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The Dynamics of the East Australian Current System: The Tasman Front, the East Auckland Current, and the East Cape Current

CHARLES E. TILBURG* Center for Ocean±Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida

HARLEY E. HURLBURT Naval Research Laboratory, Stennis Space Center, Mississippi

JAMES J. O'BRIEN Center for Ocean±Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida

JAY F. S HRIVER Naval Research Laboratory, Stennis Space Center, Mississippi

(Manuscript received 10 May 2000, in ®nal form 22 February 2001)

ABSTRACT The dynamics of the ¯ow ®eld surrounding New Zealand are investigated using a series of global ocean models. The physical mechanisms governing the direction, magnitude, and location of the East Australian Current (EAC), the Tasman Front, the East Auckland Current (EAUC), and the East Cape Current (ECC) are studied using numerical simulations whose complexity is systematically increased. As new dynamics are added to each successive simulation, their direct and indirect effects on the ¯ow ®eld are examined. The simulations have horizontal resolutions of 1/8Њ, 1/16Њ, or 1/32Њ for each variable, and vertical resolutions ranging from 1.5-layer reduced gravity to 6-layer ®nite depth with realistic bottom topography. All simulations are forced by the Hellerman and Rosenstein monthly wind stress climatology. Analysis of these simulations shows that several factors play a critical role in governing the behavior of the examined currents. These factors include 1) mass balance of water pathways through the region, 2) gradients in the wind stress curl, 3) nonlinear ¯ow instabilities, and 4) upper-ocean±topographic coupling due to mixed baroclinic and barotropic instabilities. Transport stream- functions of a linear reduced gravity model reproduce the large-scale features well but produce an EAUC that ¯ows counter to the observed direction. The residual of the mass balance of the transport through the Tasman Sea, the basinwide transport at 32ЊS, and the transport of the South Paci®c subtropical gyre east of New Zealand determines the direction of the EAUC. The 6-layer nonlinear model allows isopycnal outcropping, which changes the transport through the Tasman Sea and produces an EAUC ¯owing in the observed direction. Gradients in the zonally integrated wind stress curl ®eld determine the coastal separation points of the EAC, the EAUC, and the ECC, while a combination of nonlinear ¯ow instabilities and upper-ocean±topographic coupling contribute to the formation of meanders in the Tasman Front. Increased resolution results in greater mixed baroclinic± barotropic instabilities and thus more upper-ocean±topographic coupling and surface variability, giving a more accurate simulation of topographically controlled mean meanders in the Tasman Front.

1. Introduction and its subsequent eastward ¯ow into the South Paci®c creates one of the most complicated and dynamically The East Australian Current is the western boundary interesting regions in the world's oceans (Godfrey et al. current of the South Paci®c subtropical gyre. The partial 1980). Extending from the Coral Sea to the Tasman Sea, separation of this current from the coast of Australia the East Australian Current (EAC) system generates nu- merous eddies and has several branches including the * Current af®liation: Graduate College of Marine Studies, Uni- Tasman Front, the East Auckland Current (EAUC), the versity of Delaware, Newark, Delaware. East Cape Current (ECC), and the EAC extension. A schematic of the ¯ow ®eld (Fig. 1) reveals a boundary Corresponding author address: Dr. Charles E. Tilburg, Graduate current system that affects the ¯ow along much of the College of Marine Studies, University of Delaware, 102 Robinson eastern coasts of Australia and New Zealand. The region Hall, Newark, DE 19716. is highly variable due to eddy generation and extensive

᭧ 2001 American Meteorological Society

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FIG. 1. Schematic diagram of the major surface currents, features and transport sections (gray lines) in the Tasman Sea, and the region surrounding New Zealand. EAC ϭ East Australian Current, EAUC ϭ East Auckland Current, ECC ϭ East Cape Current, TF ϭ Tasman Front, SC ϭ Southland Current, NCE ϭ North Cape Eddy, ECE ϭ East Cape Eddy, WE ϭ Wairarapa Eddy. current meandering. Bottom steering signi®cantly af- eastern coast of New Zealand. Stanton (1976) ®rst iden- fects the surface ¯ow, and a large part of the complexity ti®ed a variable frontal jet near the Norfolk Ridge be- of the region can be attributed to the varied bottom tween 165Њ and 169ЊE. Denham and Crook (1976) topography (Fig. 2), which is dominated by the exten- showed that the Tasman Front extends from the Norfolk sive New Zealand submarine platform. Ridge to the Lord Howe Rise (ϳ160ЊE). An oceano- Unlike other western boundary currents, such as the graphic survey detailed by Andrews et al. (1980) de- Kuroshio and the Gulf Stream, the strength of the EAC scribed a front that exists along the entire northern edge varies substantially with time. The mass transport within of the Tasman Sea. Other investigations of the meanders eddies spawned by the EAC can be much larger than and variability of this front (Stanton 1979, 1981; Mul- its mean ¯ow (Ridgway and Godfrey 1997; Lilley et al. hearn 1987) at different locations demonstrated that, 1986). Composites of ship and satellite observations although the Tasman Front is highly variable, its mean show that a large portion of the EAC leaves the coast path follows meanders that are permanent features of of Australia between 30Њ and 34ЊS, while a weaker ¯ow the ¯ow ®eld. The most probable path (shown as a white continues southward attached to the coast. Godfrey et line in Fig. 2) was constructed by combining a similar al. (1980) demonstrated that, although the exact sepa- diagram by Mulhearn (1987) and a mean frontal path ration point varies seasonally, the mean behavior of the obtained from a number of cruises described by Stanton EAC ¯ow ®eld is signi®cantly different on either side (1979). Mulhearn (1987) used a series of satellite in- of a line extending south-southeast of Sugarloaf Point frared images to determine the most probable path of (32.5ЊS). Just south of Sugarloaf Point, northward shelf the Tasman Front from 153Њ to 164ЊE during 29 months currents are common and eddies are essentially circular, from March 1982 to April 1985. Stanton (1979) showed while just north of this point the currents are southward the location of the Tasman Front between 166Њ and and eddies are much more elongated. Although the ma- 170ЊE for ®ve different cruises from 1966 to 1975. Com- jority of the EAC separates from the Australian coast bining these observations reveals a most probable path and either proceeds eastward or recirculates, as much of the Tasman Front that leaves the coast of Australia as 1/3 of the original EAC exits the southern edge of at 33ЊS, 153ЊE and meanders eastward between latitudes the Tasman Sea, trapped against the western boundary of 36Њ and 32ЊS. Although the front shows substantial (Ridgway and Godfrey 1997; Chiswell et al. 1997). variability, the mean path includes several permanent As the separated portion of the EAC proceeds east- meanders. ward toward New Zealand, it crosses the northern edge Heath (1985a) provided a thorough summary of the of the Tasman Sea following a narrow, meandering path- ¯ow ®eld surrounding New Zealand. Once the eastward way. Warren (1970) ®rst suggested that this ¯ow, as- ¯ow along the Tasman Front reaches the northernmost sociated with the Tasman Front, is a zonal jet connecting point of New Zealand, it turns southeastward, becoming the EAC with the western boundary currents on the the EAUC and following the northeast coastline of New

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FIG. 2. Sea¯oor topography surrounding New Zealand. Note the large New Zealand submarine platform and the extensive ridges and troughs throughout the area. The white line represents the most probable path of the Tasman Front adapted from a similar ®gure by Mulhearn (1987) and a compilation of observations by Stanton (1979). The letters associated with the meanders are used to distinguish each meander for discussion in later sections.

Zealand's North Island. Although the majority of the the ECC. The ECC separates from North Island at ap- ¯ow gradually separates from the coast and continues proximately 42ЊS and proceeds eastward slightly north eastward, a small portion remains attached to the coast of the Chatham Rise (Heath 1985a,b). and continues to the easternmost tip of North Island, Although pathways of the EAUC and the ECC are East Cape. As the ¯ow rounds East Cape, it becomes highly variable (Laing et al. 1996), a mean dynamic

