Topographically Induced Upwelling off Eastern Australia

512 VOLUME 30JOURNAL OF PHYSICAL OCEANOGRAPHY

2000 American Meteorological Society

Topographically Induced Upwelling off Eastern Australia

PETER R. OKE

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

JASON H. MIDDLETON

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

(Manuscript received 7 October 1998, in final form 8 March 1999)

ABSTRACT

A high-resolution, numerical study of an idealized western boundary current flow over variable topographyis presented, with application to the East Australian Current (EAC). The results indicate that alongshelf topo-graphic variations off Australia’s east coast cause the EAC to accelerate over the narrowing continental shelfnear Cape Byron. This acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundarylayer (BBL), which would usually cause the EAC-driven BBL to shut down. Consequently , a region of persistent,high bottom stress was established off Cape Byron, which was responsible for driving an upwelling BBL. It isshown that the enhanced vertical mixing, associated with a low Richardson number flow beneath the EAC,reduced the local stratification. Consequently , the Burger number is decreased resulting in a long shutdowntimescale of the BBL, which enables a nearshore thermal front to be established and maintained. Such frontsare commonly observed in the region. As a part of the analysis the term balances of the model equationsare presented, comparing the dynamical balances at locations along the domain that exhibit varying degreesof topographic variability. The results indicate that the BBL dynamics were not purely geostrophic, furtherexplaining why BBL shutdown was not prevailing. Moreover, it is shown that the formation of the thermal frontwas dependent on the magnitude of the EAC’s southward transport, explaining why the occurrence of thermalfronts is greater during the spring and summer periods.

1. Introduction

The East Australian Current (EAC) is the westernboundary current of the subtropical gyre of the southPacific ocean. The EAC transports between 22 and 35Sv (Sv 106 m3 s 1) southward along Australia’s eastcoast (Ridgway and Godfrey 1997) with its strongestcurrents flowing adjacent to the continental shelf break(Cresswell 1994, 1999, manuscript in preparation ) withcurrents up to 2 m s 1 (Church and Cresswell 1986).While the shelf currents associated with the EAC aretypically weaker than the currents in the EAC’s core, itis certain that, along with other factors such as localwind forcing (Gibbs et al. 1998), the EAC will have aneffect on the nearshore zone (NSZ).

Thermal fronts have been observed in the EAC regionon numerous occasions (Stanton 1976; Godfrey et al.1980a; Cresswell et al. 1983; Tranter et al. 1986) withassociated high levels of chlorophyl biomass on the con-tinental shelf (Tranter et al. 1986). Several of these ob-

Corresponding author address: Dr. Peter R. Oke, School ofMathematics, UNSW, Sydney, 2052 Australia.E-mail: [email protected]

servations were made in the vicinity of Cape Byronduring spring and summer, away from the EAC sepa-ration zone, which is located off Sugarloaf Point (God-frey et al. 1980b) 500 km south of Cape Byron. Thefunctional importance of thermal fronts, as indicators ofhigh levels of biological activity, has long been rec-ognized (Hynd 1969; Boucher et al. 1987; Hitchcock etal. 1993; Olsen et al. 1983; Franks and Walstad 1997).The aforementioned observations typically found thatthe temperature change across the front was 2 –3 C andthat the front met the coast between Cape Byron andTweed Heads. The sea surface temperature (SST) im-ages displayed in Fig. 1, obtained from the NOAA-11satellite, show an example of a thermal front off CapeByron, which occurred in October 1996. That fronts,similar to that in Fig. 1, are a regular seasonal featureprompts the following questions: How are the frontsformed? What factors effect their formation? Is theirformation likely to result in the nutrient enrichment ofthe coastal waters? These questions are addressedthrough numerical experiments involving the configu-ration of a high-resolution regional ocean model off thecentral east coast of Australia. The experiments inves-tigate the shelf response to a continuous, steady south-ward flow, characteristic of the EAC. The results show

MARCH 2000 513OKE AND MIDDLETON

FIG. 1. SST fields of the study region for Oct 1996 showing an observed nearshore thermal front off Cape Byron. The 300-m isobath iscontoured.

FIG. 2. Schematic depicting the upwelling of a stratified flow overa sloping bottom, where I and B denote the interior and bottomvelocity (directed out of the page), respectively .

the formation of a nearshore thermal front, as a resultof a persistent upwelling bottom boundary layer (BBL),driven by the alongshelf flow. The BBL shutdown pro-cess, which would be expected to inhibit such upwelling(MacCready and Rhines 1993), was not prevailing, andan analysis of the models internal dynamical balanceswas undertaken in order to identify the dominant dy-namical processes.

Boundary layer theory of a stratified flow over a slop-ing shelf suggests that persistent current-driven up-welling is ineffective. The shutdown process is outlinedbelow. If the bottom alongshelf velocity B is nonzero,then an alongshelf bottom stress is induced such thaty

b

y 2 2C u ,D B B Bb

where CD is the bottom drag coefficient. In westernboundary current regions, such a stress will drive anupwelling BBL as depicted in Fig. 2. This upwelling

will then result in a horizontal density gradient / xbetween the upwelled BBL water ( 2) and the interiorwater ( 1) where 1 2. Under a geostrophic as-sumption the thermal wind relation

g(1)

z f x0

acts to reduce the alongshelf bottom velocity B to zeroby increasing the vertical velocity gradient / z. Con-sequently, the alongshelf bottom stress formulatedabove also reduces to zero. In the absence of a bottomstress, a frictional BBL cannot be maintained and, con-sequently, shuts down. Equation (1) is derived by as-suming that the cross-shelf momentum balance of theflow is geostrophic. Results from this study indicate thatsuch an assumption off Cape Byron, where the along-shelf topography varies considerably, is inadequate,with the vertical diffusion term playing a significantrole. It is shown through an analysis of the internaldynamical balances that the EAC accelerates, in a La-grangian sense, in the vicinity of Cape Byron in re-sponse to topographic constraints. As a result of thisacceleration the Richardson number Ri, defined as

2NRi , (2)

2 2uz z

where N 2 is the bouyancy frequency, is reduced belowthe critical value of 0.25, resulting in increased valuesof the vertical, turbulent eddy viscosity Km (Mellor andYamada 1982), which results in a persistent Ekman-likebalance near the bottom over the shelf. The strong ver-


tical mixing in the BBL causes the buoyancy frequencyto decrease, which in turn decreases the local Burgernumber S, defined as

2 2NS , (3)

2f

where is the bottom slope and f is the Coriolis pa-rameter. Boundary layer theory states that a small Bur-ger number flow corresponds to an increased shutdowntime S, where

1(4)S 2Sf

if the turbulent Prandlt number is assumed to be unity(Garret et al. 1993). The increased shutdown time ex-plains why the BBL flow was so persistent, enabling anearshore thermal front to become established. Theanalysis of the term balance for the conservation equa-tion for temperature indicated that, although an up-welling BBL was persistent, the rate of upwelling ofisotherms decreased with time. It appears as if an ‘‘up-welling limit’’ is approached, where the slope of theisotherms (or isopycnals) match that of the bottom slope,thereby allowing for along-isopycnal transport to bemaintained, uninhibited by the opposing forces of bouy-ancy.

