COASTAL AND SHELF PROCESSES (JOINT IAPSO/IABO)
Location: Watson Building G23 LTA
Location for Posters: Bridge Poynting/Watson
Wednesday 21 July AM
Presiding Chair: John Johnson (University of East Anglia, Norwich, UK)
PROCESSES AND CIRCULATION AT THE SHELF SLOPE (1)
Introduction 0900
John JOHNSON
P10/E/23-A3 Invited 0910
FORCED TRANSPORT ACROSS THE CONTINENTAL SLOPE
John HUTHNANCE (CCMS Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside L43 7RA, UK)
Ocean-shelf exchange is of interest as a component of the global cycling of nutrients and organic carbon (for example) and specifically for the fuelling of high primary production in shelf seas. Several ocean margin experiments concerned with this question have estimated significant cross-slope fluxes, especially down-slope export of particulate matter. However, large-scale flow across the continental slope is inhibited by geostrophy, and recent studies have suggested "shut-down" of the up- or down-slope Ekman transport below unforced along-slope flow.
External forcing appears to provide a means of resolving this apparent contradiction via sustenance of cross-slope circulation (in an up-welling or down-welling sense) including a bottom Ekman layer. The oceanic density field and associated pressure gradients, as well as winds, may provide such forcing. A parallel question is the form of boundary condition that the shelf and slope provide for ocean circulation.
These questions are explored with semi-analytic and fully prognostic numerical models. The models also indicate the importance of a non-uniform slope and of the coast which respectively aid and hinder cross-slope circulation.
P10/W/08-A3 0950
FLOW STRUCTURE AND SEASONALITY IN THE HEBRIDEAN SLOPE CURRENT
ALEJANDRO J. SOUZA (Centre for Coastal & Marine Sciences, Proudman Oceanographic Lab., Bdiston Observatory, Birkenhead L43 7RA, UK, email: ajso@ccms.ac.uk) John H. Simpson, M. Harikrishnan and J Malarkey (all at School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey LL59 5EY, UK).
We report on new determinations of the intensity, structure and variability of the slope current based on the 16 months of observations with ADCPs and conventional current meters on a cross-slope section at the Hebridean shelf edge during the SES programme.
After removal of the tidal signals, the mean flow over the upper slope is found to be closely parallel to the topography with speeds of ~20 cms-1. The flow extends down to depth of 500 m and is predominantly barotropic especially in winter when the flow is practically uniform between 350 m and the surface. In summer, there is a significant baroclinic component with a pronounced maximum in current at a depth of about 200 m but more than 80% of the kinetic energy is in the barotropic component. Flow in the core of the current is highly persistent with the Neumann's steadiness St > 0.8 in summer. The cross-slope profile of sea surface elevation, computed from the mean barotropic currents, shows a consistent relation to seabed topography through the seasonal cycle. Long term averages of the cross-slope components are generally small (~2cms-1) with some indication of persistent down-slope flow in the bottom Ekman layer.
Measurements with shipboard ADCP on sections at intervals along the slope show a high degree of continuity in the structure of the flow. The core of the flow appears to be related to a weak positive salinity anomaly and a depression of the 9.5_C isotherm near the shelf, but there is no strong correlation between the core of the slope-current and the core of the salinity anomaly. It is proposed that this may be due to differences in the cross-stream diffusion of salt and momentum which have different boundary conditions at the slope.
The observations suggest JEBAR as possible forcing mechanism for the slope current, but care should be brought to this assumption as other constrains, like homogeneous vorticity can produce a similar cross-shelf structure of the slope current.
P10/L/03-A3 1010
COASTAL CURRENTS DRIVEN BY WIND AND ALONGSHORE GRADIENT IN WINDCURL
DR LIE-YAUW OEY (Princeton University, Atmospheric & Oceanic Sci Program, Sayre Hall, Forrestal Campus, Princeton University, Princeton, NJ 08544, NJ 08544 USA. email: lyo@splash.princeton.edu)
When wind distribution along a coast is anisotropic, such that its cross-shore scale is smaller than its alongshore scale, the forced Kelvin wave consists of forcing by both the wind and alongshore gradient of the windcurl. The resulting evolution of the current on both the f-plane and beta-plane will be discussed, and ideas are applied to the observed seasonal variation of the coastal currents off the Southern California Bight, USA.
P10/W/09-A3 Invited 1050
TRANSPORT AND RECIRCULATION IN UPWELLING FILAMENTS
E.D. BARTON, E. Navarro Perez, R. Torres Almarza (all at School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Anglesey LL59 5EY, UK, Email oss041@bangor.ac.uk)
The offshore flow in upwelling filaments represents a plausible pathway for exchange between continental shelf and oceanic waters, mainly but not exclusively on the Eastern Boundaries of the oceans. However, compensating and possibly continuous return flows may result in no net offshore transport of nutrients or biogenic material. Two recurrent filaments, near 28N and 41N in the NW African and Iberian upwelling regimes, are examined to investigate the nature of the flow. One filament occurs in a relatively steady, possibly year round upwelling, while the other is located in a region of seasonal and intermittent upwelling. Both systems exhibit similar structure, despite having possibly different formation mechanisms, and both are associated with a well defined return flow on their equatorward flank. However, the time scales for circulation through the filament and return are significantly different in each location (7 and 20 days, respectively). This has clear implications for the transport of any non-conservative property advected around the system. In the NW African case, there is evidence that the filament interacts with offshore eddies generated by the Canaries archipelago to produce a net enrichment of island waters.
P10/W/12-A3 1130
MODELLING THE CAPE GHIR FILAMENT
Ian STEVENS and John Johnson (both at School of Mathematics, University of East Anglia, Norwich, NR4 7TJ,UK, email: i.stevens@uea.ac.uk)
Satellite imagery reveals the presence of a cool filament extending offshore from the coast of Morocco at Cape Ghir out into the Atlantic Ocean to the north of the Canary Islands. The filament appears to be a permanent feature with a seasonal variation in its zonal extent which is maximum in late summer.
A primitive equation model at 1/12° resolution covering the region between the Gulf of Cadiz and the Nort West African coast and including the Canary Islands has been constructed to investigate the generation mechanism of the filament.
The model reproduces seasonal variations in the surface temperature signal of the filament which compare well with the satellite observations, and also provides a description of the vertical structure of the hydrology and velocity field. Anticyclonic and cyclonic eddies trapped to the north and south of the Cape Ghir Plateau are produced in the model and these may play a role in the generation of the filament.
P10/W/05-A3 1150
STRATIFICATION PATTERNS AND PROCESSES ON THE INNER CONTINENTAL SHELF OF THE MIDDLE ATLANTIC BIGHT
John BRUBAKER (VIMS, School of Marine Science, College of William & Mary, Gloucester Point, Virginia, 23062, USA. Email: brubaker@vims.edu)
Patterns of density stratification and dynamic stability over the inner shelf in late summer to early autumn have been investigated in a field program in the Mid-Atlantic Bight off the coast of North Carolina. Temperature, salinity, and velocity data sets from repeated shipboard surveys over a grid of stations and from an array of moored instrumentation provided complementary perspectives. In this inner shelf environment, stratification was influenced significantly by variations both in temperature and in salinity. The relative influence was quantified through computation of the separate thermal and haline contributions to the potential energy anomaly integral, which compares the potential energy of the observed, stratified water column to the potential energy it would have if it were well mixed. In August, stratification at offshore distances < 10 km was due primarily to vertical salinity structure, whereas at distances > 15 km thermal influence was usually dominant. By October, the role of thermal structure had been reduced drastically. In addition to the vertically integrated energy measure for characterizing the water column as a whole, local stratification and stability were examined in terms of buoyancy frequency and gradient Richardson number estimates. Various spatial configurations were observed, with a thermocline and halocline sometimes distinct and sometimes merged. In August, localized velocity shear layers often coincided with sharply defined pycnoclines. With respect to Richardson number values, these stratified shear flows were usually stable, although the stability was often only marginal and some instances of likely instability in limited spatial regions were observed. In October, the regions of low Richardson number were more extensive. Key physical processes underlying the variability of stratification include fresh water influence from the Chesapeake Bay outflow and wind-driven currents, particularly at the synoptic scale on which upelling and downwelling circulation patterns redistributed the buoyancy associated with the Bay plume.
P10/E/10-A3 1210
OCEANIC INTERNAL SOLITARY WAVES:AN ANALYTICAL MODEL FOR KDV SOLITON PACKETS
John R. APEL (Global Ocean Associates, P.O. Box 12131, Silver Spring, MD 20853, USA; 1-301-460-7018 (V); 1-301-460-9775 (F); e-mail: globocen@erols.com
A new analytical model for oceanic internal solitons has been constructed that is based on (a) an approximate "dnoidal" solution to the Korteweg-De Vries wave equation, and (b) solutions to the Taylor-Goldstein equation for the vertical structure function. A finite-length wave packet results that has most of the salient properties of solitons generated at continental shelf breaks and oceanic sills: rank-ordered non-linear oscillations; a packet that propagates at speeds above the long-wave linear speed; increasing numbers of oscillations and packet lengths as time goes on; and a long-term depression of the pycnocline at the rear of the packet. The packet properties derive from imposition of a shocklike initial condition which evolves into a nonlinear undulatory internal bore that decays toward a linear state at its trailing edge. Input data include the vertical density and background velocity profiles, and specification of a numerical amplitude. The resultant solitons have properties that agree quantitatively with observed packets in the New York Bight and the Sulu Sea. The solution is extended to the full cycle of the internal tide by modeling the recovery of the "solibore" trailing edge to its equilibrium state over the semi-diurnal tidal cycle. The theory connects a wave number scale ko with the amplitude eta; from knowledge of the density and background velocity fields, and the variation of wavelengths within the packets, it is possible to construct the entire subsurface hydrodynamic field with the model. The wavelength variations come most conveniently from satellite imagery such as that from synthetic aperture radar. Thus from density data (either concurrent or historical) and a SAR image, one may arrive at reasonable estimates of the internal hydrodynamics. Predictive models can follow from the formulation. Based on a collection of images gathered from around the world, a global atlas of internal solitons is being prepared for publication.
Wednesday 21 July PM
Chair: Des Barton (University of North Wales, Bangor, UK)
PROCESSES AND CIRCULATION AT THE SHELF SLOPE (2)
P10/W/18-A3 Invited 1400
RECENT DEVELOPMENTS IN WIND-FORCED UPWELLING AND DOWNWELLING
John MIDDLETON (School of Mathematics, University of New South Wales, Sydney 2052, Australia, Email: john.middleton@unsw.edu.au)
A numerical study is made of the dynamics of the circulation that arises from forcing by a steady, uniform alongshore wind over a uniform zonal shelf in the southern hemisphere. Both upwelling and downwelling favourable winds are considered.