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reproducing the EAUC, the ECC, and the Southland Current but were unable to resolve the quasi-stationary eddies surrounding New Zealand. De Szoeke (1987) used a linear reduced gravity 3.5-layer model driven by annual wind stress encompassing the South Paci®c from 57ЊS to almost the equator. He reproduced the large- scale features well, including the EAC separation and basinwide density surfaces. His model, based on that of Luyten et al. (1983), did not incorporate cross-isopycnal ¯ux, which he suspected was responsible for weaker than observed shallow currents. Semtner and Chervin (1992) brie¯y examined this region in the context of their ½Њ 20-level global model that incorporated realistic bottom topography and was driven by the Hellerman and Rosenstein (1983, hereafter HR) monthly wind stress climatology. They found a highly variable EAC ranging from 7 to 47 Sv (106 m3 sϪ1) ¯owing southward and a highly baroclinic transport through the Tasman Sea, ranging from 3 Sv northward to 10 Sv southward. The simulated surface ¯ow within the Tasman Sea was northward, while the deeper levels were southward. Examination of the surface ¯ow ®eld reveals the presence of the three observed eddies sur- FIG. 3. Climatological dynamic sea surface height (dynamic cm) rounding New Zealand, meanders in the Tasman Front, with respect to 2000 db in the region north of New Zealand compiled and currents surrounding New Zealand ¯owing in the by Roemmich and Sutton (1998). Note the presence of the three eddies observed direction. Closer investigation, however, labeled in Fig. 1: NCE, ECE, and WE. shows that the simulated meanders do not agree with observations in terms of amplitude or phase. Ridgway height ®eld (Fig. 3) presented by Roemmich and Sutton and Godfrey (1994) compared their observations to a (1998) shows that three quasi-stationary eddies are pre- simple linear Sverdrup±Munk model of the World sent in the ¯ow ®eld. The North Cape Eddy (NCE), the Ocean by Godfrey (1989), ®nding that the linear model southern edge of which is the beginning of the EAUC, produced the observed patterns of EAC separation and is centered near 33ЊS, 173.5ЊE. The East Cape Eddy transport through the Tasman Sea. They noted that the (ECE), the southern edge of which comprises the end southward transport through the Tasman Sea was higher of the EAUC and the beginning of the ECC, is centered than observed and that the linear model was unable to near 35.5ЊS, 178.5ЊE. The Wairarapa Eddy (WE), the reproduce the eddies or meanders of the observed ¯ow. western and southern edges of which compose the ma- In this paper, we build on this earlier work by using jority of the ECC, is centered near 41ЊS, 178.5ЊE. Also 1/8Њ, 1/16Њ and 1/32Њ multilayered global ocean models present in the dynamic height ®eld is a relative low to examine the dynamics governing the ¯ow ®eld en- between NCE and ECE. Uddstrom and Oien (1999), compassing the Tasman Front, the EAUC, and the ECC. using satellite derived sea surface temperatures (SST), The EAC and the ¯ow around New Zealand are situated were also able to identify the NCE and the WE, although in one of the most diverse and interconnected regions they did not ®nd a robust ECE. of the world's oceans, in¯uenced by the Antarctic Cir- Although there have been relatively few numerical cumpolar Current (ACC), the Indo±Paci®c Through¯ow simulations of this region, both linear and nonlinear (IPT), and the global thermohaline circulation. A global models have been used to study its dynamics. Godfrey model is necessary to capture the range of essential (1973) presented a qualitative study of the EAC using dynamics. Increasingly realistic and dynamically com- results derived from a regional Bryan and Cox (1968) plex simulations are performed to determine which dy- model, an idealized ¯at-bottom six-level nonlinear sim- namics are most important in governing the different aspects of the ¯ow ®eld. The ocean model and numerical ulation using annual wind stress and differential heating. simulations used in this investigation are discussed in He found that the simulated eddies and ¯uctuations were section 2. The results and dynamical interpretation of similar to observations, but that the model was unable the different simulations are discussed in section 3, to reproduce the observed vorticity balance or season- while summary and conclusions are given in section 4. ality of the EAC due to its lack of bottom topography and use of annual wind stress climatology. Bye et al. (1979), using a diagnostic linear regional model driven 2. The ocean model by speci®ed density, wind stress, and ¯ow boundaries, The numerical model used in these investigations is obtained a steady-state ¯ow ®eld around New Zealand, the Naval Research Laboratory Layered Ocean Model

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(NLOM), a primitive equation layered formulation with Moore and Wallcraft 1998). The equations for the n- equations that have been integrated through each layer. layer ®nite-depth, hydrodynamic model are given below This model is a descendent of the model detailed by for layers k ϭ 1,´´´,n with k ϭ 1 for the top layer. In Hurlburt and Thompson (1980) with greatly expanded places where k is used to index model interfaces, k ϭ capabilities (Wallcraft 1991; Wallcraft and Moore 1997; 0 is the surface and k ϭ n is the bottom:

(Vu cos␪)ץ (Uu)ץ U 1ץ kkkkkϩϩϪV (u sin␪ ϩ a⍀ sin2␪ ␪ kkץ ␾ץ[]tacos␪ץ ϭ max(0, Ϫ␻kϪ1)ukϪ1 ϩ max(0, ␻kk)u ϩ1 Ϫ [max(0, Ϫ␻kk) ϩ max(0, ␻ Ϫ1)]uk ( h Ϫ H)ץ h n ϩ max(0, ϪC ␻ )(u Ϫ u ) ϩ max(0, C ␻ )(u Ϫ u ) Ϫ k G jj MkϪ1 kϪ1 kMkkϩ1 kkj͸ ␾ץ a cos␪ jϭ1 he cos2␪)ץ he cos␪)ץ A ϩ (␶ Ϫ ␶ )/␳ ϩϩk ␾␾kkk ␾␪ (1) ␪ץ ␾ץ[]␾␾kϪ1 k o a22cos ␪ (V ␷ cos␪)ץ ( U ␷)ץ V 1ץ kkkkkϩϩϩU (u sin␪ ϩ a⍀ sin2␪ ␪ kkץ ␾ץ[]tacos␪ץ ϭ max(0, Ϫ␻kϪ1)␷ kϪ1 ϩ max(0, ␻kk)␷ ϩ1 Ϫ [max(0, Ϫ␻kk) ϩ max(0, ␻ Ϫ1)]␷ k ( h Ϫ H)ץ h n ϩ max(0, ϪC ␻ )(␷ Ϫ ␷ ) ϩ max(0, C ␻ )(␷ Ϫ ␷ ) Ϫ k G jj MkϪ1 kϪ1 kMkkϩ1 kkj͸ ␪ץ a jϭ1 he cos2␪)ץ he cos␪)ץ A ϩ (␶ Ϫ ␶ )/␳ ϩϩk ␾␪kkk ␪␪ (2) ␪ץ ␾ץ[]␪␪kϪ1 k o a22cos ␪ hץ (V ϭ ␻ Ϫ ␻ . (3 ´ ١ k ϩ t kkkϪ1ץ

Notation that is common in oceanography is used in the discussed in detail by Shriver and Hurlburt (1997). Is- model equations. A full explanation of the parameters opycnal outcropping and vertical mixing permit over- and notation used in these equations is given in the turning circulations in the vertical, such as the ther- appendix. The model boundary conditions are kinematic mohaline circulation and meridional overturning, and and no slip. The model equations are integrated on a C allow shallow layers on basinwide scales. Isopycnal out- grid (Mesinger and Arakawa 1976) using a semi-im- cropping and vertical mixing can also affect horizontal plicit numerical scheme for the ®nite-depth simulations mass transports, which is an especially important mech- and an explicit numerical scheme for reduced-gravity anism in our region of interest, as will be seen in section simulations. 3b. Although thermodynamic versions of the model exist A modi®ed version of the 1/12Њ ETOPO5 bottom to- (Metzger et al. 1992; Heburn 1994; Metzger and Hurl- pography (National Oceanic and Atmospheric Admin- burt 1996), the versions used in this investigation are istration 1986) is used in the simulations requiring re- hydrodynamic with constant density in each layer. As alistic bottom topography. The topography is ®rst in- a result, thermal forcing and steric anomalies due to terpolated to the model grid and then smoothed twice seasonal heating and cooling are excluded. The model with a nine-point smoother to reduce energy generation does permit isopycnal outcropping via ventilation of at smaller scales that are poorly resolved by the model. model layer interfaces. This ventilation is achieved by The maximum depth of the model is set at 6500 m. The a process known as ``hydromixing'' (Wallcraft 1991). minimum depth, set at 200 m, is used as the model When any layer's thickness decreases to a certain min- boundary with a few exceptions where shallower depths ϩ imum value (hk ), water is entrained from the layer below are needed to connect semienclosed seas. The bottom at a velocity (␻k) needed to prevent negative layer thick- topography is con®ned to the lowest layer (Hurlburt and ness. Mass and volume are conserved within each layer Thompson 1980) to prevent numerical dif®culties aris- by compensating domainwide diapycnal mixing. The ing when moving layer interfaces intersect with sloping pattern of vertical mixing is determined by isopycnal topography and to greatly decrease the computer time/ outcropping and observed oxygen saturation values as model year. Two of the main reasons for including bot-

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TABLE 1. Shown are World Ocean simulations, where, in cases using realistic bottom topography, a scaling factor was used to restrict bottom topography features to layer 6. The model is run in linear mode by scaling the winds down by a factor of 1000. The model ®elds are then scaled up by a factor of 1000 to restore them to their proper magnitude. A H Model years Experiment (m2 sϪ1) (m) spanned Comments RG1 100 250/ϱ 250±325 Linear RG2 100 250/ϱ 325±450 Linear, removed New Zealand RG3 100 250/ϱ 325±518 Linear, friction patch in Tasman Sea RG4 100 250/ϱ 325±578 Linear, Indo±Paci®c Through¯ow closed FB 100 155/185/210/225/225/5500 282±299 Nonlinear, ¯at bottom RG5.5 100 155/185/210/225/225/ϱ 389±412 Nonlinear, reduced gravity RB8 100 155/185/260/375/525/variable 860±975 Nonlinear, realistic bottom topography RB16 20 155/185/260/375/525/variable 847±899 Nonlinear, realistic bottom topography RB32 10 155/185/260/375/525/variable 910±917 Nonlinear, realistic bottom topography tom topography, regulating baroclinic instabilities and been performed using 1/8Њ±1/64Њ simulations from At- forcing abyssal ¯ow to follow f/h contours, are rela- lantic versions of this model (Hurlburt and Hogan 2000) tively unaffected by this modi®cation of the bottom to- and different 1/8Њ and 1/16Њ simulations from a Paci®c pography. version of the model (e.g., Mitchum 1995; Hurlburt et The simulations used in this investigation are de®ned al. 1996; Mitchell et al. 1996; and Metzger and Hurlburt in Table 1, and the model parameters are given in Tables 1996). Other studies have used variations of the global 1 and 2. The density for each layer was obtained from model to investigate the effect of the IPT on the global the Levitus (1982) ocean climatology. In the 6-layer thermohaline circulation (Shriver and Hurlburt 1997) simulations, the depths of the two upper layers were and the role of Halmahera Island in the transport path- chosen to represent a surface layer and a layer contain- ways of Paci®c waters to the Indian Ocean (Morey et ing the equatorial undercurrent, while the mean ®fth al. 1999). interface depth was chosen to represent the boundary between intermediate and abyssal water. The simula- 3. Climatological dynamics tions were forced by the HR monthly wind stress cli- matology to statistical equilibration and were initialized We rely on the modularity of NLOM, progressing from equilibrated lower-resolution simulations. All from a simple linear reduced-gravity 1/8Њ model to high- means shown are calculated from the last four years of ly realistic 1/16Њ 6-layer simulations incorporating non- the simulation. Numerous model-data comparisons have linear dynamics and realistic bottom topography to de-