The correlation of upwellings to EAC encroachmentsonto the continental shelf was first identified by Roch-ford (1972) who suggested that, ‘‘when the core of astrong south-flowing current nears the continental slope,temperatures of mid-depth waters over the slope aremuch reduced’’ near Cape Byron. Furthermore, Roch-ford’s (1972) observations indicated that the cold, up-welled waters were typically upwelled to the surfacebetween Evans Head and Cape Byron. Similarly, Roch-ford (1975) suggested that upwellings off Laurieton, onthe New South Wales (NSW) central coast, were theresult of the EAC flow intruding onto the continentalshelf, explaining events that were seemingly unrelatedto the local winds. These suggestions by Rochford aremore fully explored by Oke and Middleton (2000),where the separation of the EAC from the coast wasshown to play a crucial role. The observations made byRochford (1975) off Evans Head during periods of up-welling showed that the isotherm slope was very closeto that of the bottom topography, consistent with theresults mentioned above.

The numerical experiments presented here involve adetailed comparison of two model configurations, a lo-cal domain model (LDM) and an extended domain mod-el (EDM). Since regional ocean modeling necessitatesthe use of open boundary conditions of some descrip-tion, then the boundary values, and their effect on themodel circulation remain uncertain (Mahadevan et al.1996). Consequently, it is argued that, if the same resultsare obtained for a small regional model and a largerregional model where the boundaries are well away from

the area of interest, then the results are robust and themodel circulation is deemed to be relatively insensitiveto the proximity of the lateral boundaries. The resultsfrom both models are shown to agree; however it isclear that there is an advantage with the larger EDMsince the imposed flow had sufficient time to adjust tothe local topography before encountering the complex,variable topography. Additional sensitivity experimentswere also performed with the EDM in order to inves-tigate the dependence of the model dynamics on thetransport magnitude, stratification, and grid resolution,and it is shown that the persistent BBL flow is dependenton the transport magnitude, which explains why morefronts are encountered off Cape Byron in spring andsummer when the EAC’s transport is typically stronger(Ridgway and Godfrey 1997).

This paper is organized with a description of the mod-el configurations and the boundary conditions presentedin section 2. The results are presented in section 3, whichincludes an analysis of a sensitivity study, and an anal-ysis of the internal dynamical balances of the modelequations is in section 4. A discussion of the results ispresented in section 5 followed by the conclusions insection 6.

2. Model configurations

The model used in this study is the Princeton OceanModel (POM). Briefly, the POM is a hydrostatic, free-surface, sigma-coordinate, primitive equation model,with an embedded turbulence submodel. For an in-depthdescription of the model equations and the numericaltechniques, the reader is referred to Blumberg and Mel-lor (1987). Details of the two model configurations usedare presented below.

a. Local domain model

The LDM extends between latitudes 29 38 S and27 23 S covering an alongshelf distance of 250 km,extending out to 125 km offshore (Fig. 3). A rectangulargrid was used consisting of 81 111 points, with cross-shelf grid spacings of 1 km over the shelf and slopeincreasing to 4 km near the seaward boundary. Simi-larly, the alongshelf grid spacings were 2 km over mostof the interior, increasing to 4 km over a distance of 25km near each of the alongshelf boundaries. The verticalgrid consisted of 31 vertical levels varying linearly overmiddepths and logarithmically over upper and near-bot-tom depths. The model bathymetr y was extracted fromthe ETOPO5 dataset (NOAA 1988) with a minimumdepth of 20 m. The maximum depth was set to 1000 min order to reduce the constraint on the time steps, whichwere 4.25 and 135 s for the barotropic and baroclinicmodes, respectively. In order to interpolate the bathym-etry onto the model grid the bathymetr y locations werefirst converted to the Australian Map Grid. The ba-thymetry was then rotated in an anticlockwise direction


FIG. 3. Study location (left) showing the extent of the local domain (filled) and the extended domain (outlined);model bathymetr y (middle) showing the 100, 200 (dashed), 500, and 1000 m isobaths; the model grid (right) for thelocal domain model.

by 10 in order to align the isobaths with the alongshelfdirection of the grid, and finally the bathymetr y waslinearly interpolated onto the model grid. The bathym-etry was then weakly smoothed in order to regulate themaximum local slope parameter r, defined by

x, yHr ,

H

which represents the ratio of the change in depth x,yH, to the average depth H over each grid cell (Melloret al. 1994). As a general rule, the maximum value ofr should be less than 0.2 in order to sufficiently reducethe pressure gradient errors associated with the sigmacoordinates (Beckmann and Haidvogel 1993). Withinthis configuration the maximum slope parameter was0.17, satisfying this criterion. The Coriolis parameterwas rotated and interpolated in an analogous fashion tothe bathymetr y so that no f -plane or -plane approxi-mations were necessary in the LDM.

The upper 200 m of the temperature field was ini-tialized with the mean temperature profile extractedfrom conductivity–temperature–depth (CTD) data col-lected near Cape Byron during October 1996 (Oke andMiddleton 2000, manuscript in preparation, hereafterOM). The spatial details of the CTD measurements werenot representative of the time averaged field due to thepresence of a highly dynamic internal tide on the shelf

at the time of sampling, and as such were not includedin the model initialization. The temperature below200-m depth corresponds to the spring, climatologicalvalues for the region as specified by Levitus (1982).The initial temperature field was defined with precon-ditioned isotherms tilting upwards towards the coast, afeature typical of a cross-shelf temperature section forthe EAC region (Fig. 4). The temperature field was ini-tially homogeneous in the alongshelf direction. For sim-plicity the salinity was set to a constant value of 35 psuthroughout the model domain.

The initial currents were defined by an idealizedGaussian jet of width 80 km and a maximum speed of0.8 m s 1 at the surface. The jet was centred over the300-m isobath, adjacent to the shelf break, and the bar-oclinic structure of the current was estimated from thethermal wind relation. The resulting southward transportwas 14 Sv, which is much less than the EAC’s seasonalaverage of 27 Sv (Ridgway and Godfrey 1997). How-ever, given the unrealistic maximum depth of 1000 m,this transport was deemed representative of a baroclinicEAC flow. The initial Burger number of the flow overthe shelf was greater than 0.29, which corresponds to aBBL shutdown time of 2 days from Eq. (4).