In the former case, the results show that most of the upwelling is confined to the region inshore of the shelfbreak. Over the slope and within the region of wind-forcing the circulation is found to be dominated by the growth of both an anticyclonic and a cyclonic eddy, an offshore flow, and downwelling. The results are fundamentally different to the upwelling described by 2-dimensional models or by 3-dimensional linear dynamics. Two new mechanisms related to the surface mixed layer are identified as being responsible for these features of the upwelling circulation. First, the offshore advection of alongshore momentum within the surface mixed layer (SML) is shown to accelerate the alongshore current near the eastern end of the region of wind-forcing leading to a sharpening of the cross-shelf gradient of sealevel, and an excess in the alongshore transport. This excess transport acts to raise sealevel, leads to downwelling below the base of the SML and the growth of the anticyclonic eddy. A simple scaling is given to show that the nonlinear effects will be important for narrow, strongly stratif ied shelves. A second important effect is argued to result from the geostrophic cross-shelf flow that is associated with the alongshore gradients of density within the SML.
The downwelling circulation obtained by simply reversing the sign of the wind stress is also examined. Over the first 10 days, when the system is largely linear, the circulation is simply the reverse of that found in the upwelling case. However, after this time, the downwelling and upwelling circulations are notably different, and the former is dominated by the development a bottom mixed layer (depth 70m) rather than the SML. Through thermal-wind shear, an Undercurrent (UC) over the slope evolves, the bottom Ekman transport becomes small and negative leading to the detachment of flow near the shelf break, localised spreading of isopycnals and further intensification of the UC. Finally, some recent modelling developments are presented for the seasonal circulation within the Great Australian Bight. The models are nested inside the Semtner-Chervin 1/4 degree output, and contain 150X60 cells in the horizontal with 40 sigma cells in the vertical.
P10/W/26-A3 1440
INTERANNUAL CHANGES IN THE EAST AUSTRALIAN CURRENT AS INFERRED FROM INFRARED SATELLITE IMAGERY
W.J. EMERY (CCAR Box 431, U. Colorado, Boulder, Co., 80309 USA) and J. Wilkin (Univ. of Auckland, SEMS, Tamaki Campus, Private Bag 92019, Auckland, New Zealand)
A five-year time series of infrared satellite images is used to compute surface currents using the Maximum Cross Correlation (MCC) technique in the area off southeast Australia. All pairs of images between 4 and 24 hours separation are used to compute surface currents. A relaxation step is added to the selection of the maximum correlation greatly reducing the errors in selecting the correct end point of the vectors. In addition next neighbor filtering can now be done in time as well as in space also improving the resulting vector velocity field. The need to prefilter cloudy pixels is a limiting factor in the application of the MCC technique. The MCC surface current fields are then to mapped using optimum interpolation (OI) constrained to yield a surface stream function resulting in a smooth surface current field dominated by eddies and variable mean flow. These OI current maps are used to compute Empirical Orthogonal Functions (EOFs) of the East Australian Current region. These EOFS characterize both seasonal and interannual variations in the surface currents. Wavenumber/frequency spectra are used to find the fundamental time and space scales characteristic of the East Australian Current region.
P10/L/06-A3 1500
THE RESPONSE OF SEMI-ENCLOSED SEAS TO SYNOPTIC ATMOSPHERIC FORCING
Christopher N.K. MOOERS, HeeSook Kang, and Lianmei Gao (OPEL/RSMAS,University of Miami,4600 Rickenbacker Cswy., Miami, FL33149-1098, USA, email:cmooers@rsmas.miami.edu,
hkang@rsmas.miami.edu, lgao@rsmas.miami.edu)
Semi-enclosed seas are under the influence of atmosheric forcing(wind stress,surface pressure,heating/cooling,and evaporation/precipitation)on a variety of space and time scales from mesoscale to synoptic scale to intra-seasonal,seasonal/annual,inter-annual,and so forth. Here, the primary interest is in the oceanic response to synoptic scale forcing. Because of the joint effects of Earth's rotation, ocean density stratification, and continental margin topography, the response of semi-enclosed seas to synoptic forcing can be rich, ranging from transient coastal upwelling or downwelling, to coastally trapped waves, storm surges, transient jets and fronts, transient surface and bottom mixed layers, convection/ventilation, and so forth.
Numerical simulations (using POM) of the response of the Japan(East)Sea to wintertime extratropical cyclone/cold front outbreaks, and the Intra-Americas Sea to summertime tropical cyclones, are used as examples. The synoptic atmospheric forcing (with mesoscale resolution) utilized is provided by relatively new sources; e.g., a spaceborne scatterometer (NSCAT) and a mesoscale atmospheric model(MM5). By comparing with the response to more conventional,lower-resolution atmospheric forcing fields, the sensitivity of semi-enclosed seas is illustrated. Conversely, the space-time-amplitude resolution and accuracy requirements of atmospheric forcing for quality simulations of semi-enclosed seas circulation are beginning to be better defined by these response studies.
Looking ahead, the new sources of synoptic atmospheric forcing inform- ation(with mesoscale resolution)offer the potential for driving simulation and predictive models of circulation in semi-enclosed seas with higher accuracy on a routine(operational)basis.
P10/E/17-A3 1520
CIRCULATION PATTERNS OVER THE PETER THE GREAT BAY SHELF AND ADJACENT JAPAN SEA AREA
Vladimir Ponomarev, and Olga TRUSENKOVA (both at Pacific Oceanological Institute, 43 Baltiyskaya, Vladivostok, 690041, Russia, e-mails archer@linkor.ru, ponomarev%dan86@poi.marine.su)
The dynamic processes associated with synoptic wind forcing and generation of topographic eddies and streamers over the Peter the Great Bay shelf and adjacent Japan Sea area are studied. The strong northern winds are typical for a cold season over the studied sea area. These winds generate upwelling and intensive water exchange between the Bay and open sea area. Satellite and CTD observations show the eddy structure of upwelling and synoptic scale multiple fronts over the shelf and continental slope. We simulated the circulation patterns in this area using the MHI numerical model developed by Naum Shapiro (Marine Hydrophysical Institute, Sebastopol, Ukraine, 1992-1998). It is the layered model based on the primitive equations and free surface boundary condition; temperature and salinity within layers and layer thickness are functions of time and horizontal coordinates. Interfaces between layers are allowed to bend up and down and layers themselves can degenerate by acquiring negligible thickness. This makes the MHI model an appropriate tool for simulation of synoptic scale vertical circulation. Numerical experiments were performed under forcing of synoptic north-western wind characterized by a strong jets directed along the axis of the Peter the Great Bay. Synoptic-scale dipole circulation patterns and streamers were obtained over the continental slope, associated with strong upwelling in the cyclonic eddy and downwelling in the anticyclonic one. Evolution of these structures and the Peter the Great current system was shown under alternative wind forcing.
P10/E/22-A3 1600
COASTAL PROCESSES INVOLVED IN THE FORMATION OF ANTARCTIC BOTTOM WATER
Ann-Marie M. WONG and Jason H. Middleton (School of Mathematics, University of New South Wales, Sydney, AUSTRALIA)
Antarctic Bottom Water (AABW) is the dense water mass found at the bottom of the ocean basins around the world. The major sources of AABW are the Weddell Sea and the Ross Sea, both of which are characterised by wide continental shelves and land barriers, confining the circulation of the water on the shelf so that it becomes very saline. This shelf water forms an integral part of AABW.
The region off the coast of Wilkes Land has been identified as a source of AABW. In the past this region was not considered as a significant source of AABW but more recent research suggests otherwise. Wilkes Land does not have the land barrier confining the circulation to the shelf which plays a significant role in the circulation in the Weddell Sea and Ross Sea. The region off the coast of Wilkes Land is characterised by deep depressions on the shelf, strong katabatic winds and polynyas which are believed to contribute to the formation of AABW.
A numerical modelling study of the coastal processes off Wilkes Land is being undertaken using the Princeton Ocean Model (POM). The model has been configured to capture the essential features of this region including the deep reverse sloping shelf, brine production, strong katabatic winds and coastal trapped waves. The effects of these factors on the formation of AABW are analysed from the model results.
P10/W/19-A3 1620
CHARACTERISTIC PATTERNS OF CIRCULATION OFF THE COAST OF CALIFORNIA
C. D. WINANT (Scripps Institute of Oceanography, Centre for Coastal Studies-0209, 9500 Gilman Drive, La Jolla, CA 92093-0209. USA. E-mail : cdw@coast.ucsd.edu)
The ocean circulation along the central and southern California coast is described in terms of a few characteristic flow patterns based on a five- year set of moored observations, drifter releases, and ship-borne surveys. The observations were carried out over the Pacific continental shelf north of Los Angeles for a distance of approximately 300 km. The characteristic patterns are similar to the synoptic patterns used by meteorologists to forecast weather. A cyclonic eddy located in the western end of the Santa Barbara channel is the most persistent feature of the observations and is present in all patterns. In spring and summer, when winds are persistently equatorward and upwelling favorable, the current pattern is equatorward as well. Whereas in the fall and winter, when the equatorward winds have relaxed, currents shift to poleward. Two indices of forcing, the local wind stress and the alongshore pressure gradient, appear to be useful predictors of the circulation pattern.
P10/W/17-A3 1640
SUMMER MARINE BOUNDARY LAYER DYNAMICS AROUND PT CONCEPTION, CALIFORNIA
Clive DORMAN (Center for Coastal Studies, ScrippsInstitution of Oceanography, University of California San Diego, La Jolla, CA 92092-0209, USA, email: cdorman@ucsd.eduORNIA)
A climatology is based upon a net of twenty four surface stations on land, islands, buoys and platforms around the Santa Barbara Channel/Santa Marina Basin in Southern California. Radar profiler and soundings sampled the upper air. Marine layer winds accelerate around Pt Conception to reach a maximum in the western mouth of the Santa Barbara Channel, then deaccelerate toward the east. This wind speed maxima is also one of the two monthly mean,summer wind speed maxima along the west coast of the United States. A Sea level pressure is lowest on the north side of the channel and on the eastern end. An air temperature inversion base in the Southern California area is lowest and strongest in the western mouth of the Santa Barbara Channel. Stratus clouds are a minimum in the western portion of the Channel. The low level atmospheric structure in the vicinity of the Santa Barbara Channel is explained as a transcritical expansion fan in the marine boundary layer.
P10/W/20-A3 1700
OBSERVATIONS AND SIMULATIONS OF THE CALIFORNIA CURRENT
Robert L. HANEY (Department of Meteorology, Naval Postgraduate School, Monterey, CA 93943-5114, USA, email: haneyrl@met.nps.navy.mil) David E. Dietrich (Center for Air Sea Technology, Mississippi State University, Stennis Space Center, MS 39529, USA, email:dietrich@nmia.com) Robert A. Hale (Department of Meteorology, Naval Postgraduate School, Monterey, CA 93943-5114, USA, email: hale@met.nps.navy.mil)
Major observational programs in the California Current (CC) during the last 15 years have greatly increased our knowledge of the quantitative nature of this important eastern boundary ocean current. In response to strong upwelling favorable winds in spring, a surface intensified baroclinic equatorward jet, with cold offshore-directed filaments and unstable frontal eddies, develops next to the coast. The jet moves offshore during summer and fall, and is replaced by a poleward Davidson Current next to the coast in winter. There is also a seasonal signal and limited offshore propagation (to only about 128W) of surface eddy kinetic energy (EKE), with the decrease of surface EKE west of 128W attributed to an unspecified dissipation process. To help interpret the new observations, long term simulations of the CC are carried out using the DieCAST regional model. In most of the simulations, the wind stress is prescribed from monthly climatology, while the buoyancy flux is computed by damping to the mean annual cycle of temperature and salinity at the surface. The simulations reproduce the main features of the annual cycle described above, however the decrease of surface EKE west of about 128W in the simulations is due to the vertical redistribution of EKE to the deep ocean, not dissipation. Adding a realistic wind enhancement offshore of each coastal headland produces both local and remote changes to the above annual cycle.
