coast. The results are compared with observations from recent in-situ measurements and drifter data with a reasonable agreement. Second, the results of a tropical storm (Josephine) surge hindcast are presented. The shelf circulation and sea surface elevation are driven by surface wind, achieved every three hours from high resolution atmospheric meso-ETA model assimilation. The model produces a realistic build up of storm surge. The results are compared with available data from tide gauge stations, moorings and drifters.

IP11ss

SUB-INERTIAL VARIATIONS OF THE ATLANTIC INFLOW TO THE NORWEGIAN SEA

Kjell Arild Orvik, Dystein Skagseth and Martin Mork

Geophysical Institute, University of Bergen, Allegaten,
Bergen, NORWAY

This study deals with variability of the Atlantic inflow to the Norwegian Sea based on long-term current measurements since April 1995 in the Sviny section. This section just to the north of the Faroe-Shetland Channel cuts through the core of the inflow where different branches merges through confluence in the shelf break. Our findings show that the Atlantic inflow occurs as a 30 km wide, nearly barotropic shelf edge current trapped over the steepest slope between depths of 200 m and 900 m. Transport estimates based on a linear regression model show an along-isobatic flow with strong variability between 0 Sv and more than 10 Sv. The fluctuations range from a few days to two months with a remarkable stable flow of 5-7 Sv on a seasonal basis, in contrast to the commonly accepted annual cycle. In this study we will concentrate on the observed oscillations with a few days period, showing properties as wind induced topographic Rossby waves. The transient velocity field will be correlated with air pressure differences between Iceland and Portugal according to the North Atlantic Oscillation (NAO) index. The results will be compared with relevant wave models and they will also make the basis for a modified model for trapped waves in the presence of a mean flow. Various wave phenomena will be discussed in the light of observations and theory.

IP11tt

Wind-Forcing and the Southern California Bight/Santa Barbara Channel Circulation

W.M.L. Morawitz and N.A. Bray

Center for Coastal Studies/Scripps Institution of Oceanography,
La Jolla, USA

The effects of wind-forcing on the Santa Barbara Channel and Southern California Bight circulation have been difficult to describe, due to the complicated coastline and bathymetry. We revisit the problem from the large scale perspective using the Calcofi data set, as well as from a Channel perspective, using moored instruments, historical hydrography, and meteorology within the channel.

From the large scale perspective, the implied Sverdrup transport from the wind-field along the south-central California coast is compared to the Ekman and geostrophic currents from the Calcofi data set. A mean of about 2 Sv. of poleward flow is observed in the inshore 200 km of the Southern California Bight, with seasonal variations of about 1 Sv. That poleward flow is consistent with the Sverdrup transport implied by the observed windstress curl over the Bight. On the other hand, the southward flow associated with the California Current cannot be explained by local Sverdrup dynamics, and we infer that it is advected into the Southern California Bight from the north.

Within the Santa Barbara Channel itself, 45 m temperature and salinity time-series, in conjunction with historical CTDs, are used to infer vertical motion within the channel. The dominant signal is seasonal. During times of strong upwelling-favorable winds in spring, upward vertical displacements on the order of 100m occur over most of the channel. Larger vertical displacements, in addition to a cyclonic circulation, are typically found in the western part of the channel relative to the eastern. Correlations between the alongshore winds and the windstress curl suggest that the vertical motions, and the cyclonic circulation, are related to the alongshore wind stress and wind-stress curl through upwelling and Ekman pumping dynamics.

IP11uu

Wind-Driven Shelf Circulation in the Northern Gulf of Mexico

Christopher N.K. Mooers and Renellys C. Perez

Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, USA

The continental shelf circulation along the northern boundary of the Gulf of Mexico is driven by vigorous atmospheric forcing, run-off from the Mississippi River system, and mesoscale eddies from the open Gulf impinging on the continental margin. The emphasis here is on the wind-driven circulation associated with seasonal, synoptic scale (weather systems), and mesoscale (diurnal sea breeze) forcing. The model output from a 10-year run of an OGCM [the sigma-level, stratified, free surface, primitive equations Princeton Ocean Model (POM)] with mesoscale resolution for the Gulf of Mexico is analyzed for a subdomain in the northern Gulf over a three-year period. To "qualify" the model output, comparisons are made of model output to observed current time series. The well-organized space-time structures in the model response to synoptic forcing are interpreted in terms of statistics and dynamics. Implications for the design of combined modeling and observational projects are summarized.