The boundary conditions implemented at the southernboundary included zero gradient and upstream advec-tion conditions for the baroclinic variables (Blumberg


FIG. 4. Initial surface velocity field (left, showing every third vector), alongshelf velocity structure (topright) and temperature section (bottom right) showing the 0.5 m s 1 and 20 C (bold dashed) contoursrespectively off Cape Byron (Y 135 km).

and Mellor 1983) and modified radiation conditions forthe barotropic variables (Flather 1976; Blumberg andKantha 1985; Palma and Matano 1998). These condi-tions allow the time averaged barotropic flow at theboundary to be imposed, or relaxed over a specifiedtimescale (2 days). Consequently, the boundary valueswere ‘‘controlled,’’ while disturbances generated withinthe domain were allowed to propagate out with minimalreflection. All variables were prescribed at the northernboundary throughout the simulations. The seawardboundary (X 125 km) was masked by a vertical wall,a feature which is unrealistic, but not critical for thepresent study, due to the absence of wind forcing.

b. Extended domain model

For the EDM the bathymetr y from the boundaries ofthe LDM were extended an additional 150 km offshoreand in both alongshelf directions. The alongshelf gridspacings were increased to 10 km in the extended re-gion, leaving the part of the domain common to boththe LDM and the EDM unchanged. Similarly, the Cor-iolis parameter was uniformly set to the LDM’s bound-ary values over the extended region. The resulting EDMconsisted of 96 141 horizontal grid points and 31vertical levels.

The rationale behind comparing the LDM and theEDM simulations was to assess the sensitivity of the

model to the proximity of the lateral boundary forcing.One characteristic of the open boundary conditions im-plemented into the configuration is that the amplitudeand structure of the volume transport was able to beimposed. The advantage of this is that a volume con-serving configuration could be obtained; however it as-sumes that the time averaged barotropic boundary val-ues were known a priori. It is therefore unclear to whatextent the model circulation will depend on these bound-ary values. In the LDM the boundaries were very closeto the area of interest, namely Cape Byron, however theEDM had the boundaries 150 km farther away in alldirections. Consequently, it is proposed that if the LDMand the EDM’s results are in agreement, then the modelcirculation is robust, and hence valid for the idealizedscenario outlined in section 1.

3. ResultsThe simulations presented below were run prognost-

ically for 15 days. All variables presented hereafter rep-resent averages over an inertial period. For the resultsbelow, fields from both the LDM and the EDM areshown for comparison.

a. Model spinup

As a first assessment of the model’s performance thetemporal evolution of the percentage change of the area


FIG. 5. Area averaged % ME0 (top) and MSL (bottom) for thelocal domain (solid) and extended domain model (dashed).

averaged mechanical energy from the initial state %ME0 was calculated, such that

1 2 22% ME g 0.5 H(u ) dA0 AT A

ME 100,0

where ME0 is the initial mechanical energy, AT is thearea of the domain, is the depth averaged density,is the elevation, H is the water depth, and u and arethe cross-shelf and alongshelf depth averaged velocitiesrespectively (Palma and Matano 1997). As an additionalmeasure of the model’s performance, the mean sea level(MSL) was evaluated, where

1MSL dA.

AT A

These properties of the simulations are presented in Fig.5, indicating that, after an initial adjustment, the me-chanical energy reached a state of ‘‘zonal equilibrium’’after approximately 7 days in the LDM and 1 day inthe EDM. Furthermore, the MSL similarly indicated thatafter an initial adjustment the volume of both modelswere conserved. These observations indicate that theboundary conditions were satisfactor y in terms of main-taining a constant energy and volume flux through themodel domain. Having satisfied these important criteria,we shall now turn to a qualitative comparison betweenthe LDM and the EDM simulations.

b. Simulated baroclinic fields

Given that the model initializations of both config-urations were equivalent, perturbations in temperatureand alongshelf velocity, and , are presented suchthat

n 0 and n 0,n n

where the subscripts denote simulation days. As we shallsee, consideration of these variables give insight intothe internal adjustments made within the model simu-lations.

The area of interest for this study was the continentalshelf off Cape Byron, situated in the center of the modeldomain. The simulated baroclinic fields on day 15 arepresented in Fig. 6. The alongshelf velocity perturbationindicates that the velocity decreased over near-bottomdepths in response to bottom friction, and converselyincreased slightly over mid and upper depths in orderto conserve the alongshelf transport. The cross-shelfvelocity fields show a shoreward BBL flow in the bot-tom 15 m of water over the continental shelf with cur-rents in excess of 0.05 m s 1. This BBL transport ex-tends across the entire continental shelf and past theshelf break.

As a consequence of this shoreward BBL flow iso-therms were advected toward the coast resulting in adecrease in temperature over the shelf and NSZ. Thisis clearly evident in the temperature perturbation fields,showing a decrease of the temperature in the NSZ bygreater than 7 C.

The results from both the LDM and the EDM werein good agreement, although the LDM shows the tem-perature decrease over a greater area on the shelf. Theabove comparison indicates that the baroclinic pertur-bations from the initial state were similar for both modelconfigurations.

c. Bottom stress

The fields of bottom stress, presented in Fig. 7, in-dicate that there was high bottom stress between TweedHeads (Y 188 km) and Evans Head (Y 80 km).The overall pattern of the bottom stress fields was sim-ilar for both the LDM and the EDM; however the mag-nitude of the stress was greater in the LDM off bothTweed Heads and Evans Head. There was also a regionof high bottom stress to the north of Tweed Heads nearthe northern boundary of the LDM. This feature is ev-idence of the model circulations internal adjustment dueto the idealized structure of the imposed lateral forcing.By moving the northern boundary farther north in theEDM and allowing the model circulation to adjust be-fore encountering the varying topography, this effectwas significantly reduced.

The region of highest bottom stress in both modelswas evident adjacent to Cape Byron, extending from thecoast to beyond the 200-m isobath. This feature explainsthe persistent shoreward BBL flow presented above asEAC-driven uplifting caused by the currents frictionalinteraction with the local topography. There is also anisolated region of high bottom stress off Evans Head.A dynamical explanation of this feature is postponed tosection 5. The similarities between the bottom stressfields in the LDM and the EDM give confidence in therobustness of the model results. The nearshore effects


FIG. 6. Cross section adjacent to Cape Byron (Y 135 km) of the , u and showing the15 15

zero velocity (bold solid) and 5 C (bold dashed) contours.

of this region of high bottom stress can be seen byconsidering the near-surface temperature field.

d. Near-surface temperature

The extent of the EAC uplifting can be seen in thenear-surface temperature perturbation field at day 15(Fig. 8). These fields indicate that the temperature nearCape Byron dropped by 8 C in the LDM and 7 Ci nthe EDM. More uplifting was simulated in the LDM,arguably due to the closeness of the lateral boundaries;however qualitatively, the results are very similar. Theregion of greatest temperature decrease in the NSZ cor-relates well with the alongshelf locations of the highbottom stress discussed above.