P10/W/29-A3 1720
USE OF MMS/SCRIPPS OCEANOGRAPHIC DATA FOR OIL SPILL RISK ANALYSIS ANDOIL SPILL RESPONSE IN THE SANTA BARBARA CHANNEL-SANTA MARIA BASIN AREA
David R. Browne
In 1991, the Minerals Management Service (MMS) entered into a Cooperative Agreement with the State of California-Scripps Institution of Oceanography (Scripps) to conduct physical oceanographic studies to determine potential environmental impacts of offshore oil and gas activities in the Southern California Bight. The result is the Santa Barbara Channel-Santa Maria Basin (SBC-SMB) Circulation Study in the primary area of active federal oil and gas leases. Field measurements extend from April, 1992, to November, 1999, with the objective of determining characteristic flow fields and their forcing mechanisms in the SBC-SMB area for MMS oil spill risk analysis and strategic planning purposes. Six characteristic surface circulation flow patterns identified through the SBC-SMB Circulation Study are discussed in terms of their relative impact on oil spill transport. This background knowledge of characteristic flow patterns, together with real-time acquisition of currents, winds, satellite imagery, and surface drifter tracks from the study’s field array is proving helpful to industry and agency tactical oil spill response efforts. During actual oil spills in the SBC-SMB area, real-time estimates of synoptic flow patterns have been used to predict, with reasonable accuracy, oil spill trajectories in this complicated oceanographic environment. Trajectory prediction performance as a result of using real-time and climactic data in past actual oil spills and oil spill drills is presented.
Thursday 22 July AM
Chair: John Middleton (University of New South Wales, Sydney, Australia)
Mesoscale circulation on the shelf (1)
P10/L/01-A4 0930
A CASE STUDY OF BORA-DRIVEN CURRENTS AND Po OUTFLOW SPREADNG OVER THE ADRIATIC SHELF (16-20 January 1987)
Gordona Beg PAKLAR (Institute of Oceanography and Fisheries, Set. I. Mestrovica 63, 21000 Split, Croatia, email beg@jadran.izor.hr), Vlad Isakov and Darko Koracin (both at Desert Research Institute, P.O. Box 60220, Reno, NV 89506-0220, USA, email:darko@dri.edu), Vassilliki Kourafalou (National Centre for Marine Research, Elliniko, Athens 166-04, Greece, email villy@erato.fl.ncmr.gr), Mirko Orlic (Andrija Mohorovicic Geophysical Institute, Faculty of Science, University of Zagreb, Horvatovac bb, 10000 Zagreb, Croatia, email orlic@olimp.irb.hr)
The bora over the Adriatic sea induces strong transient currents whose horizontal variability is mainly controlled by an orographically-induced wind-curl effect. Until now, numerical models with resolution fine enough to resolve the alongshore variability in the bora wind field were forced with suddenly applied climatological wind, and inertial effects were not considered. In the present study we use the space- and time-varying hourly winds simulated by Mesoscale Model 5 with 9 km resolution to analyze the bora wind episode of 16-20 January 1987 and to reproduce its influence on the sea with Princeton Ocean Model. Infrared (IR) satellite imaged taken after the long lasting bora event reveal a cyclonic gyre traced by the Po River cold water. Although the bora wind, being of the NE direction, may be expected to induce downwelling on the western Adriatic coast, due to its alongshore variability the bora instead drives the Po River water away from the coast. The Po River was introduced in our model as an extra volume in the continuity equation and, moreover, was assumed to have zero salinity and 5 degrees C lower temperature than the sea water. The Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature measured on 9 January was used to initialize the model, whereas temperature was assumed to be vertically uniform in accordance with some simultaneous bathythermograph measurements. AVHRR sea surface temperatures of 21 and 22 January were used to verify the model results. Initial salinity fields were obtained via a 10 days simulation, with the river discharge being the only forcing and neglecting the temperature changes. The results of the numerical simulations with space- and time-varying wing show current field resembling the cyclonic gyre visible on IR satellite images. Modeled sea surface temperatures show a smaller cyclonic gyre with lower temperature gradients than observed. In order to explain the discrepancy, in our final numerical simulations we explore a positive feedback mechanism: Po- influenced cold water enhance the stability of the overlaying atmosphere and locally reduce the wind stress which, in turn, reinforce the wind-curl effect over the Northern Adriatic.
P10/W/27-A4 0950
INTERNAL KELVIN WAVES OBSERVED AND MODELLED IN THE ZADAR AND PA_MAN CHANNELS (EAST ADRIATIC)
Ivica VILIBIC (Hydrographic Institute of the Republic of Croatia, Zrinsko Frankopanska 161, 21000 Split, Croatia, email: dhi-oco@dhi.tel.hr)
An experiment was organised during summer 1994 in the Zadar and Pa_man Channels (50 km long, 2-7 km wide, 8-50 m deep), placed in the inner Croatian waters. The experiment comprised 29 STD stations and 7 current-meter moorings (current-meters in the surface and bottom layers). Due to vertical stratification, various baroclinic processes were detected. An oscillation with 4-day period and 10 cm/s maximum current amplitude was extracted from the data, being parallel with the coastline and having bottom current which opposes to the surface one. Consequently, it is related to internal Kelvin waves, as the internal Rossby radius was a slightly lower than the channel width in its greater part. The waves were modelled in two ways: (1) using two-dimensional two-layer linear analytical model of a rectangular bay, imposing nodal line at the mouth and radiation condition on its shallowest part (head), and (2) using one-dimensional two-layer defant-like model of a bay which allowes for the variable topography. The first model successfully reproduced the phase changes in the channels, energy loss throughout the head and inclination of the current vector close to the mouth, while the second gives a more realistic along-channel distribution of the currents, being largest exactly where they are observed. The oscillation has a respectable horizontal transport for such small channel area (maximum about 5E8 m3/cycle), therefore, this should be taken into account, especially if the area is prone to the industrial pollution.
P10/E/02-A4 1010
THE CONTRIBUTION OF MIXING PROCESSES TO 3-D CIRCULATION IN THE SHALLOW ARABIAN GULF
CHERYL ANN BLAIN (Ocean Dynamics and Prediction Branch, Oceanography Division (Code 7322), Naval Research Laboratory, Stennis Space Center, MS 39529-5004, Email: blain@nrlssc.navy.mil)
Within the Arabian Gulf, mixing over the water column is achieved by way of three primary forcings: tides, winds, and evaporation. The contribution of each of these mechanisms to the 3-D, seasonal mean circulation is presented. Numerical simulations of the nonlinear, 3-D shallow water dynamics are achieved through application of the Dartmouth finite element coastal ocean model. This free surface, 3-D circulation model includes tidal dynamics, wind and buoyancy
driven flows, and level 2.5 advanced turbulence closure. An examination of mixing dynamics at both basin and localized scales is possible due to the variable grid resolution employed. Diagnostic (prescribed mass variable fields) and prognostic (with advection and mixing of the mass variables) computations in combination with the modularity of the model allow forcing mechanisms to be considered independently and together in this study of mixing over the shallow shelf water. Asymmetry of the shelf bathymetry between the bounding coasts of the Gulf lend additional complexity to the mixing and circulation patterns present.
P10/E/12-A4 1050
BOILS AND EDDIES IN TIDAL FLOWS
W.A.M.NIMMO SMITH, S.A.Thorpe and A.Graham (School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK.)
The processes which lead to dispersion by tidal flows in shelf seas are poorly understood, in spite of their great importance in the spread of oil spills and the distribution of sediment, phytoplankton blooms or larvae. Here we describe acoustic and visual observations of large ‘boils’, regions of local upwelling, in strong quasi-steady tidal currents in the well-mixed and unstratified southern North Sea. The diameter of the boils is about equal to the water depth and at any one time they affect about 25% of the water surface. The typical time for which their effect persists at the surface is at least 7 min. Comparison of these observations with measurements reported in rivers and laboratory channels, the evident presence of sediment in the boils seen at the surface, and the frequent bifurcation of sediment patches, together offer strong evidence that the boils originate as turbulent bursts and ejections from the strongly sheared boundary layer near the sea bed, and that eddies appear after the erupting water reaches the surface. Buoyant material (oil) is pushed radially outwards and accumulates into filaments around the edges of the boils. The acoustic signal from the boils is strongest principally around their up-wind edge, pointing to an accumulation or increased production of bubbles in a region of convergence with associated wave steepening and breaking along this edge. A simple calculation shows that the effect of the boils on lateral dispersion of buoyant material is consistent with other observations. The boils contribute to surface renewal, and may therefore enhance the fluxes of gases between the atmosphere and the unstratified shelf-seas. They produce a patchy structure in, for example, the colour of the water surface which may bias the measured average values of sea-surface parameters detected by satellite or other ‘remote’ sensors.
P10/W/13-A4 1110
OBSERVED TIDAL MODULATIONS IN THE EASTERN NORTH SEA.
Vibeke HUESS, Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen, Denmark. Email: vh@dmi.dk Ole B. Andersen, Kort- og Matrikelstyrelsen, Rentemestervej 8, DK-2400 Copenhagen, Denmark. Email: oa@kms.dk
During 1997 the GEOid and Sea level Of the North Atlantic Region (GEOSONAR) project deployed an off-shore tide gauge station at Horns Rev, situated 60 km west of Esbjerg at the Danish North Sea coast. The pressure gauge was employed for 8 months, before disappearing. At the harbour of Esbjerg tide gauges have been deployed for many years. The objective of establishing the pelagic tide gauge station in the eastern North Sea was to identify the influence on the existing on-shore tide gauges by the local effects such as wind set-up and wave reflections. Additionally the pelagic gauge was positioned close to a TOPEX/Poseidon crossover as well as a ERS satellite ground track, and the sea level information provided by the station is used to validate the altimetry data. The aim of this presentation is to investigate the tidal and non-tidal variations observed by the near coastal and the on-shore tide gauge, respectively. The tides in this region are influenced by non-linear tidal effects, due to the non-linear shallow water processes. Besides the higher harmonics with amplitudes up to 6 cm (M4), annual modulations of the major ocean tide constituents apparently due to interaction with the wind driven effects are observed. These annual modulations change the amplitude of the major astronomical constituents by up to 20 % in amplitude (K1), whereas the largest constituent (M2) is only modified by 5 % in amplitude.