TABLE 2. Shown are additional parameters, where the numbers 1±9 in the simulations column refer to the simulations as follows: 1, RG1; 2, RG2; 3, RG3; 4, RG4; 5, FB; 6, RG5.5; 7, RB8; 8, RB16; 9, RB32. A model parameter that is not applicable for that simulation is indicated by N/A. Parameters Value Simulations De®nition

Cb 0 1, 2, 3, 4 Bottom drag coef®cient 2 ϫ 10Ϫ3 5, 6, 7, 8, 9 ␣ N/A 1, 2, 3, 4, 6 Value that topographic amplitude with 0 5 respect to 6500 m was multiplied by 0.65 7 0.69 8, 9

Ck 0 1,2,3,4,5,6,7,8,9 kth interfacial stress coef®cient g 9.8msϪ2 1, 2, 3, 4, 5, 6, 7, 8, 9 Acceleration due to gravity ϩ hk N/A 1, 2, 3, 4 Thickness of layer k at which 50/40/40/40/40 m 5, 6, 7, 8 entrainment starts

␴k 25.45/27.55 1, 2, 3, 4 Density of layer k 25.24/26.47/26.99/27.23/27.39/27.77 5, 6 25.25/26.59/27.03/27.30/27.53/27.77 6, 7, 8, 9 ⌬␪ 1/8Њ 1, 2, 3, 4, 5, 6, 7 Latitudinal grid resolution 1/16Њ 8 1/32Њ 9 ⌬␾ 45/256Њ 1, 2, 3, 4, 5, 6, 7 Longitudinal grid resolution 45/512Њ 8 45/1024Њ 9

␻k N/A 1, 2, 3, 4 kth interface vertical reference mixing 0.02 m sϪ1 5, 6 velocity 0.04 m sϪ1 7 0.05/0.06/0.05/0.05/0.05 m sϪ1 8, 9

Unauthenticated | Downloaded 09/30/21 08:02 AM UTC OCTOBER 2001 TILBURG ET AL. 2923 termine which dynamics govern speci®c behavior in the 4b) reveals how the currents of interest are connected ¯ow ®eld. In addition, a 1/32Њ simulation is used to to the larger ¯ow ®eld. The EAC and ECC are western investigate simulation convergence with increased res- boundary currents of the innermost nested gyres. The olution. As the simulations increase in complexity, they Tasman Front is the southern edge of the innermost gyre more closely reproduce the observed features, suggest- bounded by Australia. The northern anticyclonic gyre ing that the new dynamics introduced with the new sim- east of New Zealand is bounded on the west by the ulation are responsible for the observed features. The ECC, but the southern one is unobserved. The south- inherent nonlinearity of ocean dynamics complicates ward western boundary current associated with this gyre this analysis process, since the ¯ow ®eld resulting from disagrees with the observed northward Southland Cur- the addition of a new physical mechanism may be due rent and its subsequent eastward ¯ow at the Chatham directly to the new addition or to its interaction with Rise as described by Chiswell (1996), Stramma et al. other dynamics. However, the relative roles of separate (1995), and Uddstrom and Oien (1999). Likewise, the physical mechanisms can be investigated by diagnostic simulated EAUC, the western boundary current con- calculations and careful comparison of simulations in- necting the two northern inner gyres, ¯ows opposite to volving different dynamics. Qualitative and quantitative the observed direction (northwestward in the linear sim- model-data comparisons are made throughout this dis- ulation versus southeastward observed). The simulated cussion to determine how well each successive simu- EAC and ECC both separate from the western bound- lation reproduces the observed ¯ow ®eld. aries at the observed locations (centered near 32ЊSon the east coast of Australia and near 42ЊS on the east a. Linear dynamics coast of New Zealand, respectively), while the simulated Tasman Front appears in the correct geographic location. The lowest-order estimate of the dynamics governing Although observational values of the transports are the ¯ow around New Zealand is a linear model (labeled sparse, a rudimentary model-data comparison can be RG1, Table 1), the dynamics of which are essentially made. The simulated linear EAC is 25.9 Sv, which those of a Sverdrup (1947) interior with Munk (1950) agrees well with observational values. Ridgway and western boundary layers and globally applied horizontal Godfrey (1997) estimated the ¯ow of the EAC relative friction. The ¯ow and sea surface height are calculated to 2000 db, using steric heights derived from historical using a global numerical model with realistic coastline hydrology and expendable bathythermograph (XBT) geometry and monthly climatological wind forcing. The data collected in the region, and found that the EAC model has one vertical mode and a bottom layer that is varies between 27.4 and 36.3 Sv seasonally. Chiswell in®nitely deep and at rest. Since the linear simulation et al. (1997), using CTD data gathered over ®ve cruises reveals the lowest-order response to external forcing, it between 1990 and 1994, estimated the geostrophic can be used as a baseline for the more complicated transport of the EAC relative to 2000 db to be between simulations that incorporate nonlinearities, bottom to- 22.2 and 42.2 Sv. The simulated linear transport asso- pography, multiple vertical modes, ¯ow instabilities, ciated with the Tasman Front, 10.6 Sv, also agrees well and isopycnal outcropping. with observational values. Stanton (1979), using bathy- The mean transport streamfunctions of the 1.5-layer thermograph (BT) data, calculated geostrophic volume linear 1/8Њ reduced-gravity simulation driven by the HR transport relative to 1000 db to be 7.6±8.5 Sv, which monthly wind stress climatology (Fig. 4a) reveal a sys- was slightly lower than his previous observations of 9± tem of nested anticyclonic gyres encompassing the 12 Sv (Stanton 1976). Andrews et al. (1980), using XBT Southern Hemisphere. Nested gyres were also identi®ed data, estimated the net zonal transport to be approxi- numerically by De Szoeke (1987) and Godfrey (1989) mately 15 Sv relative to 1300 db. More recent obser- and observationally by Davis (1998), using neutral vations yielded integrated volume transports measured buoyancy ¯oats. A large supergyre extends through all at 173ЊE ranging from Ϫ7.1 to 16.5 Sv over a 4-yr time three oceans following two different paths connecting frame (Chiswell et al. 1997). However, the authors hy- the Paci®c and Indian Oceans: one path extends through pothesized that the negative transport value in the Tas- the IPT (purple, dark blue, and blue contours), while man Front transport was due to an error in measurement the other travels south of Australia (light blue and blue- and should not be considered evidence of a westward green contours). Two smaller gyres compose an inner ¯ow into the Tasman Sea (Chiswell et al. 1997). The nesting: one gyre occupies the South Atlantic and Indian simulated transports from RG1 for selected sections Oceans (dark green to maroon contours) and the other along with those from the subsequent simulations and occupies the South Paci®c Ocean and extends south of observations are summarized in Table 3. The transport Australia (dark green contour bounded on west by Aus- sections surrounding New Zealand are shown as gray tralia). Three gyres, one bounded on the west by Aus- lines in Fig. 1. tralia (yellow, orange, and red contours), the other two As noted, there are some discrepancies between the bounded by New Zealand (light green contours), com- linear simulation and observations. The simulated linear pose the innermost nesting in our region of interest. EAUC (3.1 Sv to the west-northwest) is opposite in Closer inspection of the region east of Australia (Fig. direction to observations, which show part of the ¯ow

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FIG. 4. (a) Mean transport streamfunctions from the 1/8Њ linear 1.5-layer simulation RG1 for the Southern Hemisphere depicting the interbasin supergyre and the nested intrabasin gyres. The contour interval is 7.5 Sv (10 6 m3 s Ϫ1). The modeled land boundaries are chosen to be the 200-m isobath and are shown in all ®gures with the actual land superimposed over the modeled land. Flow proceeds with higher valued streamfunction to the right. Note the locations of the EAC, EAUC, ECC, and Tasman Front. The EAC and ECC separation points agree with observations. The contours are colored to allow identi®cation of different gyres. (b) Close-up of the mean transport streamfunctions from the 1/8Њ linear 1.5-layer global simulation RG1 for the region surrounding New Zealand. Note the incorrect direction of the EAUC. along the Tasman Front continuing along the northeast rent. These discrepancies suggest that linear dynamics coast of New Zealand. Heath (1985b) describes an east- are unable to explain some aspects of the ¯ow ®eld, and southeast ¯ow of 2±10 Sv, while other observations more complicated processes are necessary to fully de- (Stanton et al. 1997) describe larger transports of 11± scribe this region. 34 Sv. The simulated ECC (2.1 Sv southward) is much Since the linear model agrees with observed locations less than the observed 22 Sv (Stanton et al. 1997). The of EAC separation, we are able to use linear theory to simulated net southward transport through the Tasman investigate the baseline dynamics behind this phenom- Sea (12.1 Sv) is higher than observed values of 7.1 Sv enon. The unique position of New Zealand (directly in at 44ЊS (Ridgway and Godfrey 1997) and 6.3±9.2 Sv the path of a separating western boundary current) has at 43ЊS (Chiswell et al. 1997). Also, the southward- fueled speculation concerning the role of the island in ¯owing boundary current along South Island disagrees the separation of the EAC. Two main theories concern- with the observed northward-¯owing Southland Cur- ing its role focus on two different aspects of the ¯ow.