The temperature decrease represents the formation of

a nearshore thermal front, concentrated near Cape By-ron, extending toward Tweed Heads. This descriptionis similar to the nearshore thermal front observed byOMb and also by Godfrey et al. (1980a) in the sameregion.

e. Sensitivity experiments

As well as the comparison between the LDM and theEDM, additional numerical simulations have been per-formed in order to assess the dependence of the modelcirculation on the transport magnitude, stratification andgrid resolution. A list of these experiments, and a briefdescription of each is presented in Table 1. The resultingbaroclinic cross-sections of alongshelf velocity, across-shelf velocity and temperature fields off Cape Byron (Y


FIG. 7. Bottom stress at day 15 with contour intervals of 1 10 4 m2 s 2 showing the zero contour (bold solid) andthe 50, 100, and 200-m isobaths (dashed).

135 km) on day 15 for various simulations is shownin Fig. 9. It is found that a persistent shoreward BBLwas present in all experiments, although there was aclear dependence on the magnitude of the transport, witha weaker (stronger) BBL flow generated above the shelfbreak by a reduced (increased) transport. Furthermore,for experiment 5, which had a weak southward trans-port, the BBL flow was not maintained from the shelfbreak to the coast, unlike all other experiments. Thisdependence explains the seasonal variation of the oc-currance of nearshore fronts off Cape Byron. This ap-pears to be a result of the fact that the southward currentsin the NSZ were weaker and, more significantly, hadless vertical shear. The reduced vertical shear effectivelyincreased the Richardson number, decreasing the mag-nitude of the vertical diffusion term in the momentumequations, which is important for the BBL flow to bemaintained as we shall see in section 4. Results fromexperiments 7 and 8 with increased and decreased hor-izontal resolutions producing very similar results to theLDM and EDM results presented above.

Possibly the most significant features of the cross-shelf sections presented in Fig. 9 is that the slope of theisotherms over the shelf is very close to that of thebottom slope. This synoptic feature is consistent withthe observations made by Rochford (1972) who ob-served that upwelling near Cape Byron, that was seem-

ingly unrelated to local wind forcing, involved iso-therms originating at the depth of the shelf break beingtransported into the NSZ. When the slope of the iso-therms (or isopycnals, given that salinity is constant)match the bottom slope, along-isopycnal mixing is pos-sible, whereby water originating from within the depthof the shelf break and beyond can be upwelled to thesurface, without being inhibited by the opposing effectsof bouyancy. This implies that nutrient enrichment ofthe NSZ will be very efficient under these conditions.The isotherms are close to, but not quite parallel to, thebottom slope on day 15; however, an assessment of thetemporal behavior of the temperature decrease can bedetermined by considering a time series of the temper-ature in the NSZ and adjacent to the shelf break (Fig.10). These time series show that the upwelling of iso-therms continued well after the models had spun up;however an ‘‘upwelling limit’’ seems to be inevitable.Based on the arguments presented above, this upwellinglimit represents the situation where the isotherm slopematches that of the bottom slope, which implies thatcurrent-driven upwelling will always be limited to up-welling water originating no deeper than the shelf break.The reason for this is that beyond the shelf break theBurger number dramatically increases, so the shutdowntimescale decreases.


FIG. 8. Temperature perturbation at day 15 with contour intervals of 1 C showing the 3 C (bold dashed) contour.

TABLE 1. Summary of the numerical experiments for which sim-ulations were performed, indicating the model grid, buoyancy fre-quency N 2, southward transport (T ), and grid resolution of the cir-culation.

Exp Description

1 LDM, N 2 N , T 14 Sv20

2 EDM, N 2 N , T 14 Sv20

3 EDM, N 2 2N , T 14 Sv20

4 EDM, N 2 0.5N , T 14 Sv20

5 EDM, N 2 N , T 7S v20

6 EDM, N 2 N , T 21 Sv20

7 EDM, N 2 N , T 14 Sv, x 2 x020

8 EDM, N 2 N , T 14 Sv, x x020

4. Internal dynamical balances

The ‘‘terms’’ in the momentum equations and theconservation equation for temperature were averagedover an inertial period and extracted from the EDM atday 15 (experiment 2). The terms were then averagedover seven consecutive grid cells in the alongshelf di-rection, in order to ensure that the dynamical balancesare valid for the ‘‘zones’’ of interest. Three zones havebeen selected for analysis, and their locations are pre-sented in Fig. 11. The first is north of Tweed Heads (Y

208 km), which is located at the northern extent ofthe variable topography region of the model. This zone

was chosen to act as the control for this analysis in orderto determine the effects of the variable topography lo-cated farther downstream. The second zone selected isadjacent to Cape Byron (Y 135 km), the site of theEAC-driven BBL upwelling presented in section 3. Thethird zone is off Evans Head (Y 76 km), which wasagain a site of high bottom stress over the mid to outershelf.

We have considered the term balances in sigma co-ordinates at constant sigma levels near the surface (

0.043), at mid depths ( 0.478) and at near-bottom depths ( 0.913), after Allen et al. (1995),as indicated in Fig. 11. Based on the results presentedabove, the near-bottom level represents the dynamicsof the BBL throughout the simulation, while the midand near-surface depths represent the interior EACflow.

a. Momentum balance

The terms in the alongshelf and cross-shelf momen-tum balances include the tendency , advection, rotation,pressure gradient, vertical diffusion, and the horizontaldiffusion terms. The generic name and short-hand no-tation are presented below. The short-hand notation isnot intended to represent each term in a strict mathe-


FIG. 9. Cross-sections of alongshelf velocity, cross-shelf velocity and temperature (left to right) off CapeByron (Y 135 km) for various sensitivity experiments described in Table 1.

matical sense; rather, it is employed as a matter ofconvenience. For a fuller presentation of the modelequations, the reader is refered to Blumberg and Mellor(1987). The terms balance for the alongshelf and cross-shelf momentum equations are represented by

tendency advection rotation pressure gradient

vertical diffusion horizontal diffusion 0

v · fu P (K ) A 0,t y m z z m xx

u v · u f P (Ku ) Au 0,t x m z z m xx

where v is the velocity vector with components (u, ,w ),corresponding to the cross-shelf, alongshelf and verticaldirections (x, y, ). The operator ( / x, / y, / z);t is time, f is the Coriolis parameter, and P is pressure.Here Km is the turbulent eddy viscosity, determined bythe flow-dependent Mellor–Yamada 2.5 closure scheme(Mellor and Yamada 1982), and Am is the flow-depen-dent horizontal eddy viscosity, determined by the for-mulation of Smagorinsky (1963). All of the terms in themomentum equations were multiplied by a scaling fac-tor of D 1 t 103, where D is the height of the water


FIG. 10. Time series of hourly temperature perturbation from the NSZ (a) and adjacent to theshelf break (b) as indicated by an asterisk off Cape Byron.