P10/W/22-A4 1130
THREE-DIMENSIONAL MODELING OF TIDAL CURRENTS AND SEASONAL CIRCULATION ON THE EASTERN SCOTIAN SHELF
Guoqi HAN and John W. Loder (both at Coastal Ocean Sciences, Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2, Canada, Email: ghan@emerald.bio.dfo.ca)
Tidal currents and seasonal-mean circulation on the eastern Scotian Shelf are studied using a three-dimensional, nonlinear finite-element model with a level 2.5 turbulence closure scheme. Tidal currents are moderate over the outer shelf banks, and weak over the inner shelf except for the northeastern tip of the Shelf. The seasonal-mean circulation for four bimonthly periods (centered on February 1, May 1, August 1 and November 1) exhibits dominant and persistent southwestward nearshore and shelf-break currents, embedded with anticyclonic (cyclonic) gyres over outer-shelf banks (inner-shelf basins) and onshore/offshore exchange along cross-shelf trenches. The model transport of the nearshore and shelf-break currents shows prominent seasonal and alongshelf changes. The baroclinic component dominates the nearshore current, while the shelf-break current is sensitive to barotropic boundary inflows through Cabot Strait and on the southern Newfoundland Shelf. Tidal mixing plays an important role over the outer shallow banks, even though tidal rectification is generally weak in the entire region. The solutions are in approximate agreement with observed transports and currents for the primary flow features.
P10/L/05-A4 1150
INERTIAL OSCILLATIONS MEAR THE CRITICAL LATITUDE FOR DIUNAL RESONANCE
John SIMPSON, Tom Rippeth, (University of Wales, Bangor, School of Ocean Sciences, Menai Bridge, Anglesey, U.K., e-mail: j.h.simpson@bangor.ac.uk), Pat Hyder, Ian Lucas (Fugro Geos Ltd)
Oscillations at, or close to, the inertial frequency are widely observed in shelf seas where frictional damping is weak. In the vicinity of latitudes 30 deg. N and S, such motions may become significantly enhanced by a resonance in which the local inertial frequency coincides with that of diurnal forcing. Under these conditions, regular daily variations in wind stress tend to produce large anticyclonic motions which may extend throughout the water column as shown, for example in Craig (1989; Continental Shelf Research 9(4),343-358 and 9(11), 965-980)
In this contribution we shall examine new data sets from three low tidal energy locations close to the critical latitude including one in the southern hemisphere. The measurements have been made through the full depth of the water column with bottom mounted ADCP’s and, in one case using HF radar to measure surface currents. At each of the locations we shall identify the vertical structure of the flow and the response to forcing by windstress in the diurnal and inertial bands. Analytical and numerical models of increasing subtlety will be used to elucidate an explanation of the characteristic surface to bottom phase shift observed in all three cases.
In all three locations, we shall demonstrate that near-inertial motions make a major contribution to the total kinetic energy of the flow and must, therefore, be considered as an important candidate source for vertical mixing. Shear production of turbulent kinetic energy will be examined both directly from the observations and from the models of flow structure.
P10/W/02-A4 1210
A GENERALIZED SET OF EQUATIONS FOR COASTAL OCEAN CIRCULATION AND ITS APPLICATION TO THE EAST CHINA SEA
Shizuo FENG, Dexing WU, Hui Wang and Kai Wang (Institute of Physical Oceanography, Ocean University of Qingdao Qingdao, 266003, CHINA)
A turbulent closure three-dimensional nonlinear baroclinic coastal water system, in which tidal currents and a quasi-steady flow are dominant over the flow field and supposed to be of the same order of magnitude, is presented and examined. In view of dominant tidal currents, associated with tides as gravitational long waves, the scale analysis on the system reveals a small parameter characterizing and measuring the order of nonlinear convection, and using a perturbation approach with the small parameter the coastal water system is systematically treated. The inter-tidal transport equations for the tidally time-averaged concentration, temperature, salinity, turbulent kinetic energy and its dissipation rate are derived, in which the convective transport velocity is the sum of velocity of the dominant quasi-steady flow and the mass transport velocity and is termed a "generalized mass transport velocity". The generalized mass transport velocity as a solenoid can be viewed as a Eulerian field variable and the aggregate of such local velocities can be specified as a Eulerian field of flow of an incompressible fluid. A set of field equations governing the generalized mass transport velocity is derived and a turbulent closure problem for the coastal ocean circulation of which the velocity is embodied by the generalized mass transport velocity, nonlinearly coupled with the dominant tidal circulation through the eddy viscosity, is proposed and used to describe and understand the dynamics of the wind-, density- and tide-driven circulation and the inter-tidal transport processes. Based on the set of field equations, the nonlinear effects of the convection and the sea surface elevation of the quasi-steady flow on the circulation and, in particular, the dynamic mechanism of the nonlinear coupling between the tidal and the tidally time-averaged circulation are revealed and examined. Finally, a preliminary application of the proposed model to simulating the winter-time and the summer-time circulations in the East China Sea and to understanding their dynamics is briefly described and discussed.
Thursday 22 July PM
Presiding Chair: Chris MOOERS (University of Miami, Florida, USA)
MESOSCALE CIRCULATION ON THE SHELF (2)
P10/E/16-A4 1400
THE CONTRIBUTIONS OF NONLINEAR HYDRODYNAMIC PROCESSES TO WAVE-INDUCED NEARSHORE CIRCULATION
Mark Cobb (Sverdrup Technology, Inc. (ASGMS), MSAAP Building 9101, Door 136, Stennis Space Center, MS 39529, USA, Email: cobb@alaska.nrlssc.navy.mil) Cheryl Ann Blain (Ocean Dynamics and Prediction Branch, Oceanography Division (Code 7322), Naval Research Laboratory, Email: blain@nrlssc.navy.mil)
In the nearshore environment there are a number of nonlinear hydrodynamic processes (wave-current interactions, nonlinear bottom stress, advection, diffusion/dispersion, and tides) that play a significant role in determining the surface elevation and current velocity. A better understanding of nearshore circulation is crucial to problems concerning coastal zone management such as sediment transport, pollutant transport, water quality, and ship navigation. The contributions of each of these nonlinear processes are considered within the context of wave-induced nearshore flow. Three scenarios are investigated for the cases of a planar, colinear, and sinusoidal beach using a 2-dimensional finite element hydrodynamic model based on the shallow water equations, ADCIRC-2DDI. The three scenarios consist of applying a surface wave stress without wave-current interactions, including wave-current interactions through only the bottom stress, and examining both effects together. For each case, the nonlinear processes are analyzed both individually and collectively from offshore to nearshore and under varying wave conditions in order to determine their contribution to nearshore circulation. Within the context of this study, computed longshore currents were validated by comparison to measured values at Leadbetter Beach, CA in 1980.
P10/W/16-A4 1420
AN ASSESSMENT OF LOCAL COASTAL DYNAMICS OBSERVED WITH HIGH FREQUENCY RADAR
Colin Y. SHEN and Robert A. Fusina (Code 7250, Naval Research Laboratory, Washington, D. C. 20375 USA); Lynn K.Shay (Department of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149 USA)
An assessment is made of the dominant processes governing the circulation in a 30 km by 40 km area off the coast of Cape Hatteras, North Carolina, where surface current velocity vectors have been obtained using shore-based high frequency radars in the High Resolution Remote Sensing experiment in 1993. Although the currents in this region are constantly under the influence of winds and tides, frequent intrusion of energetic flows into the area, apparently mostly of Gulf Stream origin, is shown to have a strong influence on the current variability. Analysis of vorticity and divergence shows that the flow intruding into the area is strongly rotational with positive vorticity. This rotational motion is not in geostrophic balance with the surface pressure gradient, and an inertial oscillation is often excited as a result, with vorticity and divergence oscillating 90° out of phase. On average, the geostrophic imbalance generates a net surface flow divergence which, in turn, induces negative vorticity that more than offsets the positive vorticity advected into the region. However, vertical viscous transfer of positive vorticity from subsurface to surface is shown to be required to complete the surface vorticity balance; the same energetic cyclonic shear flows that intrude into the area are suggested as the source of the subsurface vorticity, as these flows of Gulf Stream origin are known to be subducted under buoyant shelf water.
P10/E/01-A4 1440
NEW DATA ON NONLINEAR INTERNAL WAVE EVOLUTION FROM OBSERVATIONS ON SHELF OF THE SEA OF JAPAN
Andrey SEREBRYANY (N.N. Andreyev Acoustics Institute, Moscow 117036, Russia, email aserebryany@glasnet.ru)
The process of internal tide propagation on a shelf leading to generation of soliton-like internal waves was investigated by means of special field experiment which we carried out in the Sea of Japan. During period of 24 hours research vessel with towed temperature line sensor performed 10 non-stop repeated runs along a section oriented across the shelf. These measurements were supplemented with CTD zond survey along the section made just before and after the towing and 12-hour observation by line temperature sensor in the point of the section on shallow water made from ancored vessel after the towing and the survey. On the basis of the measurements we obtained the picture describing internal wave field evolution on a shelf. It was observed that generation of intense internal wave trains took place in the shallow water zone at a distance of 5-6 km from the shore. Observed intense internal waves are soliton-like. They propagated toward the shore. The most significant fact we found was connected with an interesting phenomena of alternative displacement up and down of the internal wave train as a "hole body" by long-period internal movements on the thermocline. This process leads to the strengthening of nonlinearity of internal waves and can create for them a radically new nonlinear effect.
P10/E/14-A4 1500
A NESTED BASIN/COASTAL CIRCULATION MODEL FOR EAST ASIAN REGIONAL SEAS
Peter C. CHU, S.L. Lu, and Y.C. Chen
A nested basin/coastal model is developed to study the east Asian regional seas and shelf processes. The Princeton Ocean Model (POM) with 23 sigma levels conforming to a realistic bottom topography is use for the study. The horizontal resolution is 1 deg by 1 deg for the Pacific basin module, and 0.25 deg by 0.25 deg for the coastal module including JES. The open boundaries for the coastal module are 150 deg E and 15 deg S. The model simulates realistic circulation pattern and thermohaline structure. Comparison between the model results with observations is also given.