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TABLE 3. Shown is mean transport from selected sections, where values are given in Sverdrups (10 6 m3 sϪ1). Negative means west or south. Positive means east or north. All simulated transports are for the top ®ve layers, corresponding to the simulated thermocline. These transport sections are shown in Fig. 1 as gray lines. Paci®c Paci®c Oceana Oceanb East Australian Tasman EAUC (after EAUC (before East Cape Experiment at 32ЊS at 39ЊS Tasman Seac Currentd Fronte separation)f separation)g Currenth RG1 16.5 25.5 Ϫ12.1 Ϫ25.9 10.6 3.1 Ϫ2.5 Ϫ2.1 RG2 16.5 28.4 Ϫ11.9 Ϫ25.9 13.4 Ð Ð Ð RG3 16.8 25.3 Ϫ7.6 Ϫ25.5 14.6 Ϫ0.9 Ϫ6.5 Ϫ6.5 RG4 0.0 25.5 Ϫ28.7 Ϫ42.4 10.5 3.2 Ϫ2.4 Ϫ2.1 FB 11.7 21.1 5.3 Ϫ26.7 31.6 Ϫ13.5 Ϫ24.6 Ϫ14.9 RB8 13.6 17.1 0.1 Ϫ27.2 23.1 Ϫ3.8 Ϫ31.0 Ϫ11.7 RB16 12.2 14.1 6.4 Ϫ28.0 27.5 Ϫ8.5 Ϫ36.8 Ϫ15.9 Observationsi 5±26 Ð Ϫ6.3 to Ϫ9.2 Ϫ22.2 to Ϫ 42.2 7.6±16.5 Ϫ2toϪ11 Ϫ10 to Ϫ34 Ϫ22

a This transport corresponds to the basinwide equatorward movement out of our region of interest. Since the model excludes transport through the Bering Sea, this value can be used as a proxy for the IPT and be compared to observations of the IPT to validate the model. b This transport corresponds to the equatorward movement of the south Paci®c subtropical gyre. For mass balance, Paci®c Ocean across 32ЊS ϩ Tasman Sea ϩ Paci®c Ocean across 39ЊS ϩ EAUC (after separation) must equal the vertical mixing between layers 5 and 6 (which is very small). c Transport through the Tasman Sea at 44ЊS. d Meridional transport between the coast of Australia and 154.8ЊEat29ЊS. e Zonal transport between 31Њ and 36ЊS at 168.2ЊE. f Alongshore transport between coast and 36.0ЊS, 178.9ЊE. Positive corresponds to northwestward; negative corresponds to southeastward. g Alongshore transport between coast and 33.6ЊS, 176.5ЊE. Positive corresponds to northwestward; negative corresponds to southeastward. h Alongshore transport between coast and 40.0ЊS, 178.6ЊE. Positive corresponds to northward; negative corresponds to southward. i Observations from all studies mentioned in the text. Note that the observed transports at 32ЊS are actually observations of IPT transport.

The ®rst states that New Zealand blocks the westward- 1980; Ou and De Ruijter 1986), and the presence of propagating Rossby waves south of 34ЊS, causing the New Zealand is unimportant. EAC to separate at this latitude instead of much farther Transport streamfunctions from a simulation (Fig. 5) south (Warren 1970). The second states that the coast- where New Zealand has been removed (RG2) demon- line geometry of Australia (i.e., the bend in the coastline strate that the removal of New Zealand does not alter at Sugarloaf Point) causes the separation (Godfrey et al. the separation latitude of the EAC. In a linear simula-

FIG. 5. Mean transport streamfunctions from the 1/8Њ linear 1.5-layer global simulation RG2 with New Zealand removed from the ¯ow ®eld. Note that the EAC separation point is still between 30Њ and 34ЊS, in agreement with observations and RG1.

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FIG. 6. Annual mean wind stress curl ®eld derived from Hellerman and Rosenstein (1983). Note the large negative curl in the Antarctic region. The gray line represents the zonal wind stress curl integrated from the western coast of South America to the eastern coast of New Zealand or Australia. The white vertical line represents the zero of the integrated wind stress curl. Note that there is no zero in the integrated wind stress curl in the vicinity of New Zealand. It is always positive anticyclonic in the Southern Hemisphere with a maximum of 9.8 ϫ 10Ϫ6 (Pa ϫ 1Њ)mϪ1. tion, complete separation of a western boundary current Godfrey (1994) (12.1 versus 15 Sv) but still larger than from the coast occurs at the latitude of the zonally in- observations. Analysis of the mass balance of this region tegrated zero wind stress curl line integrated from the shows four pathways available for water to enter or exit eastern boundary to the western boundary segment. This the region (Fig. 7): the Tasman Sea, the basinwide trans- separation can be complicated by land masses that par- port at 32ЊS, the transport of the South Paci®c subtrop- tially block gyres and by gradients in the wind stress ical gyre east of New Zealand at 39ЊS, and the western that give zonal currents but do not act as outer-gyre boundary current along New Zealand (i.e., the EAUC). boundaries. There is no zero wind stress curl line (Fig. Since net vertical mixing is small compared to the hor- 6) that would cause the EAC to completely separate izontal transports in this latitude band (32Њ to 39ЊS, from the coast of Australia. A gradient, however, exists marked in Fig. 7), the residual of the mass balance of in the zonally integrated wind stress curl (shown as a the transports along the three other pathways determines gray line in Fig. 6) that causes a zonal ¯ow eastward the direction of the EAUC. If the non-EAUC transport at 34ЊS, providing a southern boundary to a nested gyre into the region (equatorward ¯ow of the South Paci®c within a larger gyre. The separation points of the EAUC subtropical gyre across 39ЊS) is larger than that out of (37ЊS) and the ECC (42ЊS) also correspond to gradients the region (transport across 32ЊS ϩ Tasman Sea) then in the zonally integrated wind stress curl. Zonal currents the EAUC must ¯ow southeastward (out of the latitude at these latitudes are seen at the same latitudes, even band) to achieve mass balance. If the transport into the after New Zealand has been removed (Fig. 5). The par- region is less than the transport out, the EAUC must be tial separation of the EAC can be explained by the re- northwestward (into the latitude band). From Table 3, sponse of the ocean to the wind ®eld with New Zealand we can see that, if the EAUC is to ¯ow southeastward present or removed, suggesting that New Zealand does in RG1, the southward ¯ow through the Tasman Sea not affect the separation point, aside from the island's must be less than 9.0 Sv [25.5 (transport across 39ЊS) effect on the wind ®eld itself. Since a reduced gravity Ϫ16.5 (transport across 32ЊS) ϭ 9.0 Sv]. Since the sim- model produces the correct EAC separation point, bot- ulated ¯ow through the Tasman Sea in RG1 is larger tom topography and coastline details of Australia must than this value and the observed value, the simulated also have little effect on the separation latitude. EAUC ¯ows in the incorrect direction. The reversed direction of the EAUC in the linear Decreasing the ¯ow through the Tasman Sea in the simulation RG1 can be linked to higher than observed linear simulation, to agree with observed values by in- southward transport through the Tasman Sea. The sim- creasing the local horizontal friction within the Tasman ulated transport is slightly less than that of the linear Sea (RG3) (Fig. 8a), produces an EAUC that ¯ows model of Godfrey (1989), discussed in Ridgway and southeastward, connecting the two northern inner gyres

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FIG. 7. Illustration of the mass balance of the region encompassing transport through the Tasman Sea, the Paci®c Ocean at 32ЊS, the Paci®c Ocean east of New Zealand at 39ЊS, and the East Auckland Current. seen in Fig. 4b. The EAUC can be divided into two ¯ow ®eld to the north of New Zealand and the meanders ¯ow regimes: 1) before separation and 2) after sepa- in the Tasman Front. ration. Before separation corresponds to the alongshore transport passing between the northern tip of New Zea- b. Nonlinear dynamics: Flat-bottom case land and 33.6ЊS, 176.5ЊE. After separation corresponds to the alongshore transport passing between the eastern A 6-layer nonlinear 1/8Њ simulation with a ¯at bottom coast of New Zealand and 36.0ЊS, 178.9ЊE, after the (FB) driven by HR winds is employed to examine ver- majority of the EAUC has turned offshore, leaving a tical structure and nonlinear dynamics. Mean surface much weaker ¯ow attached to the coast (see Fig. 1). currents calculated from the last four years of the sim- The linear EAUC from RG3 has a magnitude of 6.5 Sv ulation (Fig. 9a) reveal ¯ow meanders in the Tasman before separation and 0.9 Sv after separation, in agree- Front and the relatively persistent eddies observed by ment with the lower bound of observations. The weaker Roemmich and Sutton (1998) north and east of New attached ¯ow continues around the East Cape and joins Zealand. The simulated surface currents reveal an elon- with a westward onshore ¯ow to produce an ECC of gated eddy corresponding to the observed NCE and ed- 6.5 Sv. The separation point of the EAUC agrees well dies to the north and southeast of East Cape correspond- with observations and is situated at a sharp gradient in ing to the ECE and WE. The simulated meanders in the the zonally integrated wind stress curl (Fig. 6). Tasman Front have wavelengths (ϳ410 km) that agree Since the model domain has no transport through the with the observational values of 300±500 km (Stanton Bering Sea, all of the northward transport across the 1979; Andrews et al. 1980; and Mulhearn 1987) al- basin at 32ЊS must leave the basin through the IPT. Since though their amplitude and phase do not. estimates of the transport through the IPT vary from Stanton (1976) postulates that the wavelengths of the Ͻ5 to 18.6 Ϯ 7 Sv (Godfrey 1996), an incorrect choice meanders over the Norfolk Ridge might correspond to of a simulated IPT might conceivably affect the mass those of a westward-propagating Rossby wave, although balance of our region. However, varying the magnitude there was not suf®cient data at the time for a de®nite of the IPT, and therefore the transport through 32ЊS, has conclusion. Andrews et al. (1980) found that the wave- no effect on the EAUC or ECC. Shutting off the ¯ow lengths required for a Rossby wave with the observed through the IPT (RG4) does not change the total ¯ow propagation speed of the meanders were much larger out of the region (Fig. 8b); it merely diverts the ¯ow than those observed and suggested that nonlinearities through the Tasman Sea via an increased EAC (42.4 were affecting the ¯ow. Both Stanton (1979, 1981) and versus 25.9 Sv) with no change in the EAUC or the Mulhearn (1987) mention the possibility of topographic ECC. in¯uences on the meanders, although Mulhearn (1987) The failure of the linear model to produce the correct notes that he was unable to ®nd a clear indication of direction of the EAUC may be in part due to its inability the mechanism. to simulate isopycnal outcropping, which is discussed Several other authors have examined the dynamics in the next section. Although the lowest-order response behind stationary meanders associated with a separating of the ocean to the wind stress can explain the direction western boundary current. Ou and De Ruijter (1986) and separation of most of the major currents in our argued that the exact separation point of a western region of interest, it fails to reproduce the complicated boundary current was a function of the transport within

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FIG. 8. Mean transport streamfunctions from the 1/8Њ linear 1.5-layer global simulations: (a) RG3, in which the horizontal friction in the Tasman Sea has been increased. Note the correct ¯ow direction of the EAUC and correct separation points of all currents, except for the unobserved southward ¯ow along the east coast of South Island. The small perturbation in the southern Tasman Sea is an effect of the friction patch and not of any changes in the winds. (b) RG4, in which the IPT has been closed off. Note the increased transport through the Tasman Sea and the continued northwestward direction of the EAUC.