FIG. 11. Alongshelf locations of zones for which the alongshelf momentum balance is presented (left) andthe depths of the sigma levels representing the near-surface, middepth, and near-bottom levels.


column and t is the baroclinic time step, giving themunits of (m s 1) 103.

b. Conservation equation for temperature

The terms balance in the conservation equation fortemperature include the tendency, advection, verticaldiffusion and the horizontal diffusion terms. The genericname and short-hand notation are given by

tendency advection vertical diffusion

horizontal diffusion 0

v · (K ) A 0,t h z z h xx

where is the temperature, Kh is the vertical diffusivityfor heat, and Ah is the horizontal diffusivity for heat. Inthe POM, Kh is determined by the turbulence submodelof Mellor and Yamada (1982) and Ah is taken to beproportional to the horizontal viscosity Am, which wasdetermined by the Smagorinsky scheme (Smagorinsky1963). The turbulent Prandlt number, which relates thevalues of Ah and Am, was set to unity for the simulationsconsidered. Increasing the Prandlt number simply in-creases the horizontal smoothing of the temperaturefield. The horizontal diffusion term is meant to modelthe subgrid scale processes; however in practice it isemployed to damp small-scale computational noise(Blumberg and Mellor 1987). All of the terms in theconservation equation presented were multiplied by ascaling factor of D 1 t 103 giving them units of C

103.The resulting terms balance for the alongshelf and

cross-shelf momentum equations and for the conser-vation equation for temperature, for each zone on day15, are presented in Figs. 12 and 13, and 14, respec-tively. The near bottom balances are presented sepa-rately from the mid depth and near-surface balancessince they are of particular interest with regard to theupwelling mechanism presented in section 3.

c. Near-bottom depths

The alongshelf momentum balance to the north ofTweed Heads, in zone 1, indicates that near the bottom,the dominant balance was between Py and fu over themid and outer shelf, representing a geostrophic balance.Vertical diffusion, (Km z)z, also becomes important inthe NSZ, being balanced by Py and fu, representing anEkman-like balance. The cross-shelf momentum bal-ance was almost purely geostrophic in this zone. Theconservation balance in zone 1 indicates a balance be-tween small, negative t and positive v · , which rep-resents a weak upwelling of isotherms, representing theinitial response of the southward flow to variable to-pography.

In the region adjacent to Cape Byron, representedhere by zone 2, there was a persistent upwelling BBLflow throughout the simulation resulting in the forma-

tion of the nearshore thermal front. The near-bottomdynamical balance at zone 2 was very different to zone1, indicating that (Km z)z was the dominant term be-tween the coast and the 200-m isobath. Negative (Km z)z

was balanced by positive Py and fu. The increased mag-nitude of (Km z)z indicates that there was enhanced ver-tical mixing above the bottom due to frictional effectsfrom beneath the EAC that were acting to retard thebottom currents. Furthermore, the positive fu term in-dicates that an upwelling BBL flow was present, andthat both fu and Py were positive indicates that the flowwas not in geostrophic balance. Rather, these dynamicsare more consistent with a three dimensional, linear Ek-man balance. The balance is not totally linear, sincev · was also of secondary importance. The near bot-tom balance of the cross-shelf momentum equations wasin quasigeostrophic balance with (Kmuz)z also being im-portant. The significance of the near-bottom flow beingout of geostrophic balance has implications for the va-lidity of the MacCready and Rhines (1993) BBL shut-down theory in regions of varying topography. Thissuggestion will be further investigated below, with areconstruction of the terms in the thermal wind relationfrom the modeled fields.

The conservation balance indicates that negative(Kh z)z was balanced by positive v · . Over the shelfbreak and in the NSZ t was small and negative, indi-cating a weak upwelling; however over the midshelfregion t was small and positive. The implications ofthese features will be fully discussed in section 5. Ad-ditionally, the NSZ exhibited a high degree of com-plexity, in agreement with the inferences of OM.

Zone 3 was located to the south of Cape Byron, offEvans Head. The alongshelf momentum balance sea-ward of the 100-m isobath was similar to zone 2, withv · of increasing importance. The near-bottom fric-tional influence was isolated to the shelf-break region,which correlates well with the distribution of bottomstress presented in section 3. In the NSZ a more geo-strophic balance was evident, with a dominant balancebetween positive fu and negative Py, although (Km z)z

was also important adjacent to the coast. The advectionterm v · , became increasingly less important towardsthe coast, consistent with the conceptual views of theEAC flow as a strong, narrow jet over the shelf break.

It is clear that across the entire shelf at Cape Byronthe BBL dynamics were not purely geostrophic as in-dicated by the fact that Py and fu had the same signsin the alongshelf momentum balance and that (Kmuz)z

was significant in the cross-shelf momentum balance.The magnitude of fu in zones 2 and 3 was greater thanzone 1 indicating that the BBL flow was stronger atthese locations.

d. Mid and near-surface depths

Over mid and near-surface depths across most of theshelf the dominant terms in the alongshelf momentum


FIG. 12. Alongshelf momentum terms [(m s 1) 103] for zones 1, 2, and 3 (left to right) on day 15 fromthe EDM. The offshore distance and cross-shelf isobath locations are indicated at the bottom and top of eachpanel, respectively .

balance were Py, fu and v · , indicating that the flowwas highly nonlinear, with advection due to the along-shelf flow being important and the dynamical balancetending toward geostrophy. In the NSZ in zones 2 and3 the balances were much more complicated than inzone 1 with most of the terms becoming nonzero atsome point.

Clearly the nonlinear, advective term v · , wasmore dominant at zones 2 and 3, which is suggested tobe an indicator of the three-dimensionality , or the along-shelf advective acceleration of the flow over the varyingtopography. The cross-shelf extent of the alongshelf ac-

celeration can be inferred by this term, particularly atnear-surface depths. This indicates that at zone 1 thealongshelf advective acceleration of the flow was un-important. Conversely at zone 2, v · was dominantfrom the shelf break to the coast. This indicates that theEAC had a significant effect across the entire shelf in-cluding the NSZ. Below the regions where v · dom-inated, (Km z)z was dominant in the BBL. At zone 3the advective acceleration of the flow near the surfacewas limited to seaward of the 50-m isobath, which againcorresponds well with the regions where (Km z)z wasdominant in the BBL. This correlation between the near-


FIG. 13. Cross-shelf momentum terms [(m s 1) 103] for zones 1, 2, and 3 (left to right) on day 15 fromthe EDM. The offshore distance and cross-shelf isobath locations are indicated at the bottom and top ofeach panel, respectively .

surface advective acceleration and the high vertical dif-fusion in the BBL over the narrowing continental shelfadds weight to the argument that the accelerated EACflow interfered with the geostrophic adjustment of theBBL flow, inhibiting BBL shutdown. The change in signof v · between Cape Byron and Evans Head is per-haps an indication of a change in the rate of advectiveacceleration between the two zones.