P10/P/01-A4 1520
THE KUROSHIO EAST OF TAIWAN AND IN THE EAST CHINA SEA IN SUMMER 1997*
Yaochu YUAN (Second Institute of Oceanography, SOA, P O Box 1207, Hangzhou, 310012, China, email: yuanyc@zgb.com.cn); Yonggang Liu and Jilan Su (both at Second Institute of Oceanography, , SOA, P O Box 1207, Hangzhou, 310012, China, email: sujil@zbg.com.cn); Arata Kaneko (Department of Environmental Sciences, Faculty of Engineering, Higashi-Hiroshima 739, Japan, email: akaneko@ipc.hiroshima-u.ac.jp)
On the basis of hydrographic data obtained from two cruises during June and July of 1997, a modified inverse method is used to compute the Kuroshio east of Taiwan and in the East China Sea, respectively. It is found that: 1) The net northward volume transport (VT) of the Kuroshio through Section K2 southeast of Taiwan is about 37.5x106 during July of 1997. In comparison with VT of the Kuroshio through Section K2 during October of 1995 and May-June of 1996, this value of VT of the Kuroshio decreases obviously, i.e.: VT of the Kuroshio southeast of Taiwan decreases obviously during 1997 stronger El-Nino year. 2) There is not a branch of the Kuroshio east of Taiwan to flow northeastward to the region east of Ryuku Island during July and of 1997. 3) During July of 1997 there is an anticyclonic eddy east of the Kuroshio and a countercurrent east of this anycyclonic eddy. 4) There is an anticyclonic recirculating hyre south of Miyako Island and southeast of Okinawa Island during July of 1997. The net northward VT of the Kuroshio through Section PN in the East China Sea is about 26.3x106m3/s during June of 1997. 5) The comparison between the above computed results and the ADCP observed currents show that they agree quite each other.
P10/E/05-A4 1600
VARIABILITY IN PCYNOCLINE DIFFUSIVITY IN THE SEASONALLY STRATIFIED SHELF SEAS OF WESTERN EUROPE.
T. P. RIPPETH (School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, LL59 5EY, UK. M. E. Inall, SAMS, CCMS Dunstaffnage Marine Laboratory, PO Box 3, Oban, Argyll, PA34 4AD, UK.) J. H. Simpson (School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, LL59 5EY, UK.) John Howarth (CCMS Proudman Oceanographic Laboratory, Bidston, Merseyside, L43 7RA, UK. Neil Fisher, School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, LL59 5EY, UK.)
The determination of vertical mixing rates in the stratified water column is one of the key objectives of shelf sea Physical Oceanography with important applications in many aspects of Biological Oceanography. Recent developments in turbulence probe technology have allowed direct estimation of the vertical profile of dissipation, from which it is possible to determine the eddy diffusivity Kz via the Osborne relation; Kz = 0.2 E / N^2 ; E= energy dissipation rate;
N= Brunt-Vaisala frequency
In this paper we shall present observations of the vertical structure of the dissipation of turbulent kinetic energy, made using a Fly Profiler, over 25 hour cycles, at a number of contrasting thermally-stratified sites in the shelf seas of north western Europe. The dissipation observed in the pcynocline region varies from 10-6 Wm-3 at a low energy site in the northern North Sea station, to 10-2 Wm-3 at a station located near to the continental shelf edge during a period of intense internal wave activity. These direct measurements of E are combined with N^2 based on T and S measurements also made by the FLY, to estimate the diffusivity Kz.
The variability of the pycnocline Kz and its dependence on spatial differences in wind and tidal stirring at the contrasting sites will be considered along with the question of whether mixing rates in the stratified regime respond significantly to the springs-neaps cycle in the tidal flow.
P10/E/21-A4 1620
CURRENT PROFILES DURING THE BREAKDOWN OF STRATIFICATION IN THE NORTH SEA
John HOWARTH (Centre for Coastal and Marine Sciences, Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside, L43 7RA, UK, email mjh@ccms.ac.uk)
Two sets of current and temperature profile measurements have been made in the northern North Sea during the breakdown of stratification between September and November, in 1991 and 1998. At the two sites which were close together, about 20 miles apart, tidal currents were weak and water depths over 100 m so that the surface and benthic boundary layers were initially well separated. During both sets of measurements the thermocline substantially weakened and deepened as a result of heat loss at the sea surface. There was no exchange of heat downwards across the thermocline and by the end in neither case had the water column become fully mixed. The first set of current measurements have been analysed in terms of barotropic dynamics, inertial currents and near surface shear. The inertial currents were first mode baroclinic and were well modelled by a two-layer local wind driven model. Shears across the thermocline of 0.02 /s over 10 m were measured. Near surface shear was confined at most to the top 25 m even during the largest storms and was correlated with the wind stress. The largest shear between 2 and 25 m below the surface was 0.01 /s. During the second set of measurements turbulence dissipation measurements were also made. An interesting contrast between the two sets is that for the first the water column was homohaline whereas for the second there was a surface to bed salinity difference of more than 0.5.
P10/W/06-A4 1640
SEASONAL EVOLUTION OF THE IRISH SEA COLD POOL GYRE
K.J. HORSBURGH and A.E. Hill (School of Ocean Sciences, University of Wales
Bangor, Menai Bridge, Anglesey. LL59 5EY U.K.)
An extensive cruise program in 1994,1995 and 1996 provided observations which describe the seasonal evolution of the three-dimensional density field in the western Irish Sea. A cold, dense pool flanked by strong, nearbed horizontal density gradients was present from May until October in 1995. The trajectories of 55 satellite-tracked Argos drifters demonstrated the existence of the cyclonic circulation pattern that constitutes the western Irish Sea gyre and defined the gyre's full spatial extent. Several distinct recirculation paths were observed and the implications for planktonic organisms of the seasonal variability in circulation is discussed. Drifter speeds were in good agreement with geostrophic calculations based on the density field. The dynamical significance of strong nearbed density gradients, or bottom fronts, is highlighted. The gyre is a special case of a class of intense, organised, density-driven flows in shelf seas and the ability of numerical models to properly simulate such baroclinic circulations is critical. The results of a primitive equation model are compared with the seasonal observations. The model reproduces well the spatial pattern of heating, the residual flows and furthermore provides some new dynamical insights into shelf sea frontal processes.
P10/E/08-A4 1700
FRONTS AND WATER MASSES IN THE NORTHERN SOUTH BRAZIL BIGHT
Belmiro M. CASTRO and Luiz B. Miranda (both at Oceanographic Institute, University of Sao Paulo, Praca do Oceanografico, 191, 05508-900, Sao Paulo, SP, Brazil, email: bmcastro@usp.br)
Data from seven consecutive seasonal (summer/winter) hydrographic cruises are used to study the seasonal stratification cycle in the northern part of the South Brazil (23.4S-23.7S). Three shelf regions have been identified: Inner Shelf (IS), Middle Shelf (MS) and Outer Shelf (OS).
The IS outer limit changes seasonally: it reaches the 20-40 m isobath during summer (10-30 km from the coast) and the 50-70 m isobath during winter (40-80 km from the coast). The IS Coastal Water is separated from the lower layer MS South Atlantic Central Water (SACW) by a bottom thermal front. The presence of the SACW is associated with intrusions towards the coast of slope waters. The intrusion scale is larger during summer than during winter due to seasonal changes in the wind forcing. The MS extends offshore to the 70-90 isobaths (60-80 km from the coast) during summer, being very narrow during winter. The main characteristic of the MS is the high stratification during summer, due to the presence of a seasonal thermocline. Estimates for the thermodynamical and dynamical processes able to vertically mix the water column show that were not for the SACW intrusions during summer the MS would be almost homogeneous, not presenting the dominant seasonal thermocline. The MS is separated from the OS by a surface saline front which is located in the transition region between two upper layer water masses: the MS Coastal Water and the Tropical Water. The latter is one of the two water masses transported by the Brazil Current in the region, the other being the lower layer SACW. The OS extends from the surface saline front to the shelf break. Stratification in the OS does not vary much during the year, being a thermocline present all the time.
P10/E/15-A4 1720
THE INFLUENCE OF PATOS-MIRIM LAGOON DISCHARGE ON THE THERMOHALINE REGIME OF THE SOUTH BRAZILIAN SHELF
Peter O. ZAVIALOV (Shirshov Institute of Oceanology, Moscow, Russia, email zavialov@mi.ras.ru) Osmar O. Moller Jr (University of Rio Grande, Rio Grande, Brazil, email osmar@calvin.ocfis.furg.br)
Patos-Mirim lagoon is a large choked freshwater lagoon connected to Southwestern Atlantic through a narrow and relatively deep Rio Grande channel. It is known that the discharge from the lagoon contributes significantly into the regional heat and mass budgets (e.g. Zavialov et al., 1998, J. Phys. Oceanogr., 28, 545-562) and is among principal factors controlling the thermohaline regime of the southern Brazilian shelf. However, the Patos-Mirim plume has never been mapped in detail. A helicopter/ship CTD survey of the inner shelf south of the estuary was conducted in September, 1998. Profiling was performed simultaneosly by the helicopter and the Brazilian R/V "Comandante Varela". The total number of CTD stations was 21. The collected data revealed extremely stable salinity-controlled stratfication within the plume which largely suppressed vertical mixing. Despite of the initial southward momentum of the plume waters, south of the estuary the thermohaline signature of the plume is observable only within a distance of 5-15 km from the and separates from the coast. Thus the hypothesis that relates low salinity values observed on the inner southern Brazilian shelf south of Rio Grande to Patos-Mirim discharge is not supported by our data. Rather, this low salinity belt can be related to Plata river discharge transported by a northward coastal current.
Analysis of local historical data revealed anomalously strong secular warming trend of SST (up to 1.6 degrees C per 100 years) in the area adjacent to the estuary. This may be related to a human impact upon the freshwater discharge from the lagoon.
Friday 23 July AM
Presiding Chair: Juergen SUNDERMANN (University of Hamburg, Germany)
FLUXES OF MATTER AND PRIMARY PRODUCTION (1)
P10/W/03-A5 Invited 0850
SIMPLIFYING PELAGIC BIOLOGY FOR COUPLED MODELS
TETT, P.1, Wild-Allen, K.1 & Wilson, H.2,1 1. Department of Biological Sciences, Napier University, Edinburgh, U.K. 2. School of Ocean Sciences, University of Wales, Bangor, U.K.
Marine pelagic ecosystems contain many hundreds of species and very large numbers of individuals. Although the life of each individual and the dynamics of each population can be described by a small set of rules, marine ecologists have achieved no consensus about a way to model system dynamics. There is no ecological equivalent of physical oceanographers' basic hydrodynamic equations. Nethertheless, models must simplify the complexity of real marine pelagic ecosystems. But how much? A series of biological models with increasing numbers of state variables will be used to discuss necessary complexity and to illustrate biogeochemical (element-conserving) and ecological (semi-freely dynamically interacting) models of the marine microplankton, defined as all organisms less than 200 µm. One end-point of the series is a model with two microplankton compartments, which is able to represent the annual sequence from Spring, diatom-dominated plankton to a Summer Microbial Loop community. This model has been coupled to a depth-resolving physical framework which includes deposition to and resuspension from the sea-bed. Simulations will be shown for sites in the North Sea, and compared with observations.
P10/W/23-A5 0930
A COUPLED PHYSICAL-BIOLOGICAL MODEL FOR THE SEASONAL CYCLES OF CHLOROPHYLL AND NUTRIENTS IN THE FIRTH OF CLYDE.
JAE-YOUNG LEE and Paul Tett (Department of Biological Sciences, Napier University, 10 Colinton Road, Edinburgh EH10 5DT, UK, Email: j.lee@napier.ac.uk and p.tett@napier.ac.uk)
The Firth of Clyde, located on the west coast of Scotland, receives anthropogenic nutrients from the discharge of the Clyde and other rivers draining much of agricultural and industrial western central Scotland. As a result of these discharges, the Firth of Clyde is enriched in nutrients from agricultural fertilisers and domestic sewage.