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FIG. 9. Mean surface currents from the 1/8Њ nonlinear global simulations: (a) FB, the ¯at-bottom 6-layer simulation. The most probable Tasman Front pathway described in Fig. 2 is represented by the gray line in both ®gures. The mean currents were calculated from the last four years of the simulations. Note the mean meanders in the Tasman Front and the eddies north of New Zealand. The simulated meanders show poor agreement in phase with those observed and decrease in amplitude eastward along the Tasman Front contrary to observations. The ECC separates just north of the Chatham Rise as observed, even though there is no bottom topography to guide the ¯ow. (b) RG5.5, the 5.5-layer reduced-gravity simulation. Note the lack of regular meanders, such as those found in FB, in the Tasman Front.

Unauthenticated | Downloaded 09/30/21 08:02 AM UTC 2930 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 the current and the shape of the coastline. When the coastline slopes away from the separation point (similar to the orientation of the Australian coastline), the bound- ary current continues southward after separation and then retro¯ects to return to the latitude of initial sepa- ration, forming meanders that are then governed by con- servation of absolute vorticity. Campos and Olson (1991) and Da Silveira et al. (1999) were both able to produce mean meanders in a zonal separation jet in both barotropic and baroclinic nonlinear simulations. Both sets of authors also found that the separation point and amplitude of the meanders were greatly in¯uenced by the shape of the coastline. Holland (1978), using a qua- sigeostrophic 2-layer model with a straight western boundary did not reproduce meanders, demonstrating that some sort of asymmetry such as a sloped coastline is required to produce meanders. The simulated meanders of the ¯at-bottom model (Fig. 9a) are approximately stationary, with the ®rst meander extending southward and retro¯ecting north- ward, consistent with Ou and De Ruijter (1986), Campos and Olson (1991), and Da Silveira et al. (1999). The wavelengths of the meanders correspond to the most probable path discussed by Mulhearn (1987) and Stan- ton (1979) but the amplitudes or phase do not. The amplitudes of the simulated meanders decrease as the ¯ow proceeds away from the western boundary as would be expected in an inertial ¯ow, but in direct dis- agreement with observations. The wavelengths of both simulated (ϳ410 km) and observed meanders (300±500 km) are less than those of a stationary Rossby wave FIG. 10. Longitude±time plots of sea surface height deviations (cm) pattern or constant absolute vorticity (CAV) trajectory at 33ЊS from (a) RG5.5, (b) FB, and (c) RB8. These plots were (Haltiner and Martin 1957). For small amplitude tra- calculated from the last two years of the simulations. Note the west- jectories, stationary wavelengths in CAV trajectories are ward propagation of the signal in the reduced-gravity simulation and related to the characteristic velocity of the ¯ow by the stationarity in the two ®nite-depth simulations.

␷ a longitude±time plot of sea surface height deviation ␭ ϭ 2␲ c (4) ␤ (Fig. 10). These differences suggest that, while the me- Ί anders within the reduced gravity model can be ex- where ␷ c is the characteristic velocity at the core of the plained by CAV theory, other dynamics must be present current and ␭ is the stationary wavelength. in FB. The inclusion of the bottom boundary and, there- The observed and simulated stationary wavelengths fore the barotropic mode, allows the formation of bar- are less than the stationary wavelength (600 km) pre- oclinic instabilities involving the barotropic mode with- dicted from the depth-averaged velocity of the top ®ve in FB. The appearance in FB of meanders with wave- layers of FB (0.17 m sϪ1). According to CAV theory, lengths shorter than those predicted by CAV theory in- the simulated meanders should propagate eastward, not dicates that ¯ow instabilities are shortening the remain stationary as observed. When the bottom layer wavelength of the mean meanders via vortex stretching. is constrained to be at rest, creating a 5.5-layer reduced- Evidence of the presence of baroclinic instabilities can gravity simulation (Fig. 9b), the simulated meanders be obtained by inspection of eddies formed from the have longer than predicted stationary wavelengths and meanders within the Tasman Front. The beta Rossby 2 propagate westward, consistent with CAV theory. A number, Rb ϭ ␷/␤r , is the ratio of relative to planetary long-term mean averages out these time-dependent fea- vorticity advection, where ␷ ϭ maximum swirl velocity tures, resulting in a mean ¯ow ®eld with little or no within the eddy and r ϭ mean radius of the eddy. An meanders (Fig. 9b). Interestingly, the simulated EAC in Rb of order 1 suggests that the eddy was formed by the reduced gravity simulation separates farther south barotropic instabilities involving the ®rst baroclinic and retro¯ects northward to a larger degree than FB. mode, while an Rb of order 10 suggests that it was The propagation in the reduced-gravity model and the formed by baroclinic instabilities involving the baro- stationarity in the ¯at-bottom model are clearly seen in tropic mode (McWilliams and Flierl 1979; Hurlburt and

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Thompson 1982, 1984; Murphy et al. 1999). Eddies of isopycnal outcropping in reducing eastward baro- formed during the last two years of the simulation (not clinic transport south of New Zealand. As mentioned in shown) were characterized by ␷ ഠ 35 cm sϪ1 and r ഠ section 3a, any change in the transport through the IPT

47 km, resulting in Rb ഠ 7.9, suggesting that the eddies directly affects the transport at 32ЊS and through the and, therefore, the meanders were formed by baroclinic Tasman Sea. An increased IPT would reduce the south- instabilities. ward (or increase the northward) transport through the Baroclinic instabilities may also be identi®ed by the Tasman Sea but have little effect on the EAUC or the wavelength of the meanders within the ¯ow ®eld, since ECC. The introduction of isopycnal outcropping, how- the growth rates of disturbances due to baroclinic in- ever, signi®cantly alters the nested gyres seen in the stabilities are functions of the disturbance wavelength. linear simulations. Isopycnal outcropping in the Ant- Although the ¯ow ®eld is constantly exposed to dis- arctic region in response to the large negative wind turbances of different wavelengths, it is predisposed to stress curl (Fig. 6) reduces the transport passing south the propagation and growth of a limited range of wave- of New Zealand. The depths of the mean interfaces lengths. Within a long-term mean of the ¯ow ®eld, sta- south of Australia and New Zealand (Table 4) are at a tionary disturbances with large growth rates will dom- minimum along the coast of Antarctica, indicating sig- inate. Mean meanders produced by baroclinic instabil- ni®cant outcropping. Within the model, an outcropped ities will most likely exhibit the wavelength of these layer does not result in a zero thickness, but instead a disturbances. Tilburg (2000), using an approach similar minimum thickness. When all ®ve upper layers in the to Phillips (1954) and Haidvogel and Holland (1978), hydrodynamic model outcrop (as is the case for the analyzed the susceptibility of the mean ¯ow within FB majority of the Antarctic region), only the nonsteric to baroclinic instabilities by introducing to the mean contribution to the barotropic transport remains, result- layered vorticity equations a disturbance and solving for ing in drastically reduced transport in the upper ®ve the growth rate. He found that the average wavelength layers. The mean interface depths at the coast of New within the simulated Tasman Front corresponded to a Zealand are much shallower than those at the coast of stationary, unstable disturbance, providing further evi- Australia (Table 4), allowing less transport south of New dence that the meanders are due to the presence of bar- Zealand than south of Australia, forcing transport north- oclinic instabilities. ward through the Tasman Sea. The simulated directions and separation points of the Returning to the linear simulation of the triple nested investigated currents in FB generally agree with obser- subtropical gyre system of the South Paci®c (Fig. 4a), vations. Notably, the EAUC ¯ows in the correct direc- we see the impact of the reduced transport south of New tion, while the ECC separates at 42ЊS and forms a broad Zealand on other current systems. Focusing on the mid- band of eastward ¯ow. However, the ¯ow through the dle-nested gyre in the southeast Paci®c (represented by Tasman Sea in FB is no longer southward but is mostly the dark green contour in Fig. 4a) we see ¯ow associated con®ned to the western boundary and travels northward with this gyre passing south of New Zealand. As this with a transport of 5.3 Sv. The con®nement of a north- ¯ow is reduced, more of it must pass north of New ward ¯ow to the western boundary agrees with obser- Zealand, impacting the transport through the Tasman vations. Huyer et al. (1988), using both current meters Sea and the current system that comprises the Tasman and CTD/XBT surveys along the continental margin, Front, the EAUC, and the ECC. Since in FB the net found predominantly northward ¯ow along the Austra- transport through the Tasman Sea is northward, a portion lian shelf but southward ¯ow in the interior of the Tas- of the supergyre transport feeding the IPT no longer man Sea. The simulated surface ¯ow is northward along passes south of New Zealand but instead ¯ows north the coast and slightly southward throughout the rest of through the Tasman Sea and along the Tasman Front. the Tasman Sea, although the combined ¯ow is north- To achieve mass balance in the latitudinal band de- ward, not southward as observed by Huyer et al. (1988) scribed in Fig. 7, the EAUC now ¯ows southeastward and others (Chiswell et al. 1997 and Ridgway and God- along the coast of North Island. The EAUC has a mean frey 1997). The northward ¯ow through the Tasman Sea magnitude of 24.6 Sv before separation and 13.5 Sv combines with the EAC as it separates from the coast after separation, both of which are signi®cantly larger of Australia and ¯ows eastward, producing signi®cantly than those produced in the modi®ed linear case (RG3) larger transports along the Tasman Front (31.6 Sv) than but still within the range of observational values. the linear simulations or observations. Although some Clearly the model overestimates the effect of isopyc- transports greater than 30 Sv have been observed along nal outcropping in this region, but it does reproduce the the Tasman Front due to recirculation of the ¯ow by correct direction of the EAUC, suggesting that the dy- eddies, time-averaged transports are much smaller namics of isopycnal outcropping, if not the magnitude (Stanton 1981). The simulated Southland Current also of its simulated effect, are correct. The exaggerated ef- does not agree with observations. fect of this outcropping on the transport of the ACC has Two mechanisms may contribute to the simulated been noted earlier. Shriver and Hurlburt (1997), using northward transport through the Tasman Sea: unrealis- a lower-resolution version of this model, found that the tically high IPT transport and exaggeration of the effect simulated transport of the ACC through the Drake Pas-