The cross-shelf momentum balance in the interior andnear the surface was unremarkable in all zones, withgeostrophy clearly dominant. At some locations the ad-vection term was nonzero; however the relative mag-

nitude of this term was small compared the the geo-strophic terms.

Recall that t was positive in the BBL off Cape Byronover the mid shelf and negative over mid depths. Thisrelationship is an indication of the mutual entrainmentof the colder BBL water and the warmer interior watersubject to the strong vertical mixing resulting from theEAC’s interaction with the local topography.

This analysis was also performed for day 7.5, indi-cating the same qualitative balances. The magnitudes ofthe terms, particularly (Km z)z and (Kmuz)z in the BBLand v · and Py in the mid and near-surface depths


FIG. 14. Temperature equation terms ( C 103) for zones 1, 2, and 3 (left to right) on day 15. The cross-shelf isobath locations are indicated on the top of each panel.

were about half those presented here for day 15. Thistemporal increase indicates that the balances discussedabove were persistent and were not being attenuatedthrough the BBL shutdown process of MacCready andRhines (1993).

e. Analysis of the thermal wind relation

The finding that geostrophy was not ‘‘exclusive’’ inthe BBL off Cape Byron has implications to the validityof the MacCready and Rhines (1993) BBL shutdowntheory. As outlined in section 1, their theory assumesthat the thermal wind relation, given in Eq. (1), is sat-

isfied. In order to investigate this, the left-hand side (lhs)of the thermal wind relation / z and the right-handside (rhs), [g/( f 0)] / x, have been reconstructed inz coordinates from the sensitivity experiments 2–6, andis presented in Fig. 15. One would expect that the fieldsbe equivalent, and indeed they are closely related; how-ever the relation is clearly not satisfied for any of theexperiments. The closest balance between the lhs andthe rhs was in experiment 5, which had a decreasedalongshelf transport. Recall that for this experiment, theBBL was not continuous from the shelf break to thecoast, indicating that BBL shutdown was more dominantunder these conditions.


FIG. 15. Cross-sections off Cape Byron (Y 135 km) of lhs (left)and rhs (right) of the terms in the thermal wind relation at day 15for various sensitivity experiments described in Table 1. Contoursrange from 0.05 to 0 with an interval of 0.01.

FIG. 16. As for Fig. 15 showing the local Richardson number (left)and the local Burger number (right). The gray, shaded region cor-responds to areas where Ri 0.2 and S 0.2, respectively . Contoursrange: 0 Ri 1 with intervals of 0.25 and 0 S 0.8 withintervals of 0.2. Ri and S are greater than 1 in the interior.

Additional insight into why the thermal wind relationwas not satisfied can be gained by considering the fieldsof the local Richardson number and the local Burgernumber, defined in Eqs. (2) and (3). These fields offCape Byron on day 15 for experiments 2–6 are shownin Fig. 16. These fields show that within the BBL overmuch of the shelf the Richardson number was below,or close to, the critical value of 0.25. This characteristicof the flow explains why the vertical diffusion terms ofthe momentum and conservation equations were highin this region. For most of the experiments, the lowestRichardson number flow was located very close to theshelf break, which is beneath the strong, baroclinic EACflow. This low Richardson number flow results in en-hanced vertical mixing, which will be effective at mix-

ing BBL and interior waters, as suggested in the analysisof the conservation equation for temperature. This char-acteristic has implications for the entrainment of nutri-ents into the EAC mean flow. Additionally the highvertical mixing in the BBL means that the local bouy-ancy frequency will be reduced, which in turn decreasesthe local Burger number, in Eq. (3). The fields of localBurger number indicate that a Burger number of below0.2 was maintained in the BBL throughout the simu-lations, again with the exception experiment 5. As out-lined in section 1 the low Burger number flow corre-sponds to an increased shutdown timescale of the BBL,explaining the persistence of the upwelling BBL flow.Beyond the shelf break, the near-bottom Burger numberincreases dramatically due to the increase in bottom


FIG. 17. Cross-sections of alongshelf velocity ( 0.1 m s 1) off Tweed Heads, Cape Byron,and Evans Head showing the 1.0 m s 1 (bold dashed), 0.5ms 1 (bold dashed), and 0.0 m s 1

(bold solid) contours.

slope. For this reason the shutdown timescale is shorteroff the shelf, which indicates that a logical ‘‘upwellinglimit’’ of this type of current-driven upwelling is thatupwelling waters originate no deeper than the shelfbreak.

5. Discussion

The baroclinic variables off Cape Byron showed thepresence of a shoreward BBL flow on day 15. Conse-quently, the effect on the NSZ was to generate a thermalfront, concentrated off Cape Byron, similar to that ob-served by OMb and Godfrey et al. (1980a).

A comparison of the magnitude and structure of thealongshelf velocity fields off Tweed Heads (Y 188km), Cape Byron (Y 135 km), and Evans Head (Y

80 km) in Fig. 17 shows that the EAC intensified nearCape Byron with strong currents in very close proximityto the shelf topography, confirming the suggestion thatthe EAC undergoes an advective, or Lagrangian, ac-celeration near Cape Byron. This feature explains theregion of high bottom stress in this vicinity, indicatingthat the advective acceleration off Cape Byron increasednear the bottom currents, consequently increasing thebottom stress. The region of high bottom stress offEvans Head was located offshore of the 175-m isobath.From Fig. 17 it is evident that at these depths the along-shelf velocities immediately above the bottom were non-zero; however shoreward of the 175 m isobath, thealongshelf velocities were very close to zero, explainingthe isolation of the high bottom stress region. The reasonfor the current acceleration off Cape Byron can be un-


FIG. 18. Alongshelf variations of the cross-sectional area between the coast and the 300-misobath (left) and the shelf width (right).

derstood by considering the alongshelf variations of thecross-sectional area of the shelf and the shelf widththroughout the model domain (Fig. 18), which shows aminimum of both at Cape Byron. Consequently, it isproposed that, since the EAC is held to the continentalslope by potential vorticity constraints, the flow is anal-ogous to flow through a narrowing pipe. Such a flowexhibits its strongest flow at the narrowest point, whichfor the present study was Cape Byron. The strengthenedcurrents acted to increase the near-bottom currents, re-sulting in an area of high bottom stress, which thendrove the upwelling BBL, causing uplifting, and finallyupwelling into the NSZ.