A coupled physical-biological model has been constructed to aid understanding of the effects of these nutrients on the biota and oxygen demand. The physical model consists of a set of compartments with turbulent and advective exchanges driven dynamically by density differences and supplies of turbulent kinetic energy. It is forced by freshwater discharge to the Firth of Clyde, meteorology, and boundary conditions in the North Channel. The model was used to quantify the effect of various physical processes during the seasonal cycle, and extended to construct a budget of nutrients for the Firth.
The biological model divides organic particulates into microplankton (phytoplankton plus bacteria and protozoa) and detritus. It has two microplankton compartments, a diatom dominated (or 'diatomy') compartment and a small-cell dominated (or a 'flagellatey') compartment. It cycles carbon, nitrogen and silicon through microplankton, defecated-detritus, phyto-detritus, mesozooplankton and dissolved nutrients compartments.
Climatologically- and meteorologically-forced simulations of several seasonal cycles were compared with historic data, for dissolved oxygen, dissolved nutrients and chlorophyll, in the Firth of Clyde with the aim of using the model as a tool to predict the effects of the perturbations induced by Man or Nature. The importance of coagulation processes for the termination of diatom spring-bloom in the region has been investigated.
P10/W/15-A5 0950
MODELLING THE AGGREGATION AND SEDIEMNTATION OF PLANKTON
WILD-ALLEN, Karen (Dept. of Biologial Science, Napier University, Edinburgh, UK, Email: k.wild-allen@napier.ac.uk)
Variations in aggregation parameters were found to influence the functional group dominance of a mixed population of marine microplankton in a model system. The model cycles organic carbon, nitrogen and silica through microplankton, detritus and dissolved nutrient pools in a 1-D depth-resolving framework forced by turbulent diffusivity. The microplankton are modelled as two mixed populations of pelagic micro-organisms (<200 µm) which include photo-autotrophic micro-algae, cyanobacteria and heterotrophic bacteria and protozoa. One population is dominated by diatoms and includes silica dynamics, and the other by flagellates. This formulation allows a full range of pelagic micro-organisms to be modelled in a computationally efficient manner. Detritus is generated as a by-product of zooplankton grazing and by aggregation of diatom-type microplankton into phytodetritus which sinks rapidly (100m/day). Seasonal cycles of biomass show Spring and Autumn blooms with a shift from diatom to flagellate dominated communities as aggregation parameters increase. At higher rates of aggregation the Autumn bloom is larger and increasing amounts of nutrients in the euphotic layer remain unused. These results demonstrate the influence of aggregation processes in all parts of the pelagic system. This is important in relation to change in species balance in eutrophication, and in the export of organic material from the oceans surface.
P10/W/28-A5 1010
THE ROLE OF TURBULENCE IN DRIVING PRIMARY PRODUCTION AT SHELF SEA FRONTS.
Jonathan SHARPLES, (Southampton University School of Ocean and Earth Science, Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK, email j.sharples@soc.soton.ac.uk)
Shelf sea, or tidal mixing, fronts mark the physical transition between mixed and stratified water columns during summer in temperate shelf seas. These fronts are also known to be regions of enhanced primary productivity compared to the mixed and stratified waters on either side. This productivity requires an input of new nutrient (nitrogen) into the photic zone at the front, but the mechanisms behind this nutrient supply are not yet well understood. A simple model of the physical structure of a front, using a turbulence closure scheme to describe the temporal and spatial variability of vertical turbulent exchange, has been coupled to a model of primary production to simulate possible nutrient supply mechanisms. Two processes are investigated: nitrogen supply through the weakening stratification associated with the front, and spring-neap variability of frontal position leading to a fortnightly replenishment of surface nitrogen within the transition zone. The basic result shows that consideration solely of the cross-frontal change in vertical turbulent mixing is sufficient to explain typical observations of phytoplankton biomass, with both surface frontal and sub- surface thermocline concentrations being predicted. The addition of spring-neap tidal variability results in fortnightly pulses of frontal productivity and biomass, again similar to available observations. The model results are then used to further investigate nitrogen fluxes, nitrogen uptake, and phytoplankton growth rates within the front, leading to two important conclusions. First, for a phytoplankton with a deep critical depth, the vertical flux of nitrogen into the frontal photic zone is dominated by the flux of algal nitrogen rather than the direct physical mixing of dissolved inorganic nitrogen. This could help explain why recent estimates of dissolved nitrogen fluxes at a front are less than the estimated nitrogen requirements of the frontal productivity. Second, the total annual production at a front is altered significantly when spring- neap variability in the tidal mixing is included. For a phytoplankton with a deep critical depth, total production is predicted to be 20% less with a spring-neap cycle. For a phytoplankton with a shallow critical depth, total annual production with a spring-neap cycle increases by 50%. Consequences of these results for the design of experiments aiming to quantify annual production in shelf waters will be discussed. The conclusions can also be applied to ocean fronts in general, rather than being limited to tidal mixing fronts in shelf seas.
P10/E/06-A5 1050
HORIZONTAL TRANSPORT OF MARINE ORGANISMS RESULTING FROM INTERACTION BETWEEN DIEL VERTICAL MIGRATION AND TIDLA CURRENTS OFF THE WEST COAST OF VANCOUVER ISLAND
C.L. SMITH (Institute of Ocean Sciences, Sidney, B.C., Canada, email smithcl@dfo-mpo.gc.ca)A.E. Hill (School of Ocean Sciences, University of Wales, Bangor, Wales, U.K.) M.G.G. Foreman (Institute of Ocean Sciences, Sidney, B.C., Canada)M.A. Peña (Institute of Ocean Sciences, Sidney, B.C., Canada)
This paper examines horizontal transport of marine organisms, resulting from the interaction between diel vertical migration (DVM) and numerically generated tidal and buoyancy currents for the continental shelf and slope off Vancouver Island, Canada. DVM behaviour is simulated by migrating particles which are in the surface layer at night and near the bottom during the day. Eight tidal constituents, the tidal residual and typical summer buoyancy flows are calculated for the region using a 3-D barotropic finite element model. The interaction of these tides with migrating particles is investigated using the finite element model and Lagrangian particle tracking techniques in spatially heterogeneous tidal and buoyancy flows. DVM-tidal interaction alters the horizontal distance particles travel compared to non-migrating particles. We show that the K1 tidal constituent dominates transport patterns, producing regions of convergence and divergence, and on and offshelf transport along the continental shelf. During April and May, particles are transported from offshore onto the southern shelf and into Juan de Fuca Strait. From June to October, this pattern is reversed and there is a general offshore migration of particles. As the K1-DVM beat period is exactly 1 year, these patterns are reversed 6 months later, and do not change from year to year. When a typical summer buoyancy flow is included, the effect of DVM-tidal interactions is reduced, with buoyancy flow dominating transport of particles over most of the domain. However, cross shelf transport resulting mostly from K1-DVM interaction is shown to be important in the southern shelf in the La Perouse Bank area. This process may provide a partial explanation of observed changes in seasonal and interannual zooplankton biomass in the region.
P10/E/09-A5 1110
STARTING OF PHYTOPLANKTON SPRING BLOOM IN THE NORTHERN NORTH SEA
(FLADENGROUND)
DILEC H.TOPCU, Birgit Mieding, Uwe H.Brockmann, Venugopalan Ittekkot (all at University of Hamburg, Centre for Marine and Climate Research, Inst. for Biogeochemistry and Marine Chemistry, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany, e-mail:fg8z001@public.uni-hamburg.de)
In the Fladenground in 1976 the start and development of the phytoplankton spring bloom was studied in the frame of an international project (FLEX'76). In March/April 1996 the starting conditions of the blooming were investigated again. Along a transect from the German Bight to the Fladenground different blooming situations were found: blooming in the shallow German Bight and nutrient reduced water masses originating from the continuous production on the Doggerbank, interrupted by vertically mixed water masses without blooming indications. Only in the Fladenground chlorophyll maxima, reaching 0.15 (g/L, developed in depths between 10 and 40 m, showing during a 4 days lasting drift experiment diurnal variation, coupled with a weak vertical temperature gradient of 0.01 to 0.40C, and decreases of nitrate and silicate, and increase of particulate carbon and phosphorus, relecting the start of net production and the beginning of phytoplankton spring bloom. During this situation also a fluctuation of dissolved total amino acids (0.05 to 4 (mol/L) and to some degree also of DON (3 to 8 (mol/L) was observed, indicating the modification of water chemistry right up from the beginning of spring bloom.
P10/E/07-A5 1130
CONVERSION OF NUTRIENT ELEMENTS N AND P IN THE ELBE RIVER PLUME, GERMAN BIGHT
UWE H.BROCKMANN, Thomas Raabe, Andreas Starke (all at University of Hamburg, Centre for Marine and Climate Research, Inst. For Biogeochemistry and Marine Chemistry, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany, e-mail:brockmann@rrz.uni-hamburg.de)
In the German Bight within the propagating river plume of the Elbe during summer 1994 and spring 1995 drift experiments were performed in order to study the conversion of nutrients, dissolved and particulate organic matter. In spite of moderate changes of the water masses, marked by the drifter, the succession of nutrient uptake, formation of particulate matter in the mixed layer and modification of the dissolved organic fraction of N and P could be followed. The drift experiments lasted 9 and 10 days and were accompanied by repeated investigations of the surrounding area. By this, the extension of the river plume and the regional representativity of the development in the drifting field could be identified. During spring at the central drift station nitrate decreased with 2.5 (mol/l d, partly compensated by an increase of 1.8 (mol/l d PN, phosphate decreased by 0.05 (mol/l d coupled with an increase of 0.03 (mol/l d of PP. Due to the accelerated remineralisation during summer there was a net nitrate decrease of 0.04 (mol/l d observed but an PN increase of 0.11 (mol/l d. Phosphate decrease (0.02 (mol/l d) was larger than the PP increase (0.002 (mol/l d) at this time. However, in the bottom water increases of nutrients were observed, especially for phosphate. During summer nutrient concentrations in the bottom layer were higher than in the mixed layer, reflecting the more important regeneration mode during this season. Mainly during spring the conversion rates at the drift station in the river plume corresponded to overall net conversion in the German Bight.
Friday 23 July PM
Presiding Chair: Paul Tett (Napier University, Edinburgh, UK)
Current Poster Session
FLUXES OF MATTER AND PRIMARY PRODUCTION (2)
P10/E/03-A5 1400
DYNAMICS OF FINE SEDIMENTS IN SHELF SEAS: OBSERVATIONS, PROCESSES AND MODELLING
Juergen Suendermann (Zentrum fuer Meeres- und Klimaforschung, Universitaet Hamburg, Troplowitzstr. 7, 22529 Hamburg, Germany, email: suendermann@ifm.uni-hamburg.de
The global change of our environment is a consequence of variable matter fluxes between land and ocean. An essential part of these fluxes is represented by the transport of suspended particulate matter (SPM). SPM consists of anorganic and organic constituents, and its dynamics is determined by physical and biological processes and their interactions. Field observations show a distinct dependency of SPM concentrations on currents and waves as well as on input from rivers and land. The key processes of SPM transport are sinking, deposition and resuspension. They are controlled by the current regime (advection, turbulence, orbital motion) and by biological factors as aggregation, filtration or bioturbation. A high-resolving 3D Lagrangean tracer model will be presented containing all these components. The time scale covered ranges from hours to years. The model is applied to the North Sea, to the Bohai Sea (China), to the German Bight and to the river Elbe. The numerical results are discussed and interpreted against observational data of SPM concentrations and bottom sediments.