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TABLE 4. Shown are mean interface depths (m) at land boundaries south of Australia and New Zealand. Note that the interface depths at the coast of Australia are greater than those at New Zealand, providing greater transport south of Australia than New Zealand and forcing northward transport through the Tasman Sea. The initial depths of the mean interfaces of RB8 and RB16 are different from those of FB (i.e. interface 5 for FB is 1000 m, while interface 5 for RB8 and RB16 is 1500 m). These differences affect the extent of isopycnal outcropping on the interfaces and, therefore, the available transport south of New Zealand and Australia. Antarctica New Zealand Antarctica at Australia at Experiment Interface at 167.84ЊE at 167.84ЊE 146.92ЊE 146.92ЊE 1 51 107 51 212 2 91 268 91 383 FB 3 131 515 130 647 4 170 785 170 951 5 210 1140 210 1171 1 51 157 52 231 2 92 363 92 375 RB8 3 132 711 132 719 4 172 1234 172 1143 5 212 1718 212 1701 1 52 137 52 227 2 93 300 93 371 RB16 3 135 679 133 724 4 176 1143 174 1150 5 218 1581 214 1676 sage was much less than observed values and attributed and depth-integrated agreement with observations. The this difference to the effect of the modeled isopycnal introduction of the bottom topography greatly reduces outcropping. the northward transport through the Tasman Sea (0.1 The linear model RG1 does not reproduce isopycnal versus 5.3 Sv), while increasing the IPT transport (13.7 outcropping and therefore must be modi®ed to repro- versus 10.5 Sv). These changes combine to produce a duce the reduced ¯ow (RG3) through the Tasman Sea reduced transport in the Tasman Front (23.1 versus 31.6 and the observed direction of the EAUC. A model that Sv). Although mass balance of the region still requires incorporates isopycnal outcropping (FB) does reproduce the EAUC and ECC to proceed southward, the decrease the observed direction of the EAUC. The ¯at-bottom in the Tasman Sea transport gives a smaller transport simulation also produces meanders with wavelengths in both the after separation EAUC (3.8 versus 13.5 Sv) that agree with observations but phases and amplitudes and the ECC (11.7 versus 14.9 Sv). The simulated that do not. Southland Current ¯ows in the observed direction, re- vealing that its location is determined by bottom to- c. Nonlinear dynamics: Realistic bottom topography pography. case The major differences between FB and RB8 are the incorporation of bottom topography, which introduces A 6-layer nonlinear 1/8Њ simulation with realistic bot- a bottom layer that is constrained to follow f/h contours, tom topography (RB8) is used to investigate the role of and the change in the initial depth of the mean inter- upper-ocean±topographic coupling in this region. Mean faces. The introduction of bottom topography has two surface currents superimposed on bottom topography (Fig. 11) show that the simulation reproduces stationary major rami®cations: 1) reduction of baroclinic instabil- meanders (Fig. 10) in the eastern portion of the Tasman ities and 2) bottom steering. When the bottom layer is Front (meanders E, F, and G, in Fig. 2), the phase of constrained by topography, it cannot freely adjust to which more closely agrees with observations, revealing perturbations and become out of phase with the surface the importance of upper-ocean±topographic coupling. layer. This restriction reduces the distribution of baro- The simulated EAC separates slightly farther south than clinic instabilities and their potential for occurrence. FB (35Њ versus 32ЊS) and creates a large retro¯ection Baroclinic instabilities convert potential energy of the similar to the reduced gravity simulation. RB8 also re- mean ¯ow to eddy potential energy, effectively ¯atten- produces all three quasi-stationary eddies and the low ing mean isopycnals. Since transport is related to the observed by Roemmich and Sutton (1998) north and change in depth of isopycnals via the thermal wind re- east of the North Island of New Zealand. In RB8, the lation, this ¯attening restricts the transport south of New simulated surface ECC separates from the coast of New Zealand. The greater depth of the original interfaces in Zealand and ¯ows eastward south of the Chatham Rise RB8 (1500 m for interface 5) compared to FB (1000 m rather than north of the rise as seen in observations and for interface 5) mitigates some of the effect of isopycnal the other simulations. A summation of the ®ve top layers outcropping and provides greater potential for transport (not shown) shows a zonal current at the observed lat- south of New Zealand and Australia. The decrease of itude of 42ЊS, revealing the baroclinic nature of the ¯ow baroclinic instabilities and the increase in initial inter-

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FIG. 11. Mean surface currents from the 1/8Њ nonlinear 6-layer simulation, RB8 superimposed on the model bottom topography (m). Note the correct placement of the meanders in the eastern portion of the Tasman Front and the presence of the eddies north of New Zealand as well as reduced ¯ow through the Tasman Sea compared to FB.

(١h1, (5 ´ ١h12ϭ v g ´ face depth result in much deeper ispoycnals south of v1g Australia and New Zealand in RB8 than FB (Table 4). where v is the geostrophic velocity in layer k. The left- The relative deepening of the isopycnals allows more kg hand side of (5) is the geostrophic contribution to the transport south of New Zealand, and, consequently, de- advective term in the continuity equation (3) for layer creases the northward ¯ow through the Tasman Sea. The inclusion of bottom topography can substantially 1. Additionally, the geostrophic balance of the internal affect surface currents that do not directly impinge on mode in a two-layer model is given by (the topography. Hurlburt and Metzger (1998) found in- k ϫ f(v Ϫ v ) ϭϪgЈ١h , (6 dications of the Shatsky Rise topography affecting the 1g 2g 1 bifurcation of the Kuroshio. This steering is due to the where gЈϭg(␳ 2 Ϫ ␳1)/␳ 0. When the surface currents abyssal currents advecting upper-layer thickness gra- are much larger than the abyssal currents ( | v1g | k dients and, therefore, surface currents. | v 2g | ), which is typically the case in a two-layer model,

١h1 can be used as an approximate Conservation of potential vorticity is a suf®cient con- Eq. (6) indicates that straint to allow even deep low-amplitude topographic measure of v1g. From this observation and the combined features to steer the abyssal ¯ow. Hurlburt and Thomp- effects of (5) and (6), we see that the abyssal currents son (1980, 1982, and 1984) used the continuity equation advect upper-layer thickness gradients and, therefore, to demonstrate how upper-ocean currents can be driven steer upper-layer currents. Hurlburt et al. (1996) showed by abyssal ¯ow. In a two-layer model it can be shown that, although this theory formally breaks down in the that the two-layer velocities are linked by multilayer case, this steering effect remains when the

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FIG. 12. (a) Mean surface currents from the 1/16Њ nonlinear 6-layer simulation, RB16 superimposed on the model bottom topography (m). Note the larger meanders in the Tasman Front and the presence of the NCE and WE. The