This upwelling mechanism, first identified by Hsuehand O’Brien (1971), is similar to that proposed by Bo-land (1979) and Blackburn and Cresswell (1993) for theEAC region; however neither have linked the topo-graphic variation to the onset of upwelling. It is alsoconsistent with the explanation posed by Rochford(1975) to explain upwellings off Laurieton, on the NSWcentral coast, which were unrelated to local winds. Blan-ton (1971) inferred a similar uplifting mechanism fromobservations in the Gulf Stream region; however it isunlikely that these intrusions were topographically in-duced since the alongshelf variation of the continentalshelf width varies much more subtlely in the South At-lantic Bight. Observations presented by Gill and Schul-mann (1979) off South Africa showed the accelerationof the alongshelf current over a gradually narrowingcontinental shelf, a feature similar to that modeled here.

As outlined in section 1, a strong argument againstupwelling driven by an alongshelf flow has been pre-sented by MacCready and Rhines (1993), who argued

that subject to geostrophy, such a BBL would shutdown.It was shown in section 4 that the BBL flow was notpurely geostrophic and that low Richardson number andlow Burger number flow results in the persistence of anupwelling BBL flow. Furthermore, there was a definitecorrelation between the near-surface advection and thenear-bottom vertical diffusion terms. The results fromthe conservation balance indicated that off Cape Byronthe dominant balance in the BBL was

v · (Kh z)z,

which further explains why BBL shutdown was notforthcoming. Since the strong vertical mixing betweenthe cold upwelled waters and the warm interior watersacted to reduce the horizontal temperature (density) gra-dient between the upwelled BBL water and the interior,the vertical velocity shear that would normally act toreduce the bottom currents, by the thermal wind relation,was reduced, thus inhibiting the shutdown process. TheBurger number in the BBL was very low on day 15, ofthe order of 0.15, which corresponds to a shutdown timeof 7.5 days. If a constant upwelling BBL, with u 0.05ms 1, was driven for 7.5 days, then the isotherm dis-placement is potentially of the order of 32.4 km, far inexcess of the distance from the shelf break to the coast,which is around 21 km. These calculations indicate that,for the given Burger number, even a weak BBL flowdriven by the EAC will be sufficient to establish a near-shore upwelled thermal front.

In a numerical study involving the configuration ofa two-dimensional, cross-shelf model of the Sydneyshelf, which assumed that the topography was homo-geneous in the alongshelf direction, Gibbs et al. (1998)


argued that EAC-driven upwelling through a BBL wasinefficient. They showed that such a BBL was shutdownwithin the first few days of simulation. Their resultswere validated by observations, justifying the assump-tion of homogeneous topography for the Sydney region.However, in this study it is suggested that the alongshelftopographic variations were crucial to the simulated up-welling and the results from the idealized cross-shelfmodel of Gibbs et al. (1998) are clearly not valid forthis region.

The suggestion that the EAC does experience regionsof high bottom stress along its path is further supportedby the findings of Godfrey et al. (1980b), who used thedistribution of different sediments along the east coastof Australia to infer the typical location of the EACseparation point. Godfrey et al. argued that where thecoarsest sediment was located was an indication ofwhere the EAC typically ‘‘spins down,’’ and separatesfrom the coast. During this spindown, the sediment,which is entrained in the EAC flow, is deposited on theocean floor. Of significance to this study is that suchsediment is actively entrained in the EAC flow to thenorth of the separation point, indicating that a frictionalinteraction between the EAC and topography is a com-mon occurrance.

6. Conclusions

The numerical experiments presented in this studyindicate that alongshelf topographic variations can havea significant impact on the nearshore and shelf circu-lation. For the idealized scenario considered, involvingthe narrowing of the continental shelf near Cape Byron,it was clear that these variations accelerated the along-shelf flow through enhanced advection over mid andnear-surface depths. As a result, the alongshelf, near-bottom currents were increased, resulting in an area ofhigh bottom stress, which drove an upwelling BBL flow.This BBL flow was persistent throughout the simula-tions and was clearly not attenuated by BBL shutdown(MacCready and Rhines 1993) for most experiments.The analysis of the momentum equations indicated thatan Ekman-like balance was dominant in the BBL andthat the incidence of high vertical diffusion in the BBLwas strongly related to high advection in the ocean in-terior. Furthermore, it is clear that the advective accel-eration of the flow, along with the high level of verticalmixing between the upwelled BBL water and the interiorshelf waters, interfered with the geostrophic adjustmentof the BBL, resulting in a low Burger number flow witha correspondingly high shutdown timescale, thus ex-plaining why BBL shutdown did not prevail. It wasobserved that the isotherm slopes quickly approachedthat of the bottom slope, at which point along-isopycnalmixing was maintained, without the opposing effects ofbuoyancy. This relationship provides a mechanism bywhich nutrient-rich slope waters are upwelled and en-trained into the EAC mean flow. Such a mechanism has

been sought in many observational studies of the EACregion (e.g., Tranter et al. 1986; Hallegraeff and Jeffrey1993; Cresswell 1994) to explain how high levels ofnutrients typically accompany EAC eddies commonlyencountered along the NSW coast.

As a result of the persistent, topographically induced,EAC-driven upwelling a nearshore thermal front wasgenerated in the NSZ, with similar characteristics tothose observed by OMb and Godfrey et al. (1980a) offCape Byron. Such fronts have been observed to havesignificant biological implications, representing regionsof high productivity (e.g., Olsen et al. 1983). Since thenearshore fronts observed off eastern Australia are gen-erated by EAC-driven upwelling, they are likely to beseasonal and dependent on the strength of the EAC. Thisseasonal dependence is supported by the common re-peated observations of an upwelled, thermal front in thevicinity of Cape Byron in spring and summer periods.This dependence was verified by a sensitivity test insection 3, where it was shown that a decrease (increase)in the magnitude of the alongshelf transport, corre-sponded to a weakening (strengthening) of the shore-ward BBL flow, consequently reducing (increasing) theamount of cold water upwelled to the surface.

In addition to this mechanism being identified, a com-parison between a LDM and a larger EDM were made.Although the results from both model configurationswere in good agreement, it was clear that there are sig-nificant advantages in extending the model domain, withhomogeneous topography, where lateral boundary forc-ing is imposed. This allows the model physics to adjustthe internal structure of the baroclinic variables beforethe realistic topographic influences are encountered bythe flow.

Acknowledgments. This work was supported by theAustralian Research Council through grants to JasonMiddleton and by an Australian Postgraduate ResearchAward to Peter Oke. The Australian National Universityis also acknowledged for making available the FujitsuVPP300 Supercomputer for this research. The supportof the Australian National Greenhouse Advisary Com-mittee is also acknowledged.

REFERENCES

Allen, J. S., P. A. Newberger, and J. Federiuk, 1995: Upwelling cir-culation on the Oregon continental shelf. Part I: Response toidealized forcing. J. Phys. Oceanogr., 25, 1843–1866.

Beckman, A., and D. B. Haidvogel, 1993: Numerical simulation offlow around a tall seamount. Part I: Problem formulation andmodel accuracy. J. Phys. Oceanogr., 23, 1736–1753.