P10/W/01-A5 1420
APPLICATION OF A 3-D HYDRODYNAMIC-WATER QUALITY COUPLED MODEL TO KOREAN WATERS IN THE YELLOW SEA
CHANG S. KIM (Korea Ocean R&D Institute, Coastal Engineering Division, 1270 Sadong, Ansan, 425-170, South Korea. Email: surfkim@kordi.re.kr)
A 3-D hydrodynamic-water quality coupled model is used to simulate the impact of river discharge, nutrient loads and dynamic forcings on the variability of coastal waters. The model consists mainly of two modules; hydrodynamics and water quality parts. Both modules are directly coupled in computation scheme. The hydrodynamic model is originated from the Blumberg-Mellor algorithm and extended at Virginia Institute of Marine Science, USA to include further processes such as tidal flat, etc. The water quality module has been adapted from the CE-QUAL-ICM developed by the WES of US Army Corps of Engineers. The module deals with 25 state variables including temperature, salinity, DO, nitrogen, phosphate, suspended matter, etc. The coupled model has been applied to the waters of Kyunggi Bay, where rivers flow in with terrain nutrient loads. The model uses the curvilinear grid system with grid size ranging from 200 m to 9 km. The study area shows the maximum tidal range of approximately 11 m and wide tidal flats. This study shows the result of the model implementation for the water quality prediction in macro-tidal coastal and shelf waters off Korea.
P10/E/13-A5 1440
PROCESSES CONTROLLING THE DISTRIBUTION OF SOLID RUN-OFF OF THE TUMANNAYA RIVER (PETER THE GREAT BAY, JAPAN SEA).
Alexander Moshchenko, Konstantin Feldman ( both at Institute of Marine Biology, Russian Academy of Sciences, Far-Eastern Branch, Palchevskogo str., 17, 690041 Vladivostok, Russia, email: inmarbio@mail.primorye.ru).
Gennady YURASOV, Nikolay Vanin ( both at Pacific Oceanological Institute, Russian Academy of Sciences, Far-Eastern Branch, Baltiyskaya Str., 43, 690041 Vladivostok, Russia, email: pacific@online.marine.su)
The region studied extent is 30 km offshore and 35 km northward from the mouth of the Tumannaya River. It includes the territory of the Far Eastern State Marine Reserve. Presumable economical development of the Tumannaya River mouth area requires to study how to minimize possible anthropogenic impact on environs. That's why the processes defining natural sedimentation in the coastal area are to be important for understanding of possible unfavorable consequences for the region. Bottom sediments of the region studied are presented by a complex of terrigenic deposits from coarse sands to aleurites-pelites. The content of fine fractions varies from 1 to 45%. Their distribution have distinctive contagious character. Along with that the spots of aleuro-pelites prevailing are found closely to the mouth of the Tumannaya River and at 20-25 km off north-eastward. Results of numerical modeling of circulation on a basis of shallow water equations and instrumental observations reflect in whole the features of sediments distribution. Due to monsoon climate the maximal values of solid run-off of the Tumannaya River are observed during summer period, when the winds of the southern routes are prevailed. The schemes of circulation are defined mainly by the bottom relief features and the wind direction. For the south-western winds the transportation of suspensions and pollutants occurs north-eastward along the coastline. During the south-eastern winds an anticyclonic eddy is formed over the northern part of the region studied that also produces favorable conditions for sedimentation northward from the mouth of the Tumannaya River. But most unfavorable consequences for Marine Reserve should be expected in summer season during moderate and strong south-western wind.
P10/E/18-A5 1500
INFLUENCE OF CLIMATIC AND OCEANOGRAPHIC PROCESSES ON FORMATION OF COASTAL SUBWATER LANDSCAPES AND CONDITION OF BENTHOS ( BLACK SEA)
Alexei PETROV ,Vladimir Shalyapin and Sergey Ignatyev (all authors at Institute of Biology of the Southern Seas, 2, Nakhimov av., 335011 Sevastopol, Ukraine, email: benthos@ibss.iuf.net)
The large-scale subwater landscape/ecological investigation have been made in 1993-96 along the coastal zone of SW Crimea at 0-40 m depth. In hydrological regime of this area role of breeze effect is very significant. Daily alterations in spatial pattern of wind currents define the unsteadiness of thermocline structure mostly revealed in early summer period. Such fluctuations impact on hydrological and biological peculiarities. To differentiate landscape facies, the data on hydrodynamics, seabed microrelief and slope, grain-size structure of bottom substrate as well as quantitative characteristics of phyto- and zoobenthos were used. As a result, 7 different bottom landscapes were distinguished. In the shallow (2-8 m depth) water areas with maximum hydrodynamic load (current velocities fluctuated 0.35 to 0.60 m/s) geomorphology were represented only by cliff and rocks sloping down at 8-14o and covered by Cystoseira spp. and mussels. Within 10-15 m depth a steepness of underwater slope reduced to 4-7o and velocity of wind currents to 0.12-0.23 m/s. Mosaic pattern of grounds promotes development of landscapes with high diverse in such groups of benthos as mobile predatory ectobionts and less abundant sessile filter-feeders. Sandy and silty grounds covered by meadows of eelgrass Zostera spp. Below 18-20 m depth, where the steepness does not exceed 1-2o and velocity of currents 0.03-0.05 m/s, a fine sand factions with admixture of shell debris and silt prevailed. Phytobenthos is scarce; in zoobenthos endobenthic suspension-feeders, epibenthic mobile deposit-feeders dominate. Comparative temporal analysis of landscapes patterns and condition of biota has manifested that about 15% of coastal rocky substrates covered earlier by brown algae and mussels have been destroyed due to anthropogenic eutrophication of water areas and hydrotechnical construction and replaced by cenoses wherein indicators of eutrophication (green algae) prevail. Biodiversity of soft-bottom macrobenthos also reduced and degradation of sensitive to eutrophication indicator species has been observed. The processes of mudding leads to expand a bottom areas covered with Zostera growth that contributes to accumulation of organic matter and formation of unfavorable oxygen conditions in such sites, gradual degradation of benthic assemblages associated with meadows of eelgrass.
P10/W/24-A5 1520
THE INTERACTION OF SHALLOW WATER AND COULOMB-DAMPED POROELASTIC BED
Mian Li (Institute of Mechanics, CAS, Beijing 100080, China, Email: rxzhu@mail.c-geos.ac.cn)
In many cases of coastal engineering, e.g., constructions of coastal structure and breakwaters, piple lines, etc., Correct estimations of the dynamic behavior of seabed and propagating water wave are very important. However, due to the fact that in most cases the seabed is permeable and deforming, and the water wave, in general, is nonlinear, the studies of the interaction between the water waves and seabed are very complicated.
Now the theory of shallow water waves has been applied and the seabed is regarded as compressible and deformable. Not only the wave propagation over coulomb-damped poroelastic bed has been considered, but also the porous pressure in the seabed take into account. Nonlinear shallow water equation of boussinesq type and nonlinear dispersion relation for shallow water are derived. It is found that wave damping is agreement with the experiments, as the coulomb friction between grains is considered. Porous pressure in sand and clay has been calculated. There is an extreme value of porous pressure in some depth of clay bed and the depth is compared with the resonant depth.
Friday 23 July PM
P10/E/11-A5 Poster 1600-01
TIDAL AND SUBTIDAL CIRCULATION ON THE SOUTH BRAZILIAN CONTINENTAL SHELF
O. MOLLER Jr., D. Urbano Neto (both at Lab. Oceanografia Fisica, Depto. Fisica, Universidade do Rio Grande, 96201-900 Rio Grande, RS, Brazil; email: osmar@calvin.ocfis.furg.br)
P. Zavialov (Shirshov Institute of Oceanology, Moscow, Russia, email: zavialov@mi.ras.ru)
In order to characterize the water circulation and dynamics of Rio Grande do Sul continental shelf, time series of current and wind vectors covering a period from March to May of 1997 are analysed. A mooring station with two current meters that registered hourly values of current velocity and direction at the depths of 15 m and 45 m was deployed in the point 32 41'S, 51 27'W. The results indicate that the prevailing current direction was to NE despite of the predominantly NE winds. This indicates a geostrophic flow related to the cross shore density gradient and local sea surface elevation forced by freshwater discharge from Patos Lagoon and Plata River. The wind forcing resided mostly at the sinoptic time scale in the range between 3 and 10 days. The tidal influence was small.
P10/W/21-A5 Poster 1600-02
NUMERICAL STUDIES OF UPWELLING OFF ZHEJIANG AND FUJIAN
Yiyong LUO and Guangyao YU Institute of Physical Oceanography, Ocean University of Qingdao Qingdao, 266003, CHINA
The waters off Zhejiang and Fujian in the East China Sea have anomalously low temperatures and high nutrients during summer. The contribution of tidal current, wind stress and Taiwan Warm Current (TWC) to the anomaly of temperatures and nutrients is individually discussed by using a three dimensional numerical model as well as field observations. The result reveals that there exists a tide-induced upwelling stripe off Zhejiang and Fujian, and its large ranges appear off Zhoushan Islands and north of Fujian where the tidal currents are strong and the variations of bottom topography are large. The result also shows that there exist wind-driven and TWC-driven upwelling stripes off Zhejiang and Fujian, and their ranges are more and more smaller from the south to the north. The computed largest upwelling, 0.0005cm/s for tidal current, occurs in upper layer (10-20m), 0.001cm/s for wind stress in middle layer (20-30m) and, 0.001cm/s for TWC in bottom layer (below 30m). The combination of tide-induced, wind-driven and TWC-driven upwelling provides the mechanism for producing the observed cold water anomaly off Zhejiang and Fujian. In addition, the source of these anomalous waters is discussed by using the Eulerian-Lagrangian method. Results show that the anomalous waters originate from Kuroshio subsurface waters climbing up the continental shelf northeast of Taiwan Island.
P10/W/11-A5 Poster 1600-03
MODEL OF WATERS CIRCULATION ON A CONTINENTAL SHELF WITH THE HIGH SPATIAL RESOLUTION (ON AN EXAMPLE OF THE BARENTS AND KARA SEAS).