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®rst baroclinic and barotropic modes dominate the ¯ow in the Tasman Front suggests that the simulation has regime, a valid assumption in our region of interest. reached a convergent solution, and the ¯ow features are As discussed in the previous section, the presence of not functions of resolution. The disagreement between baroclinic instabilities in FB regulates the wavelength the two simulations in the region north of New Zealand of the meanders in the Tasman Front. The effect on the (Figs. 12a and 13) should be expected since within this meander wavelength of the decrease of baroclinic in- region both simulations exhibit a large degree of vari- stabilities due to the inclusion of bottom topography in ability and should not be considered inconsistent with RB8 is compensated by bottom steering, resulting in the convergence of the simulations. meanders that are similar in wavelength to those in FB The simulated sea surface height variability of RB16 but have a phase that now agrees more closely with qualitatively agrees with TOPEX/Poseidon altimeter observations. Inspection of the bottom topography (Fig. data (Fig. 14). The peak values of variability in the EAC 2) beneath the Tasman Front reveals a series of ridges and New Zealand region (the separation point of the and valleys, which constrain the abyssal ¯ow and affect EAC, along the Tasman Front, and directly north and the surface ¯ow. Since the presence of bottom topog- east of North Island) are colocated in the simulations raphy does not allow the abyssal layer to freely adjust and the observations and can be directly linked to phys- and create meanders solely due to baroclinic instabili- ical phenomena. The large variability in the region of ties, the wavelengths of the meanders are not directly the EAC separation is due to both the seasonal migration determined by the presence of the instabilities them- of the separation point and eddy shedding, while the selves, although the instabilities may provide a length large amount of variability along the Tasman Front in- scale for the meanders. Instead, the ¯ow instabilities dicates signi®cant movement of the meanders. The large contribute energy to the abyssal layer providing greater variability north and east of North Island can be linked bottom steering, which determines the wavelengths. to the presence of the quasi-stationary eddies and me- Increasing the resolution of RB8 to 1/16Њ (RB16) andering of the EAUC and ECC. (Fig. 12a) signi®cantly increases the amplitude of the Since the model is hydrodynamic and forced by meanders in the entire Tasman Front and produces me- monthly a wind stress climatology and the actual ocean anders in the western portion (meanders A, B, C, and is subject to seasonal thermal forcing and high-fre- D, in Fig. 2) that agree quite well in phase, amplitude, quency winds, the overall variability of the model is and wavelength with observations. Comparison of the less than that measured by TOPEX/Poseidon. Lower mean sea surface deviations of the model and the most modeled SSH variability in locations such as east of probable path from Fig. 2 (Fig. 12b) demonstrates that Tasmania and within the southern section of our region the simulation resolves each observed meander. The in- of interest is expected. However, while the simulated creased resolution in RB16 increases the northward variability north of New Zealand agrees qualitatively transport through the Tasman Sea (6.4 versus 0.1 Sv) with observations, it is signi®cantly higher than obser- with little effect on the IPT and strengthens the EAUC vations. The disagreement between the model and ob- (8.5 versus 3.8 Sv) and the ECC (15.9 versus 11.7 Sv). servations can be linked to the model's inability to ad- The quasi-stationary eddies corresponding to the Wair- equately resolve the transport south of New Zealand and arapa Eddy and the NCE are evident, but in the mean Australia. As discussed in section 3b, the reduction of the ECE and the low have merged. Uddstrom and Oien ¯ow south of New Zealand due to the model's insuf- (1999) were also unable to detect a robust ECE in their ®cient vertical resolution in that region forces northward SST ®elds, indicating a high amount of variability in ¯ow through the Tasman Sea, resulting in an unrealis- this region. The closer agreement of the simulated me- tically large current associated with the Tasman Front anders with observations suggests stronger upper- and therefore the EAUC. Instabilities and meanders ocean±topographic coupling in RB16 than in RB8. within these unrealistically large currents result in larger Preliminary results from a further increase in reso- than observed SSH variability as seen in Fig. 14. Ex- lution to 1/32Њ (Fig. 13) shows very little difference amination of the SSH variability of the lower-resolution between the 1/16Њ and 1/32Њ simulations in the vicinity RB8a (not shown) reveals less variability throughout of the Tasman Front. This simulation was initialized our region of interest and much less variability north from simulation RB16 and extended 8 years at 1/32Њ and east of New Zealand. The decreased variability resolution and, hence, is not completely equilibrated at north and east of New Zealand in RB8a is consistent this resolution but is suf®ciently equilibrated to show with our explanation of the origin of the SSH variability, the impact of the increase in resolution on ¯ow insta- since RB8a contains much smaller transports within the bilities and therefore the mean meanders within the Tas- Tasman Front, EAC, and ECC. The location of the ob- man Front. The agreement between the two simulations served ECE is a region of extremely high variability in

←

ECE has merged with the low, obscuring both features. (b) Most probable path of the Tasman Front superimposed on the mean sea surface height deviation (cm) of RB16. Note the close match between the observed and simulated Tasman Front.

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FIG. 13. Mean surface currents from a 1/32Њ nonlinear 6-layer simulation, RB32 superimposed on the model bottom topography (m). Note the good agreement between the RB16 and RB32 in the vicinity of the Tasman Front but poorer agreement in the vicinity of the highly variable region north of North Island.

the model and observations, providing a possible ex- suggests that barotropic instabilities are probable in this planation for the mixing of the low and the ECE in region. RB16. Although mixed barotropic±baroclinic instabilities Maps of surface and abyssal eddy kinetic energy are very ef®cient mechanisms for transferring energy (EKE) from RB8 and RB16 (Fig. 15) demonstrate that from the surface layers to the abyssal layers, other mech- the energy in both the surface and abyssal layers in- anisms may also drive abyssal EKE, such as vertical creases with this increase in grid resolution. The in- mixing or direct barotropic forcing by temporal varia- creased resolution in RB16 allows greater ¯ow insta- tions in the wind stress. Very little vertical mixing be- bilities and an increased rate of energy transfer to the tween the surface and abyssal layers occurs in the area abyssal layers via these instabilities. Hogan and Hurl- of the Tasman Front and New Zealand, although a sig- burt (2000) provide a clear discussion and illustration ni®cant amount does occur south of New Zealand. The of the vertical transfer of energy. A mean value of Rb collocation of EKE maxima in the surface and abyssal for eddies produced by RB16 is 6.9, which is slightly layers combined with the increase in variability with lower than FB, suggesting that the eddies' formation resolution is clear evidence that instabilities are present may be due to mixed barotropic±baroclinic instabilities. (Holland and Lin 1975) and that they, and not a baro- In fact, latitude±time plots of the meridional derivative tropic response to the seasonal wind variations, are the of the barotropic absolute vorticity (dQ/dy)at165ЊE major source of abyssal EKE (Hurlburt and Metzger (Fig. 16) show that the derivative changes sign fre- 1998). The increase in baroclinic instability in RB16 quently within our domain, which is a necessary (but over RB8 reduces the depth of the isopycnal trough not suf®cient) condition for barotropic instability and north of the ACC as a result of the increased transfer

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FIG. 14. Sea surface height variability (cm) obtained from (a) RB16 and (b) TOPEX/Poseidon altimeter data. Both show large variability at the EAC separation point. RB16 exhibits large variability of the ¯ow ®eld north and east of North Island. While the observed data show qualitative agreement in the variability pattern of the SSH around North Island, it is much lower in amplitude.

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FIG. 15. Mean surface-layer eddy kinetic energy (EKE) of (a) RB8 and (b) RB16 and abyssal-layer EKE of (c) RB8 and (d) RB16. The contour interval is 0.25 m2 sϪ2 (log). Note the change in color bar between surface and abyssal layers. The energies of both layers increase as the resolution increases and the surface- and abyssal-layer EKE are colocated, both signs of baroclinic instabilities. between the potential energy of the mean ¯ow to the pographic features in steering the surface ¯ow ®eld. Two eddy potential energy and consequent isopycnal ¯atten- separate areas merit discussion: the meanders of the ing (reduced range of temporal mean isopycnal depths Tasman Front and the apparent bottom steering of the in the region). This ¯attening reduces the mean iso- ECC over the Hikurangi Trench. As seen in Fig. 12, pycnal depths at the southern tips of Australia and New many surface features appear to be coupled with par- Zealand (Table 4), more so at the southern tip of New ticular topographic features. Topographically driven Zealand, which extends farther south (Fig. 2). In all cases abyssal currents intersecting with surface currents at the isopycnals outcrop at Antarctica (reach the minimum large angles force the surface currents to be displaced depth allowed in the model). Since all other factors are in the direction of the abyssal ¯ow where the ¯ows invariant in RB8 and RB16, the relative magnitude of overlay (Hurlburt and Thompson 1980, 1982, 1984). the baroclinic transport through sections is determined The surface currents tend to return to the same latitude by the isopycnal depth ranges across the sections. Thus, after the perturbation by the abyssal currents, consistent the results in Table 4 show greater restriction on the with potential vorticity conservation. Abyssal currents baroclinic ACC transport south of New Zealand in RB16 superimposed onto the sea surface height contours (Fig. and therefore increased northward transport through the 17) show that all meanders, with the exception of the Tasman Sea in accord with Table 2. ®rst, of the Tasman Front are associated with meridional The link between abyssal energy and upper-ocean± abyssal ¯ow on the side of a ridge or a trough. The ®rst topographic coupling can be clearly seen. Large meander is consistent with the separation overshoot and amounts of abyssal energy are collocated with bottom retro¯ection described by Ou and De Ruijter (1987). steering of upper-ocean currents such as the meanders Two meanders discussed by Stanton (1979), the large in the Tasman Front (Figs. 11, 12, and 15). RB16 with southward meander located at the New Caledonia a larger amount of abyssal energy and, therefore, more Trough (166ЊE, meander F) and the northward meander energetic abyssal currents, produces more bottom steer- over Norfolk Ridge (167ЊE, meander G) are both driven ing and larger meanders than RB8. The increased var- by meridional ¯ows along ridges. Abyssal currents also iability and barotropic±baroclinic instabilities enable drive meanders documented by Mulhearn (1987) that more upper-ocean±topographic coupling in the higher- are associated with the Dampier Ridge (159ЊE, meander resolution simulation. For this reason, we use these sim- B) and Lord Howe Rise (161ЊE, meander E). While the ulations to investigate in detail the role of several to- directions of meanders A, C, E, F, and G might be ex-

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FIG. 16. Latitude±time plot of normalized dQ/dy at 165ЊE from RB16, where Q is the barotropic absolute vorticity. Note the change in sign, a necessary condition for barotropic instabilities. pected from conservation of potential vorticity of a bar- From examination of the simulations we are able to otropic ¯ow, Stanton (1979) clearly shows that the ¯ow determine the governing dynamics behind several fea- associated with the Tasman Front is highly baroclinic. tures of the ¯ow ®eld surrounding New Zealand. Upper- Also, the southward meanders located directly east of ocean±topographic coupling via baroclinic instabilities Dampier Ridge (meander B) and on the western side of governs the phase and amplitude of the mean meanders the Lord Howe Rise (meander D) are opposite in di- in the Tasman Front. The ¯at-bottom nonlinear model rection to those predicted by conservation of potential and even the linear model with the added friction patch vorticity. However, the corresponding simulated me- produce the correct directions and separation points for anders are directly above southward abyssal ¯ow along all examined currents. The shape of the wind stress curl the ridge, which forces the surface ¯ow to form south- and not bottom topography determines the separation ward meanders. points of these currents. At ®rst glance, the ¯ow of the ECC appears to be governed by the Hikurangi Trough. It separates very 4. Summary and conclusions near to the location of the Chatham Rise and apparently follows this ridge line. In fact, the separation point in Global ocean models with increasing dynamical com- the realistic bottom topography simulations is the same plexity are used to investigate the dynamics of the East as the ¯at-bottom model. As mentioned in section 3a, Australian Current, the Tasman Front, and the ¯ow ®eld the separation is governed by the integrated wind stress directly north and east of New Zealand. The simulations curl and not bottom topography. have horizontal resolutions of 1/8Њ, 1/16Њ, or 1/32Њ for