IP11vv

Mass Flux Events in the MAB

L.J. Pietrafesa, X.F. Li, L. Xie, C. Flagg and J. Churchill

Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, USA
Brookhaven National Laboratory, Upton, USA
Department of Physical Oceanography, WHOI, Woods Hole, USA

In the Mid Atlantic Bight (MAB), the mode of transport of bottom sediments along and across the continental margin, can occur via two separate modes. One, is that of a quasi- continual bottom flux of particles with occasional increases in the intensity of the volumetric flux due to some infrequent high energy event. Here, the departures from the mean might be individually significant but in the grand scheme, contribute little to the integrated, aggregate flux. Alternatively, there is the mass-wasting mode in which infrequent, but highly energetic events collectively constitute the suite of phenomena responsible for the observed flux and measured accumulations. The NE coast of the US is known for being the recipient of energetic wintertime atmospheric storms called "Atlantic Lows" or "Hatteras Lows" or "Cyclogenetic Bombs" or "Nor'easters". These storms form principally during the months of October through April in the region between South Carolina and Virginia and generally move across the Mid Atlantic Bight bringing high seas and heavy weather to the northeast of the US. These events occur on the order of 10 to 15 times per year and have been described by Cione et al. (1992). It is of note that from June-October, tropical cyclones often venture into the area, as well. These tropical depressions occasionally reach hurricane strength, and in general constitute the summertime counterpart to a Nor'easter. This study reports on a correlation between cyclogenetic atmospheric low pressure systems which were observed to propagate north to eastward by Cape Hatteras with current meter and the sediment trap data previously reported on by Biscaye and Anderson (1995). The results of our study strongly suggest a robust storm response of sediment motion in the MAB.

IP11ww

A numerical model of tides and tidal current in the Gulf of Tonkin

Tran Phuong Dong

Marine Hydro-Meteorological Center, Hanoi, VIETNAM

A numerical model (two horizontal dimensions, vertically integrated) is used to investigate tides and tidal current in the Gulf of Tonkin which is a part of the South China Sea. The domain includes the whole part of the Gulf and continental shelf together and extends from 16ƒ N to 22ƒ N latitude, and from 105ƒ E 30' to 112ƒE 00' longitude. The bathymetry was taken from SELTOPO data set at 5’ x 5’ intervals, so that the domain has 29 x 28 grid points.

The depths vary from 5 m along the northern coastline of Vietnam to 3000 m off the shelf. The diurnal lunar tide is modelled by forcing at the boundaries and the cotidal and corange lines compare well with observations. The resulting sea levels and currents in the Gulf are discussed in terms of their dynamics, and compared with what limited information is available.

IP11xx

Dynamics of storm surges

Roger Grimshaw and Yongming Tang

Department of Mathematics, Monash University, Australia
Bureau of Meteorology Research Centre, Melbourne, Australia

Intense atmospheric events such as tropical cyclones generate a correspondingly intense response in the coastal ocean, often signalled by a large storm surge. Here we review recent advances in the dynamical understanding of storm surges, emphasising the role of coastally-trapped waves in the generation and evolution of the ocean response to strong localized atmospheric forcing of the coastal ocean on sub-inertial time scales and meso-scale length scales. Based on numerical simulations of the depth-integrated shallow water equations supported by theoretical analyses we demonstrate that outside the immediate forcing region storm surges are mainly composed of low-mode shelf waves. Inside the forcing region the parametrization of bottom stress is a crucial issue and we will describe recent work on how near-shore enhancement of the bottom stress due to the presence of wind waves can significantly affect the predicted storm surge in the near-shore region. Our results will be illustrated by a simulation of the storm surge generated by Tropical Cyclone Jane as it crossed over the North-west coast of Australia in 1983.

IP11yy

Interactions Between the Gulf Stream and Intense Coastal Storms off the North Carolina Coast

Lian Xie, L.J. Pietrafesa and E. Bohm

Department of Marine, Earth and Atmospheric Sciences,
North Carolina State University, Raleigh, USA

Intense coastal weather systems such as landfalling hurricanes and winter storms are observed to interact strongly with the Loop Current in the Gulf of Mexico and the Gulf Stream off the East Coast of the United States. Although considerable knowledge has been gained on the response of the upper ocean to intense coastal storm systems in the Gulf of Mexico and the U.S. East Coast over the past decade, and how significant two-way interactions occur between the Gulf Stream and coastal storm systems these are still poorly understood. The active 1996 hurricane season along the North Carolina Coast and the severe coastal weather during the winter of 1993 and 1995 have provided a good opportunity to address the Gulf Stream-hurricane interaction problem. In this study, a rich source of satellite, radar and other data and the unique NCSU Coastal Coupled Model are used to investigate the mutual interactions between hurricanes or winter coastal storms and the Gulf Stream off the North Carolina Coast. We will also present a schematic, conceptual model describing various scenarios of Gulf Stream-coastal storm interaction.

IP11zz

Low-frequency trapped waves in the Frame Strait

Yoshie Kasjima and Harald Svendsen

The Norwegian College of Fisheries Science, University of Tromso, Tromso, NORWAY
Geophysical Institute, University of Bergen, Bergen, NORWAY

Low-frequency waves in the Fram Strait are explored using current and temperature measurements from selected depths (two or three) at seven moorings. The results of the study show that tidal oscillations related to trapped semidiurnal and diurnal internal waves generated on rough topography constitutes a considerable part of the variability of the circulation in the Strait. Topographic Rossby waves are also present. The influence of a possible sink in the icewater frictional layer is also discussed.