SEMYONOV G.A., Bezgreshnov A.M. ( both at Arctic and Antarctic Research Institute, Bering st., 38, St.-Petersburg, Russia, 199397. E-mail: geosem@ecoshelf2.spb.org)
The calculation of currents on a grid with the high spatial resolution imposes, as a rule, restriction on the sizes of modeling area, that results in appearance of opened (liquid) borders of a model do-main. On opened borders the budget of a liquid either are unknown, that calls the inadequate results of computation of currents in shelf zones. The merging of global and regional model grids does not result in significant reduction of an error in the task of determine of the fluxes on borders of a grid with the high resolution. In order to prevent the specified above difficulties in simulation of circula-tion of waters and ice, and also for calculation of fluxes of solid substance and various pollutions for Barents and Kara seas the non-uniform model grid in spherical system of coordinates with a pole located in a point 65° N and 60° E was designed. The modeling area covers all Polar Ocean. The horizontal resolution of this grid varies from 4 km in a coastal zone up to 19 km in a strait be-tween Spitsbergen and Franz-Josef Land and reaches 120 km in Bering and Danish straits. The ver-tical resolution of the model grid is the 20 levels. For calculation of water circulation, fluxes of solid substance and pollutions is constructed the efficient numerical model. Step of integration on time can change from 0.5 till 12 hours. This model based on the complete equations of hydrother-modynamics of ocean with used of a hydrostatics and Boussinesque approximation with free sur-face, with registration of tidal component and parameterization of upper mixed layer. Also in model took into account the flux of energy from broken of superficial wind's waves.
The simulation of waters circulation in Barents and Kara seas for average year, for separate seasons and for differences state of atmosphere parameters are carried out. The analysis of water circulation in upper and bottoms layers of the specified seas is executed.
P10/E/20-A5 Poster 1600-04
PROCESSES OF VERTICAL EXCHANGE IN SHELF SEAS (PROVESS)
John HOWARTH (Centre for Coastal and Marine Sciences, Proudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, Merseyside, L43 7RA, UK, email mjh@ccms.ac.uk)
Vertical exchanges are principally controlled by the water column's turbulence characteristics. Turbulence is generated at the sea surface, by winds and waves, and at the bed, by friction. At the pycnocline turbulence levels are reduced and vertical fluxes can be inhibited. Turbulence characteristics therefore depend on and affect the water column's thermodynamics and dynamics and their interaction with the sea bed and surface.
An aim of the project is to test turbulence closure models, which are commonly used to quantify fluxes in shelf sea environmental models of, for instance, heat, of particles and of nutrients into the photic zone. Of particular importance are fluxes across the thermocline and the determination of which processes control nutrient recycling in the benthic boundary layer where a key will be to distinguish processes in the water column from those in the fluff' layer formed by freshly deposited particulates, from those in the compacted sediment.
Measurements of turbulence dissipation rate throughout the water column and intensity over a wide frequency range, and of fluxes near the sea bed are being made at two contrasting sites in the northern North Sea. The first set of measurements were made during the autumnal breakdown of stratification in the northern North Sea in 1998 and the second, at a shallow high tidal energy site in the southern North Sea, will be in spring 1999. The project will contribute to the development of robust water column plankton shelf sea models tested over a range of turbulence environments.
P10/W/04-A5 Poster 1600-05
USING SURVEYS OF AN UPWELLING JET NEAR A CAPE TO EVALUATE LAYERED MODELS OF JET SEPARATION
Andrew C. DALE and John A. Barth (both at College of Oceanic and Atmospheric Sciences, Oregon State University, 104 Ocean Admin Bldg, Corvallis, Oregon 97331-5503, USA, E-mail: acd@oce.orst.edu)
During the upwelling seasons of 1994 and 1995 the coastal upwelling jet off Cape Blanco on the Oregon coast was surveyed using a CTD mounted on an undulating SeaSoar vehicle and shipboard ADCP. The observed fields show a tendency towards intensified upwelling in the lee of the cape accompanied by separation of the jet from the shelf and flow into the deep ocean. It is clear, from SST images, that these features are typical of the region. Layered models governed by potential vorticity conservation predict the evolution in structure of a coastal jet as it encounters gradual changes in topography, latitude and coastline curvature. The applicability of such models to the Cape Blanco region is investigated in the light of three dimensional dynamical fields derived from the survey data. A significant feature of the models is that when a jet passes a topographic anomaly (such as a constriction of shelf width) its structure can smoothly distort to a new downstream state. This happens when the flow is exactly critical to long coastal-trapped waves, a condition that is especially likely to be satisfied at some time by an upwelling jet as it evolves during the course of an upwelling season. Such a critical transition can lead to strongly enhanced upwelling in the lee of a cape and may be interpreted as jet separation. The likelihood of wave criticality is here determined by direct calculation of coastal-trapped wave phase speeds for the observed stratification and jet structure. The premise of potential vorticity conservation is also investigated for the alongshore scales of interest.
P10/L/04-A5 Poster 1600-06
SHELF WAVES AS A CONTROL OVER THE FLOW, TEMPERATURE AND NUTRIENT SUPPLY IN SALDANHA BAY, SOUTH AFRICA
Grenville NELSON (Sea Fisheries, Department of Environmental Affairs, Cape Town.)
Saldanha Bay is a semicircular indentation on the west coast of South Africa centered on latitude 330 5'S with an area 1.41x108 m2 . A lightly dredged central channel drops from 12m at the centre of the bay to 40m at the mouth, which is some 6 to 7km wide. The adjacent narrow shelf supports the passage of energetic barotropic coastal-trapped waves, with a period of 5 to 6 days. The cross-shelf component of these waves acts to drive cold water into the bay below the wind influenced layer on the eastward phase. This is the dominant driving mechanism controlling temperature and nutrient supply and consequently primary productivity.
The energetic six-day period waves appear to be forced by the synoptic pressure field over the southeast Atlantic. Also present in the spectra are less energetic waves at three and four days. These are the freely propagating coastally trapped waves generated locally by wind, and when their phase corresponds to that of the forced wave, the amplitude is great enough to produce a shelf water penetration of 4km into the bay. Under these conditions, upwelling favourable winds exacerbate the inward movement of bottom water, which is entrained into the surface layer in the shallowest parts.
Tidal flow, generally of little significance on this coastline, can be an important factor in this bay at the spring tide. A tidal lagoon of area 5.3x107m2 connects with the eastern part of the bay.
P10/E/04-A5 Poster 1600-07
INDIRECT OBSERVATIONS OF DEIL MIGRATION ON THE HEBRIDEAN CONTINENTAL SHELF
T.P. RIPPETH and J.H. Simpson (School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, UK, LL59 5EY.)
In this poster we shall consider the interpretation of measurements of vertical velocity and backscatter intensity, made using an acoustic doppler current profiler (ADCP) moored on the Hebridean Continental Shelf. The existance of vertically migrating scatterers is inferred from both the backscatter signal intensity and vertical velocity data, which indicates migration rates of 2-3 cm/s, and shows consistancy between the displacement deduced from the vertical velocity and the observed movement of scatterers from the near bed to the near-surface region, evident in temporal variations in the backscatter signal intensity. Independent evidence that the observed vertical velocities are largley due to movements of the acoustic scatterers relative to the water is obtained through comparison of the ADCP vertical velocity data with vertical velocities inferred from the movement of the thermocline. The close phase locking between the motion and sunset and sunrise times has led us to interpret the signal in terms of the deil migration of zooplankton. Although no biological samples were collected during this study, previous surveys of the area have identifed species of copepod and euphausiid that are known to migrate. The 12 days of data available show an initial period, during which there was a strong vertical migration signal, and a later period, during which the signal was not clear. By combining the observed vertical velocity and horizontal velocity components in a particle tracking model, it is demonstrated that the change in the migration signal may be a result of the advection of the patch of migrating zooplankton away from the ADCP, during the later part of the observational period.
P10/E/19-A5 Poster 1600-08
ANALYSIS OF THE GEOMORPHOLOGICAL TRENDS FOR THE SOUTHERN CURONIAN SPIT LAGOONIC COAST
TATIANA A. DORMOLENKO, Boris V. Chubarenko (P.P.Shirshov's Institute of Oceanology of Russian Academy of Sciences, Atlantic Branch, Prospect Mira, 1, 236000, Kaliningrad, Russia, E-mail: chuboris@ioran.kern.ru)
The Curonian spit is the largest (near 100 km) accumulative coastal macro-form in the South-Eastern Baltic and separates the Curonian lagoon from the Baltic Sea. It is mostly covered by forest but also has the areas of opening dune and the wetlands. The sand basis of the Curonian spit determines both the high velocity of shore-line shape changing and the instability of the vegetation free areas. The objective of this work consists in the quantification and analysis of the morphological trends of the Curonian spit lagoonic littoral from 1955 till 1994 years and assessing the influence of dune stripes on these trends. Practically important problem is the evaluation of the dune cells nourishment distribution downwards the underwater coastal slope and especially alongshore from open dune cells. Such knowledge is necessary for the optimization of trees planting program aiming to dunes fixation and for the using of the open dune stripes to compensate the shore erosion. The temporal variations (1955-1995) of the coastline, depths of 1, 2, 3 meters and underwater deposit volume were numerically appraised on the basis of comparative cartometric survey for the Curonian spit lagoonic coast. The analysis was executed for the set of sequential sites of the littoral, which were selected by coastline partition with a number of shore-normal cross-sections. The main geomorphological trends for the indicated above period were quantified. The shore-line of dune cells has promoted into lagoon and the littoral of these cells has got more shallow. The coastal line and littoral of the other cells have been eroded. The influence of dune nourishment on the stability of the coastline and littoral sites adjacent to dune cells was assessed. The trends in the alongshore and downwards sediment distribution were revealed.
P10/L/02-A5 Poster 1600-09
OBSERVATIONS OS SEDIMENT RESUSPENSION AND TRANSPORT IN THE COASTAL AND BOTTOM BOUNDARY LAYERS OF LARGE LAKES WITH ACOUSTIC DOPPLER CURRENT PROFILERS
SALYOR, James H. and Gerald S. Miller (Both at: National Oceanic and Atmospheric Administration, Great Lakes Environmental Research Laboratory, 2205 Commonwealth Blvd., Ann Arbor, MI 48105; e-mail;saylor@glerl.noaa.gov)
Data from Acoustic Doppler Current Profilers (ADCP) deployed in the nearshore region of southeastern Lake Michigan provide evidence of sediment resuspension and the cross-shelf transport of these materials during episodes of winter wind storms. Significant increases (+20 dB) in echo intensity and current velocity correlate well with satellite imagery of a sediment-laden plume transporting material in a cyclonic flow around the basins perimeter. A decrease in echo intensity to near background levels following the resuspension event suggests that the larger particles settle out, leaving the very fine material that remains visible in satellite imagery for many days. In Lake Champlain, a long, narrow and deep lake that forms the boundary between the states of New York and Vermont, bottom sediment resuspension caused by surges and gravity currents in the density-stratified lake water has also been documented in acoustic profiler records. Although no concurrent measurements of Total Suspended Material have been achieved in either lake, the ADCP results show that resuspension events can be detected using these instruments. With calibration of sediment concentration and echo intensity at each site, it appears that quantitative values of sediment fluxes can be determined.