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FIG. 17. Mean abyssal-layer currents (black arrows) from the 1/16Њ nonlinear 6-layer simulation with realistic bottom topography, RB16, superimposed on mean sea surface deviations (cm) in the vicinity of the Tasman Front. Note the steering of the surface-layer meanders by the abyssal ¯ow. each variable and vertical resolutions ranging from 1.5- drup mass transport between two gyres de®ned by Sver- layer reduced gravity to 6-layer ®nite depth with real- drup ¯ow. The Tasman Front is the southern arm of the istic bottom topography. All simulations are forced by innermost gyre bounded on the west by Australia. The the Hellerman and Rosenstein (1983) monthly wind separation latitude of the simulated linear EAC agrees stress climatology and were spun up to statistical equi- well with observations, with New Zealand present or librium at 1/2Њ and 1/4Њ resolutions before continuing removed, suggesting that the separation point is due to at the higher resolutions. the shape of the wind stress curl and not the presence Analysis of these simulations demonstrates that sev- of New Zealand or bottom topography. Although the eral factors play a critical role in governing the behavior linear model reproduces the large-scale features well, it of the EAC, the Tasman Front, the EAUC, and the ECC. overestimates the southward transport through the Tas- These factors include 1) mass balance of water pathways man Sea and produces an EAUC ¯owing opposite to through the area, 2) gradients in the wind stress curl that observed, suggesting that the Sverdrup response of ®eld, 3) nonlinear ¯ow instabilities, and 4) upper-ocean± the ocean cannot fully describe the ¯ow in our region topographic coupling due to mixed baroclinic and bar- of interest. The ¯ow through the Tasman Sea is directly otropic instabilities. The increased variability of the in- linked to the direction of the EAUC. Mass balance dem- creased-resolution simulation contributes greatly to to- onstrates that the southward ¯ow through the Tasman pographic steering of surface currents that do not di- rectly impinge on the bottom topography. Sea must be less than the difference between the bas- The transport streamfunctions for a linear reduced inwide ¯ow at 32ЊS and the northward ¯ow east of New gravity model representing the ®rst-order response of Zealand at 39ЊS to simulate an EAUC ¯owing in the the ocean to the wind stress show a series of nested observed direction. Decreasing the ¯ow through the Tas- gyres in our region of interest. A supergyre spans the man Sea by increasing the local horizontal friction pro- entire Southern Hemisphere; a smaller gyre encom- duces a modi®ed linear EAUC ¯owing in the correct passes the South Paci®c Ocean; and three even smaller direction. With this decrease all examined currents ¯ow gyres are bounded on the west by Australia and New in the correct direction and separate from the coastlines Zealand. The EAC and ECC are western boundary cur- at the observed locations. However, a simulation in- rents of the gyres, while the EAUC provides non-Sver- corporating vertical structure is required to investigate

Unauthenticated | Downloaded 09/30/21 08:02 AM UTC OCTOBER 2001 TILBURG ET AL. 2941 the dynamics that modify the transport through the Tas- ing the ¯ow south of Australia and New Zealand. This man Sea. disagreement affects the direction of the ¯ow through A nonlinear 6-layer ¯at-bottom simulation produces the Tasman Sea and the magnitude of the EAUC and currents ¯owing in the observed direction and separat- Tasman Front, resulting in larger than observed SSH ing from the coastlines at the observed latitudes. It also variability north of New Zealand, complicating the anal- produces meanders in the Tasman Front, the observed ysis north of New Zealand and increasing the uncer- quasi-stationary eddies north and east of New Zealand, tainty of the conclusions. An increase in vertical reso- and isopycnal outcropping. The wavelengths of the sim- lution by the addition of more layers to the model would ulated meanders in the Tasman Front agree well with mitigate this effect. observations, but their phase and amplitude do not. The wavelengths are shorter than those predicted by constant Acknowledgments. This work is a contribution to the absolute vorticity theory, indicating that ¯ow instabil- 6.1 Project ``Thermodynamic and Topographic Forcing ities (particularly baroclinic instabilities), via vortex in Global Ocean Models'' sponsored by the Of®ce of stretching, determine the wavelength. The simulated Naval Research under program element 601153N. The transport through the Tasman Sea is northward, in dis- computations in this paper were performed on AHPCRC, agreement with observations, resulting in a transport ARSC, CEWES, and NAVO Cray T3Es using grants of associated with the Tasman Front that is larger than computer time from the Defense Department High Per- observed. This northward ¯ow occurs because isopycnal formance Computing Modernization Of®ce. The 1/16Њ outcropping in the ACC reduces the transport that passes global simulation was performed under the FY97 DoD south of New Zealand. Although the model seems to HPC Challenge project ``1/16Њ Global Ocean Model'' on overestimate this effect on eastward transport south of the CEWES T3E and using early access on the NAVO New Zealand, isopycnal outcropping does contribute to T3E. The 1/32Њ simulation was started using early access the correct direction of the EAUC. time provided by Cray on a 1536 processor Cray T3E Including bottom topography in the 1/8Њ model pro- and completed under an FY98-00 DoD HPC Challenge duces meanders in the eastern portion of the Tasman project ``Global and Basin-Scale Ocean Modeling and Front with phases that agree well with observations. Bot- Prediction'' on the NAVO T3E. CET received ®nancial tom steering of the surface currents compensates for the support for this work from the Department of Defense effect on meander wavelength of the decrease in baro- through a National Defense Science and Engineering clinic instabilities, resulting in meanders with similar Graduate Fellowship. The ONR Physical Oceanography wavelengths to the ¯at-bottom simulation. This simula- Program provides a Secretary of the Navy Grant to JJO tion reproduces all observed eddies north and east of New as the base support for the Center for Ocean±Atmospheric Zealand. It also reproduces all examined currents. Prediction Studies (COAPS). We would like to thank Increasing the resolution of the simulation to 1/16Њ Alan Wallcraft and Joseph Metzger for their model de- increases the surface variability and the amount of en- velopment work and computational assistance, Gregg Ja- ergy generated in each layer. The geographical collo- cobs for supplying the TOPEX/Poseidon-derived sea sur- cation of peaks in surface and abyssal EKE coupled face height variability, and Dean Roemmich for supply- with increased energy with increased resolution suggests ing the observed climatological dynamic sea surface that instabilities are responsible for the downward trans- height data. We would also like to thank the reviewers port of energy. Calculations of beta Rossby numbers for their helpful comments and suggestions on this man- and analysis of dQ/dy latitude±time plots show that the uscript. instabilities are most probably mixed barotropic±baro- clinic. The increased variability increases abyssal cur- APPENDIX rents and, therefore, the upper-ocean±topographic cou- pling, resulting in larger meanders in the Tasman Front. These meanders agree well with observed wavelengths, Explanation of Symbols and Notations phases, and amplitudes, but the ECE has merged with (F cos␪)ץ F 1ץ an observed low in the mean and the northward transport 1 F ϭϩ␾␪ ´ ١ ␪ץ ␾ a cos␪ץ through the Tasman Sea is increased. Preliminary results a cos␪ from a 1/32Њ resolution model qualitatively agree with ⌽ץץ ⌽2 1ץ the 1/16Њ model, suggesting that the simulations have 1 achieved a convergent solution. The separation of the ٌ⌽ϭ2 ϩ cos␪ ␪ץ ␪ץ ␾ 2a 2cos␪ץ a22cos ␪ ECC just north of the Chatham Rise seems at ®rst glance ΂΃ to be a result of bottom steering, since it follows the A ϭ coef®cient of isopycnal eddy viscosity eastward extension of bottom topography. In fact, the a ϭ radius of the earth (6371 km) separation of ECC is due to the wind stress curl ®eld. Cb ϭ coef®cient of bottom friction The vertical resolution of the model results in poor Ck ϭ coef®cient of interfacial friction agreement with observed transports in the vicinity of CM ϭ coef®cient of additional interfacial friction the ACC since all ®ve layers outcrop, severely restrict- associated with entrainment

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D(␾, ␪) ϭ total depth of the ocean at rest Gklϭ g(␳ nϩ1 Ϫ ␳ko)/␳ l Յ k, ek ϭ angular deformation tensor Gklϭ g(␳ nϩ1 Ϫ ␳lo)/␳ l Ͼ k,

,␷ H ϭ constant ץukk ץ e ϭϪcos␪ ϭϪe n ␪ ΂΃cos␪ ␪␪kץ ␾΂΃cos␪ץ ␾␾k ␶ kwϭ ␶ k ϭ 0,

,u ␶ kkϭ C ␳ 0|vkkϪ v ϩ1|(vkkϪ v ϩ1) k ϭ 1...n ץ␷ kk ץ e␾␪k ϭϩcos␪ ϭ e ␪␾k ,␪ ΂΃cos␪ ␻k ϭ 0 k ϭ 0ץ ␾΂΃cos␪ץ ␻ ϭ max(0, ␻ϩϪ) Ϫ max(0, ␻ ) Gkj ϭ gjՆ k kk k

Gkjϭ g Ϫ g(␳ kϪ ␳ j)/␳ o j Ͻ k Ϫ hkk␻Ã k ϭ 1...n. g ϭ acceleration due to gravity

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