Blackburn, S. I., and G. R. Cresswell, 1993: A coccolithophoridbloom in Jervis Bay, Australia. Aust. J. Mar. Freshwater Res.,44, 253–260.

Blanton, J., 1971: Exchange of Gulf Stream Water with North Car-olina shelf water in Onslow Bay during stratified conditions.Deep-Sea Res., 18, 167–178.

Blumberg, A. F., and G. L. Mellor, 1983: Diagnostic and prognostic


numerical circulation studies of the South Atlantic Bight. J. Geo-phys. Res., 88, 4579–4592., and L. H. Kantha, 1985: Open boundary conditions for cir-culation models. J. Hydr. Eng., 111, 237–255., and G. L. Mellor, 1987: A description of a three-dimensionalcoastal ocean circulation model. Three-Dimensional CoastalOcean Models, Vol. 4, N. Heaps, Ed., Amer. Geophys. Union,1–16.

Boland, F. M., 1979: A time series of expendable bathythermographsections across the East Australian Current. Aust. J. Mar. Fresh-water Res., 30, 303–313.

Boucher, J., F. Ibanez, and L. Prieur, 1987: Daily and seasonal var-iations in the spatial distribution of zooplankton populations inrelation to the physical structure in the Ligurian Sea Front. J.Mar. Res., 45, 133–173.

Church, J. A., and G. R. Cresswell, 1986: Oceanographic features ofSoutheast Australian Waters. CSIRO Marine Laboratories In-ternal Summary Rep. [Available from CSIRO Marine Research,GPO Box 1538, Hobart, Tasmania 7001, Australia.]

Cresswell, G. R., 1994: Nutrient enrichment of the Sydney continentalshelf. Aust. J. Mar. Freshwater Res., 45, 677–691., C. Ellyett, R. Legeckis, A. F. Pearce, and R. Boyd, 1983: Near-shore features of the East Australian Current system. Aust. J.Mar. Freshwater Res., 34, 105–114.

Flather, R. A., 1976: A tidal model of the northwest European con-tinental shelf. Mem. Soc. Roy. Sci. Leige, Ser. 6, 10, 141–164.

Garret, C., P. MacCready, and P. Rhines, 1993: Boundary mixing andarrested Ekman layers: Rotating stratified flow near a slopingboundary. Annu. Rev. Fluid Mech., 25, 291–323.

Gibbs, M. T., J. H. Middleton, and P. Marchesiello, 1998: Baroclinicresponse of Sydney shelf waters to local wind and deep oceanforcing. J. Phys. Oceanogr., 28, 178–190.

Gill, A. E., and E. H. Schulmann, 1979: Topographically inducedchanges in the structure of an inertial coastal jet: Application tothe Agulhas Current. J. Phys. Oceanogr., 9, 975–991.

Godfrey, J. S., G. R. Cresswell, and F. M. Boland, 1980a: Obser-vations of low Richardson numbers and undercurrents near afront in the East Australian Current. J. Appl. Meteor., 10, 301–307., , T. J. Golding, and A. F. Pearce, 1980b: The separationof the East Australian Current. J. Phys. Oceanogr., 10, 430–440.

Hallegraeff, G. M., and S. W. Jeffrey, 1993: Annually recurrent di-atom blooms in spring along the New South Wales coast ofAustralia. Aust. J. Mar. Freshwater Res., 44, 325–334.

Hitchcock, G. L., A. J. Mariano, and T. Rossby, 1993: Mesoscalepigment fields in the Gulf Stream: Observations in a meandercrest and trough. J. Geophys. Res., 98, 8425–8445.

Hsueh, Y., and J. J. O’Brien, 1971: Steady coastal upwelling inducedby an along-shore current. J. Phys. Oceanogr., 1, 180–186.

Hynd, J. S., 1969: Isotherm maps for tuna fisherman. Aust. Fish.,28(7), 13–22.

Levitus, S., 1982: Climatological Atlas of the World Ocean. NOAAProf. Paper No. 13, U.S. Department of Commerce, NationalOceanographic and Atmospheric Administration, 173 pp.

MacCready, P., and P. B. Rhines, 1993: Slippery bottom boundarylayers on a slope. J. Phys. Oceanogr., 23, 5–22.

Mahadevan, A., J. Oliger, and R. Street, 1996: A nonhydrostaticmesoscale ocean model. Part I: Well-posedness and scaling. J.Phys. Oceanogr., 26, 1868–1880.

Mellor, G. L., and T. Yamada, 1982: Development of a turbulenceclosure model for geophysical fluid problems. Rev. Geophys.Space Phys., 20, 851–875., T. Ezer, and L.-Y. Oey, 1994: The pressure gradient conundrumof sigma coordinate ocean models. J. Atmos. Oceanic Technol.,11, 1126–1134.

NOAA, 1988: NGDC data announcement 88-MG-02: Digital reliefof the surface of the Earth. U.S. Department of Commerce,NOAA, NGDC, Boulder, CO.

Oke, P. R. and J. H. Middleton, 2000: Nutrient enrichment off PortStephens: the Role of the East Australian Current. Contin. ShelfRes., in press.

Olsen, D. B., O. B. Brown, and S. R. Emmerson, 1983: Gulf Streamfrontal statistics from Florida Straits to Cape Hatteras derivedfrom satellite and historical data. J. Geophys. Res., 88, 4569–4577.

Palma, E. D., and R. P. Matano, 1998: On the implementation ofpassive open boundary conditions for the Princeton Ocean Mod-el: The barotropic mode. J. Geophys. Res., 103, 1319–1341.

Ridgway, K. R., and J. S. Godfrey, 1997: Seasonal cycle of the EastAustralian Current. J. Geophys. Res., 102, 22 921–22 936.

Rochford, D. J., 1972: Nutrient enrichment of East Australian coastalwaters. I. Evans Head upwelling. CSIRO Aust. Div. Fish. Ocean-ogr. Tech. Paper 33, 1-37. [Available from CSIRO Marine Re-search, GPO Box 1538, Hobart, Tasmania 7001, Australia.], 1975: Nutrient enrichment of East Australian coastal waters.II. Laurieton upwelling. Aust. J. Mar. Freshwater Res., 26, 233–243.

Smagorinsky, J., 1963: General circulation experiments with primi-tive equations, I. The basic experiment. Mon. Wea. Rev., 91,99–164.

Stanton, B. R., 1976: An oceanic frontal jet near the Norfolk Ridgenorth-west of New Zealand. Deep-Sea Res., 23, 821–829.

Tranter, D. J., D. J. Carpenter, and G. S Leech, 1986: The coastalenrichment effects of the East Australian Current eddy field.Deep-Sea Res., 33, 1705–1728.

Topographically Induced Upwelling off Eastern Australia

Documents

Transcript of Topographically Induced Upwelling off Eastern Australia